Vapor Drive Help
CZ 3A/4A (North GA area)
2001 gut reno
Vapor drive is a subject I still don’t have a full grasp on, so I’m looking for help in determining if this wall assembly in our attic is a possible moisture issue.
Our HVAC closet in the attic (attached to a finished bonus room above living space) will be air conditioned. Our HVAC guy decided to use plywood as the interior walls in that closet.
The rest of the wall assembly will likely be fiberglass batts on the attic side and Tyvek commercial wrap (leftover from the exterior) carefully detailed to be our attic side air barrier. The rest of the bonus room with be drywalled on the interior side.
Being in a mixed/hot humid climate would mean the vapor drive would be predominantly inward, correct?
If so, would that cause an issue with the interior finish of that closet being plywood instead of drywall? The closet size is about 40-50 sq ft.
I know plywood is less vapor permeable than drywall, and we don’t want to cause any potential condensation and mold risks. We came from a moldy home before and it caused severe illness in my family, so we’re doing everything we can to try to avoid those issues.
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Replies
Usually walls aren't as much of a concern as roofs. Can you talk about how your roof is insulated? "Attic" can mean a lot of different things.
The roof isn't insulated yet. We plan to make sure they follow this diagram with the blocking and baffles. As far as I know, the roof line will also have fiberglass batts, but I don't know what R-value.
The attic is vented, and there are soffit and ridge vents.
OK, the roofline is the area of concern. There has to be continuous ventilation between the insulation and the roof from the soffit to the ridge. It looks like that's what's specified, you have to make sure it gets built. Without ventilation moisture will accumulate in there and cause all kinds of problems.
Are there specific types of baffles that work best in order to create that ventilation chute along the roofline?
And as far as the walls, there is no concern for moisture/condensation with our current setup (Tyvek, FG batts, plywood)?
Any other suggestions/considerations here that we may be missing?
You may want to consider just insulating the underside of the whole roof rather than the wall and the floor. It's less area to insulate and no transitions.
I like to use 1/4" wafer board for the baffles, secured to 1x2 furring strips used as spacers to define the depth of the vent channel. This is cheap and effective, and is quick and easy to build. A finish nailer makes fast work of things here. You can also fet factory made baffles which are usually made from a semirigid plastic material or thin formed rigid foam.
I like the comment in #6 too -- why not just insulate the roof line all the way to the soffit and not worry about the wall and attic floor? Usually it's easier to get the air sealing details right when just sealing/insulating the roof.
Bill
This is probably a silly question (bear with me, I'm a newbie) - Are you referring to insulating the roof line only where the knee walls are like in this diagram? Or the entire roof, even on non-knee walls?
~3/4 of the bonus room walls are true knee walls. There is one large wall that is at our one gable end at the front of the home, so the roof line doesn't meet the top of the wall.
Also, we have a hip roof with the one gable end at the front of the home. From what I understand, if you're referring to insulating the entire roof, having a hip roof makes doing this while maintaining ventilation next to impossible. Is that correct?
BIll,
To answer your question - "Why not just insulate the roof line all the way to the soffit and not worry about the wall and attic floor? Usually it's easier to get the air sealing details right when just sealing/insulating the roof."
We have a hip roof, which from other comments and articles I've seen, makes it near impossible to do. Is that correct? Or is that information outdated? If it would be simpler to do it that way, that would be great if we can get it to work with our situation!
With a hip roof, the "outlet" vent can be an issue, since you don't have a consistent ridge line. You show a vented roof assembly in your drawing though, so you need that "outlet" vent regardless of what you do inside. The usual way to deal with this would be to place roof vents near the top of each rafter bay in the vented areas where you can't use a conventional ridge vent..
Bill
The drawing I posted is from the GBA website. I was using it as an example of how we planned to have them complete the knee walls with the blocking and everything.
We don't have any drawings of the roof or floor plan, as this is a reno, but I've attached some pictures I had on my phone before and during construction that hopefully help you get an idea of the roof from the exterior. I circled in the first pic where there is venting. I believe the front gables and the dormer window in the back are also vented at the top. Our soffits are vented all the way around the home as well (except in two 4-foot areas where our bath fans will vent through the soffit to the exterior).
I can take some pictures of the inside of the bonus room later today.
It might help to post a sketch of the entire floor with roof.