Hello everyone.  I inspect homes and produce residential construction related educational content in Alaska’s climate zone 8.  Occasionally I come across situations that I’d be very appreciative of input.  This is such a case.   I sometimes encounter older 2 story homes typically built between 1920 and 1960 with stick framed hip roofs that use attic knee walls to support the rafters at mid span.  On the warm side of the knee wall is usually a second story bedroom or a bathroom with a sloped/hipped ceiling.  These places are notorious for ice dams and air leakage related issues as they predate the insulating and air sealing building practices we use nowadays.  Often, access to the knee walls is through undersized gable end vents or a narrow panel in the knee wall if the wall is tall enough.  In close quarters like this where installing foam board on the back of the knee wall or conditioning the attic by insulating the roof is very difficult or too expensive, the prevailing practice has been to remove the exposed (and typically ancient) fiberglass batt insulation out of the knee wall from the attic side and then encase the entire knee wall in high density spray foam.  Usually the sprayers apply between 2 and 3 inches of foam directly to the drywall (or horizontal planking) and then wrap it around the individual studs at the same thickness.   The top of the kneewall area still stays open to about 1-1/2″ below the roof sheathing to maintain the roof ventilation over the living spaces. 

 Here’s my concern…..while those kneewall studs and plates are encased on 3 sides in foam, the edges of the wall on the warm side are still exposed to indoor ambient temperature and humidity conditions.    Older truss roofs and joisted attics are treated in a similar fashion.  After any wiring is upgraded, then spray foam is applied downard to fully encapsulate the joist or truss bottom chord in 2″ or 3″ of foam.  No original existing vapor retarder over the warm side edges in these cases either, but once the ceiling has been covered in foam from the attic side, an additional 10 to 16 inches of blown in insulation is added over the top which has the effect of keeping the framing that is encased in foam warmer and less likely to attract condensation from the exposed warm side edges.  Unlike sprayed attic floors,  with cramped vertical knee walls, adding a thick layer of blown in insulation to the back of the wall to keep the spray foamed portion warmer obviously isn’t an option….so is it possible that under sustained high humidity conditions (40%), the studs could absorb enough humidity over time through the exposed warm side edges to lead to dry rot?  With the framing encased in foam in a very cold climate, it’s the perfect scenario in every other respect which is why it makes me nervous.  In zone 8 we can still get sustained (2 + weeks) of -30F to -40F in the winters – and the winters are long.   I’ve looked through several of Lstiburek’s publications that relate to the subject but I’ve never seen it discussed explicitly in any of my searches through bsc, building america solution center, the various national labs, CMHC etc.   I’d appreciate other peoples thoughts on the risks (if any) that exist with encasing the attic knee wall studs in foam in cold climates.  

Thank you, 

Ilya Benesch -Fairbanks

https://buildingscience.com/documents/building-science-insights-newsletters/bsi-026-they-all-laughed

https://buildingscience.com/documents/insights/bsi-049-confusion-about-diffusion