Rigid foam vs. spray foam for walls
I’m planning a small addition to my house. I like the theory behind BSC Perfect Wall however I am a little concerned about the techniques to support heavy stone veneer overtop of 3-4” of foam board.
As an alternative I am wondering how to adopt the spirit of this wall construction technique using spray foam. Would it make sense to use the following stack?
– 1” rigid foam
– plywood
– 2×6 stud walls
– 2” spray foam
– roxul
Also, my addition is to be built on helical screws, so I have a similar insulation problem on the floor. I could do (from bottom to top):
– 2×10 pressure treated structure – 1” rigid foam
– 2×8 floor joists
– 2” spray foam
– roxul
does this make sense? Should I just delete the exterior rigid foam and accept the small amount of thermal bridging from the 2x ‘ s?
Thanks!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Does the addition need to have stone veneer? That seems like the complicating factor. Plus I don't think stone looks right floating over piers, but you weren't asking about aesthetics.
I don't see the need for any spray foam in your assemblies. Just tape the plywood for airtightness; it works better and costs less, with little change in thermal performance.
I think I need foam somewhere in order to get enough R value (31 walls, 50 floor).
Fair point about the aesthetics, in this case a cedar deck is going to hide the visual of the floating build.
I’d skip the spray foam in the walls. Assuming you’re planning on using closed cell spray foam, 2” is about R12. Mineral wool alone in 2” is a bit better than R8. There are cheaper ways to air seal than to use a layer of spray foam everywhere. I’d use either a thicker wall with batts and thinner exterior foam (due to the issue of supporting the veneer), or a thicker double stud wall with dense pack cellulose.
Thermal bridging of the studs really kills any performance advantage spray foam would have.
In the floor, I’m not so sure. I’ve never done a floor like this. I wouldn't use spray foam, I think you’d be better off with a layer of continuous rigid foam under the joists and then filling the cavities with dense pack cellulose. I’m sure you’ll get some other ideas here to try with the floor.
Spray foam is really best used where you have lots of irregular objects to seal around (like a rim joist), or where you need to adhere to something due to condensation concerns (like a non-vented roof assembly). Most regular walls and floors don’t really have much to gain from the use of spray foam over other, cheaper, methodologies.
Bill
Thanks. The more I think about it I think the floor is the easier problem to solve, with a slight caveat of the thermal bridging where the 2x10’s are. But if I end up bringing in a spray foam contractor for the walls then it’s a fixed cost i should probably leverage more.
I think the BSC method calls for at least 50% of the R-Value from foam, does it not?
Continuous rigid foam is not the same as spray foam. The spray foam contractor will charge by sprayed area and thickness too, so it’s not a fixed cost. If you do go the spray foam route, it will be cheaper to “add a little more” since they’re already onsite, and that can make it make spray foam make sense in other areas. I still don’t think spray foam is your best option in these assemblies though.
Bill
So you think it’s better to do only a thin course of rigid on the outside and more Roxul inside? Arent there different vapor barrier considerations with that strategy?
That depends on the type and thickness of rigid foam you use, and on the construction of the assembly. EPS, for example, is vapor permeable in reasonable thickness. Foil-faced polyiso is a vapor barrier (because of the foil facing). If the rigid foam Is the only exterior sheathing (no plywood/OSB), then there is less of an issue with moisture on the cold side, but you need to be able to dry towards the interior. If you put rigid foam over exterior plywood sheathing, you can trap moisture and get moldy/soggy plywood which is a problem.
Bill
Thickness would be 1" of whatever works. I understand how to do this with 2-3" foam but not with 1".
>" Should I just delete the exterior rigid foam and accept the small amount of thermal bridging from the 2x ‘ s"
Absolutely NOT! That "...small amount of thermal bridging..." is actually pretty significant, and an inch of polyisocyanurate foam will cut those losses/gains by half. The performance adder of continuous 1" polyiso is about THREE TIMES what you'd get with a full cavity fill of closed cell foam in the 2x4 wall.
The thermal value of the 2" closed cell foam in the stud bays is largely wasted due to the thermal bridging, but may (or may not) be necessary for dew point control at the foam/fiber boundary depending on your local climate. Where is this house located?
See: https://www.finehomebuilding.com/membership/pdf/184243/021269086NRGnerd.pdf
Thanks for the response. I'm in Toronto. I think the foam is needed for dew point control as you suggest.
Going back and forth with the builder and I think we've settled on a stack that works well for everyone and avoids the need for spray foam:
2x6 stud wall
OSB sheathing (complete or minimum necessary for racking)
2" foam
OSB sheathing (anchored through foam to studs, and at bottom to rim joist for shear)
WRB/drainage plane
metal lathe
stone veneer
Anyone see any issues with this?
I’m assuming you’re using mineral wool batts in the wall cavities?
If you can get a rain screen in there that might be beneficial.
You might want to look at BSC’s article about shear strength in fasteners through thick foam: https://www.buildingscience.com/documents/bareports/ba-1314-cladding-attachment-over-thick-exterior-sheathing/view
Since you have a heavy veneer going up.
Bill
Yes to mineral wool in the cavities. WRB/drainage plane (either crinkled Tyvek or Delta-Dri), is this not the same thing as a rain screen?
With respect to the shear, that is the thinking about the second course of OSB sheathing supported by the rim joist and/or top plate without having to go through foam - that should carry the shear load.
That link suggests furring strips (instead of a second course of OSB) and I have been thinking about that, but it just seems to me a 3/4" gap behind the metal lathe and stone veneer is not appropriate (like if a baseball hits that wall or someone leans on it would it push in). Maybe it would be fine though.
Wrinkled house wrap is not a rain screen.
The “3/4 gap” you mentioned that is created by the furring strips IS a rainscreen. Think of the rain screen as a ventilated open area behind the siding and in front of the sheathing/rigid foam that promotes drying. There are a lot of advantages to incorporating a rain screen if you can.
I was thinking you wanted the second layer of OSB because you needed full sheets to support the veneer you want. Most siding is installed using furring strips when thick rigid foam is used.
Bill
Would you consider Delta-Dry dimpled membrane a rain screen?
I would happily consider furring strips but have found no evidence that you can hang metal lathe and mortar on a 16” furring strip course?
Looks like they make a Delta-Dry Stone and Stucco product for exactly this application. Perfect.
I'm wondering if you guys think the second course of sheathing is adding any value - intuitively it seems like its a better way to prevent fastener bending/shear, but if those fasteners ultimately need to be anchored to the studs behind then is it really? And is it proven tested methodology?