Hi All –
I’m designing my new house and am getting different answers from my architect, my insulator, and multiple builders on two roofing questions. Some background: The house is located in New Hampshire (think cold winters) and has a metal roof. The insulation we’re planning to use is blown in cellulose, and the rafters are 2×12. The questions: #1 – do we need to have a ridge vent and soffit vents? The insulator and architect say no, while several builder friends say yes and others say no. #2 – does a membrane such as Bituthene need to be applied on the entire roof or just a 3′ section on the edge with heavy felt paper on the rest? Given that it is metal, the snow should slide off anyway I am told. I’m willing to do whatever it takes on both of the issues to make sure I am building a “100 year” house. Thanks for your advice.
PBM
Replies
I am in the process of reroofing a house that will not have attic ventilation and will be insulated between the rafters. The result is a conditioned attic. However, the rule of thumb that is most prevalent is to ventilate - studies and letters (ASHRE, University of Illinois, Florida Energy group, and a study from Las Vegas) show there are better ways of controlling the environment. You may also go to the web and search under ATTIC VENTILATION and find many of these studies. If you will forward a fax number, I will fax recent reports and letters on the subject of ventilation.
Randal
Marietta, GA
Thank you Randal. My fax is 603-485-9549. The roof in question will be a cathedral type with no attic space....PBM
Speaking of searchs, have you tryed the one here before we answer these questions all over again?
I vote for no vents. I used to be a big vent proponent, but then I got a house with unvented cathedral ceilings and reroofed it. I bought all this sheathing expecting to find massive damage, but I didn't have to replace a single sheet. Plus, the main idea behind venting the roof is to keep the surface temperature down to make asphalt shingles last longer. You got metal. No worries. I would think you're just asking for hot air to be sucked out of your interior if you put in ridge vents and soffit vents. I think that would be a big problem for you in the winter. You'd have to be SO diligent with caulking the drywall to the sills and top plates and all that carrying on. Who does all that? Going by that method, you'd ruin the whole envelope if you hang a picture with a molly bolt. I say just don't put the suction on it.
I tell you what I would definitely do if I was putting a metal roof on a cathedral ceiling, though, is that warm roof thing. Or is it cold roof? I suppose it depends on your perspective. Anyway, you got sheathing or rigid foam and then some furring strips to make an air gap, then your roofing. That just sounds like it reduces a lot of risk to me.
I know what you mean about getting different opinions on this, though. My husband can read the exact same article as me and come away convinced we should do the exact opposite thing than I got from it. It's all very subjective. Here in the South the latest literature says not to use a vapor barrier, but you can't even get fiberglass insulation without it. Science doesn't always prevail over market forces.
Good luck!
B