Greenhouse ventilation calculator

How much vent area a greenhouse needs

The rule most extension services repeat comes from ASABE: roof vents of 15 to 20% of the floor area, and side vents of the same size. UConn Extension cites it that way; Alabama Extension gives 15 to 25% and says hot Southern summers likely need the top of the range. The designer draws at least 20% of each and marks anything between 15 and 20% amber.

Why both roof and side vents

Warm air rises and leaves through the top; cool air has to come in somewhere low to replace it. Clemson Extension describes exactly that pairing of ridge vents with low side inlets, and Oklahoma State describes fresh air entering the side vents as warm air leaves through the roof. A greenhouse with roof vents only moves far less air, because nothing feeds the flow.

How the vents are counted

Each roof vent in our plans is a hinged, glazed lid that fills one 24 in rafter bay near the ridge; each side vent fills a stud bay and sits above the bench where there is room. The area that counts is the opening in the frame. The 8x12 gets 5 roof vents (23.3 sq ft) and 6 side vents (22.5 sq ft) against 19.2 sq ft needed for each. Wyoming Extension's simplest system, a continuous ridge vent 1 to 2 ft wide with inlets in the side or end walls, works on the same principle.

Openers

Oklahoma State notes that automatic wax or hydraulic openers work without electricity: the cylinder pushes the lid open as the air warms. Our plans price one opener per roof vent, or a prop stay per vent if you choose manual. The builder's plan sets each lid's weight against the opener's rated lift, so the opener you buy can actually lift the lid you build.

Sizing an exhaust fan

A fan takes over when vents are not enough or nobody is home to watch them. The rules:

• Clemson and UGA Extension: summer exhaust fans that change the greenhouse air 1 to 1.5 times a minute; Clemson says small houses need the higher rate, so the calculator uses 1.5.
• NRAES and UConn: at least 8 cfm per sq ft of floor. The calculator takes whichever is larger.
• Alabama Extension: count the height only up to 8 to 10 ft, because the air above is stagnant.
• Oklahoma State rates fans at 1/8 in of static pressure, where they move less air than in free air. The calculator uses the maker's rating at 0.05 in and shows the 0.10 in figure next to it.

The intake shutter

UGA's Bulletin 792 asks for about 1.5 sq ft of inlet for every 1,000 cfm the fans actually move; NRAES and Alabama put inlets at 1.25 to 1.5 times the fan area. NRAES wants the louvers motorized, so wind cannot blow them open while you heat. J&D publishes no free area for its shutters, so the designer counts their face and also checks that their own cfm ratings add up to the fan's.

Where it goes

Fan at one end, intake at the other, so the air crosses the whole house. NGMA adds that fans go on the downwind side where possible and must be guarded. Our plans set the fan high on the back end wall and the shutter in the door end at bench height; the Greenhouse Megastore guide puts shutters about 3 ft from the floor for the same reason. The fan, the shutter motor and the thermostat need an outdoor GFCI circuit, which J&D and our plans leave to an electrician.

Vents or fan: the cost

On the 8x12, the priced materials come to $3,813.80 with vents and automatic openers, and $3,314.45 with the exhaust fan, before the electrician. The vents, openers and door group alone is $549.42 against $513.01 for the fan version. The fan saves materials because it needs no lid framing and one sheet less; it adds a circuit and a running cost. The greenhouse cost calculator shows every line for both.

Winter ventilation

Ventilation does not stop in winter. Clemson gives 20 to 30% of the air volume per minute for winter exhaust, and recommends small circulating fans with a combined capacity of a quarter of the volume, so warm air does not pool at the ridge. If you heat, size the heater with the greenhouse heater size calculator and read how to keep a greenhouse warm in winter.