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R-49 is the common target for the roof or ceiling in cold and mixed climates. That is about 20 inches of blown fibreglass, 15 inches of blown cellulose, 16 inches of fibreglass batt or about 8 inches of closed-cell spray foam. Air seal the ceiling plane before adding any of it.
In a steel building the number that matters is not R-value, it is dew point. The inside of the panel is the coldest surface in the building, and if moist air reaches it you get water — which is why insulation that works perfectly in a stud wall fails against steel.
With these inputs:
| Result | Value |
|---|---|
| Wall area | 880 sq ft |
| Roof area | 632 sq ft |
| Total steel to cover | 1,512 sq ft |
| Order with waste | 1,664 sq ft |
| R-value at this thickness | R-13 |
| Zone target for walls | R-20 |
| Spray foam board feet | 3,025 bd ft |
| Condensation risk | Controlled — foam adheres to the steel |
Wall area = 2 x (length + width) x height - openings · Roof area = length x width x pitch factor · R-value = material R per inch x thickness · Board feet = area x thickness in inches
Because the steel panel is impermeable and cold, so it is the surface where water vapour in the air reaches its dew point and condenses. Any system that lets humid indoor air reach the back of the panel will produce liquid water, regardless of how much R-value is in front of it. That is why batts stuffed against bare steel are the classic failure — the insulation is fine, the air path is the problem.
Closed-cell spray foam applied directly to the steel, because it bonds to the panel and is its own vapour retarder, so there is no air gap for moist air to occupy. Faced fibreglass blanket installed over the frame before the panels go on is the other established approach, and it has to go in during construction. Retrofitting a building that is already up narrows the sensible options considerably.
Thick enough that the inside face of the insulation stays above the dew point of your indoor air, which depends on your climate and how the building is used — a heated workshop with people in it makes far more moisture than dry storage. The calculator shows R-value against the zone recommendation, but treat condensation control as the governing constraint and R-value as the secondary one.
Yes, and it is usually the reason people want to. The approach that works is rigid board sealed at the joints or foam adhered to the panel, plus ventilation so the moisture you generate has somewhere to go. Adding insulation without ventilation often makes a metal shed wetter, because you have made the panel colder relative to the air inside while giving the moisture nowhere to escape.
You need a vapour control layer on the warm side, and in a steel building the material doing the insulating is often doing that job too. What you must avoid is trapping a vapour barrier on both sides of an assembly, which leaves any moisture that does get in with no way out. Decide which side is warm in your climate and put the control layer there deliberately.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.