ANSWERS — HOW IT COMPARES
Active ventilation or VPD control: which does your crop need?
The house is sealed by design — both climate paths are equipment you buy. One pulls outside air through a wet wall and the length of the house; the other keeps the seal and holds a setpoint with a large AC system. One honest question sorts most buyers — and the cheaper answer is often the right one.
AMERICAN ROOTS — ANSWERED BY THE PEOPLE WHO BUILD IT · UPDATED JULY 2026
01 — WHAT ACTIVE VENTILATION IS
Three fans, a wet wall, and the length of the house between them.
Our greenhouse has no built-in vents — the structure is sealed. The active-ventilation package opens it at exactly two places: 3 exhaust fans on the door-side endwall pull outside air in through the wet wall (an evaporative pad with 5 vents, plus 1 on the ridge) on the back endwall, so every foot of the house sits in the cooled airstream. Circulation fans — 4, 6, or 8 as the house runs 60, 80, or 100 feet — keep the canopy air moving. It is one line in the book: $4,000 base plus $500 per 20 feet over 60, per bay. Configure it in the build tool.
02 — ITS HARD LIMIT
Evaporation cools toward the wet-bulb temperature — and no further.
A well-maintained pad reaches up to about 85% evaporative efficiency, never 100% (UMass Extension). What that means on a 90°F day: at 30% humidity the delivered air lands around 67°F; at 70% humidity it only reaches about 82°F. In hot, dry climates the drop can be 14 to 22°F; in hot, humid ones it can shrink to a couple of degrees (ACES/Auburn). And no ventilation system can dry the air below what the weather delivers — on a wet night, the outside sets your humidity floor.
03 — WHAT VPD CONTROL IS
The sealed house, kept sealed — and an AC system that holds the number.
The VPD path uses no fans, no vents, and no wet wall. The house stays sealed, and a large air-conditioning system — supplied by us — controls temperature and humidity together, holding your crop's vapor-pressure-deficit band regardless of the weather. A standard system runs ≈ $100,000 for the climate system alone on a standard 30×100 — an honest approximate. Your real figure is engineered to your crop, its growth stages, and your targets, which is why that quote asks for more input and more time than the build tool. Read what VPD is for why the setpoint matters.
| FACTOR | FAVORS ACTIVE VENTILATION | FAVORS VPD CONTROL |
|---|---|---|
| Summer climate | Dry heat — evaporative cooling drops the most degrees there | Hot and humid design days |
| Crop | Leafy greens, hardy nursery, cool-season veg | Cannabis, high-value fruiting, propagation |
| Quality bar | Marketable, weather-tolerant | Contract-grade consistency, year-round |
| Capital | One book-priced package, sized to length | ≈ $100,000 standard, engineered in your quote |
| Operating draw | Fan energy | A large AC system's energy, in exchange for control |
THE CHEAPER ANSWER IS OFTEN THE RIGHT ANSWER — WE WILL TELL YOU WHEN WE ARE OVERBUILT FOR YOU
04 — THE DECISIVE QUESTION
One question settles most cases.
On your hottest, most humid design day, can outside air — pulled through a wet pad — still hold your crop's required temperature and humidity? If yes, active ventilation is the honest, cheaper choice. If no, VPD control is not a luxury — it is the only thing that meets the spec.
IF YES → ACTIVE VENTILATION
Your crop tolerates tracking the weather and evaporation gets you there. Price the package live — it's one line in the book.
BUILD IT WITH ACTIVE VENTILATION →IF NO → VPD CONTROL
Your margins depend on hitting a setpoint regardless of the weather. Keep the house sealed and have the AC system engineered to your crop.
READ THE VPD-CONTROL PAGE →PLAINLY
The building is the same PE-stamped structure either way — sealed by design. Switching climate strategies later is an equipment decision, not a rebuild, so choosing the cheaper path first never boxes you in.
Is the active-ventilation package enough for my greenhouse?
If your hot days are reasonably dry and your crop tolerates tracking the weather — leafy greens, hardy nursery stock, cool-season produce — usually yes. Evaporative cooling drops 14–22°F in dry heat but only a few degrees in humid air, so the honest test is your most humid hot day, not your average one.
When do I actually need VPD control?
On a hot-and-humid site, or for any crop you sell on consistency — cannabis, high-value fruiting, propagation — where you have to hold a VPD setpoint regardless of the weather. A wet wall cannot dry air, and no ventilation system can beat the outdoor humidity floor.
Can I switch to VPD control later?
Yes. The structure is sealed by design and engineered to your loads either way, so moving from the ventilation package to a sealed VPD system later is a climate-equipment decision, not a rebuild.
What does the VPD system cost?
A standard system runs around $100,000 for the climate system alone on a standard 30×100 — an honest approximate, not a quote. Your figure is engineered to your crop and stage targets, which is why it takes more input and more time.
Price the honest answer — not the expensive one.
Start with the structure and the active-ventilation package in the build tool; a Greenhouse Expert engineers the VPD system only if your crop and climate actually need it.
WIDTH — 30 FT MODULES
OR TALK TO A GREENHOUSE EXPERT — (772) 321-6305