Sealing blocking in floor framing?
My dream house will use double stud walls over a full basement. The basement will be poured concrete with exterior insulation. It is a one floor above basement plan. The inner main floor wall will be over the concrete while the outer will be over insulation. The simplest way to support the outer wall is to ‘cantilever’ the floor framing over the concrete. Yes this creates a wood “thermal bridge” at each floor framing member but the heat loss is modest and tolerable. With the ‘cantilevered’ floor framing the conventional rim board is replaced by blocking located under the inner main floor wall. Since I plan on using a combination of ‘Trim Joist’ and ‘I joist’ (I joist on the gable ends) the blocking will need to be “notched” around the top and bottom chords of the floor framing members. Obviously the cantilevered floor framing complicates air sealing. The blocking will be a part of the primary air barrier. Can I achieve passive house tightness by using caulking on the entire perimeter of each blocking piece? What is the “best” caulking material and why? Should I attempt to “glue” the blocks in place with construction glue?
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Replies
Jerry,
You have correctly identified a problem. It's going to take meticulous attention to detail to air-seal all of those pieces of blocking -- especially if you are using I-joists.
The easiest solution to the problem is to use spray foam.
If you decide to use caulk instead, you can use a polyurethane or silicone caulk. The work is fussy. Moreover, framing lumber moves with changes in humidity. Over the course of a year, the joists and blocking will all expand and shrink. If you use lumber for your blocking, some of it will twist as it dries.
Whether the caulking will maintain a good air seal over time is uncertain.
Frankly, it's not a very good detail. My advice: use spray foam, or re-design the cantilever and air barrier.
Martin,
Thank you! I agree that using blocking and cantilevered floor framing makes air sealing very difficult. In the search for "build ability" I'll abandon this approach. Back to my search for a way to have the air barrier on the outside of the inner wall and transfer the dead load of the outer wall to the foundation. Hanging the outer wall from the roof trusses is a possibility but it doesn't work for the gable ends. Some form of "Riversong" truss may solve the gable end.
Jerry, assuming there will be insulation outside of the rim area, perhaps you could do the air sealing by placing a strip of poly under the sill and after the floor platform is done wrap the poly strip around the edges of the assembly and seal it to the platform before the walls go up. Do you have a sketch of the wall assembly?
Martin,
What do you think of this ? Use a suitable rim board with it's outer face directly under the outer
face of the inner wall which will be above it. With the full width of the rim board over the PT mud sill and concrete below. Once the rim board is in place but before installing sub flooring add 3/4" plywood pieces that, in effect, extend the floor framing to the inside plane of the exterior sheathing. These plywood pieces are the same height as the floor framing and are glued to the rim board and secured with 8 pocket screws (Kreg Jig drilled, 4 per side) . Corners will use 2 ea 3/4" plywood pieces with 45 degree miters at both ends. Then install the sub floor continuous to the inside of the exterior sheathing plane. Install the sub floor with glue and screws including 2 or more screws into the 3/4"plywood extenders. I've traded a lot of tedious sealing for a much simpler sealing challenge but added unusual framing more like cabinet making than rough carpentry.
Jerry,
I could give you some opinions, but I actually think it's time for you to talk to an engineer.
Dick,
The wall assembly is, from inside out: Drywall, 2x4 24"OC, 1/2" plywood (the air barrier), 3 1/2" space, 2x4 24" OC aligned with inner studs, 3/4" fiberboard, house wrap (high permeability), "Loxon panel, thin brick. The Loxon panels provide a 1/4"+ drainage space.
Martin,
Yes I will have an appropriately licensed professional review the plans and details. I'm not asking for that but do you see any major issues with the approach? I do have a degree in engineering and am comfortable doing stress analysis. I have calculated the worst case load, that must be transferred, per outer stud ( less than 300 pounds). Without glue and ignoring any load transferred through the sub floor. The pocket screws alone are sufficient to transfer the load with a 100%+ safety factor. On the other hand, ignoring the screws entirely the two glue joints ( to the sub floor and to the rim joist) can support 50+ times the actual load (actually 123 times with polyurethane glue's advertised strength). The worst stress in the plywood is in the sub floor under each stud and is but 15% of the usual rating. I am accepting the " thermal bridge" of the sub floor because it will add a huge amount of lateral stability to the structure.
I was more interested in a pictorial view of how you envision the joists, rim board, and sill to look. I've posted this pic before, showing the sort of thing I did. Perhaps you can adapt it for your situation to address air barrier and rim insulation issues. Remember that typically your I-joists need at least 1-3/4" bearing on the sill.
Dick,
Thank you for sharing your foundation to double wall detail. Next time my daughter visits me I'll use her laptop to scan & shrink my sketch into a PDF so I can post it. I don't have Acrobat so I'm limited to raw jpeg scans which are too large to upload. There are major differences in our basement s. I have all my insulation on the outside and it's 7" thick. Also I'm using the inner wall which sits on the floor framing that sits on the concrete as the bearing wall. My rim board is effectively 2" thick with a 2x4 horizontally at the top of the floor truss s and a vertical 2x4 under it centered over the joist flanges then 1'2" plywood over the outer face (this set up is shown in the Trim Joist brochure as one of the approved rims). I like it because it adds over 3500 pounds of load carry through capacity and is most economical. My mud sills are 2x8 so my joists have 5 1/4" of bearing with the other 2" carrying the composite rim board.