Moisture in exterior wall
I have posted questions several times regarding my double wall assembly on a house that I am in the process of building in a very cold climate. At the present time, the drywall is being finished and my air exchange system is not yet running as I don’t want the system on when there is a lot of dust. I am running a dehumidifier which is taking out a lot of moisture from the air but certainly not enough. I am heating with electric heaters, keeping the house around 64-70F. My humidity is ~50-60% as a result of the new construction and the mudding that is happening on the drywall. People have ensured me that this is typical at this phase of the project. It has been getting cold outside (20F). My wall assembly is as follows (outside to inside): clapboard siding, Tyvek house wrap, Advantech sheathing, 24″ o.c. 2×6 wall with Roxul, 2″ non-foil faced polyiso, poly air barrier, 24″ o.c. 2×4 wall with Roxul, drywall. The consensus prior to my build was that my air barrier was in a safe part of my wall assembly regarding moisture. Currently, one section of my wall does not have drywall hung as I will be putting up T&G wood shortly. Yesterday, when peeking behind the 2×4 Roxul for curiosity-sake, I noticed a miniscule amount of dampness on the inside part of the poly when I rubbed my hand on it. I understand that a new building evaporates significant amounts of moisture, as does the mudding of drywall and curing on the concrete slab, setting me up for a lot of moisture. My current air exchange situation is poor, as mentioned above. I hope to keep the humidity in the mid-high 30% once I am living in it. My thought is that the humid air is condensing on the poly and will improve as the indoor humidity lowers. And if there would be moisture, it should be allowed to dry into the house? Could I have some opinions on this situation? Thanks in advance.
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Replies
Poly is at dew temp.
So if I were to decrease the interior relative humidity while maintaining the same indoor temperature, the dew point will decrease and as a result, that surface will be dry?
The conditions at the poly need to change. That might work. You could have a cold spot/ air leak...
Many choose wood based air barriers as they don't wet so much as has been explained on this site... Bill Rose and such.
I had a similar problem four years ago when the plaster work was being done inside my double-wall house. The work was being done in winter, and the plasterer didn't want the house vented during the day while they worked, as it was affecting the workability of the skim coat. Meanwhile, I was alarmed at the humidity condensing at the edges of the triple pane windows and all over the double pane glass in doors. We reached a compromise. I'd come over to wipe down the glass several times a day, and after the plasterer was done for the day (mid-afternoon) I'd prop open a door on the front of the lower level 6-8 inches and another in back upstairs. After a few hours of natural convection flushing with low-dewpoint outside air, the humidity inside was markedly reduced. Meanwhile, I kept the woodstove cranked up to offset the drop in temperature. I was certainly glad when that infusion of moisture was over. For a few weeks afterward, I had a couple of dehumidifiers running to help as moisture kept coming off the walls.
New construction can be really wet
You're headed in the right direction.
What size dehumidifier do you have? A typical household unit is not big enough--you need to rent the type used by dry-out companies. They are large, heavy, fairly loud, and fill 5-gallon buckets quickly. Taping drywall is like D-Day from a moisture standpoint. It is hard to prevent any condensation at this stage, especially at 20F outside. It will be fine.
I have your run of the mill dehudifier. And it's on first floor where mudding is taking place. No dehudifier on second but adding one today. Will look for a commercial sized one. One more thing-wondering why moisture around windows and frames typically collect on the bottoms? Is it because its heavier and collects at the lowest point?
Thanks for the help.
Matthew,
You've gotten good advice here. This is construction moisture. The moisture on your polyethylene it will eventually dry to the interior of your house when your indoor RH drops.
It's usually cheaper during the winter to crack a few windows (and to turn up the heat to compensate) rather than run two dehumidifiers. Outdoor air is dry during the winter -- so ventilation will lower your indoor RH.