Retrofitting a Double Studwall in Barn Conversion – moisture/vapor issues
Greetings.
I am converting a roughly 40 year old barn to living space. The original build is conventional stick framing. Studwall, plywood sheathing, housewrapped and then covered with siding. On the inside the studwalls are covered with basic 2x construction lumber. There’s no insulation within those walls. It was a horse barn originally. I am creating a second non-bearing studwall set back 5 inches from the 2x covering of the original wall. In other words, I am creating a conventional double studwall in what was an uninsulated barn.
After wiring and plumbing put into the new internal wall I plan to fill both cavities with loose cellulose insulation. I am leaving the original 2x covering of the original wall in place. So there will be two separate cavities. The new internal wall will be covered moisture resistant drywall. Between the two cavities that should be 11 to 11 1/2 inches of loose cellulose.
I know from having used the barn for several years that the original wall is very well sealed for moisture. The studs and sheathing are all in good shape. But it wasn’t built as a modern air sealed structure. My question is the best strategy for vapor and moisture control. I’m trying to decide whether it makes sense to put plastic sheeting over the 2x internal side of the original wall as an extra vapor barrier between the two cavities. But I’m worried about creating areas where moisture can get trapped and lead to rot.
A couple other details. I’m an experienced woodworker. But this kind of construction/renovation work is new to me. I’ve studied as much as I can of how to execute this kind of design – different approaches to double studwall construction, the controversies about them and different approaches to moisture issues. But I’m very green (in the old sense of the word.) The original wall has modern house wrap that appears intact. (But I can’t get under the siding to examine it everywhere.) But the original build is a barn that wasn’t meant for this purpose. So there’s real limit on how much I can seal the original external wall. So I’m operating within those constraints. I realize that places some limits on the eventual energy efficiency and that’s fine. My real goal here is to do seal the renovated structure as well as possible without trapping moisture that would lead to rot in the future.
Eager to hear any advice experienced people may have.
Josh
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
You make it sound like your barn has a robust water barrier in the walls already. If so your barn is very rare almost a unicorn.
What climate zone are you in?
Did you find these articles?
https://www.greenbuildingadvisor.com/article/the-mythical-threat-to-double-stud-walls
https://www.greenbuildingadvisor.com/article/how-to-get-a-double-stud-wall-right
https://www.energyvanguard.com/blog/joe-lstiburek-s-ideal-double-stud-wall-design/
Walta
Yes, surprisingly we do seem to. I've worked in this barn for four years. I converted the second story hay loft to a woodworking shop back in 2020. I insulated it using the original walls which on the second story did not even have the studs covered. Used closed cell and then drywall over that. Water does not get into it. On the first floor when it was a horse barn there were small windows with bars. On a few areas on the edges of those windows there's a bit of infiltration. But that's basically because they didn't seem to do any flashing. But again, it was a horse barn. So yes, it seems to have a pretty solid water barrier. Again it's not a very old structure. I believe it was built in the early 80s and the look of the construction lumber and the house wrap seem to bear that out.
But everything's relative. It's robust for a barn. But whatever moisture got into the studs and sheathing in the past could probably evaporate out either to the inside or the outside. So just trying to get a sense of the best way to deal with the double wall construction now.
I've I'm going to read these again, I think I've read two of them. The key for me is that in most of these they assume you're building both walls. And they explain best practices. But in my case the existing wall is there and the kind of sealing I'd do ideally probably just isn't possible. So given that trying to see if it I should pack the two cavities, rely on the house as the major barrier and then also rely on the paint on the drywall as an additional barrier. My main concern is that I think it may make sense to rely on the imperfectly sealed outer wall as a path for moisture to escape from. I don't want to add another barrier inside the wall that may trap moisture.
What climate zone are you in on this map?
Walls that are fine in zone 2 are a total failure in zone 7.
https://basc.pnnl.gov/images/iecc-climate-zone-map
Walta
The barn is in climate zone 5a. About an hour north of New York City. So it doesn't get crazy cold here. A bit inland but not too far.
Josh,
You don't want a vapour-barrier that far into the wall. It is too close to the outside, meaning it will be cold and will attract condensation. You need a class 2 vapour-retarder close to the interior - either on the outside or inside of your new stud wall. I would use a variable-perm membrane for that. the performance and longevity of the wall is largely dependent on how well you can air-seal it. That's where I would concentrate your efforts.
Okay, want to make sure I understand. So scrap the idea of a putting some vapor barrier over the plants that cover the studs on the original (exterior) wall. And instead focus on barrier on the new (interior) studwall. My plan has been to fill both cavities with loose cellulose insulation - the already existing cavity in the original (exterior) studwall and then new cavity between the new studwall and the original wall. Would it make sense to place that interior side barrier between the new studwall and the drywall? And if I'm understanding that right, what specific kind of barrier would you recommend there? (thanks so much for taking the time with my questions). Josh
Josh,
The only downside of locating the air-barrier/vapour-retarder directly behind the drywall is that it makes it more vulnerable to damage, but that's where it goes in almost every Canadian house, and it doesn't seem to be an issue.
Like Akos I'm up here in Canada where poly is still the most common material used, so I don't have a specific recommendation for a membrane, but Scott has done a good job cataloguing the choices in this article.
https://www.greenbuildingadvisor.com/article/vapor-variable-membranes
+1 on what Malcolm said, you don't want a vapor barrier over the old wall.
Instead of a vapor barrier, you could install heavy duty house wrap (ie Tyvec commercial) and detail that as your air barrier. Since it is in the middle of the wall, it will be protected and won't have holes in it for wiring and plumbing.
You would still need a warm side vapor retarder. Regular 6mil poly behind the drywall will work just fine but one of the variable perm membranes or faced batts would allow for some extra drying capacity to the interior.
Double stud walls rely on good ventilation behind the cladding for drying. If your exterior is something like corrugated metal or vinyl you should be fine. If it is something else, you might want to rethink you assembly and reduce the overall R value (ie Mooney wall might be a good option).
Want to make sure I understand. So you'd be recommending heavy duty house wrap over the boards the cover the insider of the original wall? That's in addition to the house wrap over the sheathing on that wall? And then for the warm side barrier, is there a specific variable perm membrane you'd recommend for behind the drywall?
On the cladding, this is a concern of mine. The existing wall is studwall covered by plywood sheathing covered in house wrap and then a wooden siding attached onto that. I've examined the areas it's cut into and it seems to have held up pretty well over 40+ years. Seems to allow moisture to dry outward. See no evidence of water infiltration or rot. But I'm hoping to replace this cladding with the kind of rainscreen with furring strips that Ben Bogie recommends to allow more ability for ventilated drying and moisture not getting caught behind the cladding. Again, any pointers or advice on any of this is greatly appreciated.
The house wrap is only there as a permeable air barrier, it is not doing any WRB function. Since it is permeable it won't trap moisture like poly would in the same location.
You don't need this extra layer to be the air barrier, you can always detail the vapor retarder as an air barrier or install drywall in an airtight manner.
Both options will work just as well as long as done with some attention to detail.
This is the closest I can quickly find to your proposed wall:
https://effectiver.ca/calculator/wall.php?id=17451
Note that almost all these thick walls have vented cladding. You can search some more here:
http://cwc.ca/en/design-tools/effective-r-calculator/
P.S. Walls need to be dense packed not loose filled as the weight of the insulation in a tall wall causes the cellulose to settle over time.
Consider have the walls filled with damp sprayed cellulose. This gives you a perfect fill into all the irregular spaces and the glue resists settling.
https://www.greenbuildingadvisor.com/article/details-insulating-double-stud-wall-cellulose
Walta