This is probably a stupid question, but is there a reason not to insulate on both sides of the air barrier layer in a wall system?

For example, if I have (from interior to exterior) drywall, 2x4, OSB/air barrier, empty space, then another 2x4, sheathing (likely more OSB) and then rain screen and siding...

Originally I thought the utility chase would go in the interior 2x4 wall, but then I thought, why not put the OSB on the outside of that wall, and insulate the utility chase too, yielding an extra 3.5" of space for cellulose.

Would this lead to some problems I'm not considering?

First off - with some planning most "utilities" can go into interior partitions. Wiring will need to go in outside walls but that isn't a big thermal deal. Not sure what the "empty space" is in your stackup. It sounds like double wall construction with a space between the inner and outer wall - if that's the case I'm not sure why you need to sheath both walls with OSB (or plywood). Let the interior wall carry the drywall and the exterior wall carry the sheathing. The trick is going to be moisture control. If you have 7" or more of cellulose the inner surface of the exterior sheathing is going to be "shielded" from the heat in the interior and therefor will get quite cold. Humid air from the interior will carry moisture into the wall (and there will be some diffusion through the drywall too) and that moisture will condense on the inside face of the sheathing. Not a good thing. The best cure for that is to install a layer of foam (XPS or EPS but Dana will rightfully call you on the ungreeness of XPS) on the exterior side of the sheathing (which is also your air barrier). This takes some careful balancing of the ratio of interior to exterior R value (more is not necessarily better here). I just finished the envelope of a house (technically in zone 5 but essentially in a micro climate like zone 4) and I built a 2x4 exterior wall which taped Zip sheathing. On the interior we timberloked 2x4's horizontally to the studs to create a 7" cavity (cellulose) and then placed 2" of XPS (sorry Dana...) on the outside of the sheathing with commercial Tyvek and rain screen siding over that. This gets about an R 37 wall with very little thermal bridging (except at openings). The interior is blue board and plaster with no vapor barrier. So in essence I have insulation on both sides of the air barrier but also have no vapor barrier. I also paid particular attention to sealing the blue board to prevent as much air from migrating into the cavity as possible. I think of the blue board as the "secondary" air barrier and the zip board as the primary.

Thanks for your thoughts Birdman.


I'm in Zone 4 as well, in wet Portland Oregon. The temps don't get too cold, so I'm not quite as worried about cold moisture in the outer side of my dense packed cellulose wall, but maybe I should be.



Seems like there have been quite a few of these built lately- dense pack cellulose on double stud walls with a very air-tight envelope...



This is for a small 800sf 2 story house, there are no interior walls on the first floor. I'm currently planning on using the inner wall as the load bearing of the spaced stud wall, with a larsen truss type thing on the outside. This is so it lines up better with the ICF stem wall, and also avoids the thermal bridging at the second floor rim joist. I thought it would be easier for air seal if it wasn't at the drywall layer with all the light switches and outlets and whatnot ruining all that fancy air seal work.