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Can spray foam be sprayed directly onto radiant barrier (LP Tech Shield)?

aunsafe2015 | Posted in General Questions on

Let me start by saying I know that spraying foam directly onto the reflective side of a radiant barrier renders the radiant barrier itself useless because it eliminates the required air gap.

With that in mind, my first question is whether foam can physically be sprayed directly onto a radiant barrier (LP Tech Shield), i.e., will it stick and cure properly, or does the foil/reflective surface interfere with the spray foam?

My second question is whether it makes sense, in climate zone 4a, to insulate & seal a walk-up attic with spray foam when the attic already has a radiant barrier.  The duct work for my second story HVAC system is in my attic, and I want to bring it into the conditioned envelope of my home.  I was planning to do this by using R-38 spray foam to seal the attic.  But given that I already have radiant barrier roof decking, it crossed my mind that maybe there is a better option?  For instance, given the existing radiant barrier, would I be better off just using baffles and fiberglass?  Or, would using baffles and then spray foaming over the baffles allow the radiant barrier to still serve a useful function because the baffles would preserve the air gap necessary for the radiant barrier?  Would the ~1.5 inch of rafter depth that the baffles would consume be better used simply to have an extra 1.5 inches of foam and no baffles?

Thanks for any input!

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Replies

  1. RussMill | | #1

    Do you have soffits and ridge vents now?

    1. aunsafe2015 | | #4

      The attic is actually very well ventilated -- soffit vents, ridge vents, and 4 (yes, FOUR!) gable vents.

  2. Expert Member
    AKOS TOTH | | #2

    Aun,

    Your radiant barrier has a thin layer of aluminum on the surface, spray foam will stick to it quite well. Spray foam cures from the chemicals that are mixed inside the spray gun, if properly mixed it will cure no matter the surface.

    Generally putting more than 1.5" to 2" of spray foam between wood studs is a waste of money. The wood studs short circuit the insulation of the foam, your R38 spray foam will get you something like R24 roof.

    In your case, the cheapest might be to install vent baffles and fill the cavities with open cell spray foam. You don't want open cell foam directly against the roof as this can sometimes cause issues.

    You can also go with a mix of fluffy insulation between the rafters and foam board over the rafters if you are looking for a DIY solution.

    1. aunsafe2015 | | #5

      Thanks. I thought about DIYing with fiberglass + foam board over the rafters, but my roof is a fairly irregular shape so it was difficult for me to estimate the cost. The estimate I have for spray foam is in the $5,000 range, and my best guess was that if I DIY'd I'd be spending ~$2,000. That difference is not tiny, but the physical difficulty of getting up on a ladder to get insulation up the roof deck, and especially getting the foam boards attached to the rafters, makes me lean towards the spray foam since the price seemed reasonable (and the quote was from a company that's been in business since 1998 and their least experienced foam crew has been with them for 4 years).

  3. Expert Member
    Peter Yost | | #3

    I would not lose any sleep over negating the radiant barrier sheathing in CZ4. Your biggest challenge will be completing your air control layer where it meets your exterior walls.

    What pitch is your roof; how complicated is it geometrically.?

    Peter

    1. aunsafe2015 | | #6

      Good to know, thanks.

      I don't know for certain what the pitch is, but just eyeballing it, its probably about 8-12. It's fairly complicated geometrically, unfortunately (I did not design this house! I prefer simplicity, myself), with multiple dormers. On the bright side, it's a walk-up attic with relatively high ceilings, so even though it is complicated, it's very easy to get around in there so hopefully it won't be too much of a nuisance for the insulation company.

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