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How do I best insulate a house without an attic?

coops1mom | Posted in Energy Efficiency and Durability on

I have been researching the last six months what to do about our roof that needs replacing. We live in the Sacramento Valley, hot days and cool evenings (usually). Our house was built in 1966 and is adobe brick.

We have cathedral vaulted ceilings throughout the entire one story house – mostly NO ATTIC. Our current roof configuration is this: Tounge/groove ceiling, 1/2″ ply, 2″ Polyiso (Celotex Thermax) with reflective foil covering both sides, another 1/2″ ply, felt and finally comp shingle. We have no existing grid or frame system OR venting system. We bake in the summer and freeze in the winter.

A small section of our hallway has some attic space – so we added a whole house fan earlier this year.

Looked into building up the roof with 2x6s and then adding 5″ of closed cell spray foam but that is cost prohibitive for us. Any other ideas?


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  1. GBA Editor
    Martin Holladay | | #1

    Adding insulation to your roof will cost money; there's no way around that. However, new roof insulation will prove to be a good investment. It will lower your heating costs, lower your cooling costs, and improve your comfort.

    The method used to insulate your roof -- installing rigid foam above the roof sheathing -- is a good one. The only problem is that the original builder didn't make the insulation thick enough.

    The best way to add insulation to your roof is to remove the existing roofing, add more rigid foam on top of what you have (or on top of the top layer of plywood), and install another layer of roof sheathing (plywood or OSB) and new roofing.

    You might also consider installing a product called nailbase, which is rigid foam with OSB bonded to one side. The foam side goes down, and the OSB side goes up.

    In your climate zone (Zone 4B, I assume), the recommended minimum R-value for roofs is R-38. You now have about R-12, so I recommend that you add an additional R-26 -- in other words, about 4.5 inches of polyisocyanurate.

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