Replies

1. GBA Editor
   Martin Holladay | Mar 02, 2018 03:53am | #1

   Rob,
   Q. "For the sub-floor, can I install rigid foam (0.5 - 1") on top of the joists as an air barrier with some R-value and OSB on top of the rigid foam?"

   A. No. The rigid foam is too squishy. The tops of the joists would dig into the rigid foam. If you want to install a continuous layer of rigid foam as part of this floor assembly, it belongs on the underside of the floor joists, between the floor joists and the garage ceiling.

   Q. "Can I use rigid foam in-between the rim joist and the floor joists instead of installing the rigid foam in-between the joists against the rim joist?"

   A. For structural reasons, you need to make a secure connection (usually, with 16d nails) between the rim joist and each intersecting joist. If you want this assembly to include a continuous layer of rigid foam, the usual method is to locate the rim joist 2 inches in from the mud sill (so that the rim joist is inset from the wall framing), leaving room for 2 inches of continuous rigid foam on the exterior side of the rim joist. If you go this route, the exterior of the rigid foam will be in the same plane as exterior of the mudsill.

   There are other approaches to consider, of course. If your walls have 2 inches of exterior rigid foam, the wall foam can extend down to cover the rim joist at the exterior.

2. 
   Rob_K77 | Mar 02, 2018 10:20am | #2

   Thanks Martin. I will be adding exterior foam to the addition so I'll extend it down so it covers the rim joist. Seems like this approach would be even more efficient time wise and probably better for a air seal and insulation standpoint. As for the garage ceiling, is there a minimum that thickness for the foam against the joists? Would 1/2" suffice if I'm filling the cavity with Roxul?

3. GBA Editor
   Martin Holladay | Mar 02, 2018 11:54am | #3

   Rob,
   Q. "As for the garage ceiling, is there a minimum that thickness for the foam against the joists? Would 1/2 inch suffice if I'm filling the cavity with Roxul?"

   A. The only minimum requirements are code minimum R-value requirements for the entire floor assembly -- usually R-38 in Climate Zones 7 and 8, R-30 in Zones 5 and 6, or R-19 in Zones 3 and 4.

   A half-inch of rigid foam doesn't have much R-value -- about R-2 or R-2.5, depending on the type of foam. My own personal opinion is that if you are going to install a continuous layer of rigid foam, you might as well install at least one inch or not bother (but that's just an opinion -- it's not based on science).

   For more information on this type of floor assembly, see How to Insulate a Cold Floor.

4. 
   walta100 | Mar 03, 2018 07:06pm | #4

   Before you get to set on this idea be sure to ask your building inspector what they will require for fireproofing some places are unreasonable about this configuration.

   I am not the spray foam guy but this is one of the few places it makes sense engineered floor joists.

   Walta