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Radiant Barrier under spray foam

kramttocs | Posted in General Questions on

I see a lot of mention of both products but ‘Under’ is always used differently than I am intending.

I have a small attic area used for minimal storage. 16′ long, 8′ across, and about 5′ at the peak.
The 2×6 rafter cavities are filled with ccspf. Rafter faces are covered as well by on average 1.5″ foam.

The foam is working great but I got to wondering: would adding strapping across the rafters (provide the air gap and a place to staple to) and a radiant barrier make it even better? Can the radiant barrier be on the room side of the foam and still provide a benefit?

This area isn’t directly conditioned but it does have intake and exhaust fans (the ac infinity 12v crawlspace kind) that come on if the temp or humidity get high enough.

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Replies

  1. Expert Member
    BILL WICHERS | | #1

    Don't waste your time with a radiant barrier here -- it won't really accomplish anything. The only place I will sometimes put a radiant barrier is in a vent channel, where I can get it "for free" by using a layer of foil-faced polyiso as the vent baffle. All you get is about R1 worth of extra insulation with a radiant barrier, so it's not worth the effort or expense to add unless you can get it "for free" in some way.

    Bill

  2. Expert Member
    PETER G ENGLE PE | | #2

    First, a question: Do your intake/exhaust fans connect the attic with the interior of the house or the exterior. You don't want them going to the exterior or you significantly reduce the effectiveness of the insulation.

    In answer to your original question, No, you don't want to install a radiant barrier inside the insulation. It might help a tiny bit, but not enough to make it worthwhile. Radiant barriers work when there is a large radiant temperature difference between two surfaces. Heat flow from a hot surface (the underside of the insulation) to a cooler surface (the insulation side of the radiant barrier) is a function of the temperature difference between them. When the radiant barrier is facing a 140F+ surface and there is conditioned space below, it's working with a 70F+ temperature difference. In your case, the bottom of the insulation is probably near the conditioned room temperature. Maybe 5 degrees or even less if the spray foam is doing its job. Sure, the radiant barrier might reflect some of the remaining radiant heat, but a small percentage of those 5 degrees isn't much.

  3. Expert Member
    Michael Maines | | #3

    When you don't have any insulation, the roughly R-1 you get from a radiant barrier can make a big difference. Each time you double the R-value, you cut the heat loss in half.

    You already have a roof that is performing around R-27. Increasing that to R-28 by adding a radiant barrier will slow your heat loss by around 4%.

  4. kramttocs | | #4

    Thanks all!
    I was kind of figuring that was the case.
    Before this attic was conditioned I did have this radiant barrier on the underside of the rafters and fiberglass on the floor. So while it wasn't free, it's just rolled up right now and I thought I might as well check.

    Peter - they are internal to the envelope. The intake pulls from an adjacent conditioned room and the exhaust blows into a room downstairs. It's a bit of a hokey setup but it's working well enough.

    Unrelated but this attic is above the laundry room and is where I eventually plan to duct the intake for a HPWH from to take advantage of this heat. I know it wouldn't take long to use all the hotter attic air and then would pull from the adjacent room but that room is a bonus room so it's warmer than most other rooms in the summer anyway. The exhaust would then be ducted into the encapsulated crawlspace to provide some minimal dehumidification benefit.
    But I am holding off on switching to the HPWH to see if the all-in-one units get their noise levels down since it would be internal to the living space.

    1. Expert Member
      BILL WICHERS | | #5

      It's always a plus if you can put otherwise wasted energy (such as hot air in a laundry room) to useful use! It's unfortunate that there are so few oppurtunities to make this work economically though.

      You can limit the noise of any water heater by pluming it with flexible connectors instead of rigid lines, setting it on a suitably strong support platform that rests on vibration isolators, and drywalling the mechanical space containing the noise makers with a double layer of 5/8" drywall. There are other things you can do too, but the things I've mentioned are probably the easiest and cheapest options.

      Bill

      1. kramttocs | | #6

        Thanks Bill. Appreciate the noise/vibration ideas. Aside from the noise concern I am really excited about swapping to one.

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