I’ve got a house going up soon and the design is calling horizontal furring/strapping on the exterior walls prior to installing the sheathing to minimize thermal breaks created by the studs. Is there a loss of structural intergrity without the sheathing applied to directly to the studs? Generally, how much of a gap is provided, or perhaps I should say what thickness of lumber, is used 3/4, 1 1/2? Do I need to fill the gap or space created by these horizontal members with insulation? I would appreciate any advice you can give me. Thanks.
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Are you building anywhere there's a building code?
I don't think this idea is going to fly with any code.
The nailing schedule for the sheathing isn't going to work unless you have some good size strapping.
No shear strength with the strapping, or very little.
Guess my answer is "No".
Joe H
Edit, do a search here for "Mooney Wall" and you'll get the answer to how to do this inside.
Edited 9/18/2006 10:35 pm by JoeH
Let in diagonal bracing?
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>> the design is calling horizontal furring/strapping on the exterior walls prior to installing the sheathing to minimize thermal breaks created by the studs. <<
Who came up with that idea? Is your house 'an experiment' or what?
As insinuated above, the wall sheathing (structural panels) are nailed to the studs with specific nailing schedules to give rack resistance to the wall assemblies. Most model codes only require the structural sheathing or other means of diagonal support on each side of the house corners and every 25' for long straight walls though. Conscientious builders often like to cover the entire exterior of the house with structural panels.
If you want to minimize thermal transfer, there are other methods - like using staggered stud walls (2x6 plates and 2x4 studs) or putting the firing strips on the inside of the studs. When working with the Energy Star guys one thing I learned is that they want to minimize air pockets and therefore air currents within the wall and ceiling cavities. This is why some people face staple fiberglass insulation. I know fiberglass insulation doesn't belong in a serious discussion about extra energy efficient building techniques, but I just stated it to make a point. Anyway, it seems that minimizing air currents within the wall cavity and creating a thermal break are somewhat incompatible although if a blown or sprayed type of wall insulation is installed properly, air pockets should be minimized, especially on the exterior side of the stud bay. At least if the air currents are on the inside of the wall cavity, the air currents should be room temperature.
I think it's great though that your building budget and mindset support these kind of construction methods.
As others have said, unless there's another means of providing lateral (racking) strength to the walls (let-in braces etc.), you need to put the thermal break furring under the drywall, not under the sheathing. Get this question answered by whoever generated those plans before you construct anything.
To minimize thermal bridging/provide thermal "breaks" in my own walls, I did 2x4s on 24" alternating centres on 2x6 plates with spray foam insulation. I wouldn't recommend this approach for batt insulation of any kind, and the Mooney wall is a better approach if you're using cellulose. The alternating centres meant a lot of trimming and waste for the Icynene installer, since the same 5.5" net thickness is sprayed on the back of the exterior studs as on the sheathing and has to be trimmed off flush. The foam caused no appreciable bowing of the interior studs. The 24" exterior stud centres are no big problem if you use sheathing of adequate thickness. The foam behind the drywall gives it substantial support, but if it's not trimmed carefully you can end up with wavy drywall. Running wire was a dream- minimal drilling. I went with full 2x6s for kings and jacks and 5.5" insulated "sandwich" headers at all openings, which generated more thermal bridging. But you could do this with 2x4 framing on the exterior and just fur the opening a la Mooney wall.
Thanks for all of your advice. You summed up my feelings about racking, nail schedules etc. However, pull out your Summer 2006 Vol 179 FHB and turn to page 73 and look at the diagram. What is going here? On the left hand side, mid-way there is a call-out that reads "Exterior walls are strapped inside and out to minimize thermal bridging." I wonder how structural intergrity is maintained and what keeps the wall from racking? Can someone explain this to me?
However, pull out your Summer 2006 Vol 179 FHB and turn to page 73 and look at the diagram. What is going here? On the left hand side, mid-way there is a call-out that reads "Exterior walls are strapped inside and out to minimize thermal bridging."
Interesting. I'm going to guess that there are engineered shear walls/panels in other places to offset the strapping under the sheathing.
I'm thinking of the article about advanced framing, by Joe Listurbek (sp?) where the houses are sheathed with foam, and shear panels are inset in the wall at key points. Doesn't it make more sense to sheathe with foam, or put ridgid foam over the sheathing, rather than strapping all the exterior walls?
Seems like adding foam could help reduce air infiltration, while strapping walls will only increase the chances.zak
"so it goes"