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