Rim Joist Insulation Assembly with Exterior XPS
user-7653783
| Posted in Green Building Techniques on
I know this question has been brought up before, but I’m not sure I’ve seen a conclusive answer on the best approach. I have 1″ of exterior XPS insulation. What is the most ideal method to insulate the interior side of the rim joist? Attached is a photos. I am thinking
Climate Zone 5a- -northern New Jersey
Option 1: Air seal interior joints at rim joist/sill/floor above and install 4″ of mineral wool batt insulation pushed against the interior side of the rim joist
Option 2: 1″ of EPS insulation cut and cobbled against the rim joist and 4″ of mineral wool batt pushed against the EPS
Option 3: ?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Deleted
User ...783,
You need enough impermeable insulation (foam) to either keep the rim-joist, or the interior face of any foam you put in the inside of the rim-joists warm enough to prevent condensation.
For climate zone 5a, the R-value of the foam to do that is at least R-7.5. Since you don't have that amount on the outside you should go with option #2 to get to that R value.
Thank you!
I have the same question in another article...(https://www.greenbuildingadvisor.com/question/recommendations-on-this-basement-wall-rim-joist-detail).
For your 'Option 2' - Wouldn't 1" EPS on the inside be too little in thickness resulting in a cold interior condensing surface? And wouldn't this surface only be lowered in temp if 4" batt insulation is added?
I have 1" XPS on the outside too, and want to avoid a rim-joist-foam-vapor-barrier- sandwich, and allow drying if rim joist cavity gets wet... i also understand batt insulation alone causes issues due to its vapor/air permeance.
Bill had also recommended using EPS on the inside to allow for some inward drying, given that the exterior is likely impermeable XPS. But providing a minimum thickness may be the issue?
Maybe I am just overthinking it?
Thomas,
If there is already 1" of EPS on the exterior, adding an inch on the inside takes you above the R-7.5 necessary to keep the inside face of that foam warm enough to avoid condensation. That amount takes into account the 4" of batts. You don't end up with a vapour-barrier sandwich, as 1" of EPS is 2 to 5 perms
https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing