Wall detail – Canadian Cold Climate
Hi GBA community!
I am having a difficult time comming up with the ideal wall construction(detail) using 2×6 timber for a build I am doing this spring. There are 5500 HDD here in Northern Ontario – Canada.
This is what I have so far…working from inward to out
-1/2″ Drywall
– 6mm poly
– BIBS (fiberglass) in the 2×6 cavity
– 2″ XPS (non taped)
– 7/16″ osb
– house wrap (wrb)
– Siding and Stone
Couple of questions:
1. will the osb cause me any issues?
I have it there to affix my siding and for structural support (or can it even do that over 2″ foam affixed to studs)
2. Should I tape the XPS?
I cannot go without the 6mm vapour barrier here, the inspectors will have nothing of it
But I want to be sure I’m not creating a sandwich in which moisture if any got in can’t get out (hence the no taping)
3. Any other suggestions to go with if I have to build a 2×6 wall?
Thanks for your input
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Replies
Brian,
I'm not sure of your climate zone; I would think that northern Ontario would have more than 5,500 heating degree days, but perhaps your HDD are calculated in Celsius rather than Fahrenheit. I'm going to assume that you live in a very cold climate.
I would recommend (based on the principles explained in Calculating the Minimum Thickness of Rigid Foam Sheathing) that you install a minimum of R-15 of rigid foam on the outside of your framed walls, to be sure that your stud cavities stay above the dew point in winter. If you are using XPS, that means you need at least 3 inches of rigid foam, not 2 inches.
If you want to use both OSB and rigid foam on the exterior side of your wall, the OSB should be installed first, so that the OSB provides adequate shear bracing for your wall.
By all means, tape the seams of the rigid foam, and strive for an airtight installation.
If your local building inspector insists on an interior membrane, don't install polyethylene. Instead, install a "smart" building retarder like MemBrain.
Your reference to "siding and stone" could be a matter of concern. Installing stone on a framed wall often causes problems. I don't know if you mean field stone and mortar, adhered natural stone, or adhered manufactured stone -- but all of these types of siding have been associated with wall failures unless they are installed with excellent water management details.
Martin,
You are correct the HDD is in celsuis.
With 3" foam not only is the cost very prohibitive in this area (over $50) for a 2x8 sheet, but would it pose any window or siding installation nightmares?
I guess I will be taping the XPS if I go that route
I was hpoing to install cultured stone only about 4-5 feet from ground up as a "belt" all the way around the house and the remainder in siding. I will follow the stone installation techniques and use a rain screen to drain out any moisture if ay shoudl get back there.
My building inspector is dead set on 6mm poly...no way around it...i have even tried to show him a few articles from this site...no luck there...
5500 HDD°C converts to 9900 HDD°F
Brian: Just a question, but if the amount of foam Martin recommends is too spendy, how spendy will it be when your wall fails? That makes the foam relatively cheap, right?
Brian,
Not sure where you are exactly but I'm in NW Ontario.
Brand new rigid foam is expensive here as well.
With a little digging I was able to find some reclaimed rigid foam at a bit more than half the price of new.
That being said, if you haven't totally commited to your plan I might suggest that you explore some less foamy approaches to building.
Personally, I like the "Riversong truss".
Thanks for the comments...
My local window installers are very leary on installing over 3" of foam.
also i would need some pretty long nails for the siding? probably 4+inches
I will have do do some cost analysis on a x2 stud wall and what i would need to change in the plan.
Costs of framing vs costs of the extra thick BIBS blanket.
Plus will contractors here know what to do?
Lucas, that is quite the project you have there and great website with all those drawings and details
Did you have a hard time convincing your inspector the the XPS underslab was OK to go right to the wall and straight up and over footing? (essentially floating the slab in foam?)
I thought my inspector wanted the slab to touch the wall and to make that connection??? I might be wrong?
Brian,
Most builders install vertical furring strips, 16 inches or 24 inches on center, on top of the exterior foam. The furring strips are installed with long screws that extend back to the studs, and the siding is installed to the furring strips.
Here is more information:
How to Install Rigid Foam Sheathing
‘Innie’ Windows or ‘Outie’ Windows?
Nailing Window Flanges Through Foam
Brian,
There are many ways to build an insulated foundation and what works for me might not work for you.
I think initially my inspector was a little wary but fortunately she was also open-minded.
Any concerns she may have had were put to rest by the engineer's stamp I have on my plans.
Martin has a good article on the subject of putting your house on top of rigid foam:
https://www.greenbuildingadvisor.com/blogs/dept/musings/foam-under-footings