Hi group.. I'm still trying to design my small (1000-1200ft2) house in WI and am shooting for R-40 walls. I'd need to do 4" of XPS, but also want to do wood lap and shingle cladding, which means long screws and furring strips etc....and I'm still not sure it would be a good idea. In other words, what kind of cladding works with 4" of foam (other than EIFS)? I'm leaning towards double studs with cellulose because of the cladding issue and for the sound decoupling it offers. I'm shooting for a very soundproof house, as well as energy efficient... Thanks a ton for any comments. -michael

"what kind of cladding works with 4" of foam?"
Any type of wood siding works; we've used claps, shingles etc. I'd suggest using 4" of foil faced polyiso instead of the XPS; better for the environment, a higher R value and typically about the same price.

Hi Bob..

I considered using poly, but its more expensive here (at Menards, anyway): 4" is $2.25/ft2 vs. 4" XPS is $1.50/ft2. you supposedly get more R per inch, but I've heard it degrades in cold weather somewhat. Even 3" of poly is $1.75/ft2, but the slight price increase would be made up by shorter screws etc. But then there's fairly expensive tape for sealing the joints...

So you see no problems with fastening cedar lap on furring strips through 3 or 4 inches of foam? That's good news.

but I'm still thinking of doing double studs to save moola....

Thanks!
-m

Something to keep in mind when attaching things to foam. If the screw is loose, then you are "hanging on the end of a not well supported lever". On the other hand, if it's tight (or glued), then friction is transferring the weight/force of the siding to a larger area of the foam. In this case, foam is pretty strong (maybe 30+ pounds per sq inch of contact and 120 pounds per screw even for EPS).

This is speculative, but given the not so great permeability of thick foam, I'd consider putting it on the inside (the proper place for a vapor retarder in a cold climate).

Hi Jonr..

Yes, the long screws were my concern regarding how much weight you can hang on them. but I assume with furring strips, it should be ok. But the cost of the 6" screws has to be considered (I estimate about $400 worth for the whole exterior).

I've always wondered about putting the XPS on the INSIDE. Hard to find very many people discussing it. It makes sense to me. Problem would be elec. box mounting, as well as drywall screw pop-out (so you'd have to use furring again....) But I may use all wood paneling, so that may not affect me.

But again, I like the double studs for the soundproofing aspect..........

Thanks.

Hi Jonr..

Yes, the long screws were my concern regarding how much weight you can hang on them. but I assume with furring strips, it should be ok. But the cost of the 6" screws has to be considered (I estimate about $400 worth for the whole exterior).

I've always wondered about putting the XPS on the INSIDE. Hard to find very many people discussing it. It makes sense to me. Problem would be elec. box mounting, as well as drywall screw pop-out (so you'd have to use furring again....) But I may use all wood paneling, so that may not affect me.

But again, I like the double studs for the soundproofing aspect..........

Thanks.

Posted By 950ft2 on 24 Oct 2013 10:34 AM
Hi Bob..

I considered using poly, but its more expensive here (at Menards, anyway): 4" is $2.25/ft2 vs. 4" XPS is $1.50/ft2. you supposedly get more R per inch, but I've heard it degrades in cold weather somewhat. Even 3" of poly is $1.75/ft2, but the slight price increase would be made up by shorter screws etc. But then there's fairly expensive tape for sealing the joints...

So you see no problems with fastening cedar lap on furring strips through 3 or 4 inches of foam? That's good news.

but I'm still thinking of doing double studs to save moola....

Thanks!
-m

The R-value per inch of polyiso and XPS are about the same when it's -4F/-20C at  the center of the foam layer.  On a 4" layer on the outside of a 2x6 insulated R20 that only happens when it's truly ARCTIC cold outside. If you live north of the arctic circle, it matters. For the rest of us, the polyiso will still have a performance edge, if it's half your center-cavity R-value or better.

If you live where the binned hourly average mid-winter temp is -20C and you keep it at 20C indoors, the warm edge of the foam at center cavity averages about 0C, and the center-foam temp is about -10C.  At that temp 1.5lb polyiso that's rated R6.5/inch @ 75F for labeling in an ASTM-C 518 test will still be over R6, and XPS rated at R5/inch @ 75F center temp will have risen to about R5.4-R5.5- the iso is still out performing it.

The thinner you make the foam, the cooler the average mid-winter center-foam temp, which means at only 1" they are pretty much equivalent during January in the cooler parts of the Canadian midwest.  But in most of the US polyiso would be the better performer.

Do not skimp on the screws; they are often the 2nd most expensive part of this assembly; use GRK, Timberlok or some equivilant, and also do not skimp on the tape. There was a good article in Green Building Advisor recently about tapes. All in all double stud is substantially less expensive and works well.

Hi Bob..

Yes, I would use good screws if I went the foam route, but I'm leaning toward double studs and so wouldn't need any foam.....

Now I need to figure out if I can blow in the cellulose myself or if I'd need a contractor to get it to the right density.

Thanks.

be careful where you save money; sometimes experts are worth what they charge.

A couple of things to consider, First you said that the tape to seal ISO is fairly expensive, true, but a very good seal is delivered. Second not sure if you are on a slab or not but a Frost protected Shallow foundation (FPSF) can save you some money on concrete, and if insulated on the outside, matches up beautifully with a Foam sheet exterior. Less worry about screws holding weight.

I looked at FPSF foundations quite closely. Seems none of the contractors around here know how to do them. Also, my lot is on "disturbed soil." An engineer told me to stay away, as my lot was filled just 5 years ago. you also lose some of the cement savings in paying for the extra foam...

thanks,
-m

You may want to read this bit o' bloggery in-re the importance of getting the details perfect when going with double studwalls.

Hi Dana..

Wow, there really is a lot to think about with high-R walls in a cold climate. Seems like the experts don't all agree on what's best. An article in Fine Homebuilding "Six Proven Ways to Build Energy-Smart Walls" says "double-stud walls are tried and true." They don't even mention condensation problems under "cons." Another place I go for info:

http://www.buildingscience.com/doctypes/enclosures-that-work/etw-high-r-value-enclosure-assemblie

They seem to like the 2 x 6 Advanced Frame Wall Construction, where they skip the OSB and use the rigid foam as sheathing (except for corner bracing). but when I asked some contractors around here about this, they were clueless.

Anyway, certainly lots to think about.....

Thanks,
-michael

Exterior insulation is a better way to insulate than double stud; I think that is accepted. But, it is substantially more expensive and most buyers are price conscious and aren't willing to pay for it. Double stud is a good alternative if done well, and "done well" has to mean building a very tight house while minimizing interior moisture intrusion and utilizing proven methods to keep the interior moisture levels at the low end of comfortable and healthy so as to maintain an envelope free of condensation and mold. Cutting corners, as many DIY-ers are looking to do on this forum can be seriously problematic.

I like the part where Straube's colleague admits "We intentionally injected interior air into the walls..." (to get a moisture problem with a double stud wall). I expect that keeping humidity low, doing a respectable job of air sealing and not allowing the house to go to positive pressure (most go negative) would be more than enough to prevent any issues. But these things should be done in ALL houses anyway. If you are really worried about it, put 1/2" of taped XPS on the exterior (in additional to an inside air barrier).

I expect that crossed diagonal steel strapping is more energy efficient that corner bracing for a building with no structural sheathing.

In the latest Fine Homebuilding magazine (issue #240) there's an article on passive house design where they're using a thin layer of spray foam, then filling the rest of double stud cavities with cellulose. I haven't seen that before. I have seen "flash and batt" used in a 2 x 6 wall, but not in double stud. I wonder if there isn't an easier, cheaper way to seal the shell.... like caulk, taping sheathing joints, elastomeric paint(?)..........

Thanks,
-michael