Hello:

I found this article: http://www.jlconline.com/moisture-barriers/major-surgery-for-a-failing-fat-wall_o.aspx?utm_source=newsletter&utm_content=jump&utm_medium=email&utm_campaign=JLCNL_070515&day=2015-07-05&he=@{hashed_email_address}@

You may find it interesting, perhaps, even scary.

I opted not to use blown cellulose in my semi-super-insulated house because of the fear of 'settling' and because I could not find a contractor who could do a 'damp-spray' cellulose installation. I assumed that 'damp-spray' would eliminate the 'settling' problem - I could be wrong.

I sure am curious if there is a guaranteed way of using cellulose that eliminates 'settling' and moisture problems - any 'link' would be appreciated.

Rob.

Speaking of links, a cut and paste of that URL doens't seem to get you there... I had to try several truncations to get there.

Settling is primarily a moisture cycling & density issue, as determined by researchers at Aalborg Unversity in Denmark over the past couple of decades (primarily Rasmussen. One more complete paper not behind a pay-wall can be found here. ) Without interior vapor retarders in a VT climate the moisture peaks were undoubtedly high, and it may have settled at any installable density, but the original density is simply unknown.

The do-over using a smart vapor retarderlike Intello 3-4" from the interior and more vapor open gypsum sheathing with a vapor-open housewrap will bring the moisture cycling down by about 2 orders of magnitude, which will now be primarily a function of the wintertime outdoor temperature & moisture levels, and not interior side vapor drives.

The fatter the wall, the colder the sheathing, and the more important controlling vapor diffusion from the interior becomes.  The attitude among many high-R cellulose builders in VT has been to assume that the hygric buffering of the cellulose and 3-3.5lbs density was enough, since it always seemed to work in 2x4 retrofits.   But that's clearly not the case  when using a more vapor tight sheathing such as OSB, when you go high-R/cold-sheathing, even if using air-tight open cell foam rather than cellulose.  

I believe Bradley's solution will work.  The Intello will run about 1 perm in winter in a VT climate when located 1/3 of the way through the insulation layer (colder air at mid-insulation= higher RH= more vapor open vapor retarder) but not much higher, and the gypsum sheathing will be something like 15 perms (instead of ~1 perm for half-inch CDX), so as long as there is a rainscreen gap between the siding and WRB to limit capillary & direct wetting the peak moisture of the cellulose will be modest, and it would be able to dry in both directions fairly quickly in spring.

It has been so long since I posted on this forum that I have forgotten how to post a 'link' propery.

Sorry!  :)

I wonder what the original installed density of that cellulose was? Even dense-packed it might have settled quite a bit over 30 years without an interior vapor retarder due to very high moisture cycling, but the post-sagged density almost has to be as high as current practices dense-pack.

And it's the areas that still had insulation in the bays that had the severe rot, despite having the same hygric buffering capacity of current-practices dense-pack. So even if packed to where it wouldn't settle, it would likely still rot with ~1-perm OSB sheathing and no interior side vapor retarder.

It's worth scrolling through the 20-odd slideshow pictures to get a better look, but it looks like the average settling was about 1/3 or a maybe a bit more, so even if it was blown at 2lb density (more likely it was 2.5 or more, even if open blown from the top into an open double-wall cavity) it would be over 3lbs post-settling.  Note how the OSB is discolored where the insulation remained, but looks drier/firmer above:

I wonder how many times people will have to re-discover that vapor permeable inside and not-very-permeable outside sheathing is a bad combination in a cold climate. Also, I wouldn't be surprised if high interior humidity played a role.

AFAIK, glue (ie, wet sprayed) is completely effective at preventing cellulose from settling.

Even damp-sprayed "stabilized formula" will sag if you get it wet enough.

Damp sprayed cellulose in a 2x6 cavity with the requisite amount of exterior insulation (low perm or high) for the climate won't get that wet, since the cold edge of the cellulose doesn't stay cold enough long enough to get saturated, even without an interior vapor retarder. But in double studwalls & Larsen trusses with low-permeance sheathing it can.

Hi Dana1:

Would you mind explaining the proper "layers" required for a safe double studwall construction would be?

Here in Quebec, north of Montreal (zone 5), I was looking at doing it like this:

From outside to inside:

1. Wood siding
2. 3/4 inch vertical strapping
3. Tyvec house wrap
4. OSB sheathing
5. 2x4 studs
6. cellulose - up to 12 inches between vapour barrier & sheathing
   
7. 2x4 studs
8. 10 mil plastic vapour barrier
9. 3/4 inch strapping
10. 1/2 inch gyproc

Do I need to place insulation over the sheathing to prevent moisture problems?

Thanks,

Rob.

There has been moisture seen on the inside of double wall sheathing in late spring; as yet is not considered dangerous. Everyone is in agreement that OSB is the weakest part of the assembly as it does not handle moisture easily. We've gone back to Fir CDX which handles moisture better. It's a little pricier, though. Whether you use OSB or plywood, be sure to caulk to the studs the bottom sheet and end sheets, then tape all the joints. as this is your air barrier layer. For OSB you can buy ZIP sheathing and tape. CDX will require priming with some tapes, or use SIGA Wigluv.

If layer #8 is perfectly air tight and layer #2 is vented top & bottom it'll work just fine for wintertime moisture accumulation. If you have much of an air conditioning load you could end up with minor summertime condensation events on the polyethylene, but it's unlikely to be enough to cause a problem

A better stackup would be to swap out the OSB in layer 4 for fiberboard or gypsum sheathing to maximize the drying rate toward the exterior (OSB is only about 1-perm when dry, fiberboard is about 15perms, gypsum is over 20 perms, as is the Tyvek), and to swap out the poly in layer 2 for a "smart" vapor retarder such as Certainteed MemBrain, or Intello Plus for layer 2. (MemBrain is only ~2x as expensive as 6 mil poly, Intello is quite a bit more expensive, but more rugged.) Smart vapor retarders are under 1-perm when the proximate air is dry (as it would be in winter), but over 10 perms when at higher humidiy (if it ever got wet, or during the summertime cooling season). That will increase the drying capacity by more than an order of magnitude in both directions, while still controlling wintertime moisture accumulation. Polyethylene is cheap, but it pretty much blocks all drying toward the interior. With those two changes the assembly is MUCH more moisture resilient.

To do it with exterior foam would be pretty expensive.

BSC's Joe Lstiburek's favorite double studwall stackup uses air tight OSB on the exterior side of the interior studs as the vapor retarder, with fiberboard or gypsum on the exterior. This is more rugged than membrane type vapor retarder approach, but it's also quite a bit more expensive:

Pretty hard to get layer 8 completely tight since you'll have outlets & switches in that area.

Would you be able to use me EZ SIPS panels for your fat wall?

Posted By Bob I on 09 Jul 2015 12:05 PM
Pretty hard to get layer 8 completely tight since you'll have outlets & switches in that area.

Air sealing interior side 6-mil poly vapor barriers is standard practice (required by code) in Canada, with purpose made vapor hats over the electrical boxes (or air tight flanged electrical boxes ) taped to the poly.  That's not to say all contractors do a great job of it, but it's way better than typical practices in the US.  Air sealing to MemBrain or Intello would be done the same way. MemBrain is only 2mils thick and a bit strechy compared to 6mil poly, but it's not particularly difficult to work with. Intello is thicker/stiffer, but also not a problem. 


With a vapor permeable exterior sheathing and back vented siding minor air leakage at layer 8 would not present much of a problem.

"Permeable in both directions and more so on the cold side" bears repeating. Look at cellulose stacked 12+" deep in a vented attic for an example of this principle working well.

Also, avoid high indoor humidity. And since every building has air leaks and air leaks are typically much more important than diffusion, avoid pressurization. It's very easy to block a hvac return register and create a bad case. Or just not balance the system and get a milder case. Consider deliberate slight negative pressurization of the entire building during Winter (I do this).

I've been saying this on this site for years. Having installed I don't know how many lbs of celly over the years, I know it is almost impossible to meet the necessary settled density over the complete cavity area to ensure no settling when using larger cavity thicknesses and undefined length dimensions. In other words, give me a wall 5.5" deep by 8' high by 24" wide and I can guarantee the proper density. Do an offset stud wall and I can't for the life of me figure out how it's done.

Hello:

I have read somewhere that the only way to get this done correctly is to use the same 'fabric' that you use over the studs between the inner and outer studs as well to create individual 'cells' between each stud pair.

I don't know if this is any help but I am sure that it is more work. :)

Rob.

The fabric will allow the air that is conveying the insulation into the cavity to escape. It's still a problem with the actual weight of the material. The only way I would do a 12" cavity would be with fiberglass

The fabric increases the friction. One of the reasons fat walls settle is the weight (thickness) to wall friction ratio.
If you have 5.5" x 24" cavity with friction on all four sides, there is considerably more friction per cubic inch of cellulose then if you have 12" cavities.
If you put mesh in the middle, you return the friction ratio back close to the 5.5" cavity provided the mesh is properly tacked up.

I would think that mesh added to give it something to hold on to (friction) could be much coarser than mesh used to contain cellulose. Then it could still blow as a single cavity. I would also guess that stiffness of such a support is critical (metal mesh?).

Or just use waterproof glue and wet spray.