Moisture between Insulation and Baffle
In the process of remodeling our master bathroom. I gutted the room down to the studs, walls and ceiling. The room is 8’x20′ with a cathedral ceiling, 14′ at the peak. The original roof was framed with 2″x8″ and only had R19 insulation. I added a 2×6 to the bottom of the 2×8 so now I have 13″ bay for insulation. I installed plastic baffles on the bottom side of the roof sheathing. I then cut and installed 1.5″ Dow board vertically under the baffle on top of the wall plate to stop outside air from the soffit area from getting into the insulation bay. The baffle provides a 2″ air gap all the way to the peak ridge vent. Faced R38 insulation was then installed. After putting up 3mil plastic I used moisture resistant drywall. Today I noticed a few water spots on the drywall where the ceiling meets the wall. I pulled the drywall down in the closest area and saw that the kraft paper had some water spots. After pulling the plastic and removing the insulation I found condensation on the underside of the baffle. In some spots, I could see frost on the baffle. I live just south of Buffalo NY, current outside temp is in the teens. Inside temp is 67. I do have full vent soffits and a full ridge vent. We are not yet using this bathroom, so I don’t have excess interior moisture from showering and what not. I’m at a loss here, any suggestions.
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After a little more research. I think I'm going to pull the drywall back down (thankfully, not finished yet). I'll put up some XPS foam board and tape the seams, then firring strip and rehang the drywall. See attached detail. Question - should I pull the 3mil plastic down before putting the XPS up? I'm getting the feeling that air infiltration is my issue. Need to ensure that I get a good seal so that warm air cannot pass through to the cold surface. I'm I correct in this thinking?
Lawman89,
My guess is you are right and too much moist interior air is making its way to the underside of the baffles for the framing to adsorb and move to the vent channel. Impermeable baffles only work with very good interior air-sealing.
If that's the case you can either try and do a better job of air-sealing, or you can switch to a permeable materiel ( like house-wrap) for the baffles, which would be more forgiving.
The issue is the 1.5" rigid. Buffalo is zone 5 so for condensation control there, the rigid should be at least 40% of the cavity R value, so with R38 batts at means R25. With too little rigid, the surface will not be warm enough resulting in what you are seeing.
The option are:
-Remove the rigid baffle and replace it with something permeable (card board, fiberboard, thin OSB, house wrap, regular thin foam baffles that are not sealed also work)
-add more rigid to bring the layer up to R25.
-swap out the R38 batts for ~R10 batts.
Your proposal of rigid under the drywall won't change the condensation issue, you need to remove the thin exterior rigid layer. You can re-use that 1.5" rigid under the drywall after you take it out as you propose, any thickness of rigid under the drywall works in cold climate.
P.S. You don't need to take out all the rigid foam baffle, about 1/2 to 1/3 near the peak will do as the cavity can now dry towards this space.
Thanks for the comments, I am a little confused though. I'm using the 1.5" rigid only to close the end of the joist bay. I cut them to be 14.5" wide by 5.5" tall. I placed them on top of the wall plate standing straight up between the plate and the bottom side of the plastic baffle and them sealed around them with spray foam as per the attached detail. The plastic baffles I used have a 8"x2" channel for air flow all the way to the peak. You saying I should remove that?
Here are some pictures so you can see what I see.
Ok that makes sense. From the initial descption it sounded like you have the 1.5" rigid running from soffit to ridge. That strip at the soffit is not a problem.
The issue is still the same though. The baffles you have a vapor barriers and since they are wider than the rafter bay those flanges seal it up pretty well. The baffles are at outside temperature so any moisture that gets under them simply condenses on the surface as you see. These baffles are meant for soffit area in a vented attic with loose fill, the don't work as you have it installed.
There needs to be a path for moisture from the inside to the vent channel. The simples it to cut a bunch of 1" slots every 1 foot or so. You can also remove the ones after the soffit or you can also replace the baffles with something permeable like I mentioned earlier.
Wind washing is an overblown worry, you need proper moisture path and venting first.
All options are a PIA, the good thing is you caught it before the space was finished.
I can totally understand that. So, my plan moving forward is to remove the plastic baffles and install something more permeable (foam baffles or maybe house wrap ensuring there is at min 1" gap for air flow under the sheathing). I can reuse the R38. Then I'll put a layer of 1" rigid, tape the seams and spray foam edges to create an airtight ceiling. After applying firring strips I'll re-drywall. No need for a plastic vapor barrier between the fiberglass insulation and rigid, correct?
That sounds like a good plan. Make sure not to seal up the new baffles, about the only thing you want is to shingle them so any moisture can drain into the soffit.
With faced insulation, you don't need any further vapor barriers or retarders but the layer of foam is usually low enough permeability to also classify as a vapor barrier.
The big bonus of the rigid with taped seams is that it is a solid air barrier which is the best way to prevent condensation and snow melt issues.
If you were using conventional plastic baffles alone you would also have condensation. In theory, little enough moisture should accumulate that it can safely diffuse through the rafters into the vent space, but clearly you are experiencing higher moisture levels than that. Do you have penetrations in the ceiling?
No, there are no ceiling penetrations at all.