Insulating a ceiling. Air gap or not?
When is leaving an air gap between the top of ceiling insulation and the roof decking with ventilation a must, and when is it acceptable to completely fill the space with insulation? I’m building a small shed with 2x6s supporting the roof deck. The ceiling is vaulted. I want to install the maximum amount to insulation, but I’ve heard ventilation is necessary for longivity of the shingles. I’ve also seen where the entire cavity is insulated for maximum thermal performance. I haven’t seen a clear discuss of when either is allowable or necessary.
If local weather is a factor, I live in the peidmont of the Carolinas.
Replies
What exactly are trying to achieve with the insulation? Are you trying to keep your lawn mower cool, or is this shed going to be a workshop?
It's got nothing to do with shingle life. The ventilation is needed if there is a source of moisture in the structure that would cause moisture (if not vented) to condense in the cold space near the roof. A shed, even if occupied on occasion, probably has no real moisture source to worry about.
Now, it would be different if this shed if for your indoor herb garden.
Thanks! Another question.
Thanks for the response. I'm going to use the shed as part storage and part woodworking shop. Air conditioning is needed not only for my comfort but to keep my tools from rusting away. Articles in Fine Homebuilding have recommended completely insulating the cavity in some homes. But, I think they've only recommended sprayed foam for that situation. I don't remember any article recommending using any kind of fiber insulation. Is fiber insulation unacceptable because it's permeable to moisture?