Has anyone actually considered using Rockwool comfortboard directly on a basement concrete interior wall instead of the usual rigid foam board approach? : HomeImprovement


I’m reposting this since the last time it was posted it got taken down because of too many posts by me.

So I read a lot about using rigid foam board on the concrete basement surface to prevent water from coming through and leading the moisture mildew problems. But yet, I also read to not use foil faced rigid foam so that “some” water can be taken up by the foam and allow for drying when needed.

This is stated here in the questions/comments section: https://buildingadvisor.com/materials/foundations-sitework/basement-insulation/

“The usual recommendation is for unfaced foam on the interior of a basement wall, sealed at joints and along the top and bottom. This creates an air barrier, keeping interior basement air away from the cool basement well where it may condense, and keeping moisture from the basement wall from entering the basement interior. Concrete is highly permeable to water vapor, so it tends to pass soil moisture into the building.

Foundations are complicated because moisture can move in either direction, depending on the soil conditions, dampproofing or waterproofing on the basement walls, and temperature and humidity of the interior air.

Unfaced EPS foam is fairly permeable to water vapor with a perm rating of 5 for 1 inch and 2.5 for two inches, making it a Class III vapor retarder similar to latex paint on drywall. Extruded polystyrene (XPS) has a perm rating on 1.1 for 1 inch and .55 for two inches making it a Class II vapor retarder similar to kraft paper (for a 2-inch board).

Foil facing, however, makes any foam board virtually impermeable to water vapor like polyethylene. These are considered Class I vapor barriers.

Using an unfaced foam insulation is hedging your bets, allowing for a minimal amount of drying, in either direction, should things get wet for any reason. Using foil faced insulation could potentially trap liquid water between the foam and foundation wall. However, if there is no wood between the foam and the wall, you are correct that there is no harm to the concrete. Maybe the foil facing could delaminate depending on the adhesive used, but no great harm there.

So, bottom line: I wouldn’t worry about installing foil-faced EPS insulation directly against the basement wall the way your describing. However, it would be a good idea to seal the joints between panels and the perimeter of the insulation to keep the interior air basement air isolated from the basement wall.”

But lets think about this for a second. If “some” drying can be beneficial here, why not just use a material that would do the same, allow for water to dry and not promote mold growth. This same post mentions using Rockwool comfortboard directly on the concrete wall, as shown here:

“Yes, you can certainly use Roxul Comfortboard, placed directly on the interior side of the basement wall. Comfortboard’s main use is for exterior insulation, but there’s no reason why it will not work as well on the interior.

I’ve not heard of carpenter ants getting into interior foam on a basement wall, but it is certainly possible, especially in wooded areas or other places with ant problems. If you add a stud wall inside the Comfortboard, you can fill the cavities with unfaced fiberglass, rock wool, or any water-resistant insulation for additional R-value.

Depending on your climate, a 1 to 1.5 in. layer of Roxul Comfortboard (R 4/in), should keep the interior surface of the board warm enough to prevent condensation in warm weather.

The interior surface of the insulation board is the condensation plane for warm, moist air in hot, humid weather.”

So what are people’s thoughts on this? My main reason to even consider this is because this part of the basement isnt going to be finished, so I dont have a desire to add the rigid board, and then have to put up a wall with dry wall because of the fire hazard. And I cant find a local store that sells Thermax that can left exposed. So my options are to add the rigid foam board and then just nail the rockwool directly to the rigid foam board (no wall framing), or just skip all that and nail/tape/glue/somehow secure the rockwool comfortboard directly to the concrete wall.



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