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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.
Stud walls are sold by the bay, not the square foot. Enter your wall run, height and stud spacing and this returns the bay count, batt count and net coverage after openings.
With these inputs:
| Result | Value |
|---|---|
| Net wall area | 304 sq ft |
| Stud bays | 36 bays |
| Coverage to buy | 334 sq ft |
| 8 ft batts required | 35 pieces |
| R-value at 3.5 in cavity | R-11.2 |
| Estimated material cost | $152 |
Net wall area = (length × height) − openings · Stud bays = (length in inches) ÷ stud spacing, rounded up · Batts = coverage to buy ÷ area of one batt
A 3.5-inch cavity takes R-13 standard or R-15 high-density fiberglass, or about R-15 mineral wool. A 2x6 wall (5.5 inches) takes R-19 to R-21.
In cold climates use kraft-faced batts with the paper toward the heated side. In hot-humid climates unfaced batts plus a separate air barrier is usually the better call.
Yes — dense-pack cellulose or injected foam goes in through holes drilled between the studs, either from inside or through the exterior cladding. The holes get plugged and patched afterwards. It is the standard retrofit for a finished house, and the main limitation is that you cannot see what is in the cavity or fix what you find.
A 2x4 stud bay is 3.5 inches deep and a 2x6 is 5.5 inches, so the 2x6 holds about 57 percent more. With a material at R-3.7 per inch that is roughly R-13 against R-20, which is exactly why code in colder zones effectively pushes new construction toward 2x6 exterior walls.
Wood studs conduct heat far better than the insulation beside them, so they act as thermal bridges through the wall. In a typical framed wall the studs are 20 to 25 percent of the surface area, which pulls the whole-wall R-value well below the batt rating. Continuous exterior foam is the standard fix because it covers the studs rather than sitting between them.
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.