Replies

1. GBA Editor
   Martin Holladay | Oct 08, 2017 05:19am | #1

   Adam,
   The area you have insulated is called a rim joist or band joist. It is not a ridge beam. (A ridge beam is a beam near the ridge of a roof.)

   Here is a link to an article I wrote on this topic: Insulating rim joists.

   As I wrote in that article, "Most building codes require rigid foam to be protected with a layer of 1⁄2-in. drywall as a thermal barrier. The drywall can be screwed to the rim joist through the foam.

   "Dow Thermax polyisocyanurate, one type of rigid foam, has a facing that has passed fire-safety tests. That means that most building inspectors don’t require Thermax to be protected with a drywall layer, making it a good choice for this application.

   "Spray-foam requirements differ from those for rigid foam. As long as your cured spray foam is no thicker than 3-1⁄4 in., the International Residential Code (IRC) allows spray foam at the rim joist area to be left exposed, without any protective drywall. The relevant provisions can be found in section R314.5.11 of the 2006 IRC and in section R316.5.11 of the 2009 IRC."

   If you want to make sure that your house is in compliance with the building code, talk to your local code enforcement official. Some local inspectors interpret the provisions of section R314.5.11 broadly, allowing unprotected rigid foam if the rigid foam is restricted to the rim joist area. Others want to see a layer of gypsum drywall on the interior side of the rigid foam, even at rim joists.

2. 
   NewLaneProperties | Oct 09, 2017 09:10am | #2

   Thanks for the correction, I'm a tad embarrassed.

   I just spoke with my local inspector. I asked him if Roxul Rockwool is an acceptable thermal barrier over the rigid foam specifically around the band joist. He didn't seem to understand. He said to be mindful of the R value, but did not speak to the thermal code requirement. He said I would be fine, but I think he was talking with respect to R value. So now I don't know what to do.

3. GBA Editor
   Martin Holladay | Oct 09, 2017 09:25am | #3

   Adam,
   1. It's not surprising that a local code inspector is unfamiliar with aspects of the energy code relating to foam insulation.

   2. Mineral wool over rigid foam is a good solution. If your code official is fine with that approach, you should proceed with that plan.

4. 
   NewLaneProperties | Oct 09, 2017 09:27am | #4

   Thanks Martin, for the advice and this site. Looking forward to hearing you on FHB podcast soon.