Closed cell spray polyurethane on precast

[Michael Hurd]

I recently read an article by Joseph Lstiburek (Risky Business: High risk walls) That indicated that placing closed cell polyurethene insulation on the back of architectural precast walls in cold climates is a "no brainer" application that requires no WUFI calc's to verify acceptability of the wall performance - however, when analyzing a such a wall in zone 7 (North Dakota) I came across very high RH in the insulation at the interface of the insulation to the precast. The Dew point graphs also indicated that the insulation was within a few degrees of dew point. The RH of the insulation at the interface was running above 80% for months and the conditions of ASHRAE 160 were not met. The space I am analyzing is running high RH of around 40% - by dropping this down to 25% the above conditions drop below the 80% figure and the requirements of ASHRAE 160 are met. My question is whether or not the spray foam can handle these high RH figures for extended time periods while running close to dew point - I am concerned that the product could eventually have adhesion issues and possibly mold growth - are my concerns valid? I should say that lowering the RH is not an option due to the function of the space. Also, additional insulation thickness or a strong vapor retarder on the face of the insulation does not help. One other key point, the insulation does not show a gain in moisture over time, it cycles up and down, leading me to believe the assembly is fine.

[Michael Hurd]

Could someone please review my post from Feb 21, 2013 and comment? Really interested in getting some feedback ASAP! Thanks!

[Manfred Kehrer]

Hello Michael,

I think your concerns are valid and do not think this is a "no brainer". I do not need to run a WUFI calc. to know what results would be at the spots you mentioned. We know know that the ASHRAE 160 criteria are very conservative, however, the accumulation of moisture in the masonry may cause frost damages in a cold climate.

Mold growth is likely not a problem, as mold needs beside moisture oxygen or nitrogen to grow, which may not can get delivered in a sufficient degree in this case. Also If there is likely no way in this cases for the spores to come into the living space.

Regarding the adhesion problematic I think it is hard to get any guarantee today.

[Michael Hurd]

Manfred, as always, thank you very much for your response! I value the input from you guys! The issue you mentioned about frost damage is most likely not an issue here - this is architectural precast concrete, not masonry so I don't see an issue there. My biggest concern was mold growth (elevated importance due to the usage of the bldg) and the adhesion of the spray polyurethane over time. I would add that I modeled the architectural precast with the "drying Zone" suggested in the paper by Kunzel and Holm - which is available on the WUFI website.