Basement insulation and rim sealing questions
Hey All,
A few questions about basement insulation and rim joists as I keep wrapping my head around all the details and nuances of my build and the different types of rim joist and basement wall assemblies we have going on:
1. I know that when it comes to sealing rim joists, most people are dealing with tightening up old houses and that these areas are a major source of leaks — which is why there’s such a focus on interior air sealing at the rim joist. For new construction that uses an exterior air barrier (taped zip) over the rim joist on the exterior, are folks still caulking/foaming the perimeter of each rim joist bay on the inside? It seems redundant but maybe it’s still best practice in making sure it’s coming through the plate or anywhere else.
2. For a below-grade concrete wall in CZ 6, what is the right ratio of air impermeable to air permeable insulation (rigid foam and unfaced fiberglass batts in this case)? Above grade ratio is well documented and discussed here, but having a hard time finding details for basements. I’ve seen a lot of people and drawings reference 2″ as the amount needed — which feels a bit arbitrary. But just looking for clarity on ratio for this climate zone in a below grade wall.
3. I’ve also seen some references to leaving a gap between the foam insulation and fluffy in a stud cavity in a basement, with the idea being that if water does come through the wall, it’ll drop to the floor rather than wet the fluffy insulation. Are people still recommending/doing this? I would think you’d want to keep everything tightly sandwiched together. The foundation wall is poured concrete with no moisture signs (yet)
4. And then last… we have one area that is an insulated floor over garage. It’s floor truss assembly being blown to full depth 16″. As such, the blown in will completely cover the rim joists, which all sit on wood framing since this area is a walk-out. They are air sealed on the exterior with taped zip sheathing. Any issue with just using the loose fill fluffy insulation on these rim joists? Or should I also use rigid foam that is caulked/foamed in place?
As always, appreciate everyone’s insights here. The “best insulator in town” said I only needed 2″ of CCSF for a flash and batt on a cathedral ceiling in CZ 6… so this is why we have trust issues.
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Replies
Sean,
1. The attention to interior air-sealing is to stop the moist interior air from coming into contact with the cold rim-joist. In above ground walls that interior air-barrier is usually the drywall or a membrane. It's the difficulty of installing these in every joist bay, and the increased moisture from the concrete wall below that has made the sealed foam board the interior insulation of choice for basements. You could argue that if the top of the stem-walls are well sealed, and the ceiling above drywalled, then you could use batts as you can for the framed floors above.
2. You can find the ratios of foam to permeable insinuation for your climate zone in this article: https://www.greenbuildingadvisor.com/article/three-ways-to-insulate-a-basement-wall
3. You don't want any air-space between the two types of insulation. If you want extra protection against water moving through the wall from outside, include a dimple matt or poly against the concrete.
4. Permeable insulation is fine there, as it is on any second storey rim-joists.
Thanks Malcolm. All makes sense