GreenhouseVerdicts

Heating & ventilation / Research

Open calculations · Version 1.0.0

Floor area is not
a heating specification.

Six greenhouse sizes. Three heat-transfer assumptions. Three temperature differences. Compare 54 scenarios without hiding the inputs behind a single heater recommendation.

Published September 19, 2026 · Deterministic model · No product testing or local weather data

The practical result: in our fixed-assumption comparison, moving from 8 × 12 ft to 8 × 16 ft adds 33.3% floor area but 23.2% estimated heating demand. Total demand still rises. This is a result of the chosen geometry and model—not a promise of operating savings.
Download the 54-row CSVData, assumptions & definitionsRead the method ↓

Hold the assumptions constant

Compare sizes on the same basis.

Choose a U-factor and temperature difference to see six matching rows. A lower U-factor means less envelope transmission in this model; it is not a material label.

6 modeled scenarios shown

Every row fixes eaves at 6 ft, ridge at 8 ft, leakage at 1 air change per hour and perimeter coefficient at 0.8 BTU/(h·ft·°F). Door and vent openings stay in the gross envelope area. These are illustrative geometries, not branded kits.

Peak steady-state thermal output. kW means useful heat—not necessarily electricity drawn by equipment.
Model footprintFloor areaRoof + wall areaEstimated loadThermal outputLoad / floor area
6 × 8 ft48 ft²237.7 ft²8,771 BTU/h2.57 kW182.7 BTU/(h·ft²)
6 × 12 ft72 ft²314.5 ft²11,620 BTU/h3.41 kW161.4 BTU/(h·ft²)
8 × 12 ft96 ft²363.3 ft²13,444 BTU/h3.94 kW140 BTU/(h·ft²)
8 × 16 ft128 ft²447.1 ft²16,560 BTU/h4.85 kW129.4 BTU/(h·ft²)
10 × 16 ft160 ft²504.3 ft²18,698 BTU/h5.48 kW116.9 BTU/(h·ft²)
12 × 20 ft240 ft²661 ft²24,543 BTU/h7.19 kW102.3 BTU/(h·ft²)

Same ratio, different question

More room can mean less load per square foot—and more total heat.

The 8 × 12 model has 96 ft² of floor area and 363.3 ft² of roof and walls. Extending it to 8 × 16 increases the sidewalls and roof but leaves the two end walls unchanged.

At U 0.8 and ΔT 40°F, modeled output rises from 3.94 to 4.85 thermal kW. The per-floor-area rate falls from 140 to 129.4 BTU/(h·ft²). Neither change establishes a real greenhouse’s electricity bill.

Use this to challenge a shortcut.

A universal “watts per square foot” figure hides shape, glazing, air leakage and temperature difference. The benchmark lets you see what those assumptions do before applying them to a buying decision.

Do not choose a bigger structure just because its modeled per-area result is smaller. It still needs more total heat under this comparison, and may bring other costs the model does not represent.

Enter your own gable dimensions →

One change at a time

Which input changes the answer?

These five rows are a separate sensitivity check for the 8 × 12 ft baseline, not five additional members of the 54-row size comparison. All other inputs remain fixed for each change.

ScenarioChanged assumptionThermal outputChange from baseline
Baseline8 × 12 ft; U 0.8; ΔT 40°F; 1 ACH; perimeter 0.83.94 kW0%
Lower U-factor onlyU 0.8 → 0.5; other inputs unchanged2.66 kW-32.4%
More leakage only1 → 2 air changes per hour; other inputs unchanged4.1 kW+4%
Lower perimeter coefficient onlyPerimeter 0.8 → 0.4; other inputs unchanged3.75 kW-4.8%
Smaller temperature difference onlyΔT 40°F → 20°F; other inputs unchanged1.97 kW-50%

These percentages are arithmetic sensitivities, not measured energy savings. In this linear model, halving ΔT halves all three terms. Lowering U changes only envelope transmission; leakage and perimeter losses remain.

Reproduce the calculation

Method and units

We use the envelope, infiltration and perimeter equations described in University of Georgia Extension Bulletin 792, with U = 1/R. UGA provides the general method; the footprint choices, scenario matrix and outputs here are GreenhouseVerdicts calculations. UGA has not reviewed or endorsed this benchmark.

Q = A × U × ΔT + 0.02 × V × ACH × ΔT + P × perimeter × ΔT
Thermal kW = Q ÷ 3412.142

A is gross roof-and-wall area in ft²; V is volume in ft³; ΔT is a Fahrenheit temperature difference; ACH is air changes per hour. U is in BTU/(h·ft²·°F), and P is in BTU/(h·ft·°F). The floor is excluded from A; ground loss is represented only by the simplified perimeter term.

Roof = 2L × √[(W/2)² + (ridge − eave)²]
Walls + ends = 2L × eave + 2W × eave + W × (ridge − eave)
Volume = WL × [eave + (ridge − eave)/2]

Why these inputs?

The six footprints range from 6 × 8 to 12 × 20 ft. They are editorial examples, not a sample of sales or a ranking of actual products. U 0.5, 0.8 and 1.2 create three clearly separated sensitivity cases. We do not equate them with a particular panel thickness, construction or brand. Temperature differences of 20, 40 and 60°F are not assigned to cities or climate zones.

All heights, leakage and perimeter assumptions remain fixed to make the size comparison interpretable. In practice those inputs can vary. UMass Extension’s discussion of shutter heat loss describes how both transmission and leakage can matter at ventilation openings; our one-number leakage input does not model an actual shutter installation.

Worked baseline

For the 8 × 12 ft baseline at U 0.8 and ΔT 40°F: envelope transmission is 11,626.6 BTU/h, infiltration is 537.6 BTU/h and the perimeter term is 1,280 BTU/h. Their sum is 13,444.2 BTU/h, or 3.94 thermal kW.

Data and reproducibility

The JSON download contains full-precision calculated rows, a data dictionary, assumptions and the sensitivity cases. CSV retains full numeric precision; this page rounds for readability. The downloadable reference module uses no dependencies or network calls. Automated tests compare all 54 rows against that standalone arithmetic and the site’s existing calculator.

Download reference arithmeticRead the data dictionary →

Limits that belong beside the results

Do not multiply this peak output by an entire winter and call it an annual bill. Operating energy needs weather and runtime information, equipment input/efficiency and appropriate tariff assumptions. Structural, electrical and gas work remain outside this benchmark.

Citation and reuse

You may quote our calculated rows with the model assumptions and a citation to this page. Do not describe them as observations or product performance. This permission covers our benchmark outputs, not third-party source material or the rest of the website.

GreenhouseVerdicts (2026). Greenhouse Heating Planning Benchmark, version 1.0.0. Published September 19, 2026. https://greenhouseverdicts.com/research/greenhouse-heating-benchmark/

Change log

Version 1.0.0: initial 54-row model grid and five-row sensitivity analysis. Source methods checked September 19, 2026. Future material method changes receive a new version rather than silently overwriting the assumptions.

Use the result

Calculate your own gable heating scenario →Estimate a separate electrical running-cost scenario →Understand heat loss, heater output and input power →Check exact-model specifications and missing data →Research and corrections methodology →