R-max between drywall and insulated attic
Ralph1962
| Posted in Energy Efficiency and Durability on
I am interested in using 1/2 R-Max as a thermal break between drywall on the living side and blown insulation on the attic side. I have 2 x 6 ceiling joist and want to blow in r 38 of cellulose insulation. I would put foil side to the drywall and tape all intersections of boards to create vapor barrier.
Anyone ever done this …..this will allow to get some work done in the attic before the blown goes in, and in the end provide a more comfortable room below.The schematic attached is for dow board, but I think R Max will work better due to foil face. Thanks Ralph
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Replies
Ralph,
I'll provide a translation for GBA readers. R-Max is a brand name for polyisocyanurate insulation (a type of rigid foam).
First of all, concerning your question (shown in the sketch) about the need for a "plastic sheet," and your reference to the need for a "vapor barrier": Vapor barriers are not required by code in this location, and in general aren't needed. (Codes only require a vapor retarder; this requirement is easily met by the use of vapor retarder paint.) For more information on this issue, see Do I Need a Vapor Retarder?
Second, it's fairly common for builders to install foil-faced polyiso on the interior side of ceiling joists. If you want to do this, it's best:
1. To choose thicker rigid foam (1 inch or 2 inches thick) to do a better job of addressing thermal bridging through the joists.
2. To include 1x4 ceiling strapping, 16 inches on center; the resulting air space takes advantage of the foil facing on the polyiso, improving the assembly's R-value.
3. While there is no particular reason to worry about vapor diffusion or vapor barriers, it's always a good idea to pay attention to airtightness. That's why it's a good idea to tape the seams of the rigid foam.
Thanks. We are a little concerned about getting too thick on the ceiling. I like the idea of the strapping, but gotta pass the architect(my wife ) on the idea of losing an inch to the foam and strapping. I appreciate the response. We are also moving the duct work into a reverse soffitt to help our little ac do its job in these hot NC Summers.