Rim joists – GreenBuildingAdvisor

Replies

1. Expert Member

   MALCOLM TAYLOR | Nov 10, 2025 12:57pm | #1

   TGD,

   There are two reasons to air-seal at rim-joists: To reduce air-movement through the enclosure which will reduce energy use, and to stop moist interior air from coming in contact with the cold rim-joist and condensing. Unfortunately sealing the rim-joist with caulking only helps with the first one. You still need an air-barrier on the inside of the insulation to deal with the second.

   If you can’t get foam board into the joist bays, I wonder if faced mineral wool board might be easier to install? It would still need air-sealing at the perimeter. https://www.rockwool.com/north-america/smartrock-interior-insulated-mass-walls/

2. 

   TGD | Nov 10, 2025 02:03pm | #2

   Thanks:

   Using faced mineral wool board makes sense although getting to the perimeter to seal is problematic in many areas. Would I still want to do a perimeter seal around the raw wood joint surfaces before the mineral wool is put in place? Also, the faced side of the batt goes in towards the interior?

   Given the difficulty of all this I may opt for a closed cell foam that can be applied in multiple layers. This Kraken FastCoat Insulation Spray Foam, Closed Cell Expanding Foam

1. Expert Member

   MALCOLM TAYLOR | Nov 10, 2025 04:33pm | #3

   TGD,

   If you seal the perimeter of the insulation, sealing the framing is really just a belt and suspenders action.

   The facing must go to the interior to keep the moist inside air from getting far enough through the insulation that it hits its dew point.