Dana is right. There is a lot of point in spraying the foam rather than just solid cellulose. Regardless as to how "perfect" your vapor barrier (more properly vapor retarder) is it will still allow moisture into the wall. Once in the VR will help to trap it there. The idea of the foam on the exterior side is to have foam, which does not allow air movement (but open cell foam will be vapor permeable) covering the area of the temperature gradient of the wall where the dewpoint is likely to be. If the dewpoint is within the foam the condensation will be controlled better than if it is in the cellulose since the moisture laden air can only get to the dew point by diffusion. This is a hard concept to explain without a cross section of the wall and temp gradient to point to!!! I also can't quite fathom the construction sequence for foaming the inside surface of the wall.... how would you assemble this wall construction??

Without capability for drying the ground damp will indeed keep climbing.  It's very common in brick buildings, which is why they get built with an air cavity between the outer wythe  and the interior wall (stud or brick.)  This allows both outward & inward drying, and allows condensation to drip all the way to the bottom where it can exit via weep holes.  Without both inward & outward drying capabilty masonry will show efflorescence and spalling near the foundation over time (especially in freeze/thaw zones.) Often even WITH the cavity it can have issues. This is often dealt with by applying a sacraficial layer of mortar over the brickwork to let the mortar get crummy, to be replaced every few decades or so as it fails.

In normal studwall configuration a capillary break between the sill and foundation is now standard practice in most places. In a very thick wall this becomes even more important because the moisture has further to wick before it gets to a drying surface.

If you can't foam the outside sufficiently to keep the dew point inside the foam, it's standard practice to build it as any heaing-climate outward-drying structure, with good to near-perfect vapor retarders somewhere on the warm side.  This can be 2lb foam in the studwall, if you like, and in most of the US even 3-5" of half-pound foam would be sufficient (but not in much of Canada or Alaska.)  Be sure to use at least a semi-permeable sheathing if you foam or put polyethlene on the interior studwall (OSB would be fine, but not foil-faced iso board) and keep any housewraps vapor-permeable (most are VERY permeable.) IIRC, in my home state of MA there is no requirement for keeping the seasonal dewpoint in a foam layer in wall systems with multi-insulation stackups, but there IS in roof structures.  Foam is very good at creating a true air-barrier (and most moisture transport through walls is via air, not permeated vapor.) Foam can still be your VERY good friend, just don't punch holes in it after the fact for power & plumbing without meticulous attention to re-sealing it with foam (the slow rise stuff in cans like Great Stuff et all is just fine.)  Outward drying works very well in heating dominated climates, less well or not at all in hot-humid climates.

If you foam the interor studwall with half-pound foam have all of your plumbing & electrical roughed in before hand- they trim it to the studs so you can put on the gypsum board, and trimming reduces it's air-barrier capacity very slightly.  But if you glue the gypsum to the studs and caulk around the electrical boxes before finishing & painting it'll be come a very good air-barrier itself. You might apply polyethylene between the studs and the gypsum if you're in a 7500+ degree-day climate, but for most of the US that wouldn't be necessary.

The SIPS I use are out of NM, and around here they are almost even with the cost of a 2x6 stud wall.

When you figure in the extraordinary measures you must go to in order to gain the same low infiltration, factor in the no-cavity zero-flammability properties and the other trades - the cost-benefit ratio can't be beat. If you are hell-bent on a double stud wall, I would consider alternating 2x4 studs on a 2x6 or 2x8 plate and spraying in icynene (before electrical). But even then you'll have fire-blocking that will permit some thermal bridging.

Thank you guys for all the info. I've read extensively on the Building Science website re: temp profiles across a wall section vs. dewpoint etc. What I keep coming back to is the cold climate details they show. They have two 'typical' type sections. One will have exterior rigid foam that keeps the inside of the sheathing above the dewpoint. The second uses no foam on the exterior, and specifies that a vapor retarder may be necessary on the warm side. I take this to mean that if I have OSB sheathing and no rigid foam on the outside, then I'm not controlling the dewpoint location, and I need a vapor control mechanism. Hence, the vapor retarder on the warm side. I'm in northern Wisconsin, climate zone 7 (think, Duluth MN temps).

Please bear in mind, I'm not trying to argue, just to understand. This will be my first (and likely last) home build - so while I am a registered professional civil engineer and have a good theory background and technical understanding, the actual in the field application of home building science is new to me. However, if anyone needs a traffic signal designed, I'm your man!

Anyways - one foam contractor spraying icynene has recommended a vapor retarder paint be sprayed onto the interior of the foam (on the inside wall). I think that may be an 'ok' treatment - what do you think?

My latest wall cross section plan is this from outside to inside: 7/16" OSB / 8.5" dense pack cellulose/3.5" - 0.5 lb foam / Certainteed MemBrain SVR / 1/2" sheetrock - caulked at top and bottom plates (should I require all stud to rock locations be glued as well?) Of course will caulk all eletrical box/drywall junctions too.

I've read and re-read Dana1's posts, and I think what I'm proposing is a good solution? If foam on outside wall, you need enough to control dewpoint location - but if on inside of wall, not critical. I would definitely have all my rough electrical, etc in place prior to the .5lb foam being applied .

az - I hear you on SIPS, but the more I think about it, the less I like the structural integrity of my house depending on OSB and glue. Personal preference, please don't rain on my parade. Can you seriously match 2x6 pricing with SIP??? Not even CLOSE here!

Vapor retardent paint is a standard method of reducing the permeability of walls (including those insulated with half-pound foam.) The icynene should be an adequate air barrier, but if you don't caulk the sheet rock at all penetrations even modest amounts of air flow would defeat your vapor retardent paint- it's a necessary step. In Duluth (~9800 heating degree day climate) you DEFINITELY need an interior vapor retarder. But it sounds like you're there if you're meticulous about applying vapor-retardent paint.

In Manitoba & Ontario they'd make you use 6mil polyethylene between the gypsum and the interior studs, which is another good option. It may be more reliable (less prone to unintentional penetration) to put a polyethlene vapor retarder on the exterior side of the interior studs. With 8.5" of cellulose outside of the vapor retarder plane you won't have much potential for condensation on the interior studs, especially considering the foam can't transport much air to that plane in the first place. The foam would then be inward-drying, the cellullose outward-drying, but they'll be vapor-isolated from one another.

For an indication of relative performance of your wall to other wall systems scan down the list (or search the page) for "Larsen truss"

http://www.ornl.gov/sci/roofs+walls/articles/wallratings/index.html

Note that the Larsen truss tested by the ORNL is using 3.5" + 8" low-density fiberglass batting. With 3.5" of half-pound foam + 8.5" of dense pack celluloses yours will come in with somewhat higher clear wall & whole wall R values, as well as the thermal mass benefit of the 3lbs/cu-ft cellulose (not huge in region 7, but still some). You'll have roughly 2x the clear/whole wall R values, half the heat loss (U-values) of 6" SIPs, and the basis of a true high-performance building envelope. (All costs being equal, go for the Larsen Truss!)

With high-performance Larsen Truss walls and a well insulated roof the bulk of your heat loss will be from glazing. Think about limiting the glazed area to keep the performance high, but you need not make it a crypt. For high-performance daylighting (no view, but decent light), you might look into incorporating a few aerogel panels (Kalwall, Aspen Aerogel, et al), which are available with U values as low as ~0.05 range(!). They're pretty pricey though... Otherwise, true triple-pane insulated windows with as low a U-value as you can find, and keeping the percentage of glazed wall area low is key.

I have to wonder about the need for the open cell foam under the drywall. In terms of R value, it isn't much different from that of dense packed cellulose. As far as its use as an air barrier goes, your plan to use the airtight drywall approach (ADA) provides the sealing of the cavity against convective transport of inside air into the cavity in winter and of humid summer air the other way. With ADA, you also could eliminate the MemBrain SVR in favor of a 1-perm vapor-retarding primer on the drywall.

If you do the calcs on moisture diffusion, you find that you would need 8-9 months of sub-zero dewpoint winter air for diffusion through 1 perm drywall to cause saturation of the exterior sheathing and framing. Your winter doesn't last that long. You certainly don't want to put polyethylene (0.06 perm for 6 mil) under the drywall, as you've noted, as that would prevent inward drying.

If your cavity is full of porous insulation, you needn't worry about condensation within the cavity insulation itself. Without convective transport of humidity from inside, the moisture gradient will be fairly flat within the cavity. There will be sharp drops in concentration wherever you have a low-perm layer, such as across the drywall primer, any VR, any layer of closed-cell foam, and exterior sheathing. With the concentration within the cavity fairly even, due to its high porosity, the coldest layer, the sheathing (assuming no CC foam there) will be a dehumidifier for the whole cavity. Every other point across the porous insulation will be warmer. The miniscule transfer of moisture across the 1-perm primer will take a very long time to accumulate in the wood, even if none of it were passed by diffusion onward to the dry outside air. You have even more buffering in the cellulose, as you have noted.

Dick makes some good points- the relative value of the half-pound foam is much reduced if you use good air-barrier techniques on the finish walls.

Also, dense-packing the cellulose up against 3.5" of half-pound foam is going to apply considerable pressure to it which may cause the foam & finished wall to bow a bit. Whether you foam it or go straight-cellulose it may be worth applying an internal sheathing of OSB under the gypsum for structural strength. If you then seal/tape the seams and stagger the seams of the gypsum it makes a near-perfect air-barrier (if you caulk & foam-seal the electrical & plumbing penetrations of course.)

This guy uses cellulose-only in a ~7-8000 HDD Vermont climate, without the "Canadian style" vapor retarder 4" from the interior, 8" from the exterior he had previously been using, instead opting for air-tight drywall and vapor retardent paint. He also and has several construction tips/details to ponder before committing to a design:

http://www.builditsolar.com/Projects/SolarHomes/LarsenTruss/LarsenTruss.htm

Note, he also uses sheet copper as the capillary break between the top of the foundation and the framing lumber (a true vapor and water BARRIER), which also protects against termites:

http://www.builditsolar.com/Projects/SolarHomes/LarsenTruss/SFP%20House%20Detail.jpg

It's probably cheaper, simpler to build, and still plenty "good 'nuff" to go all-cellulose with good air/vapor sealing on the interior wall rather than monkeying around with different insulation types and sheet vapor retarders.

I have considered MemBrain for my own upcoming house project, but I have been having second thoughts about it. An air barrier, to prevent convective flow to/from the cavity, is only good to its edges. Drywall won't leak air in the field area, but of course will at its edges and at at openings for electrical boxes, etc. If a sheet material is applied as an air barrier, it must be sealed to the framing at its edges and at openings. Doesn't ADA do that sealing anyway? So if care is taken to use Tremco sealant where the wall framing plates meets the floor and ceiling and sealant is used where the drywall meets framing and covers the edges of the polypans used for mounting electrical boxes, what role would the MemBrain play?

Now, taking a different position, would it be easier to achieve and visually inspect an air barrier installation if a transparent sheet material of the right permeability is used? Pieces of MemBrain could be sandwiched between floor and ceiling and the top and bottom framingl plates, sealed with Tremco. After cellulose is blown (assumption is that some mesh material retains it, such as InsulMesh or InsulWeb, whatever it's called), and before drywall is installed, the field area of the wall framing could be covered with the MemBrain and the overlap with the earlier installed pieces tape-sealed. Then the drywall installer can do a normal install, without having to be concerned with the use of sealant.

I'm looking for suggestions here, regarding technique to use.

My mind is now spinning as im def not a building techie...

Can you guys tell me what is left out or needed in my double wall thinking..


Ill explain from the outside in...


Hardie Plank Siding
Zip System Wall sheathing - http://huberwood.com/main.aspx?pagename=zipsystemwalloverview
12" dense pack cellulose
6mil poly on exterior of interior wall
Sheetrock


Using 2x4's double wall construction, offsetting

A couple comments:
My first thought is that with the zip on the outside and the poly on the inside you may have 12" of hydroscopic material (cellulose) between two vapor barriers - this is a moisture trapping potential. I don't know the perm rating for the zip but it's OSB with another coating of some sort and taped joints - OSB has a lower perm than plywood and the coating must lower it more.

My second thought is why poly the exterior side of the interior wall - this seems like a construction detailing headache. Do you poly it first then stand it up? Do you build the interior wall first then poly it then build the exterior wall? Seems really difficult. Why not put the poly under the sheetrock (interior side of interior wall?

Birdman, I dont have the answers to your questions as im a real 'newbie' im actually trying to find the DEFINITIVE answer to my double wall construction in cold climate.

Id like someone to tell me the layers I should have. Of course ill have a hrv or erv, but curious what layers I need to make an effective, long lasting, superinsulated structure.

Actually I was thinking it would be better on the exterior side of the interior wall so as to not interfere with the electrical boxes and wires?

anyone else any thoughts on my layers and what I should change ?

actually just read this link about celluose insulation....

Curious your thoughts

http://www.regalind.com/techPDF/General%20Information/Is%20%20A%20Vapor%20Barrier%20Needed%20for%20a%20Cellulose%20Insulatied%20Home.pdf

Here's where general prescriptions are a BAD idea. Whether & where you need to use vapor retarders depends a LOT on climate zone & actual construction methods.  The weather in Spokane WA (where the 103 homes were studied) isn't REMOTELY as cold as Duluth MN, and significantly drier to boot.

That said, the wicking aspect of cellulose & very low air convection in dense packed cellulose can make up for a world of ills.  If you make the walls fat enough and wall surfaces air-tight enough you can probably build without vapor retarder just about anywhere.  Larsen truss homes get built in 7000HHD+ VT without vapor retarders and seem to do fine.  But give it 30 years- some issues don't come to light for awhile.  But if the antique (presumably not super-insulated) homes in Saskatchewan sited demonstrate just how forgiving cellulose can be. 

I'd personally opt to use air tight interior construction with vapor retardent paint and forget about it.  The practice of putting a vapor retarder on the exterior side of the interior studs in Larsen truss designs was motivated by strict Canadian building codes requiring a separate vapor retarder layer, not any actual analyis or study of issues with that layup.   Abandoning that approach probably IS the right thing to do.  With air-tight construction on the interior walls the amount of water that makes it out to the condensation plane (let alone the frost plane) of the thick cellulose wall is miniscule, and is redistributed by wicking.  Adding an interior-to-the-wall vapor retarder or air barrier seems wholly unnecessary anywhere that you have 5 months or more of above-freezing weather.  Spring/summer/fall provides plenty of drying-to-exterior time for any winter-accumulated humidity in the insulation.  (Hygric buffer==good, BIG hygric buffer==BETTER.)

Once again, I agree with Dana. I don't think a VR on the exterior of the interior suds is remotely practical - or necessary. As Dana said, airtight drywall construction will do a ton of good to keep airborne moisture out of the wall. A VR, even just under the drywall, will work in both directions ie it will block most moisture from going into the wall from the warm side but it will also inhibit drying of the cellulosic "sponge". I'm not ambivalent about this - I think a VR on the warmside of this wall is a BAD idea. I still think that you'd be well advised to do "flash and blow" meaning spraying the exterior wall with a layer of open cell foam (NOT closed cell). This will definitively seal the exterior against wind driven infiltration and, if the thickness is adequate, will keep the dew point somewhere in the thickness of the foam thus precluding condensation within the cellulose.

I think you're focusing too much on the issue of keeping moisture from getting INTO the wall and not quite enough on letting it get out, once it inevitably gets in (and it WILL get in). The open cell foam on one side of the cellulose and the airtight drywall (only) on the other side will allow drying in both directions. I would go so far as to recommend skipping the vapor barrier paint and using regular (low VOC of course) paint or even one of the clay finishes that "breathe". Also, plywood is more permeable than OSB so despite the additional cost I'd go with plywood sheathing.

As far as framing the wall I would approach it not as a true double stud wall but I would use 2x4's for the exterior side an rip 2x4's in half for the interior "wall". I'd construct an on-site jig to allow you to place a 2x4 and a 2x2 in the jig then connect them with 4 plywood gussets with a nail gun and glue. I would then use these assemblies just like studs to frame the exterior walls flat, stand them up and build the next deck on top of them. Once completely framed I'd get the roof tight, windows/doors in, housewrap the exeterior walls then foam the inside of the sheathing covering the 2x4's. Then maybe compartmentalize the cavity to allow packing the cellulose, drywall and fill the cavity from the top. I'd consult first with the cellulose installer to see how he wanted to specifically approach it so he's comfortable that he can get his density right.

Hope that helps. The really good news is you're asking all the right questions now!!!

I used to think double stud walls were dumb unless you needed a soundproof building. I once built one as a 'Jam-Hut' for a country band (it worked great). After attending some Passive House meetings and conferences lately, I'm beginning to think they're not so dumb after all. In fact, I think they're smart.

I recently attended a small Passive House meeting at Brandeis U in Waltham, Ma with Katrin Klingenberg (director of Passive House-US Institute) Mike LeBeau (an energy superhero from Duluth) MN and Marc Rosenbaum (an energy engineer from NH). Another one of the speakers was a guy who had a Passive House built on Martha's Vinyard (not super cold, but pretty extreme climate). He said it was the most comfortable house he's ever lived in "It feels like you're living outdoors because there's always fresh air and it's so incredibly comfortable" was one of his quotes.

Passive houses are extremely tight and energy efficient:

1. Airtight building shell ≤ 0.6 ACH @ 50 pascal pressure, measured by blower-door test.
   

2. Annual heat requirement ≤ 15 kWh/m2/year
   

3. Primary Energy ≤ 120 kWh/m2/year

With these kinds of specs, you don't need a big heating system. You can heat it with a dog and cool it with a beer.

We've got a case study of Katrin's home as well as a home that Mike Lebeau worked on (on Lake Superior) with cross section construction details at GreenBuildingAdvisor.com (shameless self-promotion, yes. But good information too. And the part about being swayed is absolutely true). The LeBeau house has double walls, Katrin's house was framed with I-Joists as studs and then packed with blow-in fiberglass insulation (fairly common in Germany, not so much here). Interestingly, the LeBeau house has in ICF foundation, but the R-value was too low, so thet added foam to the outside.

Your sense about vapor barriers is correct -- in very cold climates, such as yours, a vapor barrier can make sense. In fact Mike will go toe to toe with Joe L about it, but Joe will agree with him. For almost the entire rest of the country, it's more important to focus on air barriers.

I'd call Mike Lebeau and ask him about your plan to use cellulose and open cell foam. In fact, I'd also ask him about the whole plan. It sounds good, but why gamble with a web forum when you can get expert advice?

When you get it built, contact us and let's do a case study (take lot's of process photos).

Dan

Dan - I'm the original poster of this thread, but it has gotten sidetracked by newbiejohn - no biggie - but are you referencing my planned wall section or newbiejohn's wall section? I've seen the article on your site re: that house in Duluth - my bro-in-law lives in Duluth (or Dulute, if you live there :-), and he has sent me info about it as well. That house is what actually got me intruiged by the DSW concept. I would really, really appreciate it if you could pass Mike Lebau's contact info on to me!! My email is neldrick at charter dot net (spelled out to keep evil bots from spamming me). FWIW - I'd love to do a case study :-)

Funny, I was at that same Passivehouse gathering...

Mike LeBeau is from Conservation Technologies: http://www.conservtech.com/

Good folks, they know their climate well.

I guess im more surprised that there isnt or I havnt been able to find difinitive information on the web or otherwise that says in a nut shell..

You want to build a double wall constructed home in a cold climate here is what you do.....and show the layers...