Metal over Cedar shake? ventilation advised at home inspection, and more questions

Louis The
3/14/2010
I’ve been scouring thru tons of information trying to learn more about roofing, metal in particular. I have several questions. I will be as brief as I can. The home I am in is a kit home from the 70’s. At the energy audit I was told that the existing roof had an R value of like 20 (maybe it was closer to 13?). It is a cathedral ceiling throughout (tounge and groove inside) except over the garage where there is attic space. There is apparently no ventilation (except over that garage attic area). There is some old original foam insulation under the shake (what the inspector told me from looking at some plan I had from the old owner) Is roofing and ventilation handled differently with a Cathedral ceiling? Living in Alaska there is a lot more cold than hot, but I do not want my upstairs loft bedroom to get too hot from trapped heat. Is this a concern I should have? Right now I have a ton of ice dams. The old owners used “snow rakes” to deal with this. What should I be most concerned with and discussing with a potential contractor? Obviously there is a ton of heat loss going on. We do get quite a bit of snow. I know these roof’s shed the snow easy, but I also have a surrounding deck with railing that I wouldn’t want ripped of from snow slides. If we use snow brakes, does it matter how much snow sits there? From what I have read I can install metal over the old roof. Under these circumstances is that either recommended or not recommended? Why? One last thing, assuming I can install over the old rook, and put some passive layer of ventilation between the old roof and new one, doesn’t that negate the entire insulation process? Sorry for the length of this. Hopefully it helps others as well. I attached a pic of the roof.
Info @windowsonwashington.net
An informed customer is our best customer.
3/14/2010
Ventilation for roofs is designed to exhaust excess humidity and has nothing to do with heat. Most of the cooling of the roof happens on the outside surface of the roof and there are only very small temperature differentials between open vs. sealed attics in far more cooling dominated (i.e. Nevada) than Alaska. The snow melting that you experience is partially as a result of the sun and outdoor temperatures melting the snow load. Most homes that have ice damning issues are a result of heat/warm air migrating from the living space and melting the snow that is in direct contact with the roof deck. You have T&G boards that are unsealed and therefore leaking warm air through them and contributing to the snow melt/ice damning issues. The foam board, although probably inadequate for insulation value, is helping mitigate the radiant heat loss but is likely unsealed against air loss and probably not thick enough to control the radiant heat loss. In your application, I would not re-roof over the cedar shake roof. To help combat the ice damning, I would strip the roof and existing foam. I would air seal the roof deck by covering the entire roof surface with a moisture/air barrier like Ice/Water shield. You could then put a more substantial foam layer over the entire roof deck or use a vented roof system over a foam layer. Here is a link to an article that details the latter of the two options: http://danperkinsroof.com/1108_JCL_Perkins_A.pdf This is not going to be a cheap option but it will be the solution to your problem.
Guest User
3/14/2010
thanks so much for the early insight. Looking forward to others responses. Yes, I suspected that it would not be a cheep solution. At some point it was mentioned to consider stripping off the roof and using the spray foam insulation as that would take care of the airleaks from the tounge and groove. Any thoughts on that? Is the cost considerably more or less? In the end would it be worth the extra expense? It's a 2000 sq/ft home. Am I kidding myself at hoping this will be done for 40K? In the name of cost savings, does the roof over the attic area need to be done in the same manner (insulation and total sealing off) as that over the living area?
Guest User
3/14/2010
Oh, thanks for link. Much appreciated!
Info @windowsonwashington.net
An informed customer is our best customer.
3/15/2010
Spray foam will accomplish both the thermal and air barrier aspects that you are aiming to address, however, spray foam does not go on uniformly and therefore does not leave a smooth, flat finish like a rigid foam will do. As far as the roof over the attic area goes, no. It does not, nor should it really be done the same way. In that application, your attic floor is the insulation and envelope/air barrier and should be treated as such. The attic floor should be air sealed and the insulation brought up to R-50 minimums. I don't know what roofers are charging up there, but we could certainly do that roof for less than 40K.
Guest User
3/15/2010
thanks....LOL - well when I get the quotes it might be cheaper to fly you guys up then(:)). Seriously, I have no idea what it'll cost. I am just guestimating, but typically things up here are considerably more expensive due to the cost of shipping materials all the way up. A neighbor said he had a quote approaching 80K, but the slope of his roof is insanely steep. Who knows what was behind that quote or even if it was exagerated. Thanks, you've been informative and helpful. Much appreciated.
Info @windowsonwashington.net
An informed customer is our best customer.
3/16/2010
Roof quotes are like opinions...everybody has one and they can be quite different. Research and educate yourself prior to getting another quote.
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