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Community and Q&A

How Much Attic Insulation

drrenken | Posted in General Questions on

Good Afternoon,

I am building my own house outside of Atlanta and I’m about to pull the trigger on spray foaming my conditioned attic. Trying to determine how much to go with… code min is r19. I’ve got prices for both r20 and r30 open cell and there is a $1,658 difference between them.

https://www.energyvanguard.com/blog/the-diminishing-returns-of-adding-more-insulation/

After going through this article and running the heat loss numbers/my electrical cost, I’d only be saving like $62 a year going with r30. That is more than a 26 year break even time! Am I missing something in my calculations?!

I’m not seeing much reason to go more than code min right now… especially since this is one of those areas that I can always come back later and add some more foam if needed (when finances allow it).

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Replies

  1. walta100 | | #1

    You are correct spray foam is so expensive that it makes it a poor investment. I thought you need close cell anyway to keep moisture away from the cold roof.

    I understand putting the HVAC is a long-standing family tradition for no less than 12 generations. LOL

    Consider getting the HVAC out of the attic the next time is need to be replaced.

    The truth is the conditioned attic is only marginally better than the worse idea of putting HVAC in the vented attic.

    If you can air seal the ductwork, consider burying it under a thick layer of insulation.
    https://www.energyvanguard.com/blog/buried-ducts-allowed-2018-building-code/

    Walta

  2. Expert Member
    1. drrenken | | #3

      I appreciate the responses guys. I'm afraid the "anything other than spray foam" ship has sailed.

      My 2:12 shed roof is finished with standing seam metal and I also have all my plumbing lines and HVAC in the attic and my county's inspectors will not allow water lines up there unless it is spray foamed.

      I'm just wondering if my calculations of the practical differences between r20 and r30 spray foam being miniscule is correct or if I am missing something somewhere?

      1. gtholloway | | #5

        What is the overall roof assembly? Does the ceiling plane follow the roof or is it flat? Is the roof membrane vapor open or closed?

        I would forego open cell in favor of closed cell (2-3" of R6ish per inch) and then fill the rest with a vapor open blown in fill (cellulose, fiberglass, mineral wool, wood fiber).

  3. matthew25 | | #4

    Open cell is a poor choice unless you have exterior insulation? Humid air from the indoors can migrate through it and find a cold condensing surface on the underside of the roof sheathing. Closed cell is a much safer choice if you have to use spray foam at all.

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