The summer number is bigger than the winter number
Take the same 20 by 48 ft tunnel used on the heat loss page. In January, single film, holding 50 inside against 5 outside, it loses 95,153 BTU an hour. At noon in July, with 280 BTU per hour per square foot landing on 960 square feet of footprint and 70 percent of it getting through the glazing, 188,160 BTU an hour is arriving. That is 1.98 times the winter load, and it arrives whether or not anybody is ready for it.
The winter load is met by buying a heater once. The summer load is met by moving air, and moving air is a continuous cost in electricity, noise and a hole in the end wall. This is the asymmetry that catches people who built a greenhouse to extend the season and then found they could not use it in June.
How the airflow number comes out
Not all of the transmitted energy raises the temperature. Water evaporating from leaves and from wet media absorbs energy without the thermometer moving, and in a full house that latent share is large. The form takes it as a percentage because it genuinely varies from near zero on empty benches to well over half under a closed canopy, and no constant would be honest.
What is left is sensible heat, and the airflow to carry it away is the same relation used everywhere else on this site: CFM = BTU per hour divided by 1.08 times the temperature rise. At 60 percent sensible on the numbers above, 112,896 BTU an hour divided by 1.08 times 10 degrees gives 10,453 CFM. On 960 square feet of floor that is 10.9 CFM per square foot, which sits close to the old greenhouse rule of 8 CFM per square foot before its correction factors are applied.
| Shade cloth | Transmitted BTU/hr | Sensible BTU/hr | Exhaust CFM |
|---|---|---|---|
| None | 188,160 | 112,896 | 10,453 |
| 20% | 150,528 | 90,317 | 8,363 |
| 30% | 131,712 | 79,027 | 7,317 |
| 40% | 112,896 | 67,738 | 6,272 |
| 50% | 94,080 | 56,448 | 5,227 |
Read the last column as a shopping list. Forty percent cloth is the difference between three 3,500 CFM fans and two. The cloth costs nothing to run and works in a power cut, and that is the argument for it. The argument against is that you are throwing away 40 percent of the light for the whole period the cloth is on, so cloth that can be pulled back on a cloudy week is worth more than cloth that is zip-tied on in May and taken off in September.
Thin air, and why altitude is not a rounding error
A fan moves cubic feet. Heat is carried by pounds. At 5,000 feet a cubic foot of air weighs about 86 percent of what it weighs at sea level, so it carries about 86 percent of the heat, and the fan has to move about 16 percent more cubic feet to do the same job. The calculator uses the standard atmosphere density ratio, which is (1 − 0.00000687535 × elevation in feet) raised to the 4.2559 power.
| Elevation | Density ratio | CFM multiplier |
|---|---|---|
| Sea level | 1.000 | 1.000 |
| 2,000 ft | 0.943 | 1.061 |
| 5,000 ft | 0.862 | 1.161 |
| 7,000 ft | 0.811 | 1.234 |
The same correction runs the other way on the evaporative cooling side, where thin dry air is unusually good at it. If a pad wall is on the table, the pad sizing and water use calculator takes the airflow figure from this page as its starting input.
Where this stops
This is a fan-sizing calculation and not a cooling design. It says the air leaving is 10 degrees hotter than the air entering; it does not say what the temperature is at any particular bench, and in a house with poor airflow distribution the difference between the two ends can be much larger than the average implies. It also assumes ambient air is the coolest thing available, which stops being true above roughly 95 degrees outside, at which point ventilation alone cannot hold a useful temperature no matter how large the fan is and something evaporative has to enter the picture.
For a sealed indoor space with lamps rather than sun, the mechanism is completely different and the grow tent ventilation calculator is the right page — there the load is lamp watts and there is no solar term at all. For general room air changes unrelated to heat, the air changes calculator does that arithmetic.
Questions people ask
Is 8 CFM per square foot still the right rule?
It is a starting point that was written for a standard house at low elevation, moderate light and a specific allowed temperature rise, and the method it belongs to then multiplies it by factors for elevation, light intensity and the rise you want. Used bare it will be wrong in either direction depending on your site. The calculator shows it beside the physics answer precisely so you can see the gap. On the default house here the physics asks for about 10.9 CFM per square foot, which is 36 percent more than the bare rule, mostly because a 10 degree rise is a tight target.
Does shade cloth go inside or outside?
Outside does far more per unit of shade rating, because energy stopped before it reaches the glazing never becomes heat inside the building. Cloth hung under the roof has already let the solar energy into the house; it stops light reaching the crop and it intercepts some of the heat, but a good deal of what it absorbs is re-radiated and convected into the air below it. The trade is that outside cloth takes wind and weather and has to be attached to something, which is a structural question for the frame manufacturer rather than a shading question.
Why does the calculator ask what share becomes sensible heat?
Because it swings the answer by a factor of two and there is no honest default. A house packed with actively transpiring plants converts a large share of the incoming energy into water vapour, which raises humidity and costs the fan nothing in temperature terms. The same house in March with empty benches and a dry gravel floor converts almost all of it to sensible heat and runs far hotter on the same sunshine. If you are sizing for the worst case, size for the empty house, because that is the condition that happens on the first hot day of the year.
Can I ventilate naturally instead of using fans?
Often yes, and a house designed for it moves remarkable quantities of air through roof vents and roll-up sides with no power at all, driven by warm air rising and by whatever wind is passing. The catch is that natural ventilation is not a number you can size a system on the way a fan can be — its output depends on wind speed and direction, on the temperature difference at that moment, and on vent area that is usually fixed by the structure. This page deliberately calculates the mechanical case, which is the one with a guaranteed answer. A hybrid, where vents do the work most days and a fan covers the still afternoons, is usually the cheapest arrangement to run.
The calculator says I need three fans. Should I buy one big one instead?
Staging matters more than total capacity. A single fan sized for the worst hour is either off or blasting, which on a mild morning overcools the house and wastes electricity, and it gives you nothing at all when it fails. Two or three smaller fans brought on in stages track the actual load, and the house keeps running when one is down. Whether a given fan actually delivers its rating in your installation depends on static pressure from shutters, screens and any pad in the path, and that comes from the manufacturer performance table rather than from the number on the carton.