Rim joist insulation/vapour barrier questions
OronoWoodworks
| Posted in General Questions on
We are in the process of a build using i-joists, and realize that extending the poly wall vapour barrier into the joist cavity is tough with i-joists.
The plan is to use rigid foam board and use can foam around the edges, this will effectively create a vapour barrier correct?
If that’s the case, how would you detail the poly vapour barrier to foam board in the joist cavity? If foam board is used as the vapour barrier can we then add fiberglass on the warm side? If yes, would you use 1″ foam and then fiberglass?
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Replies
To better answer your question, we need to know where you are located (climate zone) and if the rim joist is in the basement (foundation type, like ICF, concrete block, pressure treated wood, etc...) or between a first and second floor.
I think it would be considered zone 6. We need to tackle this in 3 different configurations. Basement walls are ICF, ground floor walls are varied, some are icf, some are framed, second storey above. There is 1" rigid on the exterior
We temporarily had fiberglass in all the joist cavities, and definitely got condensation and ice behind it.
The ICF wall would have rigid on the outside (and inside assuming the joists are on a ledger over the foam), so you don't need to do anything there.
For the framed walls, you already have 1" rigid on the outside, so in zone 6 you can use 2x4 batts on the inside. If you want more insulation, install a total of R11 of rigid (assuming exterior R5, install R6 on the inside) for 2x6 batts.
Vapor barrier is not an air barrier, it doesn't need to be continuous. Typically the VB stops at the bottom plate and continues at the wall top plate bellow skipping over the joist area.
If you are not installing a ceiling, you need more rigid on the joist area. Roughly half you R value as rigid, the rest can be fluffy.
The floor assembly sits on the icf, not ledgered. We put batts in the joist cavities for the winter, and got a lot of condensation and ice behind them, which is why I'm asking about adding a vapour barrier in the joist cavities to make it more continuous.
Our insulation compliance package requires at least R5 on the exterior and R19 in the walls.
The basement walls will have no ceiling, the ground floor walls will.
Not sure if it matters, but our exterior insulation is micro perforated.
Fibrous batts are vapor open and will allow any interior water vapor to move towards the cold wall sheathing where it can condense. Using a rigid foam in the rim joist (as you suggested in your original question) and sealing around the perimeter with a sealant should eliminate the condensation issue in that location.
For wall cavities, R-5 continuous on the exterior isn't enough over a 2x6 (I'm assuming you have 2x6 framed walls based on the R-19 cavity insulation) wall to prevent the wall sheathing from becoming a condensing surface in climate zone 6. You either need to increase the CEI to R-11.25 (you can then move to a class III vapor retarder, painted drywall) or install a vapor retarder. In climate zone 6, the IRC allows a class I or II vapor retarder to be used. I'd recommend going with a class II (smart or responsive) product. If your air control is someplace outside (taped wall sheathing, ZIP, fully adhered WRB, etc...), you could install krafted faced insulation as the vapor control.
I'm just about to tackle this and wanted to make sure I understand.
For the rim on top of the ICF wall, the ICF wall acts as the vapour barrier, and the 2" foam does as well. Can I put fiberglass on the inside against the foam, or will that cause condensation on top of the sill? There will be no ceiling
For the joist cavities between floors, the vapour barrier will end at the top plate, then 2" foam in the cavity, same question with regards to the fiberglass, if we put fiberglass on the inside face of the foam will condensation form on the top plate? There will be a wood ceiling.
Thanks everyone!
That will work great. You can also stuff some 2x4 batts over the rigid for a bit of extra R value.
So no concern about condensation on the sill plate or top plates if we put fiberglass on the warm side of the 2" foam? There's only 1" on the exterior.