How to Keep a Metal Shop Warm in Winter: Where the Heat Goes

Where heat leaves a shop

Heat moves from warm to cold by three routes, and a winter shop uses all of them at once.

• Through the skin (conduction). Heat crosses roof panels, wall panels and doors. How fast depends on how much insulation sits in the way.
• With the air (infiltration). Warm air leaves through gaps and cold air comes in to replace it. Insulation does not slow this; sealing does.
• Into the ground. The slab edge and the floor lose heat to the soil.

A North Dakota State University extension guide to heating farm shops puts the general idea plainly: heat takes the path of least resistance, and a part of the building that is not air-sealed or insulated causes cold spots as well as wasted money (NDSU Extension, Heating Farm Shops, revised January 2025). North Dakota is colder than Kansas, so its insulation levels are not ours to copy. The route list holds anywhere, because it is physics.

Ranking the losses with worked numbers

Here is one example shop, with one set of assumptions. The heat loss through a surface is its area times the temperature difference times its U-factor, and the U-factor is 1 divided by the total resistance (R) of the layers. The first table shows the resistance of the air films that cling to every surface, which is all a bare steel panel has to offer.

The example assumes 1,000 sq ft of surface, a 50 °F gap between inside and outside air (for example 70 °F inside and 20 °F out; this is an illustration, not a Kansas design temperature), and a closed building of 24,000 cu ft.

The 0.018 in the last row is a value cited in a building-science forum thread as "0.018 Btu per cu. ft. per degree F" (GreenBuildingAdvisor Q&A, 2012). A forum is a weak source. The 0.5 air changes per hour is an assumption for the example. Nobody measured it in a shop.

The Seattle U-factor is a published default for how that roof is built, and a roof build-up has the same U-factor in Kansas. The detail behind it is on metal building insulation options compared.

What the numbers say

Bare steel loses far more than insulated steel. The bare wall loses about 12.8 times what the R-10 foam wall loses in the example (58,823.5 ÷ 4,608.3). Insulation is the biggest lever on the skin by a wide margin.

Air leakage can outweigh an insulated wall. The 10,800 Btu/h lost to leaks is more than twice the 4,608.3 from the foamed wall. Once the walls and roof are insulated, the gaps become the larger part of what remains, which is why the checklist below comes before the purchase.

A blanket can look decent on paper and still lose. The blanket roof loses 9,200 Btu/h in the example, about 2 times what the same area of foamed wall loses, though they are different surfaces, so read this as an order of size and not as a product comparison.

What these numbers leave out. The method is steady-state: it assumes the temperatures stay put. It ignores thermal bridging, where steel framing carries heat past the insulation. It ignores wind, doors opening, sun and the slab. These limits are printed on the calculator too. Treat all the figures as a way to rank problems, not as a prediction of your fuel use. They are also not a heater-sizing calculation.

Find the air leaks

A leak hunt costs little and shows up on a cold, windy day. The U.S. Department of Energy's archived Energy Savers page on detecting air leaks describes a method for houses that applies to a shop: close up, turn off the furnace, run exhaust fans so the building is under slight suction, then pass a lit incense stick near the edges of likely gaps. Smoke that is drawn in or blown shows a draft (DOE Energy Savers, "Detecting Air Leaks", 2009, archived copy). Its other tips are a flashlight swept over gaps at night while a helper watches from outside, and shutting a sheet of paper in a door to see whether it pulls out without tearing. In a shop, turn off every fuel-burning heater and any appliance with a flue first, and do not run fans against an open flue.

Look at these places:

For the eave and ridge, the closure strips are the key part. A pole-building supplier's blog notes that humid air can still enter the roof system at inside closures unless closures are placed between the fold-over and the roof steel (Hansen Pole Buildings, 2025). That post is about condensation, but the pathway is the same: an unsealed closure is an air path. We have not found a source ranking the eave as the most common air leak in steel shops, so this page does not claim it is.

How to keep a metal shop warm in winter: the fix order

1. Seal what you can see. Replace worn door seals and fill obvious gaps. This is cheap and quick, and foam will not do it for you at doors.
2. Insulate the roof, then the walls. Per square foot the bare roof loses slightly more than the bare wall in the example. Closed-cell foam sprayed on the steel insulates and seals the surface it covers, as covered on spray foam for metal buildings.
3. Seal what foam leaves. Doors, ridge closures, flues and penetrations need their own seals.
4. Size the heater last. With a smaller loss, a smaller heater is enough. The NDSU guide says older rules of thumb for heater size are outdated for well-built shops and that a heat-loss calculation is the way to size one. Use the calculator for that step, and ask the heater's maker for its sizing margin.

If you work with grinding or welding in the shop, there are rules for foam near hot work on spray foam for a heated workshop.

If your complaint is summer heat rather than winter cold, see a metal building that is too hot in summer. Whether the foam pays off is on is spray foam worth it.

What we do not do

We spray closed-cell foam on the inside of buildings, within about 35 miles of Spring Hill, KS. We do not insulate slabs or floors, repair doors, fit door seals or replace closure strips. We do not remove old insulation. If the leak list turns up a door or flashing problem, someone else fixes it, and we will say so at the visit.

FAQ

How do I keep my metal shop warm in winter?

Stop the air leaks first, then insulate the roof and walls. In the worked example above, bare steel walls lose about 12.8 times what R-10 foam walls lose, and leaks lose more than a foamed wall does. The method is steady-state, ignores thermal bridging, and is a ranking tool, not a heater-sizing result.

Does spray foam stop drafts?

Foam seals the surfaces it covers, including the laps and gaps it fills as it expands. Doors, ridge closures, flues and penetrations need their own seals. The checklist above is the list to walk.

Why is my shop floor so cold?

The slab loses heat at its edge and to the ground below, and wall and roof foam does not touch either. NDSU's guide recommends insulating around the foundation of a shop for that reason. We do not insulate slabs.

Book a free on-site measurement

If the walk-through points to the roof and walls, the next step is a visit. We measure the building and look at the surfaces, the doors and any existing insulation, and you get a written estimate after the visit. The visit is free. The way to prepare is on the free estimate 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.

Who wrote this: PolyJacks spray foam team. Last updated: October 27, 2026. How it was made: drafted with AI assistance from the sources below, then checked against each source.

Sources

• Kenneth Hellevang and Carl Pederson, "Heating Farm Shops," NDSU Extension (revised January 2025): ndsu.edu. North Dakota source; used for the general routes of heat loss and the point that heater size needs a heat-loss calculation, both of which are physics and method, the same in Kansas. Its northern insulation levels are not used.
• h2x Engineering, "Air film resistance" (citing ASHRAE Handbook of Fundamentals values): h2xengineering.com
• Seattle Energy Code 2021, Appendix A, Table A102.2.5 (default U-factors, through-fastened roofs): seattle.gov
• GreenBuildingAdvisor, "What is the correct way to manually calculate air infiltration heat loss?" (Q&A, 2012 and 2019 answers): greenbuildingadvisor.com
• U.S. Department of Energy, Energy Savers, "Detecting Air Leaks" (last updated February 24, 2009; archived copy): inspectapedia.com. A federal page for houses; air moves through gaps by the same physics in a steel shop.
• Randy Williams, "Air-sealing garage doors," GreenBuildingAdvisor (June 28, 2023): greenbuildingadvisor.com
• Hansen Pole Buildings, "Condensation Above Metal Building Roof Insulation" (2025; supplier blog): hansenpolebuildings.com