R-Value, and What Maine Actually Needs

The numbers for Maine

Maine sits in climate zone 6. Current guidance for a new build lands at R-60 in the roof and attic and R-30 in the walls. R-30 is hard to reach in a standard wall cavity with anything other than foam, which is a large part of why foam gets specified on new construction up here.

Those numbers are the prescriptive path - the one where you hit a target per assembly. There are others. A REScheck lets a building trade off against each other so long as the whole envelope performs, and a blower door test under three air changes per hour demonstrates the same thing by measurement. We have brought a house in at 1.1 air changes per hour that way, and that one was insulated with fiberglass - the air sealing is what got it there, not the R-value on the label. Which path your project uses is worth settling with your code official early, because it changes what you have to buy.

Roughly what each material gives you per inch

• Closed-cell spray foam - about R-6 to R-7
• Open-cell spray foam - about R-3.5 to R-4
• Fiberglass batt - about R-3 to R-4, when new and installed perfectly

That last qualifier is doing a lot of work. Comparison of the two foams here.

Those per-inch figures are also what turns an R-value target into a price, since foam is sold by the board foot and the thickness you need sets the cost. What spray foam costs in Maine.

Why the number on the bag misleads people

R-value is measured in a laboratory, on a flat sample, with no air moving through it. Your wall is not a laboratory. It has wires, pipes, framing, and gaps.

A wall packed with R-21 fiberglass that leaks air at every seam will perform worse in a February wind than a properly sealed wall with a lower nominal R-value. The heat is not conducting through the insulation, it is being carried out by moving air, and R-value does not measure that at all.

Losing R-value over time

Batts settle, sag and take on moisture. Damp insulation conducts heat, so a batt that started at R-21 is not at R-21 after thirty Maine winters. Foam is adhered to the framing and does not absorb water, so it holds its number.