Wall stack for climate zone (4)
I’m going to be building a new house in Tennessee, climate zone (4). The local codes use 2012 IBC but call for the 2009 International Energy Conservation Code.
I have a couple of restrictions due to the builder / community HOA … these are 2X4 walls and vinyl siding. Also restricted to a 12-4 roof and 1 story house.
I want to include a continuous insulation on the house and want to make sure my wall stack up is proper. I’ve read a lot of the articles on this site and on buildingscience and ended up with the below wall stack and materials (based on various pro/con articles as well as cost)
– Latex paint
– 1/2 gypsum board
– Blown in fiberglass insulation
– 7/16″ OSB
– WRB (Tyvek)
– 1″ XPS foam for R5
– Vinyl siding
Now for some questions.
– I’ve seen the WRB shown in two different locations. On top of the OSB or on top of the XPS insulation. Is one of these preferred for my climate zone?
– If the WRB should be sandwiched between the OSB and XPS, should I consider using Tyvek Drainwrap style to help drying should any water get to that layer?
– Is it necessary to tape any of the seams in the OSB and/or XPS or will the house wrap provide the full air seal.
– I’ve seen XPS available with a tongue and groove edge, this seems like a good choice to make a more torturous path as well as a better continuous blanket. Has anyone used this? Am I making up benefits that don’t exist?
– Any issues with the Vinyl Siding right on top of the XPS?
Thanks!
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Replies
See some of my comments I posted today on this thread:
https://www.greenbuildingadvisor.com/community/forum/green-building-techniques/96741/seeking-insulation-recommendations
Yes, you need to air seal both the foam and the OSB. (If you have to choose only one, make it the OSB, but both is better.)
Air sealing foil faced polyiso using the appropriate tapes is somewhat easier than with XPS or unfaced EPS. Polyiso will outperform XPS inch for inch, and with the air space provided by the vinyl siding the foil facer gives it a boost too. It's a greener foam, cheaper per R, and higher R both initially over the long term. If an inch is all the space you have, it's a clearly better choice.
The location of the WRB depends on how you mount & flash the windows. If the glass is roughly co-planar with the siding it's an "outie" mount, and the WRB & flashing are exterior to the foam. If the glass is roughly co-planar with the structural sheathing, the WRB & flashing are between the foam & OSB (a crinkly type WRB preferred over flat, for better capillary break & drainage.) See:
https://www.greenbuildingadvisor.com/blogs/dept/musings/innie-windows-or-outie-windows
https://www.greenbuildingadvisor.com/articles/dept/musings/installing-windows-foam-sheathed-wall
Having the vinyl siding directly on the foam is fine- the air space behind the vinyl is a reasonable capillary break for moisture that wicks around & up the interior side of the vinyl, and any bulk water that gets onto the foam dries quickly.
Stephen,
You might want to read this article: Where Does the Housewrap Go?
Thanks Dana and Martin, you have helped answer a bunch of my questions.
I am going with "Outie" windows, so that helps settle my mind on where to place the house wrap.
RE: XPS vs Polyiso my original reasons for the XPS choice were that I read that Polyiso R values begin to drop significantly with temperatures < 60. That coupled with the delta Temp (inside/outside) will be greatest in the cold months, this seemed problematic. I don't know if that's old information / obsolete product so I'll have to research that again as you make some great points about the two, plus it does not really get that cold here in zone 4a.
More than you might want to know about xps vs eps. https://www.greenbuildingadvisor.com/articles/dept/musings/thermal-drift-polyiso-and-xps
The polyiso low temperature performance drop is really only a concern in colder climates than yours. Consider youself lucky to live where it's warm enough to use polyiso without that worry, and take advantage of all the advantages Dana describes.
Thanks for the extra links on the polyiso vs xps, lots of (more) good information. Who knew the choice of the continuous insulation layer could be this confusing.
I have a couple of additional questions since now I'm leaning towards the polyiso given the additional information I've been given.
- First the foil faced polyiso has a lower perm rating than XPS. At .03 its to be close to becoming a vapor barrier. Is this something I need to be concerned with in my wall stackup? Seems like most of the drying potential would be to the inside of the structure where as the XPS at 1.1 had more outside as well as inside "flow through" potential. I don't really know the magnitude/range of the perm numbers to know if that is true or a concern at all.
- Second, since the polyiso will absorb water that it should most certainly have its seams and any exposed edges tapped, even if its covered by the WRB house wrap?
Thanks for all your help.
Stephen,
If the foil-faced polyiso is covered with housewrap and vinyl siding, there is no reason to worry that the foam will absorb moisture. The foam will stay dry. But taping the seams of the polyiso is always a good idea, because the tape will reduce air leakage.
The vapor permeance of the rigid foam is irrelevant. Walls with exterior rigid foam are designed to dry in both directions: everything on the interior side of the rigid foam dries to the interior, while everything on the exterior side of the rigid foam dries to the exterior. The rigid foam ensures that your wall sheathing stays warm and dry during the winter, so there isn't any moisture accumulation in any case. For more information on the principles behind this type of wall, see Calculating the Minimum Thickness of Rigid Foam Sheathing.