Shop Heat-Loss and Heater-Size Calculator for Metal Buildings

Shop heater size calculator: heat loss in BTU an hour

Heater size starts with how fast the building loses heat. This shop heater size calculator adds up the loss through each wall and roof area, adds the loss from air leaking in, and shows the total in BTU per hour at the temperature difference you choose. You can enter the same building bare, with foam, or with the roof as it stands, and compare. It runs in your browser; nothing is saved or sent.

It does not pick a heater for you. The heat loss is the number a heater has to beat. What the heater maker adds on top, and what output rating the heater carries, come from the maker's own sizing instructions.

Need the wall and roof areas first? Use the metal building area takeoff. For what a cold steel shop is up against in winter, see a cold shop in winter.

What you enter

Surface types:

• Bare steel wall: the steel panel with only the air layers on each face and nothing else.
• Bare steel roof: the same, with the film values for a roof.
• Foam-insulated wall or roof: you enter the foam's R-value (from the product data for the thickness) and the tool adds the air layers.
• Existing blanket roof (through-fastened): you enter the assembly U-factor. The tool does not add films on top because a U-factor already includes them.
• Other: area and a U-factor you know, for doors, windows or a wall you have a figure for.

The floor and slab are not in this tool.

The method in plain terms

Heat leaves through a surface in proportion to its area, the temperature difference across it, and how little resistance it offers. In symbols: heat loss = area x temperature difference / total R, or equivalently U x area x temperature difference, where U is 1 divided by R. A mechanical-engineering study guide gives the conduction form as "q = U x A x (T outdoor - T indoor)" (PE exam study guide, section 6.2).

Three pieces make up the total:

1. Surfaces with foam. Total R is the foam's R plus the air layers on the inside and outside faces. Steel is thin and conducts heat well, so its own resistance is left out.
2. Bare steel. Total R is the air layers only. That is why a bare steel building loses heat so fast: a thin sheet with an air layer on each side is almost no barrier.
3. Air leakage. Warm air leaving and cold air coming in carries heat. The tool converts air changes per hour into airflow and applies a fixed constant for the heat in a cubic foot of air.

The air layers

Every surface has a thin, nearly still layer of air that resists heat flow a little. Published values for these films, from the ASHRAE Fundamentals Handbook as repeated by an engineering firm's summary, are 0.68 for an inside vertical wall in still air, 0.61 for an inside surface with heat flowing upward (a roof or ceiling), and 0.17 for an outside surface in a 15 mph winter wind (H2X Engineering, air film resistance). The units are hr x sq ft x F per BTU.

So the total R for a bare steel wall is 0.68 + 0.17 = 0.85, and for a bare steel roof 0.61 + 0.17 = 0.78. Add the foam's R to either to get the foamed surface. These are Fundamentals values and describe how air behaves against a surface, so Kansas is no exception.

The roof you already have

If the building already has a rated blanket under the roof panels, do not use its label R. A blanket compressed between panels and purlins insulates less than its rating. The Seattle Energy Code appendix lists a default U-factor of 0.184 for a single layer R-10 blanket in a through-fastened metal roof without spacer blocks (Seattle Energy Code 2021, Appendix A, Table A102.2.5). That is about R-5.4 for the whole roof. Seattle's climate is not yours; a U-factor describes how the roof is built, and the adopted local code governs what a permit requires. The comparison of the options is on the best way to insulate a metal building.

Air leakage

Infiltration heat loss uses 1.08 x airflow in cubic feet per minute x temperature difference. Airflow is air changes per hour x volume / 60 (PE exam study guide, section 6.2). Folded together, that is 0.018 x volume x air changes per hour x temperature difference. The tool does not know how leaky your building is. Doors that open often, laps, ridge vents and gaps at the base all add air changes. We publish no default and no "typical" figure, because we have not opened a source for one.

Reading the heater size

The tool's last line is the total heat loss in BTU per hour at your temperature difference. A heater needs to deliver at least that, as output, at your coldest design condition. Then two things are the maker's call:

• A margin on top, and how it applies, as stated in the heater's own sizing guidance.
• Output against input. A heater's input rating and its delivered output differ by its efficiency, which is on the label.

Why not just use a BTU-per-square-foot figure? NDSU Extension's publication on farm shop heating says the old rule of thumb was about 50 BTU per square foot per hour, that modern shops commonly need only 20, and that you should do the heat-loss calculation (NDSU Extension, Heating Farm Shops, rev. 2025). Two rules of thumb that differ by more than a factor of two, from the same source, show why the calculation is better than either. NDSU is a North Dakota source; its point is a method, and it holds in any climate. We use its recommendation to calculate, not its figures.

What insulation changes in the numbers

Here is a worked example with a gable building: 2,509.7 sq ft of net wall and 2,408.3 sq ft of roof (the first gable row on the area calculator page), a 36,000 cubic-foot volume, 0.5 air changes per hour and a 50 F temperature difference. These inputs are for the arithmetic, not a prediction for any building.

Two lessons come out of it. The surfaces are the large share when the steel is bare, and after foam the leakage term is the largest single piece, so how tight the building is matters as much as the R-value once the surfaces are covered. The tool holds the air changes you entered the same in both cases; it does not assume foam makes the building tighter.

The single-surface examples tell the same story in smaller form. A bare steel wall of 1,000 sq ft at a 50 F difference loses 58,823.5 BTU per hour. Give that wall R-10 foam and it loses 4,608.3, about a twelfth.

Whether those savings justify the job is on when spray foam pays off. If you also work in the shop, welding, dust and finishing vapors are on insulating a heated workshop.

Limits

• Thermal bridging. Steel framing, fasteners and gaps carry heat around the insulation. For a blanket the assembly U-factor already includes that. For foam the tool uses the foam's R and does not model the frame, so real loss is higher than the foam row shows. How much higher depends on the building, and we have no sourced figure to give.
• Steady state. One temperature difference, no sun, no wind change, no heat stored in the slab or the contents.
• No floor or slab loss.
• Doors and windows only if you add them as surfaces.
• Moisture is separate. A building that is warm inside can still sweat; use the dew point calculator for that question.
• Foam is not a heater. Choose the heater type, venting and combustion air with the heater maker or a heating contractor, and ask your building official about requirements for the building.

Heat-loss method, films and test values

Formulas. dT = inside minus outside temperature (F), must be greater than 0. - Bare steel wall: Q = A x dT / 0.85 - Bare steel roof: Q = A x dT / 0.78 - Foam surface, wall: Q = A x dT / (R_foam + 0.85); roof: Q = A x dT / (R_foam + 0.78) - Existing blanket roof or other with U: Q = U x A x dT - Leakage: Q = 0.018 x volume x ACH x dT (equal to 1.08 x CFM x dT with CFM = ACH x volume / 60) - Total = sum, shown to 0.1 BTU/h and in kBTU/h to one decimal.

Test values

FAQ

What size heater do I need for an insulated shop?

The calculated heat loss at your design temperature difference, plus whatever margin the heater maker recommends. Use the tool for the loss, then follow the maker's sizing guidance for the rest. A single BTU-per-square-foot rule is not the answer.

How much does insulation cut heat loss?

In the examples above, an R-10 foam wall loses about a twelfth of what a bare steel wall does. For your building, enter your own areas and your own foam R-value.

Does the tool tell me what spray foam would cost?

No. It reports heat loss only. We give a written estimate after a free on-site measurement.

Want it measured on site?

A calculator cannot see your building's gaps, openings or frame. We do a free on-site measurement and give a written estimate after the visit.

Book a free on-site measurement

Pick "Spray foam insulation" on the booking form. We work within about 35 miles of Spring Hill, KS.

Spray foam insulation is a new PolyJacks service line run by its own crew and rig. We have not yet finished an insulation job, so this page makes no claims about past jobs.

Who wrote this: PolyJacks spray foam team. How it was made: drafted with AI assistance from the sources below, then checked against each source.

Sources

• NDSU Extension, Heating Farm Shops (K. Hellevang and C. Pederson, rev. Jan 2025; do the heat-loss calculation; rule-of-thumb figures): ndsu.edu
• H2X Engineering, What is air film resistance (ASHRAE Fundamentals values 0.68, 0.61, 0.17; secondary summary): h2xengineering.com
• PE exam study guide, Envelope and infiltration load methods (conduction q = U A dT; infiltration 1.08 x CFM x dT; CFM = ACH x volume / 60): open-exam-prep.com
• Seattle Energy Code 2021, Appendix A, Table A102.2.5 (default U-factor 0.184, R-10 single layer, through-fastened roof, no spacer blocks): seattle.gov