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Community and Q&A

Continuous Insulation – Mix under rafter and between rafter?

ncampbell | Posted in Energy Efficiency and Durability on

I am creating a semi-conditioned space in the attic over the top of my garage in Texas. I am using foil backed, 1 inch 4×8 sheets of polyiso foam, stacked 4 inches thick. I planned on doing an under rafter installation.

My setup:

I have a two story house. My south facing garage has a 45 degree pitch roof which starts above the garage door and continues up to the top most ridge of my house. The garage attic space is not isolated from the attic over the 2nd story … yet. I will be sealing the attic garage space off from the main attic space. The soffit vents will have clear air paths past the CI foam boards under the rafters in the garage on up to the attic over the 2nd story.

My question is:

I only have soffit vents on the north and south side of the house. When I am installing foam boards on the east or west rafters or on valley jack rafters, there is no soffit vents to provide air flow. Should I do a between the rafter installation on these rafters rather than using under rafter? What are the trapped condensation issues to worry about with both methods?

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  1. GBA Editor
    Martin Holladay | | #1

    You mentioned the word "valley." Roofs with hips, valleys, or dormers are not good candidates for the vented approach. Since the valleys and hips interfere with the venting paths, this type of roof needs to be insulated using an unvented approach. (For more information on all of your options, see this article: How to Build an Insulated Cathedral Ceiling.)

    There are two main options. You can either install rigid foam above the roof sheathing (an approach that requires new roofing) or you can install closed-cell spray foam on the underside of the roof sheathing.

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