Wall construction alternatives
House to be built is located in central OH … zone 5 cold climate … HDD of 5708 and CDD of 797. Aiming for the following R-values: basement sub-slab (R-10), basement walls (R-16), above grade walls (R-32) and attic (R-49). Planning 2″ of XPS rigid foam under the basement slab, 2″ of DOW Perimate XPS rigid foam outside the poured concrete foundation walls with a rigid foam lining the interior basement wall. Is EPS recommended over XPS or Poly-iso for the interior basement walls to aid in drying towards the interior? Looking to use blown in fiberglass to R-49 in the attic with the application of raised heel trusses.
Regarding the above grade walls, there are 2 options being considered from the exterior towards the interior:
Option 1:
– Fiber cement cladding
– Furring strips to aid in back-ventilation of the cladding
– Crinkled housewrap for hygric redistribution
– 2 layers of 1″ unfaced XPS rigid foam with taped and staggered seams (R-10)
– OSB sheathing for shear resistance
– 2×6 wall construction with R-23 blown fiberglass insulation (such as Spyder)
– Air sealing with a product such as OC’s Energy Complete
– Drywall
– Latex paint
Option 2:
– Fiber cement cladding
– Furring strips
– Crinkled housewrap
– OSB sheathing
– Staggered 2×4 studs at 24″ spacing on a 2×6 plate … essentially an offset stud every 12″
– Closed cell spray foam insulation to air seal and fill the cavity with an R-value over R-30
– Drywall
– Latex paint
Option 2 seems simpler overall from a labor standpoint, although the spray foam would be more expensive … much closer to the type of standard wall construction that the custom builders are doing in this area with the exception of staggered studs and spray foam. However, option 1 would provide for more complete thermal breaks with the rigid foam wrap, but most likely pricey as well to insulate considering all of the separate air sealing steps and layers of foam involved. I’d welcome your feedback regarding both wall options in our zone 5 climate as well as any other suggestions.
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Replies
Milan,
Q. "Is EPS recommended over XPS or polyiso for the interior basement walls to aid in drying towards the interior?"
A. Some people make that recommendation. However, I don't think that the permeance of rigid foam in this application matters much. You're not going to get much (if any) drying through the foam, and you don't need to.
Q. "Which wall alternative should I choose?"
A. Either one will work. If you don't have a preference, you might simply choose the least expensive option. There really is no answer on these wall alternative questions. It's your house, so you get to build it any way you want.
Milan,
I have also used the single plate, double stud wall system but with 2x8 plates. 2 layers of high density batt insulation will get you to near R-30 and be much less expensive than foam.
I'd side with Doug, with the exception of using a blow in insulating product as opposed to batts, but thats just me. Air sealing will greatly help the performance either way. I guess I fail to see what the learning curve for a double stud wall is to an experienced builder.
Thanks for the follow-up responses. Martin ... although you mention that both will work, does one perform better/make more sense from a building science aspect? You surprised me a bit in that I thought you might say option 1 is the better way to go as many of your articles are related to the proper details for applying exterior rigid foam. In terms of a double stud wall, is it better to include exterior rigid foam if the primary insulation is a blown-in product (cellulose or fiberglass)? Has anyone posted columns detailing the best way to construct double stud walls? Example ... single large plate or 2x4's separated by a spacer, for example. The only reason I was considering the 2 options above is that neither one takes away from the interior space if the foundation remains the same. Thanks again for your input ... it is appreciated.
Milan, take a look at the Building Science website where engineers and scientists have looked at these issues and recommended certain systems. For basements see: http://www.buildingscience.com/documents/reports/rr-0202-basement-insulation-systems
There are newer recommendations on their website as well, and many other superb articles and recommendations based on scientific testing and construction experience.
Based on the vapor permeance of EPS and XPS foam you can cover the interior basement walls with a couple of inches of foam and they will allow some drying to the inside (that is the meaning of the vapor permeance numbers). EPS has higher vapor permeance but I prefer to use XPS because it is more rigid and therefore easier to handle and apply to basement walls. Polyiso is usually foil faced and will not allow any drying so it is not recommended on basement walls unless it is used on the above grade portion of the walls.
Since you're using foam on the outside and inside of the foundation have you you thought about using ICFs for your foundation?
Good luck with your project.
Billy
William,
2 inches of EPS has a permeance of 1 perm, and 2 inches of XPS has a permeance of 0.5 perm. Since the concrete is in contact with wet soil, there is always a huge sink of available moisture on the exterior of most concrete foundation walls.
Drying through materials with a permeance of 0.5 to 1 perm is quite slow. And drying out concrete in contact with damp soil is a fool's errand. You don't want moisture to evaporate into the interior of your home in any case.
I have long disagreed with the recommendations of the Building Science Corporation on this point.
I designed a project where I did a 2x4 offset stud on 2x8 plates. The exterior surface was detailed as the air barrier, and I used ZIP sheathing, taped, to achieve this. The wall was netted and blown with r4.2 Spyder insulation. Windows were box framed out with 3/8" plywood, caulked and taped off to continue the air barrier to the window frame.
My own home will be a true double stud wall with a 3" space between the 2x4 @ 24" walls with ZIP and blown fiberglass. Same window details. Builder really liked them.
Martin,
I see your point, but 1 perm is infinitely better than zero perm. Of course the reason a low perm insulation is used is to reduce the rate of moisture driven to the interior (greatly reducing interior moisture issues) -- 1 perm allows some drying, but as you point out not a lot, and not so much that it would cause interior issues in a properly constructed house.
I have 2" XPS on my basement walls in both the old construction and new construction areas and it works great. I cut into some of it to install an egress window and there was no mold between the foam and block even though it was a fairly wet wall (we waterproofed the exterior of the wall after excavating for the egress window). With a zero permeance barrier such as plastic against the masonry I suspect there would have been mold which is the point of having some drying. No, the entire wall likely won't dry out but it apparently dried enough to prevent mold.
Anyway, in new construction there should have good foundation waterproofing, waterproofing on top of the footers, footer drains, and the slab on top of gravel, a vapor barrier, and foam so you shouldn't get much water in the wall other than the concrete curing.
I think you would agree Milan is in great shape because he has 2 inches of XPS on the exterior of the foundation which Building Science recommends and which eliminates the "concrete against wet soil" issue. He should be in great shape.
I guess you agreed with the recommendation to use XPS or EPS but disagreed with the discussion of vapor permeance? That's fair, but in this case I'll follow the recommendations of the engineers who specialize in this type of analysis. It appears there's very little potential downside in following it and perhaps greater potential downside in not following it.
Billy
Hi Jesse,
With your new wall, you're essentially gaining additional insulation cavity by separating the 2x4 studs as compared to your earlier build with a 8" plate, right? How much greater will the final wall R-value achieve as compared to the prior when using the Spider product. With double stud wall construction, does one need to install exterior rigid sheathing as well? I recall there being some disagreement on the site earlier on a different post where some felt it was unnecessary.
yes gaining additional insulation (almost 3" or r12 worth) but also allowing a thermal break at the sills of the wall.
There is the cold sheathing arguement some have with double stud walls without exterior foam. However many do it with success. I have not actually seen any evidence of issues with double stud walls and no exterior foam.
You may want to consider the LeeCor Systems non-thermal bridging SIPs. They are constructed using light-guage steel framing and EPS and are very light weight. 4x12 panel weighs approx 68 lbs.