Dealing with condensation between insulation and sheathing during winter construciton
Moisture buildup between insulation and sheathing is a problem for winter construction projects:
https://www.greenbuildingadvisor.com/article/condensation-wall-sheathing-construction
https://basc.pnnl.gov/resource-guides/condensation-control-walls-cold-weather
https://buildingscience.com/documents/digests/bsd-controlling-cold-weather-condensation-using-insulation
The obvious answers are provided in those articles: heat the interior, dry the interior, insulate the exterior BEFORE the interior.
However, what if the mistake has already been made? My plan was to get the shell weathered in before winter, do the interior through winter then finish the exterior cladding – including the exterior insulation – in the spring. With as cold as it is, if I add more heat and de-humidification I’m still likely to end up with condensation. I’d rather not be up on scaffolding in the ice and snow.
So my question to the GreenBuilding hive mind is this: depending on assembly, would painting the stud cavities with a low perm mold blocking paint cause more problems than it solves?
My plan is to peel down the insulation, remediate and dry as needed, paint the sheathing, reinstall the batts and continue interior work. The condensation will still form but it wont be affecting the OSB. Moisture will be much easier to claw back come spring when I was planning on having the drywall crew take over.
My assembly (zone 5):
2in Rainbarrier CI (R9-ish, 25-50 perm)
Mento 1000 (38 perm)
OSB, taped seams (2perm at 50%RH-12perm at 85%)
taped OSB (2-12 perm)
Sheetrock (2-3 perm painted)
(the interior OSB is an odd detail but I saw it in the field one time and liked it – hang anything anywhere and deadens exterior noise)
Once the assembly is finished and functioning correctly 1) sheathing will be able to dry to outside as needed 2)the sheathing will be warmer in the winter 3) the moisture won’t be there to condense 4) cavity insulation should be enough to prevent condensation from forming on the paint ‘internally’ between paint and OSB 5) I could add something like Intello to the mix as added insurance
Thoughts?
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Replies
Only heat the place when working. Don't heat with propane (plug in heaters, one for a stove plug and one for a dryer, works great). Crack a window when unoccupied, if interior dewpoint is the same as outdoors, you don't get condensation.
Once insulation is in, install warm side air barrier (membrane or drywall) then run heat and dehumidifier. Any moisture in the walls will dry in the spring.
Too late for your build, getting the exterior rigid up same time as the walls is great. That bit of insulation really helps to take the edge off when building in the cold.
I like to build the wall including WRB, taping, exterior rigid and WRB before standing it up. Not standard build practice but you get a weather tight wall and no need for any scaffold work except to join up sections and install siding.