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News Room: Industry News

Open-web rafters for superinsulated roofs

Wednesday, December 12, 2012   (0 Comments)
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Posted December 04, 2012 09:21 AM by Alex Wilson in Building Green.com


Open-web, parallel-chord joists with solid-wood diagonal struts for use as superinsulated roof rafters.

Open Joist Triforce rafters being installed on our house. Click to enlarge.
Photo Credit: Alex Wilson

Last week I wrote about an innovative foundation insulation material, Foamglas, that we used in our new house in Dummerston. This week I’ll talk about the open-web rafters we’re using to achieve a superinsulated roof.

First, a little background. To create highly insulated roofs there are several approaches:

When the insulation is installed in the attic floor (creating an unheated attic), it’s easy to obtain very high R-values inexpensively—it’s cheap, that is, as long as you don’t count the cost of the lost living space by creating an unheated attic. Basically, you just dump in a lot of loose-fill cellulose or fiberglass on the attic floor, filling the joist cavity and more.

I’ve heard of as much as two feet of cellulose insulation being installed in this manner, achieving about R-80. To make room for a lot of insulation at the roof eaves, it’s usually necessary to install "raised-heel” trusses for the roof framing (so that the insulation thickness at the edges is not significantly compromised.

The Triforce joists are made with solid-wood diagonal struts and glued, finger-jointed connections. Click to enlarge.
Photo Credit: Alex Wilson

If you want to insulate the sloped roof, creating living space—as we are doing—you can either install very thick rafters (14 inches or more) that can be filled with cavity-fill insulation, or you can provide more modest roof trusses or rafters and then add a layer of rigid insulation on top of the roof sheathing. An advantage of the latter approach is that the layer of rigid insulation controls the "thermal bridging” through the rafters or top chords of the roof trusses.

To keep the insulation costs down and minimize our use of foam-plastic insulation, we opted for the former option—putting all our insulation in the rafter cavities rather than installing a second layer of outboard insulation.



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