Please review our double Stud Wall over floating slab plan
Lizzieplants
| Posted in GBA Pro Help on
Since it will be a couple of weeks before GBA posts a recommended wall section, I would like to get your critique of the wall section and foundation plan drawn by my architect. We are still in the early stages of design but have a tight timeline to get the house designed and started.
Thank you
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On looking at the sketch, a few things come to mind. Where the floor trusses rest on the inner wall frame. A 2" XPS layer is shown perpendicular to the ends of the trusses, where I would have thought a rim board would be. Isn't a rim board needed for structural purposes anyway, to keep the trusses from flopping over? If the purpose of the foam board is to be an insulation dam for the cellulose cavity fill, the rim board would serve that purpose, and it can be sealed to the rest of the air barrier at its edges with acoustic sealant. True, the use of foam vs. wood as an insulation dam would improve the total R value in the rim area by a few R, but if the trusses and rim board rest entirely on the face of the inner wall frame, you still have 8.5" of cellulose (about R31) outside the rim.
Also, between floors, the notes indicate 1/2" plywood gusset plates 24" oc tying the inner and outer wall frames together. You may want to check with the local building inspector as to whether he wants to see firestopping there, which could be continuous 3/4" plywood. Some inspectors may accept dense-packed cellulose as firestopping, but the local guy rules his fiefdom. I wouldn't worry about thermal bridging through a 3/4" continuous firestop to tie the walls together; that thin slice through the wall assembly will be upwards of R12 all by itself, a fairly negligible blip on the overall heat loss.
On looking closer, it seems that the outer wall frame of the upper floor rests on the 3/4" subfloor cantilevered out past the ends of the trusses (and rim board, assuming it's there). There should be a continuous load path from roof down to foundation. Even if the inner wall frame is designated as load bearing, the outer frame shouldn't be supported by that cantilevered subfloor. Either the upper floor outer studs can run down to the lower floor outer studs, with the truss rim insulated as part of the upper floor wall cavity (and no cantilevering of the subfloor), or the lower floor studs can run up to the extended subfloor, with the rim insulated as part of the lower wall cavity. I like the latter better, as it takes care of that possible firestop at the top of the lower wall; the subfloor for the second level serves that purpose. The use of gusset plates to tie the lower floor wall frames together, if only temporary, is fine; they can be nailed to the stud faces.
Elizabeth,
I'm not sure I understand how the pressure boundary transitions from ADA to plywood in the attached snip.
The foundation section is interesting...
What climate zone are you in?
Is this to be a frost protected shallow footing foundation?
Sorry I forgot to tell you we are in Upstate NY, zone 5a. Yes it is going to be a frost protected shallow foundation. The site has a very high water table and we want to avoid digging down for footings.
Although the heated space doesn't require wings, we are hoping to add wings around the unheated garage and do a floating foundation there as well.
Elizabeth,
I'm not sure why that foundation in your climate would not require wing insulation...
Normally, heat loss through the foundation would help warm the soil but in your case the underside of the slab is quite well insulated...
Personally, I would want wing insulation to help keep the earth under your footings from getting too cold.
It might be wise to consider the insulation requirements for unheated buildings...
As the builder- a couple of comments and responses (other than thanks for the great thoughts and questions):
Dick- I like the idea of extending up the lower wall and running the subfloor to the outside. In addition to your points, it ties the floor system to the shear walls.
Lucas- The wall sheetrock would probably have to extend up to the plywood and the seam could be taped. It would have to extend up to the wall top plate anyway to do ADA. As for the foundation- the design does follow the reference you cited and wing insulation is not needed here (AFI is 1688 being conservative). The sub-slab insulation does require extra perimeter insulation- at least 3" EPS total. We are also looking at a very generous stone pad underneath the foundation.
John,
Ok, that is less cold than I thought it would be there.
A stone or gravel pad is a great idea, especially if there is a high water table.
Out of curiosity, did you use the criteria for heated or unheated buildings for this design?
The design guide doesn't really account for heated buildings with insulated slabs...
The design was made for a heated building. The heated building design can be used for up to R28 underslab insulation. Table 4 adjusts the amount of vertical insulation to account for the subslab foam
Why no foam under the footer?
John,
Thanks!
No foam under the footer because the engineer doesn't like the idea. I disagree, but I don't think it is important in this case since the only thermal bridge is up through the footing and the corner of the inner bottom plate. However, if we do go with a frost protected foundation for the garage, we will need to revisit the issue of footings bearing on foam with the engineer.
Here are a couple details from a set we're working on this week. Might be of some help. North Carolina, so termites require a serious termite shield, Cinder block foundation, floating slab, outer and inner walls tie together with a purlin at mid-wall and studs are offset just 1 1/2" so the right side of the inner wall studs is aligned with the left side of the outer wall studs to simplify the packing of the spider or cellulose insulation.
This is for a house with a 2x12 mud sill and upper top plate so the purlin will be 4 1/4" probably ripped from a 2x6. If it were a 2x10 mud sill and upper top plate the purlin would be 2 1/4" -typically I use half a 2x6 so it's really 2 5/8" and the wall has a 3/8" swell in it. we'll brace the window ROs with the same to minimize tapering the jamb extensions unless using drywall returns.
BTW: If you decide to use foam under the footing, remember that it is recommended to use an advertised psi that is about 3x what you figure you need. Ex: 20 psi load, use 60 psi foam. Look on the Dow and/or Insulfoam web sites.
Would suggest moving your plywood to the exterior of your interior 2x4 wall - this int 2x4 wall will then become your service cavity, and can connect to your roof service cavity and windows - assuring a much better airtight detail (taped plywood compared to ADA) and giving you a vapor retarder on the warm side of the insulation.
On the exterior, you can use solitex mento plus (which I happen to import at http://www.foursevenfive.com) a reinforced WRB that can be used as an exterior WRB/cellulose mesh. The exterior of your wall is now truely vapor open and thus can dry outwards when needed most.
Attached picture of wall assembly mock-up of this concept (replace TJI with balloon framed exterior 2x4 wall.
PS the balloon frame exterior allows the plywood airtight layer to be continuous from bottom plate to roof ceiling, w/o being interrupted by the floor truss.
and the images for my previous post