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Ask-The-Experts > Installation

metal panels over shingles; most energy efficient way
I usually install metal roofs (3 ft wide metal panels) over asphalt shingle roofs with only a synthetic underlayment between the metal and shingles; no furring strips. If the metal is installed this way and existing turbine vents - no ridge venting - are re-used to vent the attic, is there much gain in energy efficiency since it seems the shingles underneath the metal would get just as hot as if the metal wasn't there? The metal we use is all energy star rated but the surface of even light-colored metal gets very hot. Since the metal is only millimeters thick, it seems the heat would transfer to the shingles. I would think at a miminum ridge venting would have to be used to allow heat between the metal and shingles to be vented. Or does that even make much of a difference? I see all the time that metal roofs can lower energy bills by up to 25% but is that only possible when 1) the shingles are removed, or 2) when furring strips are installed over the shingles to provide a gap to allow heat absorbed by the metal to flow under the metal, over the shingles, and out the ridge vent? Thanks.
An additional airspace can indeed act as a thermal break and enhance the energy efficiency. And it does not require much airspace -- just something to stop the direct contact between the metal and whatever is beneath it. Many of the metal shingles have an integral airspace that achieves much of this benefit.

Additionally, good attic ventilation is helpful.

The federal tax credit that was in place for metal roofing, though, was for products with reflective pigment in their coatings. Reflective pigment blocks radiant heat which passes through materials normally and warms airspaces.

I have a video up on energy efficient roofing that you may find helpful. http://www.youtube.com/watch?v=MoaIqhMXPag
Thanks for the reply. Most of my customers here in NE Arkansas would rather have a metal roof because of their longevity and their greater resistance to hail and wind. We have quite a few tornadoes, hail, damaging wind each year, usually. Metal roofs cost more, though, than asphalt shingles, but if we install the metal directly over the shingles, the cost is usually only 10-20% more than asphalt shingles. The higher initial cost is offset, though, through energy savings but only if the metal roof is actually more energy efficient than asphalt shingles. I tell customers what I've read about metal roofs: energy savings up to 25%, but more likely 10-20%, can be obtained by installing a metal roof. My question is this: is that true if metal is installed directly over shingles and no ridge vent is used? I am thinking it's not. I've been looking for published research but have found none.
The use of reflective pigment metal roofing, all other parameters the same, will result in improved efficiency compared to standard shingles. I would say that your 10 - 20% number is probably accurate.

However,metal works very well with various types of ventilation and that ventilation will enhance the efficiency considerably.
Good video Todd.

The better approach would be the use of some over deck venting and the installation of the new roof panels on purlins.
I have a very old hunting cabin in the Upper Penn. of Michigan that badly need a roof job. It currently has about 4 layers of rolled and single shingles. Is it possible for me to install the metal roofing over the top of what is there? The building is over 100 years old, and made of iron wood (solid as a rock). There is no attic, or valleys or gables. Just two sloping surfaces sharing the common peak.

We're in our mid to late 60's so the chore of removal of the exhisting roof is a concern. As a result, we're considering the "easy way out". Any suggestions or comments will sure be appreciated.
There obviously may be some things I don't understand but what you have described to me sounds like something that could be installed over if that is your preference.
Thanks for the speedy response. I guess that settles it, we'll put the steel on top of everything currently up there. We hoped we could do it that way and just needed the professional opinion before committing to the job. Thanks again.
Do I have to use purlins on my shingle roof ?
There are a few metal roofing products which MUST be installed with battens, Many which can be installed with or without battens, and also some which must be installed without battens. So, it all depends upon your product selection.
i am applying the 29 gauge metal roofing over asphalt shingles. Should I use furrong strips?
You can. It probably is not essential though.
I have asbestos shingles currently on my house. Would it be safe to install a metal roof over the asbestos shingles or would I need to contact a contractor to first remove the asbestos shingles in a "safe" manner?
I would suggest checking local building codes as well as with your state EPA office. I have always found that the EPA suggests encapsulization (sp?) rather than removal and disposal. Then you need to check with your metal roofing supplier and see what they say. I always suggest underlayment over the old shingles, and also trying a few test fasteners before beginning the installation, to make sure your fasteners do not cause the old shingles to crack badly.
i am thinking on using metal shingles or panels to be installed over existing asphalt shingles. My roof has four open valleys. Most metal shingle designs interlock to each other and valleys. Because of the valleys would it be required to have a tear off of the existing roof so that a new valley could be installed that has the interlock feature? If panels are used would a tear off be required due to the valleys?
Chuck S, most metal shingles would install over your existing if you wish. Underlayment should be please over the old shingles and then the new metal shingle system including pre-formed valleys and other accessories.


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