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Radiant barrier, best location

brtlmj | Posted in General Questions on

I am planning to install radiant barrier in my attic.

Now, let’s get a couple of things out of the way first:
– I live in California; it gets hot here
– I have AC ductwork in the attic. (The duct insulation is 4.2 and I _will_ be replacing it, but that’s a separate project.)
– I cannot convert the attic to a conditioned one, as a good part of it is not accessible
– I am also air sealing the attic (well, at least the accessible parts).

I estimate that I am going to need <$200 in material and a weekend of work. Can’t go (very) wrong, right? The question I have is: where does the radiant barrier go? I can: 1) Staple it directly to the underside of the roof sheathing. 2) Staple it to the rafters, leaving an air gap between the barrier and the sheathing. 3) Do both – at least in the critical areas (south side). Ideas, suggestions, comments? Thanks! Bart

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Replies

  1. User avatar
    Dana Dorsett | | #1

    Staple it to the underside of the rafters, so that the low-E surfaces are not in close contact with the roof deck.

    To go one better, a silvery low-E paint should go on the underside of the roof deck and sides of the rafters before installing the RB. Products such as Lo Mit II Max, or HeatBlock Ultra are north of 80% IR reflectivity, and are very low-E (sub- 0.2) and will make a measurable difference. Nano -sphere ceramic paints with magic mouse milk don't do anything worthwhile- the paint needs to be silvery and shiny. Anything with a reflectance over 75% or emittance under 0.25 would be worthwhile, if you can't find the aforementioned. A short list lives here: https://cdn.shopify.com/s/files/1/0542/2337/files/radiant-barrier-paint.pdf?1710353271856735281

    In most CA climates you'll be better off keeping soffit-to-ridge venting rather than closing it up, but making the RB reasonably air tight to the interior to limit air handler driven infiltration when the AC is running. In some parts of CA it may be worth using a perforated aluminized fabric type RB. Even though they will leak more air, they have sufficient vapor permeance to allow moisture to escape the attic into the vent channel. Perforated RB should still be detailed as an air barrier to limit air infiltration- the perforations are very tiny, and it takes many square yards to add up to a square inch of air leak, whereas one poorly sealed seam can be several square inches.

  2. brtlmj | | #2

    Are there really any low-E paints that are worth the money and effort? I was under the impression that they all were scams.

    Also, I am certainly not trying to seal attic vents. I am sealing the leaks between the conditioned space and the attic. Wall headers, electrical boxes etc.

    Thanks!

  3. Norman Bunn | | #3

    Follow the link Dana provided and you will see there are four more (out of 16) that have an acceptable emmissivity (sp?) in addition to the two he gave.

  4. User avatar
    Dana Dorsett | | #4

    The two low-E paints mentioned test at a sufficiently low emissivity to meet the legally regulated definition of "radiant barrier".

    The others don't, though some on that list are pretty close, and (as a friend of mine used to say), "...good enough for the kind o' girls I go with..." :-) Several others aren't even close.

  5. User avatar GBA Editor
    Martin Holladay | | #5

    Dana,
    Radiant barriers are defined in ASTM C1158  ("Standard Practice for Installation and Use of Radiant Barrier Systems (RBS) in Building Constructions") as surfaces have an emittance of 0.1 or less.

    As I noted in one of my articles (‘Insulating’ Paint Merchants Dupe Gullible Homeowners), no paint on the market meets the legal definition of a radiant barrier.

  6. User avatar
    Dana Dorsett | | #6

    I stand corrected!

    The paints with emissivity of 0.25 or less meet the ASTM C 1321, definition of an Interior Radiation Control Coating System, not radiant barrier. The Heat Bloc Ultra and Lo/Mit-II have decent margin on the IRCCS spec, but are still way over radiant barrier spec. A few others still meet the IRCCS spec, but not by much.

  7. brtlmj | | #7

    In general, I am mostly concerned about the bang I get for my buck. I would go with paint - even if it is not nearly as good as metallized foil - if it was significantly cheaper, easier to install, etc.

    Lo/Mit-II Max seems to be sold in 5 gallon buckets and costs $374.95. The claimed coverage is 2000 sqft. I can't tell if it requires a primer or not; if it does, I would need to spend another $100 or so.
    This buys me emissivity of 0.14.

    My $100, 1000 sqft roll of radiant barrier foil arrived yesterday. Declared emissivity - 0.03. I think the winner is obvious.

    Now, there are areas where I cannot (the very edges of the attic) or should not (the top of the attic, where the vents are) install the foil. I could be tempted to try a low-E paint there, if I could buy it in smaller quantities.

  8. User avatar
    Dana Dorsett | | #8

    It's easier to install the paint (with a sprayer), but it's more expensive than aluminized fabric RB. But there is synergy in doing both. When a low-E surface (the painted roof deck & rafters) is facing another low-E surface, the radiated heat transfer is much reduced.

    If you're only going to do one, the roll of RB is the right choice.

    I hear you about only being able to by low-E paint 5 gallons at a time...

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