Double Stud Wall Insulation
I’m in the planning stages of a 2 story, approximately 3500 sq ft home in climate zone 7 (Colorado Rockies, elevation just over 9000 ft). Cold climate with relatively low humidify/dew point. I am not a contractor, but have reasonable building experience, including to the studs renovations and new construction of another home in Colorado).
I am leaning towards double stud wall assembly for various reasons (as opposed to exterior rigid insulation on a single stud wall), not the least of which is the limited exterior building season in the high rockies (May – October). I’m planning to get from breaking ground to at least dried in during May – October 2025. The wall plan would be to build the outer stud wall per ‘normal’ construction, use a vapor-permeable WRB, furring strips, rain screen as needed, then siding. The siding will be a mix of stone veneer, cedar board and batton, and vertical nickel gap wood (thermal treated ash, locust, sassafras, cedar or other).
The question i have is on the space in between the stud walls. Lstiburek’s Ideal Double-Stud Wall Design includes installing OSB sheathing to the outside of the inner stud wall – “a layer of OSB or plywood on the outside of the inside wall … That extra sheathing serves as the primary air control layer as well as the vapor control layer.”
I’m looking for opinions of replacing the OSB layer on the inside wall with a layer foil faced polyiso foam board installed on the inside of the outer stud wall, and taping the seams. The polyiso would serve the as air and vapor control layer. The outer stud wall should get very little moisture from the inside, and be able to dry to the outside. The inner stud wall would dry to the inside.
Inside construction order on this would be fairly straight forward:
1: insulate the stud cavities of the outer wall
2: Install layer of foam to outer wall (likely with plastic cap roofing nails), and tape all seams and nail heads
3: Build inside stud wall
4, “Normal” wall completion (electrical and plumbing, insulation, drywall)
Another advantage to this is once i have steps 1 and 2, the walls would half way insulated and air tight so i could keep the home warm-ish in the winter construction months (November-April).
I appreciate any opinions on this wall make up.
Tracy
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Tracy,
The main downside is it uses a lot of foam board, which most double stud walls avoid, but as you say it has a lot of upsides, and will perform well.
One thing to look out for is that the insulation in the outer wall and foam add up to a high enough ratio of the total wall R-value that you don't get condensation on the inner-face of the foam, which is effectively a vapour-barrier. The rule of thumb is that those two outer layers should be 2/3 of the total, but it really needs calculating for your climate.
It is much easier to detail the exterior sheathing as your main air barrier, a proper mid wall air barrier would still have floor joists poking through it which is much harder to air seal. I really don't see much benefit of moving it inboard.
In zone7, the condensation control ratio is 45%:
https://buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies
You could build with a 2x6 exterior load bearing wall and insulated that faced batts and build the interior 2x4 stud wall say 3.5" gap. This would put the facer at about the location so it could be your code required vapor retarder for you wall. This would not require any foam, the whole thing could be insulated with three layers of cheap batts (faced R22+ two layers of R14 HD batts) and would still get you some thermal comfort in the winter.
You could even simplify even more and build a load bearing 2x8 wall and an interior 2x4 wall with offset studs without a gap. Slightly lower assembly R value but won't really budge your yearly energy use much. You can do similar insulation option, R28 faced batts for the 2x8 and unfaced R14 for the 2x4.
With either assembly it is still a good idea to detail the drywall in an air tight manner as your secondary air barrier. You would also still have watch construction moisture sources such as unvented propane heater.
I feel like foam would be easier to get an air tight barrier. The reason Joe Lstiburek likes OSB in the middle is the sturdiness of sheet goods versus 6mil plastic.
That said, I do prefer to not use foam because of the environmental impact of foam. For the previous house i built, i scored a semi load of inexpensive, reclaimed polyiso sheets for exterior rigid. However the reclaimed poly i find is usually removed from the roof of industrial buildings, so is not foil faced. Unless i found reclaimed, foil faced polyiso, avoiding foam is preferred.
My current energy building loss calculations have the worst case BTU per hour loss through the walls is about 6000 btu's/hour, with the walls spec'd at r45. My initial discussion with the structural engineer called for 2x6 studs at 24" OC.
Based on achieving ~r45:
1. Batts at r3.5 per inch puts me at ~13" of insulation.
Assuming staying at 2x6 structural wall, puts me with a 4" gap between walls.
If i did this, i would probably move the vapor retarder to the inside of the outside wall. More on this below.
2. Batts with a central 2" layer of of poly i puts me at just over 11". With the 2x6 structural wall, this basically puts the gap at the 2" of foam. The foam would never move as it would be sandwiched tight between the stud walls.
The extra 2" thickness on the batts only wall would decrease my interior sq footage by a negligible amount, so not a factor in my decision.
The biggest factor in my decision is achieving a better secondary air barrier as you mention. I am planning to cover all ceilings in 6 mil plastic, mainly to reduce moisture from diffusing into the attic. It might be overkill, but it seems like not a bad idea to block moisture from the joist bays. I'd be interested in opinions on plastic under the floor joists.
On the idea of faced batts, i think those are most effective with drywall directly over them to help the edges of the facing seal up to the studs. If they are on the inside of the outside wall, i feel like there would be significant risk to a leaky vapor barrier.
So if i go with batts only, i would likely do all unfaced, and then put 6 mill plastic on the inside of the outside wall to act as vaper barrier and secondary air barrier (the primary being the WRB outside the sheathing) . In which case i would take your suggestion of building the structural walls with 2x8 framing, putting the percentage of insulation outside the vapor/air barrier at 0ver 45% (factoring in degrading of r value for thermal bridging in only the structural framing).
Advantages include:
Air barrier installed between the stud wall allows for more margin of error for detailing drywall around outlet and switch boxes.
2x8 framing adds about 30% to the framing material cost over 2x6, much less less than added cost of a foam layer.
Installing the plastic on the inside of the outside wall seems less susceptible to damage and installation defects than trying to install it to the inside of the inner wall before lifting the wall in place.
The wall plastic can be taped to the ceiling plastic which seems like would improve the vaper barrier to the attic.
As to the rim joist (planning to use I-Joists) - Because i am not strictly following Joe Lstiburek's ideal wall, (my structural wall is the outer wall), the rim joists are an area i am looking for a good solution. My current thinking is with the 2x8 framing, i can see if the structural engineer would spec that the wall cantilever over the floor joists by 1"-2". Then fill the cantilevered area with insulation to break the thermal bridge, sheathing straight down from the wall plane to cover the cantilever, and of course the WRB. From the inside, the ends of the joist bays are easier to insulate and get airtight. I'm still thinking about the insulation and vapor barrier on the sides of the floor joist. Maybe just fill the joist bay with insulation, and go with the idea of plastic on the bottom of the floor joists as a vaper and air barrier.
As addition info, i am planning to use minisplits as the primary heat source. And likely a smallish, high efficiency natural gas boiler tied to radiant in the basement slab and a small snow melt area at the front entry and garage apron area. Power in the mountains tends to go out more often, and my small propane generator would be able to power the gas boiler to keep the house from freezing. There will be a couple direct vent gas fireplaces, not because i like them, but because it is expected in homes in the area. So moisture sources in the house would be mainly cooking and human and pet respiration. And a thermostat controlled steam humidifier run in the winter. Interior humidity levels at a mostly unoccupied home in the area range from around 20% in the winter to 40% in the summer.
Tracy
You seem to be mixing the use of the air barrier and vapor barrier, and vapor retarder terms.
I can tell you from experience that getting 6 mil polyethylene detailed as your primary (or secondary) air barrier is not as easy as you'd think. It's very easy to tear and puncture the stuff during installation, and EVERY FASTENER will punch a hole that will stretch out and leak. These little holes aren't as big a deal for a vapor barrier as they are for an air barrier. Air barriers are easier to get right when made from some sort of panel product, be that drywall, plywood, OSB, rigid foam, etc. Note that fiber faced polyiso will work just fine as an air barrier.
A vapor barrier in the middle of that wall doesn't seem to gain much. I'm be much more inclined to put a vapor barrier on the interior side of the wall (i.e. behind the drywall), since moisture drive in heating dominated climates is primarily from the interior, conditioned space towards the outdoors. Putting the vapor barrier in the middle in this case just creates a potential condensing surface within the wall. My preference would be a smart vapor retarder on the interior, behind the drywall, to keep moisture from getting into the wall in the beginning in this case.
Kraft facers are a sort of poor man's vapor RETARDER, so less of an issue that polyiso in terms of the potential to cause problems down the road. You really can't detail that kraft paper as an air barrier though -- there is no way to avoid gaps and tears during installation.
Bill
In my mind, an air barrier is by default also a vapor barrier. (Maybe i'm wrong there). And i am mostly talking about putting an air barrier in the middle of the wall assembly , and implying it also acts as a vaper barrier.
I agree with you on the paper faced batts. Most remodeling projects I've done involving taking out drywall on exterior walls revels missing batts, kraft facers not attached to the studs and/or big tears and gaps in the paper.
I do take your point of keeping the moisture out of the wall to begin with. My biggest reason to put an air barrier in the middle of the wall assembly is that some where down the road, someone is going to put holes in the drywall for adding speaker wire, ethernet wire, new electrical, etc. By having the air barrier in the middle of the wall would keep the air barrier in tact as long as they did no punch all the way through. And i like the idea of an air barrier as a back up in case of any defects in the wrb installation. I feel like as long as i stay well above 45% of the insulation to the outside of that air barrier, then condensation should not be an issue. I grew up in a drafty farm house, so maybe i'm being over concerned about air infiltration from the outside.
I have not put up plastic sheeting in this way, but i believe you on the frailty of getting a well sealed air barrier.
Tracy
Your poly behind the interior stud wall should work. Since it is not your primary air barrier, no need to go overboard detail it as such. If it is mostly sealed it will do its job which is to prevent convective loops inside the wall. Its main job is a vapor barrier and as long as it mostly covers the area, it will work as such.
The ceiling poly is definitely a good idea. Since it is the primary air barrier in this case, it should tie into the exterior wall air barrier. You can look at the discussion here for options:
https://www.greenbuildingadvisor.com/question/air-barrier-continuity-at-sheathing-top-plate-ceiling-transition
Your canti idea for the rim joists is the simplest. Since the sheathing is well sealed there, there is no need to do any more air sealing around the floor joists. Insulate the space with fluffy and call it a day.
Wall mount mini splits are not the best idea for a new build. They don't have proper air filtering and it is hard to get air conditioning into bedrooms. The best is a ducted heat pump or a slim ducted mini split if your loads are low enough.
Floors in well and air sealed basements are not cold. Unless you have a large walkout, I would skip the floor heat. Non electric propane heat of some kind are defiantly needed if you have spotty power. I would add a through the wall unit to the mechanical area to keep your place from freezing. Generators tend to fail when need them, you want a heat source that will keep your house from freezing no matter what. For the snow melt, if the area is not large, you can get a larger water tank with a big burner and use a plate HX off that.
Many materials that are air barriers are vapor open. Drywall and plywood come to mind immediately.
Tracy,
The main knock against poly is it is completely vapour-closed, which can cause problems is some situations, but it can perform very well as an air-barrier. It's use is ubiquitous in Canada, and the tightest house ever tested used poly.
Air barriers ARE NOT ALWAYS vaport barriers, but they can be. A few common materials as examples:
Plywood/OSB: Air barriers, but vapor permeable
Polyethylene sheet: Air barrier AND vapor barrier
Tyvek house wrap: Air barrier, but vapor permeable
Foil faced polyiso: Air barrier AND vapor barrier (because of the foil facer being a vapor barrier)
Fiber/kraft faced polyiso: Air barrier, but vapor permeable (the facer in this case is vapor permeable)
Drywall: THE classic air barrier, but surprisingly vapor open
etc.
Note that vapor barriers like poly that have holes in them are no longer air barriers. I supposed a "mostly sealed" sheet of poly is a sort of air movement "retarder", but not a real air barrier. If you take care to seal the poly at ALL penetrations, so that there are NO holes, then it's a good air barrier. Drywall tends to be easiest to detail as an air barrier, because the taping and mudding you're going to do anyway also acts to airseal the assembly.
If you want to detail poly (or smart vapor retarders, which is what I usually use) air tight, I would suggest you run a bead of polyurethane sealant over the perimeter of the framing that the poly will be installed against, then staple the poly THROUGH the bead of sealant, then roll the poly over the sealant with a tape roller. This way the sealant seals all the staple holes, and you end up with a good perimeter air seal. I've done this many times and it doesn't add as much time to the project as you might think. I seal electrical boxes to the sheet with flashing tape after cutting clearance holes (i.e. the boxes and wiring go in first, THEN the membrane, then you can see the boxes through the membrane and cut holes for them with a knife. I tape the perimeter of the box to the membrane as a last step.
Note that I do always recommend two air control layers, usually with the interior drywall as the primary and the exterior sheathing as the secondary. You can put poly (or rigid foam) in the middle of your wall assembly and detail that as your secondary air barrier, but I'd still recommend detailing the interior drywall airtight.
Bill
Thanks all for the time and thought to respond to me on this. It has given me a bit to think about, and i have a few months to think about it. Most of the opinions i have seen GBA and FHB is to put best effort into keeping inside vapor from diffusing into wall cavities and vented attics.
I like the suggestion to use polyurethane sealant and roll in to seal the polyethylene layer to the studs or ceiling joists or trusses. In addition to helping seal, it seems like it would help prevent tear out at the staples.
I still like the idea of a layer of polyiso in the middle, but that will probably depend on if i can find a truck load of reclaimed in Denver.
On the building heating comments, i plan to post some questions here after i give that a bit more thought. My previous home build I installed two Chiltrix air to water heat pumps, a buffer tank, and zoned in floor radiant. I also put in a dedicated full house filtration/humidifier air circulator. I gained a lot of good experience installing those system.
Tracy
My experience sealing the membrane (I'm usually using Certainteed's MemBrain here) is that the polyurethane alone is ALMOST enough to hold things up without staples. If you were careful, you could probably just staple a few places around each corner, maybe a foot past in either direction, and the sealant would probably take care of the rest -- especially once it was rolled out. I've never tried this myself though, I've always stapled periodically.
BTW, a pneumatic staple gun that can shoot the little Arrow staples is awesome for this kind of work. I got one off of Amazon for maybe $20 or so years ago and it makes tacking up membranes like this -- and faced insulation -- way, way easier. Helpful hint: put a swivel air fitting on the thing, and use a lightweight 1/4" air hose (I like Flexeel). Do this and the hose won't fight you while you work.
Bill