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Insulating scissor truss

Hi, I am building my first house, what a project that has turned out to be but that's for another day. Anyways, our great room has scissor trusses for a nice pitched ceiling. I would like to have at least 24" of insulation to keep things nice and toasty in the winter. There is a 4' overhang outside on either end. I nailed a 24" high plywood leaving a vent space above where the red is on the layout. My question is what material and how to go about it. To use the high density cellulose is has to be contained by some mesh? Any input appreciated.

truss22.jpg1.19 MB
Asked by Tom Smith
Posted Sat, 07/19/2014 - 02:32

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3 Answers

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Tom,
First of all, here is some general advice to all GBA readers: you should nail down your insulation details before you specify and install your roof trusses. Insulation decisions shouldn't be last-minute affairs.

It can be tricky to insulate this type of scissors truss, which is why I am not a fan of scissors trusses. Many types of fluffy insulation like to slide downhill, and the braces on the trusses make it difficult to install InsulWeb or similar membranes to contain cellulose.

Gravity can work against you. It's always easier to insulate a flat ceiling than a sloped ceiling.

There are several possible solutions, and the solutions depend to some extent on the steepness of the ceiling slope and the clearance available between the ceiling and the roof sheathing.

One approach is to build an unvented roof assembly. This can be insulated above the roof sheathing with rigid foam insulation, or below the roof sheathing with spray polyurethane foam.

Another approach is to use cellulose insulation, and to mound the insulation higher than usual, especially at the top of the slope -- in other words, install extra insulation -- in hopes that the extra insulation will make up for any slumping.

Answered by Martin Holladay, GBA Advisor
Posted Sat, 07/19/2014 - 07:33

2.
Helpful? 0

All trusses need to be braced together. Do so installing 2x4 bracing perpendicular to all scissor trusses at the bottom of both W5s to help contain some of the insulation slide as well. 4/12 pitch is not that steep, so you should be safe with cellulose. Use minimum 5/8 ceiling drywall to hold the weight of the insulation.

Answered by Armando Cobo
Posted Sat, 07/19/2014 - 09:35

3.
Helpful? -1

Vented roof
bottom of trusses rigid foam taped.
fill trusses with cellulose

Answered by aj builder, Upstate NY Zone 6a
Posted Sat, 07/19/2014 - 22:03

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