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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.
Basement walls need R-10 to R-15 of continuous insulation on the concrete, and 2 in of XPS foam (R-10) runs about $0.84 per sq ft in material or about $2.04 installed. Enter your wall perimeter and height for 4×8 sheet counts, furring strips, R-value and the cost to insulate your basement.
With these inputs:
| Result | Value |
|---|---|
| Net wall area | 1,012 sq ft |
| 4×8 sheets required | 32 sheets |
| R-value achieved | R-10 |
| Furring strips (2 ft OC) | 64 pieces |
| Material cost | $850 |
| Installed cost (pro) | $2,064 |
Net wall area = (perimeter × height − openings) × (1 + waste %) · Sheets = net area ÷ 32 sq ft per 4×8 sheet · R-value = R-per-inch × thickness
Not directly. Concrete wicks moisture and batts hold it against the wall, which grows mould. Put rigid foam or closed-cell spray foam on the concrete first, then frame and batt in front of it if you want more R.
Most codes want R-10 to R-15 continuous below grade. Two inches of XPS gets you R-10; polyiso hits R-12 at the same thickness.
Against bare concrete it is a poor idea. Concrete is damp and cool, and batts held against it let humid room air reach a cold surface where it condenses, which over time means mould in the cavity. The established approach is rigid foam or closed-cell spray applied directly to the concrete, which keeps the wall's surface warm enough that condensation cannot form.
The whole height in most cold climates, because the top of a basement wall is the part closest to outdoor temperature and it loses the most. Some codes allow insulating only the upper portion in milder zones. The rim joist at the top is the highest-value few square feet in the whole basement and is very often missed.
Not an interior polyethylene sheet in most cases. A basement wall can be damp from the outside, and a sealed interior barrier leaves that moisture nowhere to dry to. Rigid or closed-cell foam manages vapour as part of its own makeup, which is why it is the usual recommendation rather than batts plus plastic.
At the calculator's rates, 2 in XPS foam (R-10) costs about $0.84 per sq ft of wall in material and about $2.04 per sq ft installed. A basement with a 120 ft perimeter and 8 ft walls, less 40 sq ft of windows and doors, is 920 sq ft: roughly $770 in foam DIY or about $1,880 installed, before waste. Polyiso reaches R-12 at the same thickness for slightly more.
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.