Simple one on thermal bridging
jklingel
| Posted in Energy Efficiency and Durability on
Say you build a “normal” double-wall house, and run the sub-flooring out to the exterior sheathing; thermal bridge. OK. If you stop the sub-flooring 1″ short (2×6 stud, so plenty of bearing) of the exterior sheathing and spray foam the gap, will that suffice to kill the bridge? Should one also install 1″ of foam board on the exterior (taped, etc)? Thanks. john
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The whole wall contains a series of thermal bridges...aka studs. The sub flooring is only a small part of it. You would still need to use exterior foam on the outside or an actual "double-wall" system meaning two 2x4 walls spaced a few inches apart running parallel to each other in place of the standard 2x6 wall. The space would then be filled with insulation thus reducing the thermal bridge.
Walla: Sorry for my incompleteness. The double-wall will be 2x6 exterior, and 2x4 interior, separated by dense packed cellulose. I believe it was Robert R who mentioned the thermal bridge of the sub-floor, which I had not thought about. As I think about this, I guess I'd also have to go a tad deeper and look at the floor joists which run out to the rim joist. Maybe I better just plan on 2" of foam on the outside, all the way around, and narrower double-walls. Either that, or one of the truss wall designs.
John,
The subflooring is an insignificant thermal bridge, and an inch of foam exterior to the 3/4" deck won't make any difference. The significant thermal bridge and air barrier interruption is the floor frame (joists and rim joist), which is always challenging to seal and insulate well.
There should be no reason to support a floor assembly on the outer wall if the whole system is properly designed. Roof bears on outer wall, floors on inner wall. That's the only sensible way to design a double wall house.
Robert: Thanks... for confusing me! "Roof bears on outer wall, floors on inner wall." That is new to me. I will have to look into this. I have been looking at a Build It Solar web page that has, I believe, your truss wall discussed (or the Larsen, and a link to yours; looking at all, again). I am starting to get my head wrapped around this outer-truss wall system... I think. Are you telling me with the above quote that your roofs rest on that ballooned 2x3 exterior wall? In the pics on that web page, I see the floor bearing on the interior wall (which nicely eliminates the rim joist T-bridge issue), but the roof seems to bear on both. No?
John K,
I think your concern is very similar to Daniel Ernst's
It might help to use Dick Russell's illustration posted (11/18 I think)
on this thread
https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/18889/airtight-sheathing-thermally-isolated-double-
I believe Robert has stated elsewhere that this 'strong outer wall' approach would be needed in marine and seismic environments. As opposed to the 'strong inner wall' of the Larsen and Riversong truss approach.
I think I understand the walls but I'm wondering what the best way would be to deal with roof ties which might well have to pass through conditioned space. Accept the thermal bridge? or tuck them between ceiling and floor and insulate the gap? I guess it might depend on whether or not you were going to use the attic as living space.
In the previous post, by "ceiling and floor" I meant "ceiling and floor above". I presume that the gap between ceiling and floor below is full of people and stuff.
John,
Sorry for the confusion. I was responding to your question about "double wall" systems, and I differentiate between a truss wall (like mine) and a double wall, which (for 30 years) has meant two independent walls tied together at some point(s).
All my early experimentation and experience with super-insulation was with various iterations of the double stud wall, which all had the disadvantages of ambiguous load paths, excess materials and thermal bridging (and air/vapor/thermal barrier interruption) of the floor assembly.
I've lately designed several double wall houses (including one that's being finished up right now) that eliminate the floor assembly interruption and ambiguity of load paths by using a wide enough foundation so that both inner and outer exterior walls can have bearing to ground, having the floor assembly rest on the inner wall and the roof on the outer, sheathed wall. This also addresses the issues of exterior wall wind and seismic resistance that many builders are facing in various code jurisdictions (and uses all grade-stamped KD lumber and plywood sheathing).
I hope that's more clear.
John,
If you go to the Airtight Sheathing thread, there are lots of illustrations of variations of the double wall with inner floor bearing.
OK. I thank all of you for the posts, and esp the pics. I am starting to see what is going on here, and I like it. I think I can pull this off now. john