Request for comments on this wall mock-up.
CZ 6B – SW Colorado – 7000′
In the attached photo of this exterior wall mock-up there is no WRB. I plan to install some kind of WRB on the 2×4 exterior frame over the Roxul then apply the 1×3 strapping and corrugated steel siding (the actual siding will be wavey with a grayish weathered patina). My concern is heat on the southern exposure although most of it will have roof overhangs for summer shading. Should I use roofing felt (15 or 30#), or housewrap (Tyvek or cheaper HD everbuilt) ? The vertical corrugations allow for air movement cooling and there is no contact conductance but the heat radiation might be too hot for the felt? Also – perforated L flashing and ICF protection Z flashing will be installed at the bottom even though it’s not on the mock-up.
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Replies
Chuck,
It certainly looks to me as if the wall is (a) unbraced and (b) lacks an air barrier.
Building codes require wall bracing to prevent racking. I suppose that you could argue that the steel siding provides bracing, but I can't imagine that any code official would agree without an engineer's stamp.
I advice anyone using air-permeable insulation to include two air barriers: an interior air barrier and an exterior air barrier. I suppose that you could install interior drywall complying with the Airtight Drywall Approach, but you haven't told us about any plan for an exterior air barrier. Perhaps you intend to buy a fancy European WRB membrane and tape the seams?
Martin, I should have labeled the picture or explained it better. The 2x6 interior wall is the structural wall and has 7/16 OSB on the outside taped with Siga Wigluv or equivalent as the air barrier which I wanted to have buried in the wall for protection and stability. To aid structural bracing I am also considering metal strapping in addition to the OSB. How does this sound?
Chuck, your wall in similar to this one: http://ourhouseuponmoosehill.blogspot.com/p/details.html checking that blog out might be of interest to you.
With the OSB sheeting on the exteriror side of the structural studs it's already adequately braced.
If the foundation is only under the 2x6 structural studwall what you have is essentially a Larsen Truss wall design. You may need some plywood or OSB or 2x gussets between the structural studs and the exterior side studs to support the weight of the siding when the non-structural exterior side studwall is cantilevered off the foundation.
http://www.sensiblehouse.org/images/con_truss_wall2.gif
You still need a WRB detailed as an air barrier at the exterior side of the exterior rock wool.
There is no appreciable cooling provided by the convection behind the siding, but it DOES provide both a capillary break and high drying rates toward the exterior.
In a 6B climate with more than 1/3 of the total R outside of the OSB, you won't need an interior side vapor retarder. In your case it looks like you have about 2/3 of the total R outside of the OSB, with huge drying capacity toward the exterior, so from a moisture resilience POV you're fine even with minor air leakage from the interior (though air-tight is always right from both moisture & energy perspectives.)
Chuck, you won't have any problems with deterioration if you decide to use building paper if it isn't in contact with the steel siding. That wall does a lot of things right. It is simple, with the air barrier protected from both moisture and mechanical damage. Detailing openings and penetrations may be fussy. I second Bill's recommendation to look at Lucas Durant's blog for tips.
Dana, this would have looked better sitting on the actual foundation which is taper top ICF so both stud walls rest on the concrete with treated bottom plates and air barrier transition detail from slab to OSB. There is additional outer layer of EPS to compensate for the thinner ICF at the top. Question - would asphalt paper work as WRB on the outside (not sure how to detail as air barrier)?
Malcolm, yes the window bucks will be interesting, I'll check the blog for tips. Thanks.
Chuck, is that white sleeve something tying the two walls together? Unless it's more than one story there isn't any need to.
#15 felt is really hard to make air tight, but it's a good air barrier. A layer of housewrap over the studs but behind the girts would be easier.
Chuck - It seems like you could use 2x4's on the inner,structural wall to save money, and go thicker with the roxul between the studs, where there is no watering down of R-value because of framing. Not sure you need to tie the 2 walls together structurally ( the white pipe in the photo?).I like the wall,and am contemplating doing something similar, but with the outer wall being the structural one (sheathed on the inside) so that the house can be closed in and weather tight fairly quickly.Is dense pack cellulose available to you? I'm thinking that might be good on the inside wall, for some water vapor buffering. You may not need another air barrier if you did this ,but I defer to the experts on that.Best of luck to you with your project
Kevin, when you move your plywood to the inside of the outer wall it now has at least 2/3 of the insulation on the interior side. Depending what role the plywood plays in your assembly, as an air barrier or vapour retarder, this may pose practical and code problems.
Malcolm, that pvc pipe has a 10" TimberLok through it which I may only use one midway on the southern walls which are 13.5' tall (shed roof), but I'll see how the middle space measures all around and use them to tighten it up if there's too much air gap between the Roxul.
Dana, yes, you are right the housewrap would be easier and I'll probably do that after making sure the building inspector is ok with the overall wall design.
Kevin, I could go 2x4 structural framing on the 8' inner wall on the north side with 5.5" Roxul in the gap but the 13.5' south wall with windows needed the 2x6 framing. the inner walls sheathed on the outside are very normal to framing crews around here so they shouldn't be too puzzled by that part (I wanted to frame it myself but it would take too long). There is a dense pack cellulose contractor in the area but they are busy and expensive because it's the great new thing and what I like about Roxul is it's easy too work with, I can do it myself, and if I want to remodel, add on, or just take it apart to inspect the wall inside I can easily do that without special equipment or contractors. That is why I went with Roxul.
Malcolm - I know, you're right, and I may use 2x6 on the outside wall to partially offset this problem. These" rules of thumb"came about due to using foam on the outside of the studs, which does not allow much drying to the outside,so you've got to keep the sheathing warm so that condensation does not occur. Roxul or cellulose WOULD allow drying to the outside, making these rules a bit more "bendable". I feel that my plan is certainly an improvement( as is Chuck's plan) over a simple double stud wall, whose sheathing would get considerably colder and therefore have more tendency to get wet,especially the north side. The plywood would be my structural sheathing as well as my AB. Thanks for your comments,I always appreciate them.
Chuck,
For more information on walls similar to yours, see The Klingenberg Wall.
Kevin, I don't know how much of the wet sheathing problem is theoretical. I guess we will find out as Building Science Corp and others continue their testing.
Our code up here still requires the vapour retarder to be close to the inside of the wall assembly. What difference it makes in practice seems to be a pretty open debate.
What appeals to me about having the inner wall load bearing is that you can stop your floor system at that point and run your outer studs past providing continuous insulation at the rim joist. For one story houses on slabs those advantages disappear.
Chuck wrote:
"that pvc pipe has a 10" TimberLok through it which I may only use one midway on the southern walls which are 13.5' tall"
Makes sense. As I said: looks like a resilient, simple wall that should be easy to build and perform well.