U-value (thermal transmittance) measures how quickly heat passes through a material in BTU per hour per square foot per °F difference. Lower is better — double poly and twin-wall polycarbonate have much lower U-values than single glass, meaning less heat escapes.

What is the temperature differential (ΔT)?

It is the difference between the desired indoor temperature and the coldest expected outdoor temperature. For example, keeping 60 °F inside when it is -10 °F outside gives a ΔT of 70 °F.

Why is a 25% safety margin added?

Real-world factors like air infiltration through vents and gaps, thermal mass of soil and benches, and extremely cold snaps can push demand above the theoretical minimum. The safety margin ensures your heater can keep up.

Does this account for infiltration losses?

The base formula covers conductive heat loss through glazing and walls. Infiltration (air leakage) is approximated by the 25% safety factor. For very leaky structures, add an extra 10–20% manually.

What glazing should I choose for the coldest climates?

Triple-wall polycarbonate (R-3.3) or double-layer glass with a low-E coating offer the best insulation. Double-poly film with air inflation (R-1.5) is the most cost-effective upgrade for hobby growers.

Can I use this for a high tunnel or hoop house?

Yes. Select "Gothic / Quonset" as the roof style and "Single Poly Film" as glazing. A high tunnel typically has no supplemental heat, so the output tells you what a heater would need to supply if you added one.

Greenhouse Heating Calculator

Name: Greenhouse Heating Calculator
Availability: InStock
Author: Nham Vu

Calculate the BTU/hour heating requirement for your greenhouse based on dimensions, glazing type, and desired temperature.

Greenhouse Details

Length (ft)

Width (ft)

Wall Height (ft)

Roof Style

Glazing Material

Inside Temp (°F)

Outside Low (°F)

Fill in the details on the left and click Calculate to see your greenhouse heating requirement.

Summary

Calculate the BTU/hour heating requirement for your greenhouse based on dimensions, glazing type, and desired temperature.

How it works

Enter the greenhouse length, width, and sidewall height.
Select the roof style (gable or Gothic/Quonset).
Choose the glazing material (e.g., single glass, double poly, twin-wall polycarbonate).
Set the desired inside temperature and the coldest expected outside temperature.
The calculator multiplies total surface area by the glazing U-value and the temperature differential to produce BTU/hour.
A recommended heater size with a 25% safety margin is also shown.

Use cases

Sizing a new furnace or boiler for a hobby greenhouse.
Comparing the heating savings of upgrading from single-glass to double-poly or twin-wall.
Planning heating costs before building a commercial greenhouse.
Determining whether an existing heater is adequate after adding a greenhouse extension.
Estimating heat loss for season-extension high tunnels.
Checking compliance with energy design standards for new greenhouse construction.

Frequently Asked Questions

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Last updated: 2026-05-29 · Reviewed by Nham Vu