Zone 7 (Saskatchewan) need help with selecting an energy effective wall will proper air sealing
Hello eveyone,
I have been reading through this site for a couple of years now and I must admit that I am somewhat more confused now than when I started out.
We are building a new house in Saskatchewan (Zone 7B I think) and we were hoping to add a bunch of extra insulation and at the same time employe some advanced air sealing techniques. However, we will be working with a builder who is familiar with building to code which in our province means 2×6 walls with cavity R20ish (not effective) insulation and an interior poly vapour / air barrier.
I understand that the code is poor and I want to build a better house than this code prescribes. However, I don’t want to get in to trouble with my builder trying to instruct him to do things that he is (a) not familiar with and (b) lacks knowledge with. I am worried that I will end up with a problem house.
Our priorities are air tight and as much insulation as feasibly possible. I have been leaning towards a 2×6 wall with some sort of batt or dense pack cavity insulation and 2 – 4 inches of continuous rigid foam on the exterior. My builder should be able to handle this as he has built homes with 1″ exterior foam in the past so I am hoping he can extrapolate to 2 layers of 2″ but I am concerned about all the detailing especially windows and doors. I am especially concerned about air sealing and moisture in the walls as my builder only know interior poly behind the drywall acting as both air and vapour barrier.
I hate the concept of using poly behind the drywall for many reasons. My main concern is that the poly is never tight given all the screws and penetrations that go through it. And if I wrap with exterior foam (2 layers) thereby limiting my wall’s ability to dry to the outside am I just opening myself up to problems given that moisture will get through from the inside but at the same time the interior poly will greatly reduce the walls ability to dry to the inside.
I guess my real question to the community is what would you do if you were in my shoes. Keep in mind that I am limited to choice of builder so getting a builder out to my site with knowledge of thick tight walls is probably not an option. Would it be safe to ask my builder to give me a 2×6 wall with R2x cavity insulation and 2 layers of 2″ rigid exterior foam knowing that he is probably not going to build the tightest walls?
Thanks for you interest and opinions.
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Replies
rtp130,
I agree that your best chance of success is to choose assemblies builders in your area are familiar with, and keeping that in mind this is what I'd suggest.
- There is a big difference between the detailing necessary when you go from 1" to deeper exterior foam. I would try and avoid that, choosing instead a wall assembly which had more depth, but could be detailed conventionally. Maybe 2"x8", double-stud or Mooney-wall framing. Then you can leave the doors and windows in the same relationship they usually are to the rest of the wall components.
- The big knock on interior poly is that it can create a wrong side vapour-barrier in a cooling climate, not that it isn't an effective air-barrier. I wouldn't worry about it from that perspective at all. Most builders experienced in standard Canadian construction techniques turn out pretty tight houses using it - and all the other trades are familiar with what needs to be done to achieve that.
- Consider strapping your house to provide a rain-screen cavity. It builds in a whole lot of resilience against potential moisture problems from both the exterior and inside.
Thanks for the comments Malcolm! I believe that keeping it simple is probably my best option. I also like the rain-screen concept and will be asking my builder to add one to the exterior.
Here is an easy way (not the only way, but easy to understand and to construct). First a little information so that we are all speaking the same language - climate zone 7 is a pretty cold climate - this means that a relatively high R-value is appropriate - with today's technology this means thick insulation - not too much of a problem in the roof, generally you just use raised heal trusses and blow in as much as desired. But, it is a problem in the walls - there are only two ways to get that R-value up - one is to use a high-tech solution such as vacuum panels or closed cell spray foam (both are really expensive and have some not well publicized downsides) - the other is to use thicker insulation (which means thicker exterior walls). Most people choose thicker walls.
A second problem in cold climates is moisture accumulation in walls, floor ceiling, etc. - this is usually a result of condensation out of the air moving through the insulation. If it occurs it leads to all sorts of bad results (mold, rotting wood, and loss of heat through the wet or damp insulation). The solution I'm about to provide eliminates the moisture problem, provides a high R rating, is easy to construct (relatively easy - all house construction is hard) and keeps the total wall thickness down a little.
First let's deal with the exterior walls - I recommend double wall construction with a layer of foil faced Polyisocyanurate insulation sandwiched between the two walls. the easy way to construct this is to construct the exterior of the double walls first as if it were to be a single wall house and then add the insulation between the studs followed by attaching the polyiso to the interior face of the studs. Last build the interior of the double walls up against the polyiso and insulate between its studs. This means that the interior of the double stud wall is constructed just like any other interior wall in the house - simple! Now, what's the moisture and R-value situation? First it is necessary to caulk the seams and caulk and/or tape the polyiso to both the floor and ceiling before construction of that interior wall, this will provide a continuous air/moisture barrier. Note that the polyiso in the walls is the air/moisture barrier (the best that money can buy!) for the walls. How about the R rating for the walls? In your climate an R rating above R-40 is desirable - this means a 2x6 exterior of the double wall and a 2x4 interior side with mineral wool (Roxul to most canadians) between the studs in both sides. The final R-value will exceed 44 with 3 inches of polyiso or about 50 with 4". This means that even with only 3" the temperature of the interior foil surface of the polyiso will only be around 6C with 3" or 8C with 4" of polyiso at -37 C outside (assuming you keep the inside temp. around 21C). Looking at A dew point calculator (there are dozens of them on the web) we find that you aren't even close to the dew point in either case inside the wall so there will be no condensation in this wall at any outside temperature you're ever likely to reach. The advantages to this type of wall are many - it can and will dry to both the exterior and the interior if any moisture ever does get into the mineral wool portions, the air/moisture barrier (the polyiso) is protected from physical damage fire and insects by the double walls - it's a little thinner than a wall insulated with all fluffy insulation and the moisture barrier is in the right place ( approx. 1/4 of the thermal resistance from the warm side into the wall). I haven't said much about floor or ceiling construction, that's really up to you and depends on the style of house desired and the nature of your building site. Good Luck!
Hi Forrest, I’ve seen you propose this wall a few times and I really like it. I thought it may be easier though to instead of building another interior 2x4 wall, you could just use 2x4 strapping to hold the polyiso in place. This would give you an inch and a half service cavity. Obviously foam thickness would need to be greater for the same r value, but this seems easier to construct. Any thoughts?
Inch and a half service cavity seems like too tight a space to me....can't fit electrical boxes etc. in that very easily. Uses almost the same amount of wood as a wall but provides less than half as much space for insulation. And if it's vertical strapping you have no top or bottom plate to nail the finish drywall (or whatever) to. If it's horizontal you have a slightly unconventional arrangement of studs which can also complicate mounting stuff like cabinets and electrical boxes.
Kyle_R: Thoughts - it requires a little less wood, a little more insulation, and probably a little less labor - easy to construct yes - less or more expensive??? Might complicate the installation of electric boxes. Oh yeah ! One other thing if you are going to fasten these it's going to require really long nails or screws to go through 5 or 6 inches of foam plus 1.5-inch lumber and 2 inches or so into the exterior wall lumber - these will be expensive!
Thanks Forrest and Jim who is also recommending a very similar wall. I do like the concept of a double wall as it should be somewhat familiar to a standard builder and the deviations should be fairly straight forward to figure out. I also like the idea of protecting the vapour/air barrier between the 2 walls.
The double wall still allows thermal bridging at the rim joist and the top plate unless I am missing something. How would you deal with this thermal bridging?
Our house design is fairly straight forward, standard gable roof with no vaults just flat ceiling and a conceret (maybe ICF) basement.
I am wondering how to tie the polyiso wall vapour/air barrier into the ceiling? Also what are thoughts on strapping the ceiling out to keep the electrical inside of the vapour barrier?
rtp_130: Let's talk about tying the polyiso to the ceiling first. I don't have any brilliant new ideas here, but I can tell you what I have done in the past and a few things I have seen other people do. I have seen polyethylene sheet fastened to the bottom of the roof trusses and then 2x4 strapping attached (perpendicular to the roof trusses) which is then covered with sheet rock leaving a 1.5-inch gap between the polyethylene and the sheet rock (this is fairly common in the New England area in the U.S.). The gap is used as a wiring chase, the polyethylene can then be taped to the moisture barrier in the walls This is a rather expensive solution. I have also seen the polyethylene sheet covered with sheet rock without any strapping (much less trouble and expense) but not as moisture resistant - and I have seen one person install plywood rather than sheet rock and put several coats of paint on, claiming that the paint is sufficient as a moisture barrier. In all cases the moisture barrier in the walls must be taped to the barrier in the ceiling to be truly effective. The last time I built one, I used the second of these and installed the ceiling sheet rock before constructing the interior walls (including the interior stud wall of the double wall assembly) so the polyiso in the walls was taped to the polyethylene and the interior stud wall fastened to roof trusses through the ceiling sheet rock.
The polyiso in the walls butts up against the bottom of the roof trusses and is continuous down to the floor -so I don't see any thermal bridging at the top plate. The R-value at the top plate would be the combined value of the wood for the double wall (R-10) plus the value of the polyiso (approx. 5.6 times the thickness in inches) - (for 3 or 4 inch polyiso the totals = 26.8 or 32.4), but the top plate is only 1.5 inches (I know it requires a double top plate on the exterior stud wall but it's a single on the inside stud wall) so you have 1.5" strip all the way around the house that has an R-value somewhat lower than the rest of the double stud wall. This doesn't amount to much in heat loss.
The last house I built had a floating slab floor system, I didn't have a rim joist. SO once again, no new and brilliant solutions. I can only recommend the solutions commonly proposed on this site such as "cut and cobble" or spray foam for rim joists Once Again - GOOD LUCK!
I live in Whitehorse, Yukon (Climate Zone 7b). It seems like a lot of people up here are building a double stud wall with mid-wall foam, more or less as follows (outside to inside): siding, strapping for a rainscreen cavity, building wrap, sheathing, 2x6 stud wall with batt insulation, 1.5 "or 2" faced EPS such as Halo Interra, 2x4 stud wall with batt insulation, drywall. The advantage of this system is that it is conventional on the inside and outside for finishing purposes. The Halo Interra is a continuous insulation layer and continuous air/vapor barrier when taped. You have approx 2/3 of your insulation outside the vapor barrier, which is about right for this climate to prevent condensation on the warm side of the vapor barrier in winter. Your wall can dry to the inside or outside. You can run all your wiring through the 2x4 wall without having to worry about penetrations in the vapor barrier. Also no worries about puncturing the vapor barrier with misplaced drywall screws, etc. The main thing for a builder is figuring out the detailing around windows and doors. Aside from that it's just two conventional stud walls. I looked very seriously at this type of wall when building a new house in Ontario (Climate Zone 6b) but our builder there was more comfortable with a conventional double-stud wall filled with dense-packed blown fiberglass.
I guess I wrote that at the same time as Forrest Stanley was writing his reply. Exactly the same wall....Polyiso versus eps is the only difference!
Jim,
It sounds a lot like the walls on his house build Stephen Sheehy blogged about here on GBA a few years ago, except instead of foam, the mid-wall air/vapour barrier was a membrane. Do you see much advantage to using foam?
Malcolm,
I'm not a builder but I see the main problem with a membrane is trying to install it between the two stud walls. Ideally it would go on the exterior side of the interior 2x4 wall to ensure sufficient insulation (2/3) on the exterior side of the vapor barrier. That seems like a tricky thing to install compared to just slapping up a bunch of foam boards and taping them. If you are using batt insulation with a gap between the two walls you also have to figure out how to put continuous insulation in that gap. I did help a friend install Halo Interra in this fashion and it was pretty easy.
Jim
Jim,
I'm warming the idea more and more. It's an interesting alternative to using interior poly as an air-barrier/ vapour-retarder. My reservations are mainly around:
- Whether it makes sense to add the complexity and materials associated with double-studs and rigid foam to one project.
- That compared to the ease of situating the air-barrier at the continuous exterior sheathing, air-sealing still has to deal with the tricky rim-joist situations.
- The disruption to the established sequence of constuction - meaning the framing and insulating crews have to come and go.
- And that unlike double-wall assemblies where the outer one is continuous, you lose that additional insulation at those vulnerable points.
Zone 7 would put you somewhere around 11000 heating degree days (6000 HDD C).
Base wall (R5 rigid+2x6 with batts) is around R24.
So over a heating season 1000sqft of wall would loose:
1000 sqf * 11000 HDD*24H/R24=110 Therms.
Assuming you are heating with a cold climate heat pump (this can work even in zone 7), you would probably average a COP of around 2.5 over the heating season. Your electricity consumption to provide the heat for that wall than would be
110 Therms *29.3/2.5=1289kWh
Assuming your electricity is around $0.15 that is $200/year for each 1000sqft of wall.
Bumping up the assembly to around R40 drops that to around $120, so not all that much. Better walls will generally provide more comfort and tend to be more robust, none the less you have to really sharpen your build pencil to make sure the build costs still has some reasonable ROI.
I would be hesitant to build the double stud walls with the inner foam suggested above as it requires special work flow outside of normal build as both insulation and framing guys have to come twice to the site.
My suggestion would be to keep it simple.
Bumping up the studs to 2x8 24" oc with R30 batts doesn't add all that much while keeping it a standard build.
Going for 1.5" of permeable polyiso instead of R5 rigid is also a small change and you can still nail most siding up directly through the foam.
You can also spec one of the smart vapor retarders instead of poly as your warm side vapor barrier to allow for a bit of extra drying.
Anything outside of this (that is double stud walls, thick exterior rigid) would probably add too much cost for a builder that is unfamiliar with advanced building techniques.
The important item is to have the builder tape the seams of your exterior sheathing and use a proper sill gasket or quality tape for the wall to foundation connection. Air sealing is probably the single biggest energy saving measure you can make in the build that can be done for minimal extra cost.
Hello Akos, thank you for the excellent details / calculations. I agree that we should be thinking about this project from an ROI mindset. Although it is hard to put a price on comfort, as you point out a more well insulated house should be a more comfortable house.
I have asked my builder about 2x8 walls and this option does seem to keep it simple. Although it would add some cost to building the wall as they tell me it is more difficult to work with the 8" lumber. Also I thought polyiso was NOT permeable, am I misunderstanding something here? I would think that a 2x8 wall with and 1.5" of rigid would be the way to go if you want to let the wall dry to the outside?
I asked my builder about taping (sealing) the exterior sheathing and he said that they purposely leave a gap between all exterior sheets to allow for drying. It seems to me that the building codes / practices of the past are making it difficult for average folks to build an energy efficient house. My biggest concern is asking my builder to do something, like tape and seal the sheathing, that he is not comfortable with educated on. I feel that if I were to have troubles down the road with the wall that it would be hard to get him to warranty the build.
Anyway thanks for the comments you have giving me more to think about and talk with my builder about :)
I think it’s common to think an air barrier will prevent drying when it’s really the vapor permeability that matters most.
Your building / assembly can both breath and be air tight.
For foam/insulation permeability, check out this article from Building Science Corporation: https://www.buildingscience.com/documents/guides-and-manuals/gm-guide-insulating-sheathing/view
Unfaced poly iso is a Class III, which is semi permeable.
Getting a solid air barrier is your #1 goal. This is something that you can't skimp on in zone 7. A well insulated but leaky house can easily loose more energy than a well sealed R14 house.
Your builder is right about gaps between sheathing. This is needed to allow for movement with humidity. There is however absolutely no reason why you can't cover those gaps in the sheathing with tape. Since at best the tape covers couple of percent of the OSB area, you are barely affect drying ability of the sheathing.
Taping is simple. A quality tape (Zip, 3m 8067 or Tescon Vana) doesn't cost all that much, takes maybe a couple of hours to tape up the seams of a side of the house while the walls are on the ground. This really is the lowest cost and best performing air barrier.
If your builder is dead set against taping, price out flashing the stud bays with about 2" of closed cell spray foam. The cc SPF combined with about R5 exterior rigid would provide sufficient condensation control for R20 batts allowing you to skip the warm side vapor barrier. Your inspector might still require it though, in which case adding it in doesn't really change anything.
As for permeable polyiso you can look at IKO enerAir. Fiber faced roofing polyiso (used for commercial flat roofs) tends to be cheaper and is also permeable. Roofing polyiso can't be left exposed, so the WRB needs to go over the foam not over the sheathing. Since this not a standard build flow for your builder, EnerAir is probably a better fit. If the polysio is too expensive you can also go with unfaced EPS or GPS, little bit less R value but still permeable.
About the only hard part with building with 2x8s is they don't come pre-cut. There is really very little difference in building with it. 2x8 24" OC is also about the same weight as 2x6 16" OC, so standing up the walls is not any harder. Lumber cost is slightly more but close enough to call it the same.
I tend to prefer a 2x6 or 2x8 with continuous exterior insulation over a double stud as well.
TStud is a more advanced product that could improve cavity insulation effectiveness as well.