I am renovating a 60's ranch bank house in NC to meet energy star guidelines, and the interior walls have been stripped down to the studs.  Since I cannot practically add an external building envelope / insulation, I am trying to find the best solution.  Cost is less important than energy efficiency, air quality and moisture control.

I have read the building sciences research, particularly the Charlotte profile which is applicable to us.  Of concern is their note about drying mechanisms - that the wall insulation must be permeable to allow interior and exterior drying.  I am split as to whether closed-cell or open-cell is the more appropriate for air sealing and insulating the walls.  The closed-cell insulators tout the low permeability as a feature, that any moisture reaching the plywood from the outside would be stopped and would dry back to the outside.  The open-cell insulators tout the high permeability as a feature, that any substantial moisture problem would be detected so that it can be corrected.

It seems to me that closed-cell will be more energy efficient but may cause vapor control problems, and open-cell is less energy efficient but may have less vapor control problems.  And I interpret the Charlotte Profile that closed-cell is simply not appropriate for us, though that has been my preferred choice up until now.

Open or closed?  I would appreciate any insight.

Also, I have substantial thermal bridging due to massive headers and support beams surrounding enormous windows (some are adjacent 6x6) and glass doors (12x8).  Some of these beams are 4x6's and 2x8's exposed directly to the exterior (I can see light coming through the gap between the beams - this will be caulked).  All recommendations for addressing thermal bridging appear to be external insulation, but is there a reason why I cannot glue .75/1" EPS styrofoam to the studs on the inside, then glue the gypsum on the styrofoam?

Thank you.

You could put rigid foam on the inside of the studs, but it's harder to get continuous coverage through floors and ceilings. If you put the foam on the outside, you can span the rim joists. But you've got brick on the outside, so exterior foam is out of the question.

If you put foam inside the studs, it's probably silly to put open cell foam between the studs because XPS is less permeable than open cell. Use closed cell foam between the studs and skin a layere of rigid foam inside if you're really worried about thermal bridging. But in North Carolina, winter doesn't last that long -- even in the mountains, so my guess is that thermal bridging is less of a worry than say New England or Montana. Maybe a good compromise would be a strip of fanfold (3/8 in.) on the face of the studs (the first R is the biggest).

Hope that helps,

Dan
TheGreenBuildingAdvisors.blogspot.com

I am anspray foam installer in Norther California.  We deal with areas that have very high humidity such as the coastal regions and high elevations.  We install both open and closed cell spray foam.  Our preference is to use open cell foam in these areas.  Closed is a true vapor barrier and will work the same as putting 10 mil plastic on the walls.  Open cell is an intermediate vapor barrier, 1 to 10 perm, and will allow the moisture to dry from either side.  It will work like the new smart vapor barriers.

I hope this helps.

We are a New England Foam-In-Place insulation company that uses an injection foam installed from the outside like cellulouse. It has an R value of 5.1 per inch and a perm rating of about 16 like open cell foam. We get the best of both worlds, as we need it for our winters.
Good luck

Thank you for the feedback. Exterior insulation is not an option, so I think I will go with open cell which will allow any trapped moisture to dry out in both directions. I plan to use 0.5" EPS sheathing cut to cover only the face of the studs and have the open cell foam cover the sides of the studs. This should reduce thermal bridging while still retaining permeability between the studs for drying.