Controlling Vapor Drive Inside Soon-to-be-Renovated Walls
We made a mistake years ago when repairing rot and structural damage to our house and I need some help in finding the best solution and understanding some of the behavior of the building materials in question. I’ll do my best to get to the point without rambling as I have much respect for the experts who put their time in to help people here.
House: Built 1988, bought 2006, located near Albany airport (good weather data from station). Huge rot and structural damage found due to improper flashing and nothing installed between cedar siding and OSB. Repaired about 2010.
Repair: Removed all siding and replaced almost every door and window. Installed exterior rigid foam, fiber cement siding, pvc trim, windows pushed out, flex wrap door and window pans, typar flashing, custom metal flashing, calked flanges and trim, etc.
Repaired wall assembly (outside -to- inside)
Fiber cement lap siding
1″ Foil faced polyiso, taped seams
Tyvek house wrap
7/16 plywood where replaced (OSB where not)
5.5″ Mineral wool bats where opened (fiberglass where not)
6 mil poly sheet
Now that we are finally renovating the interior and I am educating myself about building science, I understand that what we did was wrong for our climate. The outside walls are sealed up tight as a drum, but can’t breath out. 6 mil poly keeps them from drying in. Only 1″ of exterior foam means we are below the dew point at the sheathing during the winter.
I would prefer not to remove the siding, trim, etc etc and break all the sealing we did just to add another layer of foam. I would also prefer not to spend a huge amount of money on a renovation only to find my walls rotted 10-20 years later. If I’m spending money, it needs to be on fixing any performance issues first.
During renovation many exterior walls will already be opened and so I am OK with opening ALL the exterior walls, from the inside. I have come up with some ideas, but I have yet to speak with anyone who can give me any kind of confident answer that we will be successfully addressing the problem.
To start, I will probably be using SIGA’s Majrex vapor control layer (which is directional) behind the drywall. It ranges from .16-1 perm inward, but up to >4 perms outward. In other words, it lets vapor in 10x faster then 6mil poly, but it lets it out 250x faster than 6mil poly. Vapor still goes where it wants.
I discussed flash and bat with someone (1″ open cell for breathability, but keeps air from the sheathing) but I was advised that it is a huge PITA in an occupied home because of the application mess.
The better option seems to be something like 1″ of EPS in each stud bay against the sheathing, cut about 1/4-1/2″ short all around. Then the gap filled with great stuff. This should keep the inside face of the foam above the dew point (almost all the time) but allow the OSB to dry inward if it needs to.
While I understand most aspect of vapor drive, I don’t understand the mechanics of moisture saturating something like EPS. If the temp at the interior of the sheathing drops below dew point, then vapor pressure = saturation pressure and moisture will flow from the wall cavity through the EPS.
If it encounters the condensation point inside the EPS does it condense and block the pores? If so, when its hot and AC is running and direction is reversed, if dew point is again in the EPS, does it stop the OSB from drying inward?
Does it keep pushing through and saturate the OSB?
Does it wick into the OSB?
How much water can EPS absorb?
If the Perms of the OSB are higher than the VC layer (inward) does it really do anything to help?
Is it more of a belt and suspenders in case of air infiltration in the cavity?
I should also say we have a ground source heat pump and humidity in the winter is 15-25%. Summer is low too (50% maybe) because of the level of dehumidification with this type of HP. Spring and Fall make me nervous. We will be installing an HRV as we tighten up the house.
Another idea is to install an active dehumidifier to force our indoor RH to track with outside temps to make sure sheathing inside is never below dew point (or in mold conditions).
Anyone ever apply mold killing paint in stud bays as a preventative measure?
Thanks in advance for your help. Sorry for the long post.
Also: Am I just crazy?
P.S. I opened a bathroom wall this mid-winter to put thermocouples inside to track the sheathing and insulation temp. The wall has been untaped, unfinished drywall since the repairs in 2010. It did have the PE sheet all that time. No mold, no water, no wetness yet. I didn’t measure the wood with a moisture meter but I will soon.
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