greg dickson
9/11/2017
I have a two story gambrel style roof house located in North East Alabama, it has an asphalt shingle roof that is old but in decent enough shape. I want to install a metal roof using unpainted galvalume. One of my main goals is to reduce heat build-up in the house during the summer. I was told by one contractor that the metal roof could be installed over the existing shingles, since they were in good shape. I was told by a metal roofing distributor (who knew I had a contractor and would only be buying the metal from him) that if I did not remove the shingles I would still have the heat build up. That even though the metal would quickly cool, the shingles would retain heat and heat the house long after the sun went down. I thought the shingles would act a little like insulation under the metal roof. If not, I would assume that the metal roof would keep the shingles from getting AS hot as being directly exposed to the sun, but will I still have the heat to deal with? Should I remove the shingles to get the benefit of the cooler metal roof? thanks, greg
Todd Miller
Classic Products, Inc.
9/11/2017
Gambrel roofs often have dormers and other protrusions which must be considered in terms of how the thickness of a metal roof with battens, etc will impact things. Installing a metal roof that either has an integral airspace or can be installed with one (such as would be accomplished by using battens) can enhance energy efficiency. Many of the metal shingles have integral airspaces while other products can be installed on battens. The airspace acts as a thermal break, stopping or reducing conductive heat transfer. Additionally, you may wish to look for an Energy Star listed metal roof.
Guest User
9/11/2017
So, if battens are used creating an air space, will that make a difference on the asphalt shingles being heated? Would it be better to remove them or would it not make that much difference if the air space is created
Todd Miller
Classic Products, Inc.
9/11/2017
There is no test data on this subject but it is my opinion that the R Value of the asphalt shingles is a positive thing, as long as there is a thermal break (airspace) between them and the asphalt shingles. Now, one other option ... you can cross battens (vertical battens and then horizontal battens) and actually vent the resulting vertically oriented chambers, to help get any gained heat back to the outside ... and that is hugely effective.
Guest User
9/11/2017
Thank you for your help and the quick response. I am not skilled in roofing or physics, but it seems logical that keeping the asphalt shingles ON would help in all respects. However, I have found that logic does not always point to the correct answer. thanks, greg
Info @windowsonwashington.net
An informed customer is our best customer.
9/12/2017
Keeping all the thermal mass on the roof (i.e. thermal storage battery) will have the opposite effect when it comes to transferred heat into the attic/framing assembly. Even with some cross battens and above deck venting, the plywood deck (i.e. without asphalt) will absorb and transfer less heat into the structure. If heat is the pressing issue, remove the deck and incorporate some above deck venting and radiant barriers.
Guest User
9/14/2017
Hi, did a little reading and had more questions. This is not arguing with a professional, cause I don't know enough to argue. I would really like to keep the shingles, for a lot of reasons, but if it is best, I will remove them. IF I leave them on and use Roofing foil will that stop heat build up on the shingles and roof deck? I would use vertical 2x4s on the shingles with battens on top of them for the air space. That way, the hot air would have the vertical space between the 2x4s to travel up the roof and out into open air through the ridge vent. Whether the shingles stay or go, would it be a good idea to put rigid insulation up? If I wanted to put 1" rigid foam insulation, how would that be installed with the foil or on the shingles? Would it cut and attach to the underside of the battens (facing the foil or shingles while leaving an air gap)? Or could the rigid insulation just be laid on top of the 2x4 vertical runners with the battens on top of it, holding the insulation, then attach the metal roof to the battens? thanks, greg
Info @windowsonwashington.net
An informed customer is our best customer.
9/14/2017
Why do you want to keep the shingles? Lets start with that first and then figure out if this is a good fit for you. If the aim is to make the roof deck cooler, getting rid of the thermal mass and providing for some above deck venting (as well as some radiant barrier) is the best combo. The shingles do not really serve as an insulator in this case. I like your cross batten approach and putting some foam between the 2x4s on the vertical side with a foil facing would make for a very good barrier and would really move some convective air flow. The only thing not taking off the shingles is doing there is saving you on the labor and debris removal. Between all the additional wood and technical details, I would prefer (i.e. on my home) to be installed directly to the deck of the home. That is just me.
Guest User
9/14/2017
Still hard not to think of the shingles as protection/insulation, but I see your point. I guess the only real reason left is the cost and mess of tearing the shingles off. Years down the road I guess I had rather have the best installation. When you say "foam between the 2x4s on the vertical side with a foil facing" are you talking about the foil facing the deck or foil facing the underside of the metal roof? I was looking at RoofingFoil™ radiant barrier to go on the deck facing the bottom of the metal roof. If the shingles are removed, where should the rigid foam and the RoofingFoil™ radiant barrier go? thanks, greg

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