Attic trusses – insulation question
jmckay@grenergy
| Posted in General Questions on
Followup for clarification on at ICF home with attic trusses. I do plan to insulate the attic truss floor for two reason. !st is for noise reduction and the other is that this area will only ocassionally be occupied and will have reduced temps most of the time. Can I get away with spry foaming the roof trusses soffit to soffit with 2″ and then putting in batt insulation to fill the cavity. I do plan to very soon finish this area so do I also need to insulated the side walls of the room if the rafters are already filled with insulation.
Jim
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Replies
Jim,
The R-value of 2 inches of spray foam depends on what type of foam you use. Two inches of closed-cell spray polyurethane foam has an R-value of R-12 or R-13. Your suggested approach is called "flash and batt." You proposed approach will only work if you live in climate zone 1, 2, 3, or 4C. If you live in climate zone 4A or 4B, or anywhere colder, you need more spray foam.
These provisions are detailed in section R806.4 of the 2009 IRC. Everything you need to know is explained in the article I linked to before, Creating a Conditioned Attic, which I suspect you may not have read. If you need more information on this issue, check out How to Build an Insulated Cathedral Ceiling.