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Unvented Low-Slope Roof With Radiant Barrier

h4x | Posted in General Questions on

Good evening everyone,
I was reading through this article here:
Insulating Low-Slope Residential Roofs 
and this got me thinking…say you have a 2/12 mono slope shed roof…I get that you would need a significant air gap to properly vent that roof assembly, but what if the purpose of the air gap was just for the radiant barrier?

For example, consider a 2/12 mono roof that connects to your house’s sidewall, zone 4, cathedral style.  The roof assembly is a mix of fluffy insulation internally and rigid foam insulation externally. On top of the rigid foam, you have a layer of roof decking covered with everyone’s favorite self-adhesive ice and water shield. Furring strips are added to create an air gap…not for venting but for a radiant barrier. A second layer of decking is installed with foil facing toward the air gap, and additional underlayment and shingles are applied to wrap things up. The air gap would likely be open to the soffit below, but there would be no vent at the sidewall at the top of the mono slope.

Is there something wrong with this approach?

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Replies

  1. GBA Editor
    Kiley Jacques | | #1

    I’m giving your question a bump. While you wait for builders to chime in, look at this roof assembly: The Evolution of an Energy-Smart Roof. Your situation made me think of it. It’s a sealed attic with an elevated roof cover over a ventilated deck. The idea is to promote above-sheathing ventilation. The architect uses a radiant barrier in his hot-humid climate. The angled furring seemed like a smart innovation.

    1. h4x | | #4

      Thanks for the response, Kiley. Yeah, the premise is the same: elevate the roof cover off the decking. However, my concern is whether there will be other moisture issues due to the low pitch.

  2. Expert Member
    Peter Engle | | #2

    The gap between layers of decking and radiant barrier might work a little, but at great cost for the extra materials. The net radiant effect will be low because there will be so much convective heat transfer through the gap. Without any meaningful ventilation, the air in the gap will approach roof surface temperature with or without the radiant barrier and small local convection cells will efficiently move heat across the gap. Radiant barriers are most effective when they separate surfaces of significantly different temperatures. I'm not sure that will happen much in your assembly. You could get the same effect (if any) at much lower cost by using foil-faced polyiso insulation with taped seams. Cover that with furring strips, then sheathing. Or, use foil-faced radiant barrier sheathing. The two layers of sheathing plus ice&water are expensive overkill. And if you're going to the trouble of creating an air gap, vent top and bottom. This may or may not improve the thermal performance on a low-slope roof, but it's pretty cheap to do. And, upgrading the gap by using 2x3's instead of 1x3's would also help.

    1. h4x | | #5

      Thanks for the info, Peter. Based on the initial article, part of my concern was that a low pitched roof like this would need something greater than 2x3's for the air gap...2x3's seem like the simple solution that doesn't break the bank. Couple that with the foil faced polyiso and that takes out two birds with one stone.

      My primary concern was what other problems (water and thermal) I would run into by having an elevated roof covering from the decking if it wasn't being vented...and if that was going to be worth the trouble of doing so.

  3. Charlie Sullivan | | #3

    Your proposed design would work. You would get perhaps R-5 out of it, for heat flow downward. The question is whether there's any reason to do that vs. make your rigid foam that you are already installing one inch thicker. The foam would be the same in the summer and better in the winter. The gap+foil might be cheaper in materials, but the extra labor probably negates that benefit.

    If you like building stuff like that, with foil and fussy details instead of foam, you could go all out, and use 8 layers of foil, with furring strips between each each, so you have 8 air gaps, and get R-40 out of it. But do you really want to do that?

    1. h4x | | #6

      Don't tempt me with fussy details! lol
      You bring up a good point, and one I hadn't really considered. Up till this point, I was planning to vent the roof, top and bottom...but when I ran across the article about venting a low pitch roof, that called the design into question. However, if 2x3's for an air gap are sufficient for a 2:12 roof, then I'll continue with that plan.

      The roof design is definitely a belt+suspenders approach (and some might argue an elastic waistband, too). Three principals were at play in the design...1) vented roof on a cathedral ceiling. The air gap would allow drying to the outside, and also serve as the radiant barrier (as Peter noted above, venting is needed to help mitigate the heat transfer via convection)...2) I am extremely paranoid about roof penetrations on the roof covering. At the very least, if there was any significant water penetration through that layer, it would likely drain off/dry out in the air gap...3) We were initially going to have a metal standing seam roof but have since changed the roofing material...the air gap was there to help with condensation on the underside of the metal. However, even though we aren't using the metal roof anymore, the air gap still remained in the design.

      All that said, I hadn't considered yet the effective R-value provided by the radiant barrier...if I need to add another inch of foam and make the roof assembly unvented, then that is a simple solution.

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