Easiest way to insulate under shingles?
Last Post 08 Jul 2015 04:52 PM by richa-774. 15 Replies.
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Dan KegelUser is Offline
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27 May 2015 01:54 PM
When I proposed polyiso panels, my roofer grimaced and said that was hard to put on because you needed to top it with plywood. He said "Why not just use that roll stuff? You can get away without putting plywood on top of it." After digging a bit, I think he was referring to http://www.low-e.com/thermasheet.html which appears to be hokus pokus bunk; its actual R-value is 1.3, but downhill with a tailwind, if installed just so, it acts as a radiant barrier, too. (Never mind that their pictures show it being installed over plywood or felt, where it can't possibly act as a radiant barrier.)

But it brings up the question: if all you want is a cheap way to reduce the amount of heat soaking into the attic from a hot roof, what are your options? Sure, there's Energy Star shingles, and we'll be using them (we have to, the city requires it).

Not all Energy Star shingles are identical; I guess I could go shopping for ones with higher reflectivity.

But is there also some R-2 rolled foam product that a roofer could roll out over the plywood and under the shingles without risking rot?

Bob IUser is Offline
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27 May 2015 02:03 PM
I'm still trying to wrap my head around the idea of insulating above vented attic. If the heat can bypass the insulation, what's the point? The whole idea of insulating above the sheathing is to keep the attic cooler, which will not happen if you vent the attic. And yes, you need to install plywood above the foam, and it should be screwed into the rafters. It's really a job for a carpenter, not a roofer.
Bob Irving<br>RH Irving Homebuilders<br>Certified Passive House Consultant
gosolarUser is Offline
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27 May 2015 03:39 PM
go shopping for ones with higher reflectivity.
arkie6User is Offline
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28 May 2015 12:24 AM
Posted By Dan Kegel on 27 May 2015 01:54 PM

But it brings up the question: if all you want is a cheap way to reduce the amount of heat soaking into the attic from a hot roof, what are your options?

One option is foil faced roof decking.  The cost for foil faced OSB decking is typically ~$3/4x8 sheet more than comparable thickness regular OSB decking in my area.  I haven't priced the difference on foil faced plywood vs. regular plywood, but the difference should be comparable to OSB.

Note that the radiant barrier foil side of the decking faces in toward the attic.  It seems counterintuitive that this would work, but it does.

http://www.buildgp.com/thermostat-r...er-plywood

http://www.norbord.com/na/roof-sheathing/solarbord

http://www.homedepot.com/p/Unbrande.../202106242



jonrUser is Offline
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28 May 2015 07:26 AM
Insulation under the shingles of a vented attic will keep the attic cooler (and the shingles hotter). But typically you care about the heat transmitted into the house and for that, just add more attic insulation (typically blown in cellulose).
arkie6User is Offline
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28 May 2015 07:31 AM
As jonr mentioned, just adding a few more inches of insulation, assuming an infrared blocking insulation such as cellulose rather than fiberglass, will be as effective at keeping heat out of the home envelope as installing the radiant barrier decking I mentioned. That is unless you have your HVAC equipment and/or ducts in the attic. In this case, keeping the attic cooler will help the HVAC performance and reduce losses.
AltonUser is Offline
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28 May 2015 12:04 PM
I think the radiant barriers that have the foil side down are Low-E instead of radiant. Sales people tell me radiant but engineers tell me Low-E. To be radiant, the foil should be on the top side with an air space. I have used this type of roof decking with mixed results, as far as the homeowner is concerned. Some owners think it works well and others say they cannot notice any difference in temperature between old and new portions of their home.
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Dan KegelUser is Offline
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28 May 2015 01:54 PM
The attic will have R30 on the floor, but alas, HVAC equipment and ducts are in the attic. Yes, I have read http://www.nrel.gov/docs/fy10osti/48163.pdf ("Ducts in the Attic? What Were They Thinking?"), but that's just the way the house is.


I think the cheapest legal (i.e. energy star) shingles are $75/square, and ones that reflect about 50% more are about twice that. But whether the wife will go for them is another matter, none of them look brown enough for her.


The path of least resistance for me may be to just go with whatever shingles the wife likes, skip deck insulation, get the duct guy to use metal bends per http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/53540/Keep-Your-Elbow-Rigid-A-Lesson-in-Flex-Duct-Installation, make him verify the ducts are nice and tight, and then wrap them with R-19 myself :-)


I suspect people who claim good results from "low-e" roof deck layers are experiencing the placebo effect.

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28 May 2015 03:20 PM
Posted By Alton on 28 May 2015 12:04 PM
I think the radiant barriers that have the foil side down are Low-E instead of radiant....
I doesn't matter which way the foil is facing, up (with an air gap) or down (with an air gap), it is still considered a radiant barrier.  And to be considered a radiant barrier, it must have a low emissivity (low-e) coating.  The foil faced roof decking is a radiant barrier having a low emissivity aluminum coating.  It functions essentially the same whether the foil is pointing up or down.

From:  http://en.wikipedia.org/wiki/Radiant_barrier

"All materials give off, or emit, energy by thermal radiation as a result of their temperature. The amount of energy radiated depends on the surface temperature and a property called emissivity (also called "emittance"). Emissivity is expressed as a number between zero (0) and one (1) at a given wavelength. The higher the emissivity, the greater the emitted radiation at that wavelength. A related material property is reflectivity (also called "reflectance"). This is a measure of how much energy is reflected by a material at a given wavelength. Reflectivity is also expressed as a number between 0 and 1 (or a percentage between 0 and 100). At a given wavelength and angle of incidence the emissivity and reflectivity values sum to 1 by Kirchhoff's law.

Radiant barrier materials must have low emissivity (usually 0.1 or less) at the wavelengths at which they are expected to function. For typical building materials, the wavelengths are in the mid- and long-infrared spectrum, in the range of 3-15 micrometres.

Radiant barriers may or may not exhibit high visual reflectivity. While reflectivity and emissivity must sum to 1 at a given wavelength, reflectivity at one set of wavelengths (visible) and emissivity at a different set of wavelengths (thermal) do not necessarily sum to 1. Therefore, it is possible to create visibly dark colored surfaces with low thermal emissivity.

To perform properly, radiant barriers need to face open space (e.g., air or vacuum) through which there would otherwise be radiation."

arkie6User is Offline
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28 May 2015 03:28 PM
Posted By Dan Kegel on 28 May 2015 01:54 PM

The path of least resistance for me may be to just go with whatever shingles the wife likes, skip deck insulation, get the duct guy to use metal bends per http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/53540/Keep-Your-Elbow-Rigid-A-Lesson-in-Flex-Duct-Installation, make him verify the ducts are nice and tight, and then wrap them with R-19 myself :-)
Why not just use all metal rigid duct to start with? 

Whatever you use, air sealing needs to be meticulous since it will be located in the attic.
Dan KegelUser is Offline
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28 May 2015 03:33 PM
I assumed flex was ok and cheaper for long straight runs. I could ask how much more all-metal would cost. Maybe that would be less prone to developing leaks later.
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28 May 2015 03:57 PM
Posted By Dan Kegel on 28 May 2015 01:54 PM

I suspect people who claim good results from "low-e" roof deck layers are experiencing the placebo effect.



There is test data to support the claims.

http://web.ornl.gov/sci/ees/etsd/bt...et2010.pdf
Dan KegelUser is Offline
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28 May 2015 05:17 PM
I was referring to people who install it without the requisite air barrier.
jonrUser is Offline
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30 May 2015 03:18 PM
Reflectivity and emissivity are very different things, but when a barrier is using reflectivity, it should be called a "reflective insulation system". In such cases, Kirchhoff's law is misleading, because the system is unlikely to be at thermodynamic equilibrium (due to things like convective air movement).
Dan KegelUser is Offline
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30 May 2015 03:22 PM
It's still a scam.
http://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/30705/why-reflective-insulation-still-being-sold-hd
richa-774User is Offline
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08 Jul 2015 04:52 PM
Can you just use blocks of cut foam under the roof deck? They can be installed with a gap for venting.
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