I’m building a house with a cathedral ceiling ( 18 inch wood I-joists with 5/8 sheathing). I am using metal roofing on top of purlins which will give an air space between the metal and the sheathing. Should I then be concerned about ventilating between the insulation and the underside of the sheathing?
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In nearly six years of hanging out here I've seen more disagreement on this topic than anything but religion, politics, and which side the blade should be on.
But here's my 2 cents anyway. <G>
"should" is a loaded term. As in "should I ventilate"
1) Check you local codes. You might even talk with the inspector ahead of time. If no inspectors or codes, see 2).
2) Buy Joe Lstiburek's book on moisture control for whatever climate you are in. Joe is one of the few people who can accurately, intelligently, and painlessly provide you with enough information and knowledge to arive at a decision that is suitable for the building, the climate it is in, and the intended purpose. Check out http://www.buildingscience.com
3)The dynamics of thermal and moisture control in the modern, energy -efficient building envelope involve a lot more than any pat answer can provide. Except for the very driest of climates, you must assume that some moisture is going to get into unheated space and that it may condense. If it does, you need to plan for some way for that moisture to be reliably dried out of that space.
Depending upon the climate, construction, and use of the building, that drying can reliably occur to the outside, the inside, or if you make a mistake, not at all.
DRC
Thank you Dave, I'll get that book!
additional ventilation not necessary, IMO
Book Good!
Excellence is its own reward!