Hi all.. I'm finally ready to build my little house in WI (7700 HDD) and I think double-stud is the way I want to go. Walls would be about 11" thick. My concern is doing it right, while staying within budget. I've read that there are concerns about condensation and potential mold on the interior surface of the sheathing, as it will can get quite cold. Flashing it with a few inches of foam would cost too much. 1) Do I really need to worry if I'm careful with air sealing and keeping interior humidity low in winter? 2)Is there enough borate in the cellulose to prevent mold growing on the OSB? 3) Is there some easy way to treat the OSB so it doesn't rot? (some sort of primer or borate)? 4) what about using rock wool or denim instead of cellulose? (more expensive, but easier to install) Thank-you very much! -michael

Michael,
I'll offer my double stud wall concept. The primary air barrier is plywood (plywood is a bit more costly than OSB but is far less sensitive to being moistened occasionally) on the outer face of the inner 2x4 wall in addition to being an air barrier it serves as a "smart" vapor control as well, eliminating the need for air tight drywall. I have found that finding a competent installer for dense packed cellulose is near impossible so I've chosen Roxul comfort bats for my insulation the material cost is very close to dense packed cellulose and it is far more DIY friendly. Also the Roxul gives about a 10% improvement in "r" value as well. I've designed my walls around the standard thickness of Roxul so I have 3 1/2" space between stud walls. I plan to use 24" OVE framing for both walls. The outer wall is erected first without any sheathing, then the inner walls are built with sheathing and stood up. Once the roof is on the air barrier is detailed through the open outer wall and the 2 layers of outer Roxul are installed followed by an outer sheathing of fiber board, then a vapor open WRB and siding over a vented rain screen. FWIW I'm using thin brick on "Loxon" panels which is not a low budget exterior. To minimize the condensation risk the studs should be aligned. My walls calculate as r41+ with all wood framing accounted for. With the sheathing having 2/3 of the insulation on it's outside 40 % indoor humidity and -5f outside temperature will not cause condensation. What kind of foundation are you putting under the walls? FWIW I'm using a "PWF" basement which will also be r40+.

Hi there.. I considered doing the sheathing/air sealing on the outer side of the interior studs, but it seemed to get complicated. Also, what kind of "fiberboard" are you using? How much$/sheet? Is it any less susceptible to moisture/mold (if moisture WERE to get in the wall)? I also think it would be easier (for "my crew")to frame up the outer shell and get it dried in quickly, then go back and add the inner wall. I'm not sure about cost of Roxul being on a par with cellulose. I estimate my DIY cellulose cost to be under $1000 vs about $3500. I would, however, rather deal with batts than blowing cellulose all over the place. And I do wonder if I can get it to the proper density.... Thanks for the discussion.... -m

Lliebler's method sound's pretty good. Double walls & related insulation method are generating a lot of discussion in the trades these days; two excellent recent articles are 1) in the current Fine Homebuilding - Architect Steve Baczek's discussion of his double walls on a Passive House, and 2) in the March issue of Journal of Light Construction, a description of a Roxul installation by Passive House Builder Lee McGinley. We've built 10" double walls in several houses and are moving toward adding an Intello air barrier on the interior of the interior wall, then a 1-1/2" wiring chase, to provide better air sealing in the cavity.

Hi.. Yes, I've been following the Fine Homebuilding project. That guy is spending a LOT of moola on those walls. 4" of spray foam? I wonder if that's the only solution? Isn't it possible to get a good tight seal with poly and drywall techniques etc? Thanks... -m

First, these are Certified Passive Houses. Second, he mentioned the SF cost at a seminar, and it wasn't much higher (if any) than typical custom homes in that area.

Third, No this is not the only way to get a good seal; it is possible to get good seals with the Airtight Drywall Approach if you follow the guidelines, or with air barrier products like Intello, or with other robust techniques. Fourth - poly has no place in a typical wall assembly.

"'What kind of fiber board are you using"  I have only chosen the thickness, 25/32 at this time.  There are several brands in the market, some are International Build-Rite, Georgia Pacific, Blue Ridge.  I will use the "structural" grade from whichever maker my lumber supplier stocks.   I did a quick web search to find a budgetary price and found the 1/2" structural grade of Build Rite in a 4'x9'sheet  at Menards for $9.99.  As part of the framing lumber package I would expect the 4x8 sheets of 25/32 to be  around $13.

The question about moisture tolerance and mold resistance  relative to OSB or plywood is more difficult to cite an authoritative source but It is my impression, based on comments in Inspectapedia and other web sources, that it tolerates moisture at least as well as plywood which is far better than OSB and is somewhat less likely to support mold than either plywood or OSB.  It's advantage is it's high moisture permeability so it will stay relatively dry if protected by a WRB and rain screen.
 

Hi Liebler..

I did a little research on fiberboard siding. I did see the BildRite stuff at Menards. I wonder how much it gets used(?) Seems like a good idea, and costs about the same as plywood.

Do you plan on taping the exterior sheathing? Or are you focusing on sealing with the inner sheathing etc? Maybe doing both would be good.?

My thoughts are to use poly (local code) and seal it well, along with sealed drywall on the inside, then use either plywood or OSB or fiberboard with taped seams etc on the exterior. I wouldn't use OSB unless I treated it with something. I thought rolling on a coat of Tim-Bor or other borate treatment might be cost effective...??

Thanks,
-m

M,
I've done more shopping, at "Menards" and found cellulose at $6,99/18.1# which at 3.5 #/cu ft for r=3.8 calculates as $0.0349/rsqft. The Mineral wool offered by Menard's 's is Thermafiber instead of Roxul but it is priced similarly and calculates as $0.054/rsqft so the cellulose is definitely more economical if it can be installed properly. Menard's also has a price on another alternative for the exterior sheathing, that being "Denseglass" a fiberglass faced gypsum with permeability comparable to fiber board but it is much stronger and more rugged, but also significantly heavier. The "Denseglass" costs $17.99/sheet, the same as 4 ply plywood. Based on this it's worth considering DIY dense pack cellulose in a conventionally built double wall with 1/2" Denseglass" on the exterior and 7/16" OSB on the outer face of the inner wall. Framed 24" OC using OVE with a 7/16 OSB top "tie" plate can the outer cavity, successfully, be DIY dense packed with a "rental" blower?

Hi again, L...

Denseglass is new to me. I would think plywood would be good enough, as most builders around here use OSB on everything. But then, a lot of them don't know what they're doing, especially with a thick wall.

I'm also rethinking the poly. Did some reading on Intello, but not sure I need it, as I don't so a lot of cooling in the summer. It's also not cheap. There are so many things to consider. I'll be talking to my potential builders about all this, but I doubt I'll learn much...

I did speak to the company that makes the cellulose blowers at Menards. He said the'ye are same machines the pros use, but you have to use a restrictor nozzle to get the higher density.

I guess code says I need 2 x 6's on main floor if I want to go 24" centers (mine's a 2-story). No big deal...a few more $100??

-m

m,
DenseGlass is the brand name for Georgia Pacific's fiberglass reinforced Gypsum exterior sheathing. There are several other vendors with similar products. They offer vapor permeability of about 20 perms and are used extensively in commercial construction. As I said previously they are significantly stronger than fiberboard but also heavier. FWIW none of the fiberboard makers recommend installing their products on 24"OC framing while 1/2" DenseGlass is rated to withstand 155 mph wind on 24" OC framing if the sheets are installed with the long dimension horizontal. Now that I'm aware of it I'll definitely use DenseGlass as exterior sheathing. FWIW My new wall's layers from outside to inside are 1/2" thin brick, Loxon panels (1/4"thick formed steel), GreenGuard DC14. Tyvec, 1/2" DenseGlass, 2x4 24"OC with r15 mineral wool, 3 1/2" mineral wool (r15), 7/16" OSB caulked to studs and plates of 2x4 24" OC spaces filled wit r15 mineral wool (framing aligned with outer wall), 5/8" Drywall With the 24" OVE framing I should have a framing to insulation ratio of 19% or less and this calculates to r40+ at 19% wood.

I was too quick to say "I'll definitely use DensGlass" ! I had been planning to use a metal substrate (panel) system to attach the "thin brick" and use GreenGuard DC14 over house wrap as a "rain screen. DC14 is a textured fan fold XPS that is specifically designed to be used behind "moisture reservoir" types of cladding. DC 14 is 1/4" thick and adds r1 to an assembly using it. Well it turns out that at least one vendor of metal substrate for thin brick has tested their panels attached to 24"OC framing over 1/4" of "insulative foam". The tests show a "safety factor" of 2 with wind speeds of 130 MPH. So it'll be studs, Tyvec, DC14, Ambrico EZ Wall, thin brick. NO EXTERIOR SHEATHING! And my r40 + BRICK walls are 12 5/16" thick!