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How to best insulate a new truss roof?

I am going to build a new home in northern Idaho, zone 5. It will be a ranch house with walk out basement. Most of the house will have scissor trusses, 8/12 pitch upper and 4/12 pitch lower. With the exception of two shed dormers that will be stick framed. I originally thought of closed cell spray foaming the entire underside of the roof deck for an unvented attic space.
There will be a few areas which will most likely have 9' flat ceilings such as baths & closets.
Would this be the optimal approach as opposed to using batts and blown in on top of that and venting the attic?
How much of a difference between the two methods would it be if the foam proves to be too cost prohibitive?
Also the garage roof will be walled off from the main house under the roof line. That demising wall will be insulated how ever I do the main roof. The garage will be built with attic trusses for storage over the garage. The garage will probably be heated most of the time to be just above freezing unless we have work to do in the garage for shorter periods of time. For the garage ceiling I was thinking of 1" foam and 5/8" FC drywall under that and batts above for the ceiling area all around the outer perimeter of the storage area above. And then encapsulating the storage area with 1" foam, drywall and batts and then venting the garage part of the roof above the storage area. Does this sound workable?

Asked by James Mills
Posted Mon, 02/24/2014 - 18:08


2 Answers

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Forget about using foam. Instead, buy raised heel trusses which provice, at a minimum, 26" below rhe sheathing at the eaves. Install and seal a vent chute and 18"-20"of blown cellulose. (allow at least 10-15% for settling) You might need baffles to keep the insulation from sliding down, but your installer should be able to figure that out. You can frame the flat ceilings under the scissors, so they'll be warm storage.

Answered by Bob Irving
Posted Mon, 02/24/2014 - 19:18

Helpful? 0

Scissors trusses can be tricky to insulate, because loose insulation materials don't generally like to be installed on a slope, unless they are fully encapsulated by rigid materials.

You should talk to your insulation contractor about your options, because there is no use in recommending a method that your local contractor can't perform.

In general, any method of insulation can work, as long as the venting details (if required) are well executed; as long as attention has been paid to airtightness; as long as the R-value of the insulation meets or exceeds minimum code requirements; and as long as the installation is performed meticulously, so that there are no voids.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 02/25/2014 - 09:12

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