Greenhouse Heating Calculator

Greenhouse heating goes wrong at the first step, because the instinct carried over from house heating is to size on floor area, and a greenhouse is nothing like a house.

A 10 by 12 foot gable greenhouse has 120 square feet of floor and about 434 square feet of glazing. Heat leaves through the envelope, so the relevant area is the larger one — and it is 3.6 times the number most people start from. On a taller or narrower house the ratio is worse.

Compounding that, the envelope has almost no thermal resistance. Single polyethylene at U 1.15 is R-0.87. Single glass is barely different. A code-built house wall is R-20 and its ceiling R-49, so a greenhouse skin offers something like a twentieth of the resistance across three times the area — roughly sixty times the heat loss per square foot of floor.

That arithmetic is why a small hobby greenhouse can cost more to heat through a northern winter than a bedroom, and why season extension in a cold climate is an expensive hobby unless the set point is kept low.

Everything else — heater type, fuel choice, thermostat brand — is a rounding error next to the set point and the number of glazing layers.

Heat loss is directly proportional to the temperature difference. Holding 60°F when the outside averages 38°F is a 22 degree difference; holding 40°F is a 2 degree difference. That is not a 33% saving, it is a 91% one, and it is available for nothing but a decision about what to grow. Hardy salads, brassicas, overwintering perennials and rooted cuttings all do perfectly well in a frost-free house.

Adding a second glazing layer with an inflated air gap takes the U-value from about 1.15 to 0.70 — a 39% cut in every heat loss figure on the page, permanently. A film kit and a small blower cost a fraction of one season’s saving on a heated house, which makes it one of the clearest paybacks in horticulture.

The cost of the second layer is light. Double poly transmits about 78% against 87% for a single layer, and in a northern winter with short grey days, light is frequently the constraint on growth rather than temperature. That trade is worth thinking about deliberately rather than optimising heat alone — it is also why triple-wall polycarbonate, which cuts loss furthest, is the right answer less often than its running cost suggests.

Several techniques cut the bill without touching the heater, and most cost very little.

The peak load figure sizes the heater and the seasonal figure pays for it, and they answer genuinely different questions.

Size for the design night with a margin. A heater that just meets the calculated peak has nothing left for an unusually cold night, a torn film or a door left open, and the one night it fails is the night the crop is lost. Twenty to twenty-five percent over the calculated figure is a reasonable allowance.

Use the local 99% design temperature rather than the record low. That is the temperature exceeded on all but about 88 hours of the year, and it is what heating engineers size to — designing for the record low buys a much larger heater for a handful of hours a decade.

On fuel, the choice between vented and unvented combustion is worth understanding. An unvented heater is thermally 100% efficient because nothing goes up a flue, and it releases its combustion water and carbon dioxide into the house. Plants use the carbon dioxide; the condensation encourages fungal disease, and incomplete combustion produces carbon monoxide and ethylene, which damages plants at very low concentrations. Most growers vent for those reasons, and anyone running unvented needs a working carbon monoxide alarm without exception.

Finally, put a maximum-minimum thermometer in the house and check it. Calculations are estimates; the thermometer is the measurement, and a greenhouse that dropped to 28°F on a night the model said would be 40 is telling you something about infiltration that no formula will.