Q:
The examples and pictures in your article “Why Flash and Batt Makes Sense” (FHB #217) all reference wall insulation. Would you recommend the same techniques for attic insulation? I need to upgrade my current insulation, which is blown cellulose. Removal could be troublesome. Any suggestions?
David Beckel, Washington D.C, None
A:
Michael Maines, an architectural designer, responds: Flash and batt or flash and fill can work just fine in an attic floor. A common approach is to spray a flash coat of closed-cell foam and then spray as much cellulose or chopped fiberglass as you can, ideally to R-60. The flash coat needs to be only thick enough to block air, about 1 in. or 11/2 in. In a heating climate, there are no concerns with condensation if the foam is the only vapor retarder and is on the warm side of the assembly.
Flash and fill also can be combined with foam on the cold side. On some renovations, I have removed the existing insulation, installed rigid foam above the floor joists, then dense-packed the cavities full of cellulose. Because the foam is not vapor permeable, it needs to be thick enough to eliminate the chance of condensation. Extra attention is required at the ends of the joist bays to ensure there are no air leaks.
While you might think that you could just install rigid foam over existing joists and insulation, there is a good reason not to. Dense-packed cellulose is installed at about 3.5 lb. per cu. ft., which prevents it from settling and ensures good contact with the rigid foam on a new installation. Loose-blown cellulose is only about 2 lb. per cu. ft. That means you will have a little gap between the cellulose and the foam. If the ends of the joist bays are not sealed, that little gap is a giant thermal short circuit, rendering the foam nearly useless. In addition, you likely have at least some air leaking from the living space into your attic. Simply adding foam on top, without addressing those air leaks, will mean moisture problems in the future.
It’s no fun to get all the cellulose out of an attic, and while insulation installers have big vacuums that can do the job, they usually don’t want to risk ruining their equipment with hidden debris. Unfortunately, removing the cellulose is the only way I know to do the job right. My firm has had good luck using small dust collectors meant for woodshops. When you’re done, seal any air leaks with canned foam, or do a full flash coat between joists.
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