Wall Assembly Help for 3A/4A Climate Zone
I’m in the North Georgia area, on the line between 3A and 4A climate zones (depending on which climate zone map you look at, it could be classified as just zone 3). We have made some decisions regarding our exterior remodel, and now that I’ve been exposed to more of the information on this site and other like-minded sites, I’m questioning our decisions.
For context, it’s a 2001 home that originally had vinyl siding on OSB (no WRB, insulation board, or anything underneath). Our current plan is to:
1. Remove the vinyl siding and windows
2. Replace any damaged OSB
3. Wrap the home in Tyvek Commercial wrap and tape the seams (this WRB chosen for its durability and 28 perm rating – permeable without being too open)
4. Install new Pella fiberglass windows – integrated with the house wrap and flashed appropriately
5. Install Benjamin Obdyke Rainscreen Max
6. Install Hardie siding with satin paint
On the inside, our plan was (wood stud wall):
1. Install Rockwool in wall cavity
2. Install drywall with latex paint
My concern is that we may not be thinking about this correctly from a moisture standpoint. The LAST thing we want is a condensation & moisture issue down the road. My initial thinking was to keep it so that everything could dry to the inside and outside, but I’m wondering if this plan will work the way I hoped.
We’re set to begin next week, so there isn’t time to pivot too much on the exterior (adding big changes like exterior insulation isn’t an option at this stage). But we can still make minor adjustments and also have a little time to make interior changes. I’m hoping you all can provide some much-needed guidance here!
Would taping the OSB seams on the exterior be a good idea to prevent air leakage, or should taping all Tyvek seams, including the top and bottom, take care of that?
Do we need a class II smart vapor retarder like Intello on the interior?
Are there any other potential issues and (more importantly) solutions you see here?
Truly appreciate any and all help I can get ASAP!
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Replies
You should tape the seams of the OSB sheathing to achieve some air tightness and reduce heating and cooling loads. Mechanically-attached WRB products like Tyvek do not make good air control layers, but they can work fine as water control layers. It is definitely not an air-tight product.
Your proposed assembly can currently dry in both directions so you should be fine from a vapor and condensation standpoint. However, keep in mind the 2021 IECC does require exterior insulation in CZ 4. See IRC Table R402.1.3. You can do either R-20 in the stud bay with R-5 exterior, R-13 in the stud bay with R-10 exterior, or nothing in stud bay and R-15 exterior. Since you probably only have 2x4 studs, the first option is not really viable for you. Since this is a remodel you probably don't have to meet this requirement, and your city/county may not even have adopted the 2021 IECC code anyways.
I think you'll be fine from a moisture standpoint especially considering the assembly can dry in both directions.
Thanks so much for your reply!
What tape would you recommend I use for taping the OSB seams?
And should I have them tape all of the Tyvek seams including the top and bottom? Or leave the bottom open?
Leave the bottom open, following Tyvek's suggested installation methods. You don't have to tape any seams of the Tyvek. For the OSB seam tape, you have some options:
- Siga Wigluv 60
- Zip Flashing Tape
- 3M 8067
- Tescon Vana
https://www.dupont.com/content/dam/dupont/amer/us/en/performance-building-solutions/public/documents/en/commercial-wall-2-tyvek-wrb-under-ci-43-d100808-enus.pdf
Thanks so much ! My only concern with taping the OSB seams is that the home wasn't originally built to be air-tight. I have heard of issues being caused by making a home that wasn't built to be air-tight into an air-tight home. I've also seen that OSB needs to be able to expand and contract with weather, deeming it necessary to keep the seams unsealed. Any thoughts on this?
OSB is vapor-permeable and you are using a vapor-permeable house wrap. So drying out isn't really a concern in this situation. Since you are replacing the windows, you should be able to install them with the correct flashing so that no bulk water is making it into your wall cavity. You are reducing the drying potential compared to what your wall use to have, but it still has some decent drying ability so I wouldn't be worried about it.
Some things you may want to worry about when sealing up old houses are CO2 concentrations (might warrant adding an ERV or other fresh air system), any combustion appliances you have and possible CO buildup, and relative humidity levels now that you have less airflow to clear that humidity buildup. You might need an ERV or a dehumidifier (or possibly a venting dehumidifier although it won't be as efficient as the two separate appliances).
Taping OSB seams is a long time accepted method of air sealing a house so I don't think there are any concerns with seasonal movement. The modern high quality tapes allow for some movement. Keep in mind you are installing a WRB on top of it so it shouldn't be getting very wet. And it can dry fairly quickly if it does.
Appreciate your quick replies! Will taping the OSB seams make it tight enough to warrant those appliances (ERV, dehumidifier, etc.)? The home has a vented attic if that makes a difference (soffit vents + continuous ridge vent).
Also meant to ask what issues we may run into if we decide NOT to tape the OSB seams and go with our plan as-is? Trying to weigh the pros/cons of each option. Thanks so much!
tjones,
Your house may not have originally been designed to be airtight, but with your new renovations it will be. There is no downside and big benefits to taping the sheathing, and air-sealing wherever you can.
tjones,
Your plan will work fine. Tape the new sheathing as your primary air-barrier. You don't need an interior vapour-retarder.
Appreciate your reply!
What tape would you recommend I use for taping the OSB seams?
And should I have them tape all of the Tyvek seams including the top and bottom? Or leave the bottom open?
You mention an exterior remodel, but also installing insulation and drywall. Are your walls not insulated or dry walled currently?
Sorry for the confusion there. We are doing an interior remodel as well. The entire home is gutted down to the studs, so there is no insulation, drywall, etc.
Mineral wool is great for DIY because it’s not itchy, but if you are hiring it done I would see if you can save some money on high density fiberglass batts. They are R15 for 2x4 walls and R21 for 2x6.
+1 for fiberglass. I would also get a price on blown-in fiberglass instead of batts for a custom-fit of maximum insulation value. The benefit with the blown-in method is that you don't have any paper facer that will reduce the wall cavity's ability to dry. Also, no paper to promote mold growth. Rockwool is usually more expensive and has higher embodied GHG emissions than fiberglass. Cellulose is another option that's even more green but has a slightly lower R-value than fiberglass.
Matthew,
Why not use fiberglass batts with no facers?
Yes I forgot about that option!