Question about radiant roof insulation
Hello everyone. I was recently contacted by a prospective client who asked me about radiant/reflective roof insulation. He said that he had seen a roof cross-section that showed the roof sheathing, then a layer of reflective insulation, then felt, then the shingles. I am unfamiliar with this method. I do not understand how adding insulation directly below the shingles is beneficial. In fact, I can see it causing more problems than it solves.
What I currently use reflective insulation for is on wall surfaces. I have been very pleased with R-Tech brand insulation. I use it over top of my house-wrap just beneath the siding. I do realize that this is only applicable for certain siding materials and not others. Anyways, is anyone able to offer suggestions or procedures? Thank you.
Replies
I've never seen a radiant barrier installed on top of the sheathing. Typically radiant barrier would be installed on the underside of the sheathing. There are 2 methods: one is buy it in a roll and roll it out before sheathing goes on and leave it hanging loose(about 2" of air space) under the sheathing. The other is to use an LP OSB sheathing that has the foil applied directly to the underside of the sheathing(I forget the name). I've used both options and presumably they both work about the same.
O.K. here is my big question about these 2 options. The manufacturers of the rolled radiant barrier and a few other sources say it won't do its job without the air space but the LP product doesn't have an airspace at all because the foil is applied directly to the sheathing surface. I probably haven't asked the right people but I haven't been able to come up with a reason for the difference.
I don't believe either of these options will do much for you if you will be using spray foam insulation applied to the underside of the sheathing.
The radiant barrier surface needs to "look" to open air (3/4").When it is applied to sheathing the shinny surface is face down into the attic air. In that case the qualify is that that it is a reflector of radiant energy. But rather a low emissive surface so that it does not re-radiate the heat energy.Most (all?) materials that have high reflectivity to radiant energy are also have low emisivity..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Thanks for the input. Here is a link to the LP OSB product http://www.lpcorp.com/radiantbarrier/radiantbarrier.aspxAlso, I agree that it would be doing any good if using icynene foam for insulation.
With a properly insulated ceiling and well ventilated attic, there should be no need for a reflective barrier at the roof sheathing. right?
Wrong. The idea behind a radiant barrier is that especially in the summer it should reduce attic temperatures. By quite a bit in some cases. Whether you like it or not the temperature of the attic does affect the cooling load on the whole house. Same in the winter the idea being the radiant barrier will help radiate some amount of heat back towards the living space lightening the heating load a little. Every little bit counts right?
For radiant barriers to work, they need at least an inch of free airspace in front of them
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Can I jump in here with a question?
How about this spec: sheathing, foam board insulation, Tyvek silver, 1" strapping, another layer of sheathing roofing felt, galvalume metal roofing.
Curious as to opinions on Tyvek Silver.
Edited 5/5/2008 9:43 pm by talkingdog
A radiant barrier's only purpose is to reflect infrared radiation. Most rough or dark surfaces absorb infrared quite well, so if you put one of those materials in contact with the radiant barrier film, it will keep it from working as intended.As the previous poster said, there needs to be an air space on one side of the barrier film.
>there needs to be an air space I have got an airspace there with the 1 inch strapping. I think this is otherwise known as a cold roof. The second layer of ply is supposed to provide soundproofing against oil canning in the metal roof.Originally, I had thought of using silver galvalume roofing as the radiant barrier, but I am afraid of claims from the neighbors up the hill. Better to keep it black and put the barrier inside.
Well, with this build-up, it is not only expensive to procure materials but also expensive and time consuming to install. The way I view it is both cost prohibitive and zero return on investment. Let me qualify that by saying this; the goal was to reduce infrared radiation from heating the attic. With the proposed built-up system, you may get a well insulated attic, but not the originally intended purpose.
With a build up like this, a SIP would be much more cost effective. But then back to the issue of radiant heat reflection. Maybe we can get them to build a SIP w/ a reflecting surface built in.
For radiant barriers to work ... they need:
1) air space ... as pointed out
2) a reflective surface - while you may install bright shiny mirrors for a radiant barrier ... what happens to that surface over time (e.g. just before the house is occupied) ... it gets dusty as heck ...
just say no to most radiant barrier applications ... they give you marginal benefit for the money and effort. And don't EVER buy into that notion of 'effective increase in R-value' ... it is BS.
My opinion.