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Thread: ceiling insulation

  1. #1
    DIY Junior Member 7echo's Avatar
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    Default ceiling insulation

    I have no attic. There is a 6.5 inch air space from the inside of the ceiling drywall to the underside of the roof decking. There is no insulation, just some foil stapled to the ceiling drywall. 1957 low slope.
    I will have some access to most of the space during a remodel. If I blow insulation into the air space will the roof deck over heat? The roof is a torch down product.
    I would like to insulate if possible. When I run a load calc for hvac the tonnage drops from 3 to 1.5 with the addition of 4" of insulation.
    Thanks in advance!

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Depending on the climate, a radiant barrier (the foil) counts as about R-19 by itself. FOr it to work, it needs at least 1/2" airspace on either side (it can't for example be stapled directly on a panel - if is it, then it will conduct and be of little use). Lots of air flow could compromise some of it's effectiveness.

    If there aren't any vents to allow air flow under the deck, then the radiant barrier might be plenty. There are various independent studies you can read if you search about the effectivness of radiant barriers, and under what conditions they work. I put one in in my attic, and the attic temp in the summer dropped about 25-degrees on a hot summer day. Even with R40 on the ceiling, prior to this, the ceiling would be hot because everything was heat soaked. After, the ceiling was the same temp as the interior walls.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Architect Spaceman Spiff's Avatar
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    The best option would be spray in polyisocyanurate and fill the whole void. Insulation right under the membrane is fine and is how most flat commercial roofs are done. The only drawback is cost. The other option is rigid polyiso boards. I'd try for 4" but 2" would be better than nothing. Keep the board as low as you can and the radiant barrier will help. You need to either have the foam in contact with the roof or have a space where air can circulate.

    If it were me I'd do the spray in foam. Many other insulations won't be as effective in this case. Hope this helps.
    Spaceman Spiff aka Mike

  4. #4
    DIY Junior Member 7echo's Avatar
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    Default

    Thanks for the replies.

    I don't think the radiant is any good in my house. It is stapled to the joists and then drywall was installed against the material. I was thinking I could blow in, but I bet the foam would be better and easier. The concern is that 1)any air flow would go away and 2) the roof deck would over heat. I guess a lot of people are spray foaming in the roof cavities now, but this would end up being solid foam between the roof deck and ceiling, like what Spaceman Spiff suggests. The climate is humid and hot, coastal georgia. What about condensation forming on the ceiling? Or inside the cavity if any room is left in there?

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