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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.
A pole barn has no stud walls. Posts sit at wide centres with horizontal girts between them, so insulation spans the girts rather than filling a cavity — and the biggest single decision is whether you insulate at the ceiling or follow the roofline.
With these inputs:
| Result | Value |
|---|---|
| Wall area to insulate | 1,480 sq ft |
| Ceiling area | 1,200 sq ft |
| Total surface | 2,680 sq ft |
| Order with waste | 2,948 sq ft |
| Girt runs per wall | 6 runs |
| R-value at this thickness | R-19.5 |
| Zone target for walls | R-20 |
| Spray foam board feet | 8,040 bd ft |
| Roofline costs this much more | 65 sq ft |
Wall area = 2 x (length + width) x eave height - openings · Ceiling area = length x width · Roofline area = length x width x pitch factor · Girt runs = eave height in inches / girt spacing · Board feet = area x thickness in inches
The ceiling is less area and therefore cheaper, and it lets the roof space stay vented, which handles condensation under the steel. Following the roofline is what you do when you want the full height of the building as usable conditioned space — a shop with a lift, say. The calculator shows the extra square footage the roofline costs so the decision is a number rather than a guess.
Not the way you would in a stud wall. Post-frame buildings have posts at wide centres with horizontal girts, so there is no 16-inch cavity to friction-fit a batt into. The usual approaches are spray foam directly against the steel, rigid board fitted between or over the girts, or a fabric-faced blanket system designed for post-frame. Batts pushed against bare steel will hold condensation.
Nothing structural — the terms are used interchangeably for post-frame construction, with 'pole shed' more common in some regions and usually implying a smaller building. Both use posts set in or on the ground carrying horizontal girts, rather than a continuous stud wall on a foundation, and both insulate the same way.
You need to control moisture, and in a steel building the condensing surface is the underside of the panel itself. Closed-cell spray foam is its own vapour retarder and solves it by adhering directly to the steel with no gap for air to reach. Any system that leaves an air space against the steel needs that space deliberately vented rather than sealed, or you get water dripping off the roof on cold mornings.
It scales with surface area rather than floor area, which is why tall buildings cost disproportionately more — a 40 x 30 barn with 12 ft walls has more wall than ceiling. The calculator gives the areas and, for spray foam, the board feet; get quotes on those figures rather than on the building's footprint, which is the number contractors are most often given and the one that means least.
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.