Do I need an interior vapor barrier of some sort?
The house was built in 1991 and is located in SE Mich, Zone 5?
It currently has a wood composite siding, no housewrap, and a mixture of plywood and double foil-faced foamboard (no discernible pattern). The studs are 1x4s. On the inside, it had fiberglass with a plastic vapor barrier. The windows are flangeless wood windows that I plan on keeping and restoring.
I have removed the interior drywall and replaced the fiberglass with mineral wool insulation. I plan on removing the exterior siding and all foamboard, replacing it with semi-solid stained cedar and OSB. I plan on staining all sides of the cedar.
I do not believe the windows and doors were flashed correctly, or at all, and this has led to mold issues directly beneath the windows, which has been remediated.
I have several questions:
1) Should I wrap the house in Tyvek or #15 or #30 felt?
2) Is a 1/4″ rain screen/drainage plane enough?
3) Do I need an interior vapor barrier with that mineral wool? If I do, can I just staple kraft paper to the studs and drywall over it?
4) Is there a way to flash the windows without removing them entirely? Or is the only way to flash them to remove them and build a sloped sill pan (the windows themselves have a slight slope, but the rough opening is flat and caulked with great stuff currently). Do I flash with stick and peel membranes, flash with asphalt, or do I need metal sill pan things? I am okay with having an innie window, but I do not understand it enough to explain it to my contractor.
Thank you for any help you can provide!
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Replies
Hedy,
Q. "Should I wrap the house in Tyvek or #15 or #30 felt?"
A. Yes. Every house needs a water-resistive barrier. For more information, see All About Water-Resistive Barriers.
Q. "Is a 1/4" rain screen/drainage plane enough?"
A. Yes. For more information, see All About Rainscreens.
Q. "Do I need an interior vapor barrier with that mineral wool?"
A. No. No building codes require an interior vapor barrier (although codes require cold-climate homes to have an interior vapor retarder, which is different from a vapor barrier.) For more information, see Do I Need a Vapor Retarder?
Q. "Can I just staple kraft paper to the studs and drywall over it?"
A. Yes, but it is vitally important to pay attention to airtightness.
Q. "Is there a way to flash the windows without removing them entirely?"
A. Not really. However, if your windows are protected by a generous roof overhang, you may be able to get away with an imperfect flashing job.
Q. "Is the only way to flash them to remove them and build a sloped sill pan (the windows themselves have a slight slope, but the rough opening is flat and caulked with great stuff currently)?"
A. That is certainly the best way -- especially if your house shows signs of water entry.
Q. "Do I flash with stick and peel membranes, flash with asphalt, or do I need metal sill pan things?"
A. Every rough sill needs to be flashed. Careful workmanship matters more than the materials selected. Either a commercial sill pan or a site-built sill pan can work, as long as the workmanship is high quality. I don't know what an asphalt sill pan is, however. For more information, see Window Sills That Won't Rot.
1. To some degree this is a preference, however Tyvek lies flatter on the wall meaning less chance it will contact the siding across the rain screen cavity and create a capillary bridge. When taped and sealed, sheet WRBs like Tyvek also act as air barriers - although imperfectly.
2. A 1/4" drainage plane will almost invariably in some places have either the WRB or even the framing touching across the gap. It also is really only acting as a capillary break as it is too narrow for any air movement allowing drying. 1/2" performs much better.
3. You need a vapour retarder not a barrier. Much more important is to air-seal the wall. Most of the moisture problems in walls occur because of air leakage not diffusion of water vapour. Your interior drywall can act as both if diligently sealed at the perimeter. A "smart membrane" also works.
4. I'll leave this to others as I have no experience with flangeless windows.
Hedy,
One more point: if you are planning to remove the siding, you have a golden opportunity to improve your home's performance by adding one or two layers of exterior rigid foam. It would be a shame to miss that opportunity to address thermal bridging through the studs. For more information on this topic, see:
Roofing and Siding Jobs Are Energy-Retrofit Opportunities
How to Install Rigid Foam Sheathing
Thanks for the quick reply!
I have interviewed several siding companies - none of them knew how to install siding with exterior rigid foam. My house is located in a rural area that is a lengthy trip from larger construction companies and most were not willing to make the trip. Therefore, I just dropped that
Only one siding company knew what about furring to create a rain screen, and they were only willing to do 1/4" gap because they were unsure how to modify window sill and trim if it was larger? I am not clear on the reason for their refusal.
So for the interior, is it necessary to add the kraft paper and seam seal it? Or is the mineral wool enough of a vapor retarder? Can I simply seal up the interior drywalls at the perimeters?
Thank you for the answer about the windows. I was afraid that they needed to be removed. None of the siding companies I interviewed seemed willing to do that portion of the job; they wanted to simply add a retrofit sill to the windows? Again, I am unclear about the details.
Hedy,
If you are unclear about how to flash a window rough opening, I don't recommend that you come up with the job specifications and scope of work. Instead, you really need to find a competent contractor familiar with energy retrofit work.
To find such a contractor, I suggest that you contact local energy raters (for example, energy raters certified by BPI or RESNET -- visit those two web sites for lists of raters in your area) and ask the raters to recommend contractors who understand these issues.