Question 1

What R-value do I need?

Accepted Answer

It depends on climate zone and assembly. The DOE recommends R-13 to R-21 for walls and R-30 to R-60 for attics across US zones. Check the DOE map for your zone rather than using a national average.

Question 2

What is the R-value of a 2x4 wall?

Accepted Answer

A 3.5-inch fibreglass batt is nominally R-13 to R-15. Once you account for the wood framing, the whole-wall value drops to roughly R-11, which is why continuous exterior insulation has become common.

Question 3

Why is whole-wall R-value lower than the batt rating?

Accepted Answer

Because studs conduct heat around the insulation. Wood is about R-1.25 per inch against fibreglass at R-3.2, and framing occupies 15–25% of a typical wall. The batt rating describes the cavity, not the wall.

Question 4

Does 24-inch stud spacing improve insulation?

Accepted Answer

Yes, measurably. Advanced framing at 24 inches on centre reduces the framing fraction from about 15% to 10%, which raises the whole-wall R-value even with identical insulation in the cavities.

Question 5

What is the highest R-value per inch?

Accepted Answer

Closed-cell spray foam at R-6 to R-7 per inch, followed by polyiso board at R-6, XPS at R-5, and mineral wool at R-4. Fibreglass and cellulose sit around R-3.2 to R-3.5.

Question 6

Should I use faced or unfaced insulation?

Accepted Answer

Faced batts include a kraft vapour retarder that faces the warm side — interior in cold climates. Use unfaced where a vapour retarder already exists or where the assembly is designed to dry inward, which depends on your climate zone.

Question 7

Can I compress insulation to fit?

Accepted Answer

Compressing a batt reduces its R-value roughly in proportion to the loss of thickness. An R-19 batt squeezed into a 3.5-inch cavity performs at about R-13, so you are paying for performance you do not get.

Question 8

Does insulation stop air leakage?

Accepted Answer

Fibreglass and cellulose do not — air passes straight through them, which is why air sealing matters as much as R-value. Closed-cell spray foam is an air barrier in its own right. In most retrofits, sealing leaks returns more than adding insulation.

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