Vertical Farming and CEA HVAC Duct Guide — VPD Management, Dehumidification, CO₂ Enrichment for Indoor Agriculture

Why vertical farming HVAC is fundamentally different

A commercial office building runs at roughly 50 to 80 watts per square metre of cooling load on a peak summer day. A vertical farm leafy-greens grow zone runs at 400 to 700 watts per square metre, year round, 18 hours a day. A cannabis flower room runs at 800 to 1200 watts per square metre, with ten times the latent load of an office. The HVAC architecture cannot simply be scaled comfort cooling — it has to manage four interlocking variables simultaneously, on tighter tolerances than any other commercial sector, and it has to do it for crops that turn HVAC failure into immediate yield and revenue loss.

Those four interlocking variables are temperature, humidity, carbon dioxide concentration and air movement. Get any one of them wrong and the others stop working as designed. Vapour Pressure Deficit (VPD) — the headline number in modern controlled-environment agriculture (CEA) — is a derived value from temperature and humidity, and it shifts the moment either changes. CO₂ setpoints fall apart if the air movement does not mix the room. Air movement at the canopy is what allows the leaf to actually offload water vapour and absorb CO₂, regardless of what the bulk room reading says. The HVAC ductwork is the physical transport layer for all four of these variables, which is why CEA HVAC duct design is its own discipline rather than a vertical of comfort HVAC.

This guide consolidates the design framework SBKJ engineers use when consulting on ductwork specification for vertical farms, greenhouses, cannabis cultivation facilities and tissue-culture propagation rooms. It sits alongside the related pharma and biotech cleanroom guide, the cold storage and cold-chain guide, and the duct insulation guide. Each of those interacts with CEA design at the boundary — propagation tissue-culture rooms run at cleanroom standards, post-harvest processing runs at cold-chain standards, and CEA insulation tolerances are tighter than comfort cooling because thermal bridges drive condensation events.

The CEA segments and why they each need different HVAC

Controlled-environment agriculture is not a single industry — it is a family of related industries that all use indoor climate control to grow plants, but the HVAC profile of each one is distinct. Designing a vertical lettuce farm and a cannabis flower facility with the same duct system fails both. The four major CEA segments are:

Vertical farms — leafy greens, herbs and microgreens

Multi-tier rack systems, typically 5 to 12 tiers high, growing lettuce, basil, kale, microgreens and culinary herbs under LED at 200 to 400 micromoles per square metre per second PPFD with photoperiods of 14 to 18 hours. Cycle times are short — 14 to 35 days from seed to harvest depending on crop. The HVAC profile features moderate sensible loads (400 to 700 W/m²), high latent loads from dense canopy transpiration, and a strong demand for tight CO₂ distribution because the leafy crops are CO₂-limited at moderate light. Operators in this segment include AeroFarms (US, UAE), Plenty (US), Bowery Farming (US), Infarm (Germany), 80 Acres Farms (US), Square Roots (US), Mirai (Japan), Crop One Holdings (US, with the Bustanica facility in the UAE — the world's largest vertical farm at 30,845 square metres), Stacked Farm (south-east Queensland, Australia), SproutStack (NSW, Australia) and InvertiGro (Sydney, Australia).

Greenhouses — tomato, cucumber, capsicum, strawberry

Single-storey glasshouse or polytunnel structures, typically 2 to 30 hectares per facility, with supplemental LED or HPS lighting and active climate control. The HVAC profile features lower volumetric heat density than vertical farms (because the structure has more area and more glass), but much higher solar gain variability and dehumidification challenges driven by transpiration plus envelope leakage. Most modern greenhouses run hybrid systems with pad-and-fan evaporative cooling for shoulder seasons and refrigerated dehumidification with sensible heat recovery for tight VPD windows. Tomato and cucumber prefer 0.5 to 1.2 kPa VPD and tolerate wider temperature ranges than vertical-farm leafy greens.

Cannabis cultivation — recreational and medical

The most HVAC-intensive segment. Cannabis flowering runs 12-hour photoperiods at 600 to 1000 micromoles per square metre per second PPFD, demanding tight 0.8 to 1.5 kPa VPD windows that vary across the 8 to 10 week flower cycle. Total cooling loads of 800 to 1200 W/m² are typical, with latent loads peaking at 1.0 to 1.5 kg of water per hour per square metre during weeks 4 to 6 of flower. Odour control via carbon filtration on exhaust is non-negotiable in most jurisdictions because terpenes are detectable at parts-per-billion. Australian licensed cultivators include Little Green Pharma in Perth, Cann Group in Melbourne, Aphria-Tilray's Mildura facility, and Cannatrek in Shepparton — each with multi-zone HVAC architectures sized for high-PPFD flower rooms. North American operators face the same engineering envelope at much higher facility scale.

Tissue culture and propagation

The cleanroom-adjacent segment. Tissue culture lab plus propagation chambers run at HEPA H13 filtration, ISO Class 7 to 8 cleanroom standards, with low-PPFD lighting (50 to 100 micromoles per square metre per second) and very tight temperature control (plus or minus 0.5 degrees Celsius). The HVAC ductwork specification overlaps significantly with the pharma cleanroom guide — sealed seams, terminal HEPA modules, controlled cascade pressurisation. Tissue culture is the parent stock for everything downstream so contamination risk is the dominant design driver, not energy efficiency.

Vapour Pressure Deficit — the central HVAC challenge in CEA

Vapour Pressure Deficit (VPD) is the difference between the saturation vapour pressure at leaf temperature and the actual vapour pressure of the air around the leaf, expressed in kilopascals. It is the single most important environmental parameter in modern CEA because it drives transpiration directly. A correctly held VPD means the plant transpires at its target rate, taking up nutrients, regulating leaf temperature, and growing on the breeding curve the variety was selected for. A VPD that drifts too high (dry air) drives transpiration past the root supply rate, causing leaf curl, calcium deficiencies and tip burn. A VPD that drifts too low (humid air) shuts transpiration down, causing nutrient stalls, condensation events and disease pressure from Botrytis, powdery mildew and Pythium.

The HVAC system controls VPD by simultaneously holding dry-bulb temperature within plus or minus 0.5 degrees Celsius and relative humidity within plus or minus 3 percent of setpoint. That is a tighter envelope than most comfort cooling systems can deliver, and it has to be held continuously across photoperiod and dark-period transitions when the latent load can swing by 70 percent in 30 minutes. Typical VPD targets across CEA segments are:

• Lettuce and leafy greens: 0.6 to 1.0 kPa during photoperiod, 0.4 to 0.6 kPa during dark period. Romaine and butterhead are at the lower end, baby leaf and microgreens at the higher end.
• Basil and culinary herbs: 0.7 to 1.1 kPa during photoperiod. Higher than lettuce because basil is heat-tolerant and benefits from stronger transpiration.
• Tomato: 0.5 to 1.2 kPa, with seedling stage at the lower end and fruit-bearing at the higher end. Greenhouse operators run wider tolerances than vertical-farm operators.
• Cucumber: 0.4 to 0.8 kPa. Cucumbers are humidity-loving and a low VPD reduces fruit cracking.
• Capsicum and pepper: 0.7 to 1.2 kPa. Similar to tomato but tighter on the upper limit during flowering.
• Cannabis seedling and clone: 0.4 to 0.8 kPa. Low VPD because root systems are not yet developed enough to support high transpiration.
• Cannabis vegetative: 0.8 to 1.1 kPa. Mid-range to drive vigorous growth without stressing the plant.
• Cannabis flower (early, weeks 1–3): 1.0 to 1.3 kPa. Higher VPD initiates flower set and reduces stretch.
• Cannabis flower (peak, weeks 4–6): 1.2 to 1.5 kPa. Highest VPD of the cycle because transpiration is at peak and Botrytis pressure is highest.
• Cannabis flower (late, weeks 7–9): 1.0 to 1.4 kPa. Slightly relaxed as plants mature and transpiration declines.
• Tissue culture and propagation: 0.3 to 0.5 kPa. Very low VPD because explants and seedlings have minimal root systems and rely on humidity for survival.

The duct system role in VPD control is twofold. First, it has to deliver tempered, dehumidified air at the volume rate the load calculation demands without short-cycling. Second, it has to maintain that air mix at the canopy via HAF fans and induction nozzles so the bulk room VPD reading actually reflects what the plant experiences. The most common failure mode in CEA HVAC is a sensor reading 1.0 kPa VPD across the room while the canopy boundary layer is sitting at 0.3 kPa because the air is not being mixed. The duct design has to anticipate this and specify HAF distribution alongside the primary supply.

Lighting heat load — the dominant cooling driver

For modern horticultural LED operating at 2.5 to 3.5 micromoles per joule efficacy, roughly 50 to 70 percent of input electrical energy converts to sensible heat at the canopy. The remaining energy splits between photosynthetically active radiation absorbed by the plant (which becomes latent heat through transpiration plus stored chemical energy in carbohydrates) and far-red and near-infrared wavelengths that pass through or reflect off the canopy.

For a leafy-greens grow zone running 200 to 300 micromoles per square metre per second PPFD over an 18-hour photoperiod with high-efficacy LED fixtures from suppliers such as Fluence, Heliospectra or Signify GreenPower, expect 250 to 450 watts per square metre of canopy heat load. A cannabis flower room running 800 to 1000 micromoles per square metre per second canopy PPFD generates 600 to 900 W/m² of lighting heat alone. These numbers drive the HVAC sizing directly. Daily Light Integral (DLI) ranges from 12 to 17 mol per square metre per day for lettuce, 17 to 25 for basil and herbs, 22 to 30 for tomato fruiting, 35 to 45 for cannabis flower — and the cooling load scales linearly with DLI for any given crop area.

The duct system has to deliver enough cooling air volume to remove this heat at the design temperature differential. For a typical cannabis flower room at 800 W/m² sensible load and a 6 degrees Celsius supply-to-room delta-T, the supply air rate works out to roughly 110 cubic metres per hour per square metre of canopy. For 1000 m² of canopy that is 110,000 m³/h or roughly 30 air changes per hour for a 4-metre-high room. Vertical-farm leafy-greens rooms run at lower air-change rates per square metre of canopy because the canopy is stacked vertically and the actual room volume is dominated by the rack structure, but the per-tier air movement requirement is similar.

Air movement at the canopy — the parameter most installers miss

Bulk room air change rate is not a useful design number for CEA. What matters is the air velocity at the leaf surface, measured in metres per second, integrated across the entire canopy. The target is 0.3 to 0.5 m/s at the leaf, because:

• Below 0.2 m/s, the boundary layer of saturated air over the leaf surface becomes thick enough to stall transpiration. The plant cannot offload water vapour even though the bulk room reads correctly. This is the classic "sensor says 1.0 kPa VPD, leaves are wet, plants are not transpiring" failure mode.
• Above 0.7 m/s, mechanical leaf damage starts to show — wind burn on leaf edges, especially on tender crops like baby leaf lettuce, basil seedlings and cannabis clones. Transpiration is also driven past root-supply rate and you see calcium deficiency symptoms within 7 to 10 days.
• The 0.3 to 0.5 m/s target has to be uniform — a hot spot at 0.6 m/s next to a dead spot at 0.1 m/s averages to a sensor reading of 0.35 m/s but neither plant population grows correctly.

The duct system role here is to deliver primary tempered and dehumidified air at room scale, then hand off to dedicated horizontal airflow (HAF) fans that handle leaf-level mixing. HAF sizing is typically 50 to 100 m³/h per square metre of canopy. HAF fans run continuously, 24 hours a day, regardless of HVAC load — they are the main mechanism for VPD uniformity and for delivering CO₂ from the supply plenum to the leaf. In multi-tier vertical farm racks, HAF fans are typically mounted at each tier shelf because the rack structure blocks room-scale airflow from reaching the canopy.

Induction nozzle diffusers — sometimes called swirl diffusers or jet diffusers — are increasingly common on vertical farm primary supply ductwork because they entrain room air into the supply jet at high ratios (5:1 to 10:1) and deliver mixed air to the canopy at controlled velocity. They cost more than standard linear or perforated diffusers but eliminate cold spots near the supply and dead spots away from it.

CO₂ enrichment — the third axis of CEA HVAC

Ambient air is roughly 420 ppm CO₂. Most CEA crops are CO₂-limited at full PPFD, meaning photosynthesis rate plateaus before light intensity does. Boosting CO₂ to 800 to 1200 ppm during photoperiod increases yield by 15 to 30 percent for leafy greens, 20 to 35 percent for tomato and cucumber, and 25 to 40 percent for cannabis. CO₂ enrichment is therefore standard practice in commercial CEA, and the HVAC duct system is the distribution layer.

CO₂ sources fall into three categories. Liquid bulk CO₂ with onsite vaporiser is the cleanest and is industry standard for cannabis and high-value vertical farm operations. Gas bottle banks (manifolded high-pressure cylinders) are common for smaller facilities. Burner offgas — captured CO₂ from natural-gas combustion — is the cheapest source and is widely used in greenhouse operations, though it requires careful scrubbing of NO_x and ethylene. Bottled and vaporised CO₂ is the only source that meets food-grade purity standards for leafy greens being marketed as premium produce.

The injection point matters more than most installers realise. Industry best practice is to inject CO₂ into the supply air plenum upstream of the diffusers, not via dedicated CO₂ lance lines into the room. Plenum injection rides on the existing air movement and self-mixes. Lance-line injection creates plumes that overshoot setpoint at one end of the room and undershoot at the other. CO₂ sensors should be located at canopy level, not in the return air duct — the return reads several minutes behind the canopy and the control loop oscillates.

For multi-tier vertical farm racks, CO₂ distribution is genuinely difficult because each tier shelf creates a separate microclimate. Dedicated horizontal airflow fans at each tier, sized 50 to 100 m³/h per square metre, force lateral mixing so the CO₂ setpoint of 800 to 1200 ppm holds within plus or minus 50 ppm across the canopy. Tight TDF flange duct seams matter here — leakage rates above 3 percent create CO₂ setpoint instability and waste expensive bottled or vaporiser-sourced CO₂. A CEA HVAC duct system with seam leakage at SMACNA Seal Class B (3 percent at 250 Pa) is acceptable for office cooling but unacceptable for CO₂-enriched grow rooms — Seal Class A (less than 1 percent at 250 Pa) is the CEA standard.

Cooling load calculation — the four-component model

Total cooling load in a CEA grow room is the sum of four components. Each component has its own time profile through the day-night cycle and the design has to accommodate the peak case for each.

Component 1 — Lighting heat load (sensible)

As covered above: 250 to 900 W/m² depending on crop intensity, present only during photoperiod. For a 12/12 cannabis cycle this is on for 12 hours and off for 12 hours. For an 18/6 leafy greens cycle this is on for 18 hours.

Component 2 — Transpirative load (latent)

The plant takes up water through roots, transports it to leaves, and evaporates it through stomata. This converts sensible heat in the room (which would otherwise raise temperature) to latent heat in the air (which raises humidity). For a fully canopied leafy-greens room transpiring at 0.5 to 1.0 kg/h/m², latent load is 340 to 680 W/m² at 2450 kJ/kg latent heat of vaporisation. Cannabis flower rooms at 1.0 to 1.5 kg/h/m² generate 680 to 1020 W/m² of latent load. This is the dominant dehumidification driver.

Component 3 — Envelope and infiltration (sensible plus latent)

Heat conducting through walls, roof and slab, plus any air infiltration through doors and seals. For well-insulated CEA buildings (R-3 walls, R-5 roof, R-2 slab insulation, vapour barrier continuous) this is typically 30 to 60 W/m² of canopy. For converted warehouses without proper envelope upgrades it can hit 100 to 150 W/m².

Component 4 — People, equipment and process loads (sensible)

Workers in the room generate roughly 100 W per person of sensible heat plus 50 W per person of latent. Pumps, fans, irrigation controllers and supplemental equipment add 20 to 50 W/m². CO₂ burner offgas (if used) adds heat too — a 50 kW burner generating CO₂ for a 1000 m² facility adds 50 W/m² of sensible heat that has to be removed.

Summing these for typical CEA scenarios:

• Vertical farm leafy greens, 250 µmol/m²/s PPFD: 350 sensible + 510 latent + 50 envelope + 30 process = 940 W/m² of canopy. Most of this is latent — dehumidification dominated.
• Vertical farm basil and herbs, 350 µmol/m²/s PPFD: 500 sensible + 600 latent + 50 envelope + 30 process = 1180 W/m².
• Cannabis flower, 800 µmol/m²/s PPFD: 800 sensible + 850 latent + 80 envelope + 50 process = 1780 W/m². The headline number cited for cannabis flower of "800 to 1200 W/m²" is typically the sensible-only figure or a partially averaged number — the full latent load doubles it.

Dehumidification — the harder half of the design

Most first-time CEA designers underspecify dehumidification by 30 to 50 percent because comfort-cooling rules of thumb suggest the cooling coil will deliver enough moisture removal as a side effect of sensible cooling. In CEA this is wrong. The latent-to-sensible ratio in a grow room is 0.5 to 1.5 — comparable to a kitchen or pool deck — and refrigerated cooling coils sized for sensible heat alone cannot reach the dewpoints required to hit 0.4 to 0.6 kPa dark-period VPD targets.

Industry-standard CEA dehumidification architectures fall into three categories:

Refrigerated dedicated outside air systems (DOAS)

Standalone air-handling units with cooling coils sized 30 to 50 percent above sensible-only sizing, plus reheat (electric or hot-gas) to deliver dry, tempered air. Most vertical farm and cannabis cultivation operations use refrigerated DOAS as the primary dehumidification stage. Typical specification is a coil delivering 3 to 5 degrees Celsius leaving air at the design point, reheated to 18 to 22 degrees Celsius for room delivery. The duct sizing has to handle the supply rate at full latent design — typically 60 to 100 m³/h per square metre of canopy.

Desiccant wheel assist

For VPD targets below 0.6 kPa (tissue culture, cannabis seedling, dark-period leafy greens), refrigerated coils alone struggle to reach the required dewpoint without ice formation. A silica-gel or lithium-chloride desiccant wheel adds capacity by adsorbing water vapour onto the wheel medium and regenerating with hot air. Desiccant wheels are common in pharmaceutical cleanrooms and the same architecture works in CEA. The duct system has to incorporate the regeneration air loop and specify the right gasket material because regeneration temperatures hit 90 to 130 degrees Celsius.

Standalone fan-coil dehumidifiers

For smaller cannabis cultivation rooms (under 200 square metres canopy), distributed fan-coil dehumidifier units mounted in the room are common. Each unit handles a defined zone, with condensate drained to a common header. This architecture trades capital simplicity for a worse VPD uniformity profile — the rooms have hot spots near each unit and cool spots away from them. For commercial-scale CEA the centralised DOAS approach is more capital-efficient and delivers tighter VPD.

The duct ramifications are significant. Dehumidification ductwork carries cold, near-saturated air (at 5 degrees Celsius and 100 percent RH the absolute humidity is 5.4 g/kg; at 20 degrees Celsius and 50 percent RH it is 7.2 g/kg). Any thermal bridge in the duct insulation creates a condensation point that drips. CEA duct insulation has to be specified as continuous closed-cell with no fibrous gaps — the duct insulation guide covers this in detail. Rectangular duct corners need particular attention because the surface-to-volume ratio at the corner is higher and the corner is the first to drop below dewpoint.

Filtration and pathogen control

Filtration grade in CEA is driven by the segment more than by general HVAC convention. The four levels of filtration commonly specified are:

MERV 13 to MERV 15 supply filtration (general grow rooms)

MERV 13 (equivalent to ePM1 50% under EN ISO 16890) is the baseline for most leafy-greens vertical farms and tomato/cucumber greenhouses. It removes pollen, mould spores and most pathogenic bacteria. MERV 14 to 15 is the preferred specification when the facility is in an urban area with significant outdoor particulate (PM2.5 above 25 µg/m³ annual average). The duct system sizing has to account for the filter pressure drop (typically 100 to 200 Pa clean, doubling at end-of-life) — undersized fans will fail to deliver design airflow at filter end-of-life.

HEPA H13 (tissue culture, propagation, clean rooms)

Tissue culture lab and propagation rooms run at HEPA H13 (99.95 percent at MPPS) terminal filtration. The duct system upstream of the HEPA should be sealed to a high standard because any leakage downstream of the pre-filter and upstream of the HEPA bypasses the filtration strategy. Stainless-steel ductwork is preferred in tissue culture spaces because it is wash-down compatible and does not shed galvanizing dust. SBKJ supplies spiral round in 304L stainless for these applications. Cross-reference the pharma cleanroom guide for full cleanroom duct architecture.

Carbon filtration (cannabis cultivation, terpene odour control)

Cannabis cultivation operates under nuisance odour regulations in most jurisdictions because terpenes are detectable at parts-per-billion concentrations and travel kilometres on prevailing winds. Standard practice is granular activated carbon filtration on the exhaust air from flower rooms, with carbon loadings of 50 to 100 kg per 1000 m³/h of exhaust. Replacement intervals are typically 6 to 18 months depending on terpene loading. The exhaust ductwork has to be sized for the carbon filter pressure drop (300 to 500 Pa) and the duct material has to handle the slightly elevated humidity at the exhaust point.

UV-C in-duct (pathogen knockdown)

UV-C lamps installed in the supply duct downstream of the cooling coil add a pathogen-inactivation stage. UV-C output of 0.5 to 1.5 W per cubic metre per hour of supply air is industry standard for pathogen knockdown on Botrytis, Pythium and powdery mildew. The duct has to have an inspection port for lamp replacement, and the inner duct surface near the lamps should be UV-resistant (galvanized G90 is fine; some painted finishes degrade). UV-C is increasingly common in commercial CEA as an additional layer of pathogen control without resorting to chemical fumigation.

Pest exclusion and pressurisation cascade

Insect intrusion is one of the leading causes of unrecoverable crop loss in commercial CEA. Thrips, aphids, whitefly, spider mites and fungus gnats all enter via outside air intakes, door openings and unsealed envelope penetrations. The HVAC duct strategy for pest exclusion has three layers:

• Insect screening at outside-air intakes: 50 to 100 mesh stainless steel screens upstream of the OA filter bank. Suppliers like Crop Defenders and Phytotronics specialise in CEA-grade intake screens. Mesh selection is a trade-off — 50 mesh excludes thrips but adds 30 to 50 Pa pressure drop; 100 mesh adds 80 to 120 Pa and excludes smaller pests but doubles fan power consumption.
• Pressurisation cascade: grow rooms held at +5 to +15 Pa relative to corridors, corridors at +5 Pa relative to outside, exhausted areas (composting, waste rooms) at -5 to -10 Pa. The pressure cascade ensures that any leakage flow is from clean to dirty, never the reverse. The duct sealing class has to be tight enough to maintain this — Seal Class A or EN 12237 Class C is standard.
• Airlock entries: double-door vestibules between corridor and grow room, with both doors interlocked and a 30-second pause between cycles. The HVAC ductwork has to deliver enough makeup air to the airlock to recover positive pressure within the door cycle.

Cannabis cultivation adds a fourth layer — pheromone monitoring and integrated pest management protocol. The HVAC design influences pest pressure indirectly via humidity control (high humidity favours fungus gnats and powdery mildew), and the duct surfaces should be smooth and easily inspected so that pest infestations in the duct interior can be spotted and treated. Internal duct lining is generally avoided for this reason — bare metal is easier to inspect than fibreboard or fabric.

HVAC architecture options — room-based, rack-mounted and hybrid

The duct architecture for a CEA facility falls into three main families. Each suits different facility scales and crop types.

Room-based HVAC with overhead supply

The simplest architecture. A primary DOAS unit feeds rectangular supply trunks running overhead, with linear or induction-nozzle diffusers delivering tempered, dehumidified air at the canopy level from above. HAF fans handle leaf-level mixing. Return is via low sidewall grilles or floor returns. This architecture works well for cannabis flower rooms (single canopy layer) and greenhouse operations. It is the dominant architecture for facilities under 2000 square metres and for any facility where the canopy is one or two tiers deep.

SBKJ machinery for this architecture: SBAL-V galvanized auto duct line for the rectangular trunk, SBTF spiral tubeformer for round branch ducts, and TDF flange for sealed connections. Insulation is closed-cell elastomeric on the cold side, FRK-faced for vapour barrier integrity.

Rack-mounted dedicated airflow per tier

For high-density vertical farms with 6 to 12 tier rack systems, room-based HVAC cannot deliver airflow to the inner tiers — the outer tiers shadow them aerodynamically. The solution is dedicated supply ductwork running vertically through the rack system, with induction nozzles or perforated supply at each tier shelf. AeroFarms, Plenty, Bowery, 80 Acres and Bustanica all use variants of this architecture. Each tier is treated as its own microclimate with localised supply, return and HAF.

The duct sizing is fundamentally different — round spiral runs vertically with branch take-offs at each tier shelf, sized for 6 to 10 m/s velocity. The spiral tubeformer is essential here because rectangular ductwork creates flow disturbances at the tier branches that hurt distribution uniformity. SBTF spiral plus a high-quality flange system delivers the airflow profile that multi-tier rack farming requires.

Hybrid architecture

Most large commercial CEA facilities use a hybrid: primary DOAS at room scale providing tempering, dehumidification, CO₂ enrichment and outside air dilution, plus per-rack or per-zone recirculation fan-coils handling sensible cooling and leaf-level mixing. The hybrid splits the design problem into two stages — the DOAS handles the slow-moving variables (humidity, CO₂, fresh air) and the recirculation handles the fast-moving variable (temperature). Each stage can be tuned independently, which is the main reason hybrid architecture has become dominant for commercial-scale operations.

Cannabis cultivation specifics

Cannabis cultivation has the most demanding HVAC envelope of any CEA segment. Beyond the high heat loads and tight VPD windows already covered, there are four areas where cannabis HVAC duct design diverges from leafy greens:

Odour control and exhaust treatment

Terpenes (myrcene, limonene, caryophyllene, pinene and others) are detectable at single-digit parts-per-billion concentrations and are subject to local nuisance odour regulations in most jurisdictions. Standard practice is granular activated carbon filtration on the exhaust air from flower rooms. The exhaust duct system sizing must accommodate the carbon filter pressure drop (300 to 500 Pa fresh, 600 to 800 Pa end-of-life) and the duct material at the terpene-saturated exhaust side must be either galvanized G90 with confirmed gasket compatibility or 304L stainless. EPDM gaskets degrade over 24 to 36 months in terpene-saturated exhaust ductwork; silicone is preferred.

Pest management and integrated protocol

Cannabis crops are vulnerable to spider mites, thrips, hemp russet mites, fungus gnats and root aphids. Pesticide options are tightly regulated, so prevention via HVAC and envelope is the dominant control strategy. The duct system has to support integrated pest management — smooth interior surfaces, no fibrous lining, accessible inspection ports every 6 metres on supply trunks, and exclusion screens at all OA intakes and exhaust make-up paths.

Power budget and electrical infrastructure

A 1000 square metre cannabis flower facility runs at roughly 1.5 to 2.0 MW electrical demand at peak (lighting, HVAC, dehumidification, controls, irrigation, processing). The HVAC fraction is 30 to 45 percent of total electrical demand. Designers have to budget tightly because most cannabis facilities operate on demand-charge tariffs where peak kW drives 50 percent of the electricity bill. Duct sizing for low pressure drop (3 to 5 m/s on trunks rather than the 6 to 8 m/s typical in office HVAC) trades capital cost for fan-power saving — for a 1000 m² flower room this typically pays back in 18 to 36 months.

Materials and sanitation

Cannabis is a regulated agricultural product and most jurisdictions require the cultivation facility to meet GMP-adjacent sanitation standards. The supply ductwork should be inspectable and cleanable, which favours larger trunk sizes (over DN 400 round equivalent) and removable diffusers. Some Australian, Canadian and US-state cannabis regulators have moved toward requiring 304L stainless steel for any duct segment downstream of a humidifier or in contact with any nutrient-bearing aerosol — a specification that drives unit cost up roughly 4x compared to galvanized but is non-negotiable in those jurisdictions.

Materials selection and gasket compatibility

The default duct material for commercial CEA is galvanized G90 steel — 0.6 mm to 1.0 mm gauge depending on diameter and pressure class. G90 (275 g/m² total zinc coating, both sides) is the standard for HVAC applications and provides 15 to 25 years of service life in typical CEA grow-room conditions. The exceptions where higher-grade materials are required:

• 304L stainless steel: Required for any duct segment downstream of an evaporative humidifier, in cannabis cultivation rooms with peak humidity above 75 percent RH for sustained periods, in tissue culture lab spaces (cleanability), and in fish farming or aquaponics integrated rooms. SBKJ supplies SBTF spiral tubeformer in 304L for round duct, and SBAL-V auto duct line in 304L for rectangular.
• 316L stainless steel: Required for marine-coast CEA installations (within 5 km of saltwater), high-chloride municipal-water humidification, and aquaponics fish culture rooms with sustained chloride loading above 200 ppm.
• Galvanneal: An alternative to G90 for paint-finish requirements (rare in CEA but appears in some greenhouse architectures with painted exterior finishes).
• Aluminium: Occasional for weight-sensitive rack-mounted ductwork in vertical farm tier installations, though corrosion resistance is poor in high-humidity grow rooms and most operators prefer galvanized.

Gasket and sealant selection is more complex than most installers appreciate. CEA grow rooms have specific contamination concerns that drive material choice:

• Silicone gaskets: Preferred over EPDM in cannabis cultivation because EPDM outgasses low-molecular-weight hydrocarbons that interact with terpene profiles. Silicone is also preferred in food-grade vertical farm applications under BRC certification.
• EPDM gaskets: Acceptable for general leafy-greens vertical farms and greenhouse operations. Lower cost than silicone, 10-year service life under typical grow-room conditions.
• Butyl mastic: Used in TDF flange sealing. Confirm low-VOC formulation for any food-grade or cannabis facility.
• Fibreboard or fibrous gaskets: Avoid in CEA. Fibre-shedding contaminates produce and creates pathogen harbourage points.

Energy efficiency and heat reuse

CEA HVAC is energy-intensive — 30 to 45 percent of total facility electricity use, and roughly 0.4 to 1.2 kWh per kilogram of leafy greens produced (vertical farm) or 1.5 to 4.5 kWh per gram of dried cannabis flower. The financial pressure on this load is significant and drives several energy-recovery strategies that influence duct design.

Chiller waste heat recovery

The condenser side of the chiller serving the cooling coil rejects roughly 1.2 to 1.4 times the cooling delivered. That heat is at 35 to 55 degrees Celsius and can be reused for reheat in the dehumidification stage, for hot-water demand in the facility, or for heating the propagation lab during cooler months. The duct system role is to integrate the reheat coil into the supply path with appropriate insulation and condensate management.

Mechanical Vapour Recompression (MVR)

For high-condensate operations (cannabis flower at peak transpiration generating 1500 kg/day of condensate), mechanical vapour recompression can recycle the latent heat of vaporisation from the condensate back into the system. MVR is more common in greenhouse operations than vertical farms but is starting to appear in large commercial vertical-farm DOAS designs.

Night-time free cooling and outside-air economiser

In Melbourne, Sydney, southern NSW, central Victoria and most of New Zealand, the outdoor temperature is below 18 degrees Celsius for at least 4000 hours per year. During these hours, an outside-air economiser can reduce cooling load to near zero — the duct system just has to filter, dehumidify (or humidify, depending on outdoor dewpoint) and deliver outside air directly. The economiser duct sizing is typically the same as the primary supply (since you have to handle peak load with mechanical cooling) but the OA damper and pre-filter sizing become critical because you are running at full OA fraction for thousands of hours per year.

Integration with HVAC&R refrigeration

Some commercial CEA facilities co-locate cold-chain post-harvest cooling with growing operations. The post-harvest cool rooms reject heat to the same condenser water loop as the grow rooms, and the integration can deliver 10 to 15 percent system COP improvements. The ductwork has to support the dual operation — supply ductwork to grow rooms, separate exhaust ductwork from cool rooms, and condensate drain integration. The cold storage and cold-chain guide covers the cold-chain side of this integration in detail.

Vertical farm operators globally — facility profiles

CEA is a young industry but the operator landscape has consolidated rapidly over 2022 to 2026 as the first wave of unprofitable concept facilities exited and the second wave of proven operating models scaled. The leading commercial vertical farm operators globally:

• AeroFarms — US-headquartered with operations in Newark NJ (the original 6500 m² facility) and Abu Dhabi (Al Quoz, opened 2022). Aeroponic mist delivery, 12-tier rack system, leafy greens and herbs.
• Plenty — California-based, with the Compton vertical farm at 8400 m² serving Southern California retail. Vertical tower architecture with rotating canopy. Strawberry expansion underway in Virginia.
• Bowery Farming — New Jersey-based with sites in Maryland, Pennsylvania and Texas. Indoor smart farms growing baby leafy greens and herbs for major US grocery retail.
• Infarm — Berlin-headquartered with modular farm units distributed across European supermarkets and central farms in Germany, Denmark, France and the UK. Restructured 2023 toward central farms.
• Crop One Holdings — Boston-based, joint venture with Emirates Flight Catering operating Bustanica in Dubai — at 30,845 m² the world's largest operating vertical farm, supplying Emirates inflight catering and UAE retail.
• 80 Acres Farms — Cincinnati-based, with the Hamilton Ohio facility at 6500 m² automated end-to-end. Leafy greens and tomatoes for US Midwest retail.
• Square Roots — Brooklyn-headquartered with farms colocated at Gordon Food Service distribution centres across the US Midwest. Modular shipping-container farm architecture.
• Mirai — Japan-based pioneer of commercial vertical farming. Multiple farms across Japan, with the Kashiwa facility producing 10,000 lettuce heads per day from 2300 m².
• Farm.One — New York-based specialty herbs and edible flowers operator serving Manhattan restaurants. Smaller-scale operation, ultra-premium positioning.
• Bustanica — Dubai, operated by Crop One Holdings and Emirates Flight Catering. Largest vertical farm globally as of 2026 at 30,845 m², producing over 1000 tonnes of leafy greens annually.

Australian vertical farm operators:

• Stacked Farm — South-east Queensland, leafy greens and herbs for east-coast Australian retail. Proprietary multi-tier rack system with automated seeding, transplanting and harvesting.
• SproutStack — New South Wales, modular vertical farm pods serving Sydney metropolitan retail.
• InvertiGro — Sydney-based vertical farm technology and operations company, with R&D and pilot facilities in metropolitan Sydney.

Australian licensed cannabis cultivators with significant indoor cultivation:

• Little Green Pharma — Perth, WA. Medical cannabis cultivation with indoor flower rooms.
• Cann Group — Melbourne, VIC. Indoor cultivation and processing for medical cannabis market.
• Tilray (Aphria-Tilray) — Mildura, VIC. One of Australia's largest licensed indoor cannabis cultivation facilities.
• Cannatrek — Shepparton, VIC. Hybrid indoor-greenhouse cultivation and extraction.

Each of these operators has a different HVAC profile depending on facility scale, crop mix, climate and capital strategy. The duct architecture varies but the underlying engineering envelope — VPD control, latent load management, CO₂ distribution, leaf-level airflow — is consistent across the industry.

Greenhouse versus vertical farm cooling cost comparison

The economic case for vertical farming versus greenhouse production is highly dependent on cooling cost, which is highly dependent on local climate, electricity tariff and crop pricing. The key trade-offs:

• Greenhouse cooling load: 200 to 600 W/m² peak summer, varying with solar gain, latitude and crop. Annual cooling energy 80 to 200 kWh/m². Primary cooling via pad-and-fan evaporative (cheap) plus refrigerated dehumidification (expensive).
• Vertical farm cooling load: 400 to 1200 W/m² continuous, with no diurnal variation. Annual cooling energy 2500 to 7500 kWh/m². Cooling is 100 percent refrigerated.
• Greenhouse heating load: 100 to 300 W/m² peak winter at temperate latitudes. Annual heating energy 100 to 400 kWh/m². Often gas-fired.
• Vertical farm heating load: Effectively zero in commercial operation because the lighting waste heat exceeds envelope loss for most of the year — the building runs in cooling mode continuously.

The duct sizing implications: greenhouse HVAC trunks are sized for peak summer dehumidification rather than peak winter heating, with seasonal turn-down on the OA economiser side. Vertical farm HVAC trunks are sized for continuous peak operation with no seasonal turn-down. Vertical farms therefore have higher duct capital cost per square metre of canopy but more predictable operating profiles. The choice between the two formats is dominated by land cost, electricity tariff and target retail price — not by HVAC engineering preference.

SBKJ machinery for CEA HVAC duct production

Three SBKJ machine families are particularly suited to CEA HVAC duct fabrication. Each addresses a different segment of the CEA duct system requirement.

SBAL-V galvanized auto duct line

The SBAL-V auto duct production line in galvanized G90 is the workhorse for general CEA grow-room rectangular supply trunks. Single-shift output of 800 to 1500 metres of duct per day depending on size mix, with TDF flange formed integrally to the duct length. Tolerances meet AS/NZS 4254 Class A, SMACNA HVAC duct standards and EN 1505. Standard wired for 380V/50Hz Australian grid; export configurations for 480V/60Hz also available. The TDF flange seal class achieves SMACNA Seal Class A (less than 1 percent leakage at 250 Pa) which is the CO₂-retention standard for CEA grow rooms.

SBTF spiral tubeformer

The SBTF spiral tubeformer handles round duct production for high-volume CEA branches and trunks. Spiral seam construction is preferred over rectangular for CEA round duct because the spiral seam is inherently tighter (Seal Class A baseline) and the round profile delivers better airflow uniformity for vertical-farm rack distribution. SBTF range covers DN 80 to DN 1500 round in galvanized G90 or 304L stainless. Single-shift output 200 to 600 metres per day depending on diameter.

TDF flange roll former

For CO₂-retention applications, TDF flange (also called Mez flange) is the preferred connection because the formed flange profile and gasket retention groove deliver SMACNA Seal Class A by default. SBKJ ships TDF flange machinery integrated into the SBAL-V line or as a standalone unit for retrofit installations. Confirm gasket selection — silicone for cannabis and food-grade applications, EPDM for general leafy greens and greenhouse use.

Construction phase and food-grade fitout

Most commercial CEA facilities are built in two phases. Phase 1 is the shell-and-core construction — foundation, structure, envelope, primary mechanical and electrical infrastructure. Phase 2 is the food-grade or pharmaceutical fitout — duct interior cleaning, sanitation surface installation, BRC or GMP-compliant finishes, validation and commissioning.

For BRC food-safety compliance (most premium leafy-greens vertical farms target BRC Global Standard for Food Safety), the duct system specification has additional requirements: smooth internal surfaces, no fibrous insulation in contact with supply air, removable diffusers for cleaning access, condensate drains piped to dedicated waste systems (not commingled with hand-wash drains), and documented cleaning schedules for the duct interior. The construction sequence has to plan duct interior protection from the moment of fabrication — sealed end caps on every duct length, no on-site storage of open ductwork, and final-clean validation before commissioning.

For cannabis cultivation under GMP-adjacent regimes (US-state regulated, Australian TGA, Canadian Health Canada), similar principles apply with additional requirements for personnel access controls, pesticide-residue testing zones and chain-of-custody documentation. The duct system role is mostly to support the sanitation and access requirements through smooth interior surfaces, accessible inspection ports and removable diffusers.

Validation and commissioning — the last 10 percent that determines success

A correctly specified CEA HVAC duct system can still fail commissioning if the validation steps are skipped or rushed. The five validation activities every CEA project should run:

Psychrometric mapping

Measure dry-bulb temperature and relative humidity at a 9-point grid per zone, at canopy level, at three points in time across the photoperiod-dark cycle. Document VPD at each grid point. Acceptance criterion: VPD within plus or minus 0.1 kPa of setpoint at all 9 points across the cycle. Failure modes: hot spots near supply diffusers, dead spots away from HAF fans, condensation events at the dark-period transition. Remedy: HAF fan repositioning, supply diffuser balancing, dehumidifier setpoint adjustment.

CO₂ distribution mapping

Measure CO₂ concentration at the same 9-point grid as the psychrometric map, plus 3 additional points at canopy edges and corners. Acceptance criterion: CO₂ within plus or minus 50 ppm of setpoint at all points during steady-state photoperiod operation. Failure modes: lance-line stratification, plenum injection imbalance, return-air sensor lag. Remedy: switch to plenum injection if not already in place, relocate sensors to canopy level, increase HAF fan capacity in dead zones.

Light recipe verification

Measure PPFD at canopy level using a calibrated quantum sensor (LI-COR LI-190R or equivalent) at the same 9-point grid. Acceptance criterion: PPFD within plus or minus 5 percent of setpoint at all 9 points. The HVAC duct system intersects this validation because hot spots in the room can affect LED driver thermal protection (some drivers throttle at 50 degrees Celsius) and produce uneven canopy PPFD that the lighting design did not anticipate.

Duct leakage testing

Pressurise each duct trunk to 250 Pa above atmospheric and measure leakage rate. Acceptance criterion: SMACNA Seal Class A (less than 1 percent leakage), or EN 12237 Class C (leakage less than 0.009 m³/s/m² at 250 Pa). Failure modes: TDF flange gasket misalignment, transverse joint sealant gaps, hanger penetrations. Remedy: re-tape and re-seal at fail points, replace failed gaskets. Document the leakage test as part of the commissioning record — this becomes critical evidence in any later dispute over CO₂ setpoint instability.

Operational integrated commissioning

Run the full crop cycle for a minimum of one growth cycle (14 to 35 days for leafy greens, 8 to 10 weeks for cannabis flower) with all systems operating, and document yield, quality and energy consumption against design targets. Adjust setpoints based on actual crop performance — most facilities tune VPD and CO₂ setpoints by plus or minus 10 percent over the first three crops to dial in the optimal envelope for the specific cultivar and facility geometry.

Where to take CEA HVAC duct procurement next

If you are designing or building a vertical farm, greenhouse, cannabis cultivation facility or tissue culture lab and need HVAC ductwork sized, specified and supplied to engineering standard, the SBKJ engineering team has commissioned over 5,000 ductwork machines globally and consults regularly on CEA HVAC duct specification. We are based in Box Hill North, Victoria, Australia, with field engineers covering the Australian and New Zealand market directly and the North American market through partner distribution.

Ask us for a duct sizing review on your specific project — VPD targets, latent load assumptions, CO₂ distribution architecture, and material specification. The earlier in the design phase the duct review happens, the lower the rework cost in commissioning.

Talk to an SBKJ engineer about your CEA HVAC duct specification →

FAQ

What is the typical heat gain from LED lighting in a vertical farm?

For a modern horticultural LED at 2.5 to 3.5 µmol/J efficacy, roughly 50 to 70 percent of input electrical energy converts to sensible heat at the canopy. Leafy-greens grow zones at 200 to 300 µmol/m²/s PPFD generate 250 to 450 W/m²; cannabis flower rooms at 800 to 1000 µmol/m²/s generate 600 to 900 W/m². The HVAC supply must remove this load while holding temperature, VPD and CO₂ simultaneously.

What VPD range should I design for in a leafy-greens vertical farm?

For lettuce, basil and microgreens the typical operating window is 0.6 to 1.0 kPa during photoperiod and 0.4 to 0.6 kPa during dark period. The HVAC must hold dry-bulb temperature within plus or minus 0.5 degrees Celsius and relative humidity within plus or minus 3 percent of setpoint to keep VPD inside this window.

How much condensate does a vertical farm generate?

Transpirative water generation runs 0.5 to 1.0 kg/h per square metre for leafy greens and 1.0 to 1.5 kg/h per square metre for cannabis flower. A 2000 m² vertical farm at 1.0 kg/h/m² generates 2000 litres per day — the dehumidification ductwork and drain piping must handle this without flooding.

Do I need stainless steel ductwork in a cannabis cultivation room?

For most cannabis cultivation, galvanized G90 is acceptable provided the supply does not contact growing media or nutrient runoff. Use 304L stainless for condensate pans, dehumidifier housings and any duct downstream of evaporative humidifiers — volatile fatty acids and chloride content can pit galvanized in 18 to 36 months. Confirm gasket selection — silicone preferred over EPDM for low-VOC requirements.

How do I distribute CO₂ evenly across a multi-tier vertical farm?

Inject CO₂ into the supply air plenum upstream of the diffusers rather than via dedicated lance lines. Plenum injection rides on existing air movement and self-mixes. Per-tier HAF fans force lateral mixing so the 800 to 1200 ppm setpoint holds within plus or minus 50 ppm. Tight TDF flange seams matter — leakage above 3 percent destabilises CO₂ control and wastes injection gas.

What air velocity should I target at the canopy?

0.3 to 0.5 m/s at the leaf surface. Below 0.2 m/s the boundary layer saturates and transpiration stalls. Above 0.7 m/s you get mechanical leaf damage. Use overhead supply ductwork plus dedicated HAF fans sized 50 to 100 m³/h per square metre to handle leaf-level mixing.

What filtration grade should I specify for vertical farming HVAC?

MERV 13 to MERV 15 (ePM1 50% to 80%) is industry standard for general grow rooms. Tissue culture and propagation step up to HEPA H13. Cannabis runs MERV 14 supply plus carbon filtration on exhaust. UV-C in-duct downstream of the cooling coil adds pathogen knockdown — increasingly common in commercial CEA.

What is the cooling load for a typical cannabis flower room?

800 to 1200 W/m² total cooling at 800 to 1000 µmol/m²/s canopy PPFD, comprising 600 to 900 W/m² sensible from lighting, 100 to 200 W/m² latent from transpiration, and 50 to 100 W/m² envelope and people. A 1000 m² flower room is roughly 800 to 1200 kW cooling — the equivalent of a 30,000 m² office building.