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TOPIC: Insulation Concerns

Insulation under metal roof

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Hello, I am currently looking into adding insulation to my roof while I re-roof the house this summer. The house has a T&G ceiling with exposed rafters. My only option for insulation will be to install it on the top of the roof. It currently has 1" of foam with asphalt shingles. I was thinking about adding 8-10" of foam and using a metal roof. I am in the beginning stages of researching this and found this site to be very helpful! The layers right now are T&G wood, foam, plywood sheeting, shingles. Would it be possible to leave the foam and plywood and add the foam and metal roof over these existing layers? What needs to be between the metal roof and the foam? Is the cost of the standing seam cheaper than the metal shingle style roof? There a roofing product sold at my local box store made by Midwest Manufacturing, is that product any good? I attached a picture of the house, do you think a standing seam roof would look good on this style of a house? Sorry its the only picture I have right now.
Guest User

12/15/2010
Yes. You will be creating a SIP (Structurally Insulated Panel) in effect. Depending on what foam you are looking at, 8-10 inches might be overkill. I would also advise that you seal up that first layer so as to complete the envelope layer of the home. T&G wood is very leaky and will allow a bunch of the conditioned air out of the home.
Eric Novotny
An informed customer is the Best Customer!

12/16/2010
What would be the best way to seal this T&G wood up? I did notice another thread that said to use the ice and water shield that is sticky. If I used polyisocyanurate board I would still need around 7" in order to get my R-49 rating for the roof right? I checked out Dan Perkins Roofing and found some very useful information on insulating over the roof. He stacked 2x4's and insulated in between then, if I was going to add 7" of insulation to my roof how would I accomplish the framing? And finally, I included a very quick sketch of my house and the layout of the T&G and attic space. If I was to remove the current insulation and decking over the T&G to tighten up the air gaps, I would be at a different height than the attic space roof. Does anyone have a recommendation on what to do to get the heights to match? Thanks for the help!!!
Jeff Frantz

12/22/2010
If you get the deck air tight (Peel and Stick Ice/Water down as an air barrier), the reality is that you don't really need a full R-49. Foam is much better and performing as its job as an insulator as compared to fiberglass that an R-30 of foam probably equals out to an R-49 of fiberglass. If you are going for a full 7", you are going to need to layout the grid and sink some 9-10" Sips screws to attach a plywood topper to that foam. That is also quite a tall step and I would prefer to see a grid pattern of 2x4s laid out like that Perkins roof that you read about because I would be suspect of the attachment of that plywood topper to the real roof.
Eric Novotny
An informed customer is the Best Customer!

12/23/2010
So the best bet will be to take the roof down to the T&G and cover with ice sheild, then I will have to build this up in order to match the decking height of the attic side of the house. I will then layer the 1.5 inch foam. I was thinking 3 layers would get me right around the R-30 number.
Jeff Frantz

12/23/2010
That is the only way to seal up the T&G if you are trying to stop the air loss. R-30 in foam is quite good at that point.
Eric Novotny
An informed customer is the Best Customer!

12/23/2010
If I was to build the roof insulation up like Perkins does over the cathedral ceiling with 2x4's and foam board, could I just run the 2x4's over the attic area without the foam board? This would leave a big open space between the rafters and sheating and the new roof height. This would help save money on the foam board in areas that are attic spaces. Attached again is the drawing of the cathedral ceiling area and the attic space.
Jeff Frantz

12/28/2010
Yes, as long as you have an insulation layer that is below. In that case, the insulation layer and your envelope layer must line up so that is the area that must be sealed.
Eric Novotny
An informed customer is the Best Customer!

12/28/2010
In a sense I will be creating a vented space over the attic space, this should only help the attic stay cooler correct? There wont be any issues with this open space, like moisture problems?
Jeff Frantz

12/30/2010
Attic ventilation is only to control humidity. I would not concern myself will trying to keep attic temperatures down if you have proper insulation.
Eric Novotny
An informed customer is the Best Customer!

12/30/2010