I'd love to get a professional opinion on what the absolute minimum spacing between wall studs in a double wall would need to be to create a functional thermal break. In other words, say you were to build a 2x8 wall with 1-2x4 tight to the outside sheathing, another on the same plane, but tight to the inside finished wall, leaving you with about a 3/4" thermal break. If that 3/4" were filled with rigid foam, would there be any benefit to having the thermal break closer to the warm living area rather than between the exterior stud and cooler sheathing. I'm working on a design for a wooden panel yurt, 27 panel, with lots of corners, and lots of thermal bridging, I'm just trying to come up with the most cost effective, highly insulated stick frame. Just a hypothetical question. Any opinions

Take a look at http://www.truprotect.com/. They have a product that can be applied to, let's say, a 2 x 6 stud prior to the sheathing, that will create a thermal break without going to a double wall. Infrared photos show that it does work!

Thanks, It looks like an impressive product, I'm waiting for a quote.

Radiant barrier? Oh please.

richntiff,

I was "impressed" with the product, because so far its the only rigid foam board with reflective foil that I've seen, that is safe to use for an interior wall application, without another sealed material over it (drywall). not saying this foam would be my finished interior, but its nice to know that this stuff is safe enough to install in a living space, If I could use this rigid material to hold blown or loose insulation in the cavity, then I could drywall the interior walls and wood trim the 27 interior corners rather then mud and tape every tedious corner. (taping is not one of my strong points) So for my application, this product has its perks over other radiant barriers.

Do you have any constructive opinions on this post? My first thought was that I might be able to get a batt of r-30 in this wall cavity, but I'm finding that might not be possible. I'm more interested in finding out if the location of the thermal break has any affect on the performance of the wall.

Thanks for any feedback.

Gj: No idea what the min dist is, but I'd suggest looking at this a tad differently, for cost sake. I used double 2x4 walls on my house in '80, w/ R-19 batts stacked between them. 2x6 is too spendy, and 2x8 is even worse. This has worked well for me; I burn about 9 btu/sf, including heating water, in Fairbanks. My new house will be similar, but larger "free batts" and an empty inner wall for wires and pipes. I don't know if this is applicable to a yurt, but it is right along w/ what you have planned and avoiding 2x8s may save you some $$. Email me at [email protected] if you want a jpg of the new wall design. I am no expert, but having lived here since 1980 and using WAY less oil than anyone I know leads me to believe that I may be onto something here. A word on the reflectivity of things in walls: apparently it works off the bat, but eventually gets dusty and provides little, if any, help. It is not recommended where dust could be an issue; whether or not that applies to you is your call. Good luck w/ whatever you use. j

In almost all cases the thermal break is most effective on the exterior of the framing. This places most of the thermal mass on the interior of the insulation and smooths out the ambient temperature variations.

With rigid foam under your siding you can get a thermal break as thick as you want without the hassle of a double wall. Some siding manufacturers will limit how thick a layer they will allow (Hardie board says 1" max). If you could go 6" thick, you could get about an R-30 with no cavity insulation at all. This would be about twice as effective as a typical framed wall with R-19 cavity insulation.

You need to be careful where the dew point is in the wall assembly when doing this or you can get condensate on the panel. I believe in cold climates if there is foam on the outside the stud bays should be empty which means you need more foam than usual to get your r-value but necessary to keep the dew point in the foam.

I tried this stuff....

http://building.dow.com/na/en/sis/index.htm?DCSext.dbs=www.dowsis.com

R-5.5 at one inch thick, vapor barrier, thermal break for the whole wall and I think there is some kind of foil behind the styrofoam. They claim it is good for structural walls. It costs $1 sqft roughly. It went up with no trouble. We'll see.....

That DOW SIS looks like a great product. Cant wait for your review !!

One thing you can do along the line of thermal break w/ double wall: use 2x6 sill and plate, and 2x4 studs, alternately meeting the outside and inside faces @16".

If using 1/2" DOW SIS sheathing in place of 1/2" OSB on the outside of a 2x4 or 2x6 wall, would this provide enough of a thermal break, or would there still be a chance of moisture reaching dew point and causing condensation within the wall cavity? If 1/2" isn't sufficient, would 1" SIS work? I would probably use closed cell foam between the studs. BTW, I live in Michigan.

Thanks everyone for the feedback, I've taken a little time off from my project, but I'm back on the computer and finalizing the design for our 30' wooden panel yurt. I was able to get a sample of the 1/2" DOW SIS panel. It pretty rigid, and looks nice, but the more I research this product, the more I realize how new it is to the residential market, and I don't think I want to be the "guinea pig" this time.

I'm leaning towards the ZIP sheathing w/ 1/2" foam. I'll feel better having the shear strength of the 1/2" sheathing vs. the 1/4" of paper product that seemed to separate easily when I left in a bucket of water over night. I'm sure they have tested this stuff, and its probably great, but I cant afford to be replacing this stuff in ten years, if they find out its not all its cracked up to be. Just my opinion. Thanks again for everyones feedback.

IIRC Dow Thermax (foil faced iso) comes in a fire-rated version that requires no additional thermal barrier.  But even the standard stuff is pretty safe.  It takes a HELL of a fire to ignite it, but it'll eventually go on it's own. But at some lower flame level there are toxic smoke issues, ergo the mandatory thermal barrier.  Iso tolerates temps much higher than other foams.  (It's a heluva lot more thermal break than R2 you get out of the Truprotect stuff as well.)

The location of the thermal break is not critical- performance is equivlant whether it's exterior, center-structure, or interior.

But if you're using foil-facers, placement counts- be mindful that unperforated foil is a Class-I vapor retarder, and improper placement can create as many problems as proper-placement solves.

The goods that I'd think most appropriate for unconventional framing apps would be aerogel stud-strips like Aspen's InsulCap:

http://www.aerogel.com/markets/c_framing.html



You get about an R4-5 thermal break out of 1/2" of depth, and you can stick 'em up about anywhere.