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Community and Q&A

Vapor barrier for existing ext. house wall (to become 4th wall for new adjacent sunroom addition with hot tub & indoor pool)

Dan Theman | Posted in Energy Efficiency and Durability on

We’re planning to add a stick built addition to our home which will contain a hot tub and small therapy swimming pool. We’re going to include negative pressure exhaust fans and a dehumidifier system.

The three new walls will be built with Zip-R sheathing and 3″ of closed cell spray foam and will therefore be very tight. The fourth wall will be the existing exterior wall of our house (currently 2*4 wood studs with fiberglass batt insulation, OSB Sheathing, tyvek house wrap and vinyl siding).

Our plan is to remove the siding and house wrap and install a vapor barrier and drywall over it. I’m trying to decide what would be the best way to accomplish this.

This will probably be there least airtight of the four walls and if the room becomes pressurized or if there’s vapor drive because of high humidity, I don’t want it going into my house through this existing wall which has fiberglass Batts and isn’t built to deal with moisture.

The good news is that the addition will be conditioned space, so this will no longer be an “exterior” wall and there shouldn’t be huge temp differences on the two sides which should mitigate condensation risk, but there could still be vapor drive with greatly differing relative humidity levels on both sides so I want to be careful.

I’ve considered applying a peel and stick vapor barrier membrane on my existing sheathing and drywalling over that, but I’m concerned about using a total barrier that wouldn’t ever let the wall dry to the sunroom side (in case there was ever a situation where my interior humidity was higher than the sunroom and that wall needed to dry to the exterior) and about having a flat enough surface for a good drywall install.

Another possibility would be to put a layer of the Zip-R Sheathing over my existing sheathing (foam side facing existing house sheathing) and drywalling over that. It would create a good surface to apply drywall over, the foam would act as a vapor retarder and it would just add another buffering layer between the rooms to help restrict airflow across that wall assembly.

I’d greatly appreciate any thoughts on the best way to handle the detail for this existing house wall. I’m in Maryland. Thanks in advance!

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  1. Stephen Sheehy | | #1

    If the pool and tub have reasonable covers, you may not get much moisture. You'll want good covers to save energy anyway.
    Keep the covers on unless the tub or pool are in use and run an exhaust fan for a while after use. My (unconditioned but insulated) hot tub room doesn't get humid at all.

  2. Expert Member
    Dana Dorsett | | #2

    A studwall with the 3" of closed cell foam is an incredible waste of closed cell foam, and packs a powerful environmental hit. Despite the R20-ish center cavity R the thermal bridging of the 3" of framing lumber reduces the net thermal performance to little more than a 2x4/R13 wall, and is less than 2x6/R19 wall.

    The money is better spent on thicker ZIP-R, or going with standard sheathing + exterior foam using open cell foam for the cavity fill. A full 3.5" of open cell foam uses less polymer than a mere 2" of closed cell foam, and uses water as blowing agent instead of HFC245fa, an extremely powerful greenhouse gas. The facer of the foam side of ZIP-R is a vapor barrier, and is not damaged by moisture- it protects the OSB from interior side moisture drives. If standard OSB is used, 3" of exterior polyiso (R18-R20) would bring the performance up to code-min without any cavity fill, but with that much exterior foam R13 of open cell foam would not be particularly risky, even in a pool/spa room (but would be for a steam room.) Even 2" of exterior polyiso would be enough for R13 open cell foam cavity fill, as would the 2.5" thick ZIP-R (labeled R12.6).

    The now interior partition wall (formerly exterior wall) would be fine with peel & stick impermeable membrane, but that's really overkill unless you're creating a steam-room. Stripping it to the sheathing (OSB? CDX?) and taping the seams & caulking all penetrations with a polyurethane caulk to make it air tight would be enough, assuming there isn't a true vapor barrier on the other side of the wall(?). OSB & CDX are "smart" vapor retarders, and will rise to 5 perms or more if it's humid enough in the pool/spa room to grow mold, which presumably you would limit with your ventilation(?). (If you don't the studs of your new studwall will rot.) Under more normal high humidity conditions the sheathing will have a vapor permeance less than latex interior paint, which would keep moisture diffusion rates low enough that the assembly would dry to the other side of the wall into the air-conditioned space.

  3. Dan Theman | | #3

    Stephen and Dana, thank you both for the quick responses! I do plan to keep them both covered whenever they're not being used, so hopefully this is all belt and suspenders, but I'd like to be thoughtful in the way I'm designing the room just in case.

    Dana, we initially designed the addition with the more recommended methods for a "real" indoor pool enclosure, using a continuous vapor membrane and all exterior insulation (very similar to a REMOTE building), but unfortunately this is a small residential project and I was unable to find any local residential contractors in MD with experience building that way (or who were even comfortable giving it a shot without charging me a huge premium to account for unknowns and their learning curve). I still wanted to try to have a continuous air and vapor envelope to keep moist air from moving across and into my wall cavities and given what I know about poly and other more traditional vapor barriers, I figured that 3" of closed cell spray foam would be the most fool proof way to get a good air and vapor seal. I know it's wasteful and bad environmentally and an certainly open to doing it differently if there are better ways, but my main concern is long term durability of the building (and second, energy efficiency...unfortunately environmental friendliness is third on my list but this is a small project so hopefully won't be a huge negative impact if I end up using the spf). I'm planning to use the 1.5" Zip R on the exterior in order to provide additional air and vapor sealing, but mainly to reduce the thermal bridging and the risk of condensation on cold studs in the wall cavity - we're planning to use 2*6 construction with the 3" closed cell foam, so parts of the studs would be left exposed in the wall. Does that help at all with your concerns about thermal performance of that wall? I'd strongly prefer to use open cell foam if it would be a good enough air and vapor barrier, but I think I'm pretty much limited to Zip-R Sheathing rather than regular exterior rigid foam. I know it may sound strange with so many folks on these boards using those methods but I literally couldn't find anybody who had ever used exterior rigid foam and they were very confused when I brought up some of the door and window flashing issues and some of the other details required when the exterior foam gets thicker...I may be able to get away with some small thickness of exterior foam, but I'm in a mixed humid environment and am guessing about how well I'll be able to control humidity in this room and won't know if I'm right until after it's built, so I'm nervous using a little exterior foam and hoping it's enough. I didn't know that the foam facer was an actual vapor barrier, thought it was just semi impermeable. If that's the case, maybe I would be okay with open cell spray foam and the Zip-R? Do you know how think the Zip-R would need to be? I know you said that the 2.5" would be enough, is that with R13 open cell inside? With 2*6 walls I could go thicker than 3.5" of open cell if that would be helpful (or maybe it would make the sheathing colder and have the opposite effect)?

    As far as the partition wall goes, it's currently osb. I'm not planning to create a steam room and will keep it as well ventilated as I can - hopefully I'll mainly only have elevated humidity levels for a couple hours a day when the covers are off, during which I'll run the dehumidifier (residential portable unit, 70pints) and the exhaust fans and then I'll put the covers back on, let the dehumidifier and fans run for a little while and everything will dry back up. Again, that's my plan but I won't know real world conditions until after it's built so I'm just trying to be careful in case some of my assumptions are off or my kids somehow get in there and leave covers off for an extended period, or there's a power outage where fans and dehumidifier won't work, etc. That being said, I'm okay with a little bit of overkill - the peel and stick on one wall shouldn't be too expensive or difficult. If that's the case do you think it's generally a good other words, no major downsides or risks other than just being overkill? If so I'll probably do it, but if you think it would be better to leave the possibility of drying to either side I'll just leave the osb and tape, caulk and seal as you suggested should be sufficient.

    Thank you again for your help with this, I really appreciate it!!

  4. GBA Editor
    Martin Holladay | | #4

    I agree with Stephen and Dana. There won't be much vapor drive through the wall you are concerned about, and the wall won't have any cold surfaces that might encourage worrisome condensation. So stop worrying. The OSB is already a vapor retarder.

    Just focus on achieving an airtight thermal barrier for the new room (the floor, three walls, and roof assembly).

  5. Expert Member
    Dana Dorsett | | #5

    "I figured that 3" of closed cell spray foam would be the most fool proof way to get a good air and vapor seal."

    The facer on the ZIP-R foam is a less than 1 perm (which meets Canadian code definition of "vapour barrier"), which is also MUCH lower permeance than the OSB side if the OSB has a moisture content high enough to support mold, so what do you need the low-perm spray foam for? OSB isn't going to rot- it'll dry toward the exterior, and the polyiso won't either.

    So it's really about the air seal, which is achievable with just the ZIP tape, but could be backed up with open (or closed) cell foam, or polyurethane caulk. It will also need caulking under the bottom plates, and between any doubled up headers & other framing is necessary, even with foam cavity fill.

    "I'm planning to use the 1.5" Zip R on the exterior in order to provide additional air and vapor sealing, but mainly to reduce the thermal bridging and the risk of condensation on cold studs in the wall cavity - we're planning to use 2*6 construction with the 3" closed cell foam, so parts of the studs would be left exposed in the wall. Does that help at all with your concerns about thermal performance of that wall?"

    It will beat code min performance, but having the 2.5" nominal air space with 5.5" deep 2x6 studs and 3" fill doesn't buy you anything. In some locations the empty cavity behind the wall board may require fire-stop blocking to meet code (that's true in many locations), which increases the framing fraction. A 2x4 OSB or CDX sheathed wall with 2" foam Bonfiglioni-style edge strips (see: ) and a full 6.25" cavity fill of open cell foam would have comparable or better thermal performance. (Ripping some 2.5" ZIP-R into 1.5" edge strips would also be comparable.) A cheap 4-6 mil polyethylene vapor barrier behind the wallboard would guarantee that the cold OSB is never wetted from moisture diffusion, the air-impermeable open cell foam guarantees there isn't convective moisture transport to the sheathing, and you'd have to air condition the spa-room to some ridiculously low temperature for summertime moisture to accumulate in the open cell foam behind the polyethylene. (Are you planning on keeping it colder than 65F in the spa room during the summer?)

    With 2" foam in the Bonfiglioni edge strips the 6.25" of half-pound foam would have about the same polymer content as 3" of closed cell foam, and less polymer overall due to the polyiso only being in the edge strips, a center-cavity R of about R23, which is somewhat lower than 3" closed cell + R6.6 ZIP-R, but a significantly higher thermal break on the framing fraction compensating for the lower center-cavity R. Six inches of open cell foam costs about the same as a mere 2" of closed cell foam, and the edge strips (labor included) will probably be substantially cheaper than full sheets of 1.5" ZIP-R. The OSB could be ZIP ( not "-R) or CDX (recommended) or a cheaper OSB, though CDX/OSB would also require a WRB layer under the siding whereas ZIP does not.

    The OSB on the pre-existing wall is fine- just detail it as an air barrier and call it a day. If you'll lay awake at night without a specific vapor barrier layer, paint the OSB with "vapor barrier latex" primer before putting up the wallboard, and it'll always be below 1 perm, but could still dry into the spa room if it ever saw a bulk water incursion (which it couldn't with a peel'n'stick membrane or 6 mil poly vapor barrier.) Unless there is foil or vinyl wallpaper or a polyethylene vapor barrier in the original exterior wall stackup, the risk to the partition wall is very low even if you did nothing but put up painted wallboard on the spa room side, even if you didn't even bother to air seal. Err to the vapor-open side for that one, but making it air tight to prevent convective transfer to the air-conditioned side during the your high-humidity spikes makes that already low risk lower.

  6. Dan Theman | | #6

    Dana, thank you again for your extremely helpful responses and suggestions!

    My responses and a couple of follow up questions:

    1) Bonfiglioni strips - those look great. Never heard of that before and might give it a try. The installation looks pretty straight forward to me and doesn't look like it will greatly complicate door or window flashing so should be do-able. That being said, it's very "different" so if the contractors I'd like to use balk, what would you think about keeping the current sheathing plan of 1.5" Zip-R (or even going down to 1" if possible) and just filling the entire 5.5" in my 2*6 wall using open cell instead of doing 3" of closed cell like I had been planning? That way I'd still have the vapor barrier with the Zip-R foam facer and although it wouldn't be the full 6.25" of open cell, I'd also have the foam on the Zip-R to add R value. I think my only hesitation with your suggestion is the poly vapor barrier... My understanding is that they almost always end up having tons of holes from drywall fasteners and every other wall penetration and that they're not very easy to seal up and make really vapor proof unless the construction work is close to perfect, which I cannot expect. Do you think I'm being overly cautious about it and that it should be "good enough", even with an average install job? Believe me, I love the idea of saving money which I'm sure will happen if we can pull off your recommendation. I do not plan to cool the room to 65 in the summer so the reverse vapor drive shouldn't be a big concern.

    "paint the OSB with "vapor barrier latex" primer before putting up the wallboard" - this is a great suggestion! I was also thinking about using the vapor barrier latex primer on the drywall (planning to use paperless Dens Armor Plus for the whole room) that will go over the osb on that partition wall. My thinking there is that it would then also protect the drywall from the room moisture, but on the other hand if moisture came through the wall from the house side (can't really imagine that happening but who knows) I suppose it could get "locked in" to the drywall if it had a vapor barrier painted on it. Do you recommend painting it on the osb or the drywall or does it not matter? Drywall would probably be slightly easier...

    "Err to the vapor-open side for that one, but making it air tight to prevent convective transfer to the air-conditioned side during the your high-humidity spikes makes that already low risk lower." - I'm sorry but I got a little lost here. Would you mind clarifying what you meant by "Err to the vapor open side"? I'm planning to use the Dens Armor Plus (breathable) throughout the room with regular latex paint on the walls so that they're somewhat vapor open and I don't create any double vapor barriers (I'm already planning to use the vapor barrier latex primer on the ceiling and that would be the only vapor barrier that I use there... Just planning blown cellulose insulation above that - open to suggestions for those two details too if you don't think that's a good approach). If we paint vapor barrier latex primer on the osb of the partition wall I'll definitely make sure to keep the drywall and latex paint that goes over that wall vapor open to ensure no double vapor barrier - is that what you meant?

    Thanks again, I can't tell you how helpful this has been and I'm very excited about the prospects of not using closed cell spray foam!

  7. GBA Editor
    Martin Holladay | | #7

    Dana wrote, "So what do you need the low-perm spray foam for? OSB isn't going to rot- it'll dry toward the exterior, and the polyiso won't either."

    I just want to be sure we are clear on a few points:

    1. This hot tub room / indoor pool room may or may not be unusually humid. If the room ends up with high indoor humidity -- not uncommon for rooms with indoor pools -- then the room's thermal envelope has to be detailed with care. The indoor humidity level depends in part on occupant behavior and thermostat settings.

    2. I've had conversations with Joe Lstiburek about this type of room. He's been called in as a consultant on a lot of envelope failures -- rotten roofs and walls -- surrounding indoor pools. Opportunities for disaster abound. Joe told me that his current recommendations for pool rooms is to stick to the PERSIST approach -- putting all of the insulation on the exterior side of the sheathing, with a peel-and-stick membrane between the exterior sheathing and the rigid foam insulation.

  8. Dan Theman | | #8

    Thank you for your input Martin, much appreciated! I agree, this is not intended to have "steam room" humidity levels, however, because of costs I will not be able to include a "proper" indoor pool dehumidification system and will be relying on covers, exhaust fans and a portable room dehumidifier to keep humidity levels in check. Hopefully it won't be bad and most of this planning will have been for nothing, but I won't know real world conditions until it's built and everything is up and running, so I feel like I need to build to prepare for worst case scenario (within reason and within budget). My preference would be to use PERSIST/REMOTE building methods (and we already created plans to do it that way), but as I mentioned above we couldn't find any willing local residential builders who were comfortable doing it that way without charging a small fortune to account for their learning curve. Since that wasn't an option, we figured that the next best thing would be closed cell spray foam (a somewhat foolproof way to ensure a tight vapor barrier seal) combined with Zip-R sheathing to reduce thermal bridging and the risk of cold studs as a condensing surface in the wall.

    Dana, when I was asking if that helped at all, I just meant the Zip-R to helping with thermal bridging, not the leftover 2.5" of airspace... The purpose of that is twofold - wanted to use 2*6 construction to reduce the amount of lumber required in the wall (again, to reduce thermal bridging and allow more space for insulation) and to leave room for electrical and anything else that may need to go behind the drywall without having to puncture the foam (and without having to shave the foam flat to get the drywall on, which I understand can potentially increase the vapor permeability of the foam, which would defeat the purpose of using closed cell in the first place).

    Assuming it could unintentionally end up being more like a steam room, do you both still think that the partition wall isn't of great concern and could just be air sealed and possibly painted with vapor barrier latex primer? Dana, would you still have the same recommendations for the other walls or do you think it would be safer (strictly in terms of long term building durability) to go with our original plan of 1.5" Zip-R and 3" of closed cell foam? I know it's terrible stuff and I hate to use it but this is 3 small walls and I think I'd prefer that lesser evil than the possibility of my family spending time in a room with possible mold in the walls or having to rebuild the room if everything rots.

    Thank you both again for your thoughts and comments.

  9. Dan Theman | | #9

    Martin and/or Dana,

    Just wondering if you had any other thoughts on this topic?

    Thanks again for your help!

  10. GBA Editor
    Martin Holladay | | #10

    If you can't find a contractor who is willing to implement the PERSIST approach, then your plan to install closed-cell spray foam on the interior side of the sheathing is probably the next best approach. I think it will work.

  11. Dan Theman | | #11


    Thank you very much, I really appreciate your feedback. I was really liking the possibility of using open cell foam but after further consideration I think closed cell is probably the "safer" route so I think I just need to stick with it. Thanks again!

  12. Charlie Sullivan | | #12

    If you want to stick with closed cell foam, but don't want the severe climate impact of the blowing agents that are normally used, two of the major foam chemical suppliers how have versions available with "HFO" blowing agents that eliminate that problem, while also providing similar to slightly better R value. Lapolla Foam-Lok 2000-4G and Demilec Heatlok XT HFO are the two I know of. See

  13. Dan Theman | | #13

    Thank you Charlie, that's fantastic! I will absolutely call around and find out if any of the reputable insulation contractors in my area have either of those available. Thanks!

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