Ask the Architect

What’s the right insulation?

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Q. I am confused about the insulation that was just put into my attic floor and cathedral ceiling. The guys who did the carpentry told me the roof and floors were totally over-designed, and they could have saved me a lot of money with 2x8 instead of 2x12 lumber everywhere. Then I read somewhere that the insulation was supposed to be much more than what they put in. When I read the paper cover on the rolls, it said R-21, but one of the people I interviewed, but didn’t hire, said he uses foam, which was much more expensive, to get to R-49, which I found out after is the actual requirement. What should I do now? How does the insulation pass inspection, and was I supposed to have R-49?

A. This is one of those questions that you needed to run by one person who could have guided you properly before all the “experts,” confident as they were, got involved. You should have asked either your local building plans examiner or your design professional, and they would, hopefully, have shown you the building residential codes, or the Energy Codes, called the International Energy Conservation Codes (IECC), to verify and clarify your issue.

By looking at the table, N1102.1.2 and N1102.1.4 you would see that the code requires, for a ceiling, R-49, which is 14 inches thick, or the alternative of R-38, which is 10½ inches thick. Why they differ is that one table refers to insulation in between the roof rafters and ceiling joists, while the other table refers to the whole construction assembly of wood, insulation, ceiling board, etc. The worst advice you got was from the carpenters, who may understand framing connections and cutting wood, but they showed a limited view of the whole picture.

Every so often we get a call asking why we used such large lumber for the framing, and I have to try and explain that you have to meet all of the requirements, not just a few. You would never get the lower value of R-38 to fit into a 2x8 ceiling joist or roof rafter cavity, because fiberglass or mineral wool insulation is R-21 or R-22 (depending on the manufacturer) in that size cavity, and you need to have air space for air flow to remove moisture. So the carpenters gave you bad information. If you chose to spend the money, approximately 60 percent more for the foam insulation, you could have come near the R-49 with R-47, corresponding to the first table in the codes that I referred to, and that would have been acceptable, because it would have been more than the assembly-required R-38. As confusing as this all sounds, you should wonder, even a little, as I do, if all this technical stuff ever actually makes it to the construction. So often I see installers who are not doing the right job and enforcement that lets the problem go, even after all the calculations. Stay warm.

© 2015 Monte Leeper. Readers are encouraged to send questions to yourhousedr@aol.com, with “Herald question” in the subject line, or to Herald Homes, 2 Endo Blvd., Garden City, NY 11530, Attn: Monte Leeper,

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