Attic insulation and moisture management
I am in Chicago (Climate zone 5) and have a deep energy retrofit on my hands. The 111 year old building has a “low-sloped roof” and we recently finished insulating the attic. We packed 10 ½ inches of rock wool batts between the roof joists and mounted 4 inch thick XPS sheets to the bottom of the roof joists. The joints between the XPS sheets were sealed with a thick bead of closed cell SPF. This should render the system pretty airtight.
I have to assume, however, that some moisture will still migrate into the insulation assembly and roof structure. To let that moisture dry out, I plan to ventilate my roof deck by mounting two by two’s parallel to the roof slope onto the existing deck. A layer of ½ plywood over the two by two’s will be my new roof deck. The ventilation intake and exhaust will be along the low and high point edge of the low-slope roof.
I have read the GBA Blog posts “Insulating Low-Slope Residential Roofs” and “How to Build an Insulated Cathedral Ceiling”, which were very helpful, but have two questions which keep nagging me:
1) Am I kidding myself by thinking that ventilation will keep my cold roof deck and roofing structure sufficiently dry?
2) I don’t quite trust the venting rule of 1 square foot of net free ventilation area for every 300 square feet of roof area. Shouldn't there be a minimum airflow requirements that must be met?
Posted Mon, 12/02/2013 - 23:04
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