Wednesday, October 1, 2014

A Simpler, Foam Free "Perfect Wall"?

I’m still researching wall and roof/ceiling assemblies because I haven’t been totally happy with anything I’ve tried yet.

in some areas of the country, especially in the Northeast, insulation contractors have been dense-packing unvented rafter bays with cellulose for years.”

Then I discovered a project by David Posluszny in Shirley, MA, that used a simple, very easy to build double wall section.  From inside to outside:   Drywall, netting, 12” of cellulose, plywood sheathing, (Henry Blueskin), furring, wood siding.

Since intra-wall condensation was a concern, he did some moisture testing after an extreme cold spell: “With the help of Bill and Jim from National Fiber, we tested my wall assembly. We took moisture readings in North, South, West and East walls, checked the moisture gradients, lofts, and crawlspace. We are pleased to find all the moisture levels came back well within comfort zone. This wall assembly works.”

David actually used Grace Ice and Water Shield as the air and weather barrier, but as he mentions in the video, Blueskin would accomplish the same tasks, is cheaper, and is vapor open.

This is an extremely tight house.  It’s possible that this wall section could develop moisture problems in leakier house.  However, David and the insulation suppliers emphasize that dense packed cellulose mitigates moisture problems in two ways:  It retards air movement and redistributes moisture because it is hygroscopic. So this could be a very robust assembly that can tolerate sloppier air sealing than what David has done.   (0.1 ACH 50 might be some sort of record)?  

This assembly also solves the tricky problem of how to air seal at the eaves where trusses meet walls.   The trusses don’t include the eaves, which are built after air sealing.  By laying 2x4s flat on top of the first layer of roof sheathing and covering  them with another layer of plywood, he ensures that shingle nails will never penetrate the air barrier and the cold roof has generous venting:

Some more features of this wall:

1.  Since all the air sealing is done at the exterior weather barrier, no air sealing is required for the electrical work.
2.  The air and weather sealing is done in one layer, which simplifies everything and reduces cost.
3.  The self adhesive air barrier has no penetrations due to staples.

The Problem

The theories behind vapor diffusion in walls would show that this is a risky assembly.  In practice, however, the wall seems to work.  Apparently cellulose can pull moisture from the inside of the exterior sheathing and release it back into the house as fast as the condensation occurs.

The next step would be to prove this in the lab and in the field.  This would be worthwhile because I don’t think there is an easier or cheaper way to build an R42 wall.


  1. I'd still want to be careful. This puts your air barrier on the outside of the assembly, yes? So even if it worked for this guy, another installation contractor might not be as good, and might leave air gaps in the wall, and you get warm indoor air hitting a cold air barrier (in winter) and condensing. It's true that the hygroscopic nature of cellulose helps here, but it's still going to be tempting fate a bit.

    Now, I wonder what would happen if you moved your sheathing/air-barrier to be just outside the inner-wall studs, and put insulation on both sides, possibly cellulose throughout. Then your air barrier would be much warmer in winter, and you'd have a much less risky assembly.

  2. Your caution is well-placed.

    I plan to use Blueskin VP100, which will allow drying to the outside.

    I wouldn't move the air barrier because the whole point of this wall is to combine the air barrier and the weather resistant barrier, and make it easy to inspect just before cladding.