Where to place air gap on framed wall insulation
user-7359166
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Hi. I’m building an indoor sport court in Minnesota that has 12 foot concrete foundation walls on two sides. It is framed with a 2×8 exterior wall and a 2×4 interior wall, with about 1.5 inches between the two walls. I just finishing applying 2 inches of spray foam on the wall. Next, I plan to put up 3.5 inch (15R) mineral wool batts, and then finish it with CertainTeed MemBrain.
My question is… should I place the 3.5 inch mineral wool batts on the 2×4 wall that is closest to the inside and where the drywall will be hung? Or, should I place the batts all the way against the spray foam on the 2×8 wall?
Thanks for your advice.
-Gary
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Replies
Hi Gary.
Is it too late to build the interior wall so that the foam and mineral wool batts fill the wall completely? That would be ideal. Otherwise, I think it is best to install the mineral wool tight to the foam and to make sure to have an interior air barrier.
Yes, it's a little too late. I just finished the insulation this weekend and will be putting up the vapor barrier shortly. I went ahead and installed the wool batts tight to the foam.
If I would of realized this challenge ahead of time, I might have filled up the cavity with more spray foam. Right now I'm limited to 14R of batts to keep in line with the 50/50 ratio of spray foam to batts as recommended by the following publication by Johns Mansville. https://www.jm.com/content/dam/jm/global/en/building-insulation/Files/BI%20Toolbox/Technical%20Bulletins/Technical%20Bulletin_BIT17_007_USA.pdf.
An air gap between insulation layers is a "thermal bypass" that cuts into the thermal performance of the assembly, since any air convecting around the batts moves heat (and moisture) via the air. Installing the batts tight to the foam prevents that.
While there would ideally be a continuous air barrier on the interior facing side of the batts, the thermal performance of high density R15 batts isn't going to suffer much without it. Air tightness of the MemBrain becomes critically important if it isn't installed tight to the batts. Rock wool is extremely vapor permeable despite it's air-retardency, and air leaks around the MemBrain could result in periods of condensation (or frost) and dampness where the rock wool meets the foam.
Is there good documentation you can point me to for installing the MemBrain? Do you recommend taping the seams, using acoustical sealant, or both? Do you start with the ceiling or walls first?