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Thread: Radiant barrier paint for attics

  1. #1
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    Radiant barrier paint for attics

    I have heard of aluminum foil radiant barriers to reduce heat gain and loss in a roof. They sell roof plywood with foil bonded on one side. I know some people, say it does not do much. living in a desert I think it can't hurt.
    So today I found found some legitimate paint companies make what they call radiant barrier paint.
    Would be easy to apply to the bottom of the roof deck with my airless sprayer and a long extension pipe. Not sure if it is watertight so the roof deck would not dry to the inside or not.
    Of course the companies selling the aluminum foil say the paint is a gimmick and you should buy their products instead. You do have to install air chutes near the eaves either way to keep blown insulation from blocking soffit vents.
    Bil lD.

    http://104.130.6.56/roofing/reflective-coatings/liquidfoil-attic-barrier

    on edit: the Henry product says it allows water vapor to escape
    It is 84% reflective compared to 97% for foil.
    Last edited by Bill Dufour; 01-17-2021 at 11:12 AM.

  2. #2
    Radiant heat transfer is a function of the temperature difference to the fourth power. The surface also influences it but the primary thing is the temperature difference. In the real world, what we need to worry about in houses is convection (leakage, movement of air from outside to inside or vice versa) and conduction (movement through materials). A lighter colored roof helps some but most of the roofs around here are dark because we think it looks better. Foil could help by blocking convection of the heat from the roof to the inside directly and forcing it to conduct through the foil. That isn't a huge conduction barrier but it helps because convection is much more efficient heat transfer than condution. Painting the underside of the roof is not going to do much of anything.

  3. #3
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    Where I live it does not rain for about 8-9 months. Summer is often over 100 with no cloud cover for weeks. So the radiant heating is extreme on the south slope of a roof here. I agree it would not do much in cooler climates or in winter.

  4. #4
    In theory a radiant barrier should work, and they were all the rage about 15-20 years ago. I think what they found out was that once the radiant barrier got the inevitable coating of dust, it lost its reflectant abilities, kind of like how you can't see yourself in a dirty mirror. At that point, they became heat absorbent material rather than heat reflectant material.

  5. #5
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    The foil covered roof sheathing really works but it attracts lightening and current flows across it causing fires. The paint is not as good but doesn’t attract lightening.

    http://www.texasinspector.com/files/...-Causation.pdf

  6. #6
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    The paint I linked too says it does not affect cell phone or tv signals so not electrically conductive. maybe the paint insulates the aluminum flakes from touching? lightening is not an issue here.
    Bil lD

  7. #7
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    I spend some time in attics and am a big fan of the foil radiant barrier. It may be my own subconscious bias but it seems like the radiant barrier attics are cooler. As far as cell service, 10 or 15 years ago it could create serious signal problems. In my neighborhood it was very common to see people outside in the heat of the day doing business calls. I used to mention this to home buyers when purchasing a house with the barrier. Things must have improved on the cell phone side, at least where I live, because it is no longer an issue.

  8. #8
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    Quote Originally Posted by Bill Dufour View Post
    The paint I linked too says it does not affect cell phone or tv signals so not electrically conductive. maybe the paint insulates the aluminum flakes from touching? lightening is not an issue here.
    Bil lD
    That was a very interesting paper!

    My experience with and understanding of the radiant barrier paint is that it's 'ceramic' rather than 'metallic'. The ceramic aspect is what makes it reflective. I can attest, through person experience and use, that RBS does in fact lead to lower temperatures in attics than those without RBS (25 degrees or more). Since all roof decking is supposed to be spaced ~ 1/8" apart, on both the long sides and the short sides (we used to use a framing nail as a spacer), maybe the answer to the conductivity transference issue is to change the standard H-shaped clips that are used to align and space the sheets along the long edge between the rafters to a non-conductive material rather than the metal they're currently (no pun intended) made of?

    My $0.02
    Don't let it bring you down,
    It's only castles burning,
    Just find someone who's turning,
    And you will come around

    Neil Young (with a little bit of emphasis added by me)

    Board member, Gulf Coast Woodturners Association

  9. #9
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    I see these magic ceramic spheres to add to paint on amazon. Much cheaper then the magic paint.
    Bill D.

    https://www.amazon.com/ThermaCels-In...8ZATYE3FYTERSG

  10. #10
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    I believe California requires all new roofs to use a "cool roof" rated products. Not exactly sure but it means light colors not black.
    If I where to use the magic paints I would use a coat or two of cheap primer first. That would soak into the bare wood and reduce the amount of expensive paint needed to get full coverage.
    Bill D.

  11. #11
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    LP (and others) has a roof and wall sheathing product that has a reflective back for thermal control. That's obviously not great for retrofit unless the whole roof is being torn down to the rafters, but is still an indication of support in the industry for better energy efficiency. The paint sounds interesting, but must be "fun" to apply to an existing structure, especially if there's already insulation (particularly blown in) in place.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  12. #12
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    I had full-length ridge vents installed the last time my composite shingle roof was replaced (due to hail damage). I already had adequate soffit venting, and marginal non-powered roof venting (the latter was removed when the ridge vents were installed).

    This made a HUGE improvement in attic temperature. Should have done it a long time ago. They can be installed on an existing roof, but you will need ridge-row shingles over the top of it.

    -- Andy - Arlington TX

  13. #13
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    For what it's worth...whether already in the paint or as a dry-ad to the paint, the "magic" beads are tiny, lightweight, hollow, borosilicate beads (the manufacturer 3M prefers to call them 'bubbles'). The air-filled borosilicate beads lower the rate of heat transfer (because they are air-filled). The heat transfer rate is proportional to the bead concentration in the paint and barrier thickness. Higher bead concentration (more air) and greater barrier thickness - lower rate of heat transfer. The bead's extremely low density keeps them suspended in the paint until the paint dries (Stokes Law). The same beads are used in the manufacture of lightweight plastics, like car dashboards. The oilfield industry uses these and other beads as an admixture to reduce cement density.
    Spray-foam attic insulation uses the same concept...'air-filled', non-convective (because the air in the system is static), high R-value, barrier.
    Loran Galey
    Do good and be good and you will be happy.

  14. #14
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    For primer I would use stain not paint so the wood can breathe. I would have to use a webster or leaf blower to get dust and cobwebs to a reasonable level before painting.
    The local utility used to offer a $3.00 rebate on the foil covered sheathing sheets. This was just enough to cover the slight extra cost over regular sheets. The foil has pinholes so any moisture could breathe out of the wood to the inside. I used it to do the roof in my woodshed to make it brighter inside. I think it helps
    Bill D

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