Adding AC to House
I’d like to add air conditioning to our house (currently has forced air furnace with flexduct throughout). The walls are painted GWB/untaped poly vapor barrier/2×4 studs with R13 unfaced fiberglass batt/ply/tyvek/LP lap siding. My question is whether or not adding air conditioning will create any condensation (and potentially mold) to the outside face of the poly (therefore inside the wall) under the GWB. My limited knowledge of building science leads me to believe that maybe because the poly isn’t tape and the wall can dry to the outside things would be okay, but I wanted to get the opinion of the GBA community that knows much more than I do about it. The house is in Oregon (zone 4C) where the summers are getting hotter, but there’s not much humidity.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Given your dry climate I don’t see a problem.
Walta
Agree with Walta, but want to add one comment. The idea that the poly not being taped is what saves this is likely not the case. (Low humidity is.) If you did the same exact thing in a very humid environment, it'd be possible to get lots of condensation on the back of untaped poly.
Think of it this way... the lack of tape isn't going to allow all that vapor to magically sneak through the untaped laps and through the drywall and paint. The vapor is going to hit a big brick wall at the below-dew point poly and condense.
Thanks to you both for your responses.
I may be thinking about this wrong, but could you figure out a way to track the outdoor dewpoint temperature and ensure your set point is always higher? Assuming this is sufficient to know for sure condensation wont occur (I'm not sure if there is any reason the moisture content of air could increase was it moved through the wall) it could give you piece of mind (A quick look at a Portland shows a max DP for August last year of 22.2 C)