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Community and Q&A

Best exterior wall design within standard 2×8 dimensions

Saskatchewanite | Posted in Energy Efficiency and Durability on

I have been reading several of the GBA posts and articles and I don’t seem to locate the answer to my question so I wanted to drop you a line by email.

I am located in Saskatchewan, Canada and currently have a house under construction that is well under way. That being said I won’t over complicate it with detail but the house is not a high efficiency design and I am not and don’t pretend to be knowledgeable on a lot of points that are covered on GBA.

Hhowever, this is what i have

Construction detail:

Southeast corner of Saskatchewan, Canada 15 miles north of North Dakota
2000 sq ft per floor – main floor and partial walk out (south side walk out)
ICF 8” insulated concrete basement walls
House is south oriented with high SGHC glazing on all windows, although there is very minimal north facing and only one west facing windows, the south glazing is approximately 50% of the south wall sq ft with overall glazing approx. 20% of exterior wall sq ft

Back to the question(s):
The situation I am in is i recently discovered the GBA wealth of knowledge and the basement is already constructed with main floor framing ready to commence.

Because of this I am limited to the 2×8” framing wall thickness for the main floor
The construction originally called for 2×6 walls, which was altered because I was wanting more insulation value I opted to go to 2×8 exterior walls.

However, now after much reading of your very informative articles I am posing the following question:

Would I be better to go with a double 2×4 stud with offset studs to help reduce thermal break?
They typically insulate the 2×8 with a R22 fiberglass batt, reading your posts would you recommend a cellulose blow in and what method would you recommend?

I have discussed with the contractor regarding cellulose and he says they use the fiberglass batts as the fiberglass has better air flow at the same R value.
Isn’t he wrong in his thinking here? His concern is vapor in the wall assembly without sufficient airflow within the wall.
From your site, I am lead to believe this is poor theory but reading comments and posts I am unable to determine what the right answer is, other than cellulose is better but why?


Current construction design if i don’t intervene
2×8 framing with 6mil poly air barrier, R22 fiberglass batts, 7/16 exterior OSB sheathing, Tyvek regular house wrap, James Hardie siding

My suggested alternative
2×4 double framing on 2×8 plate with offset 2×4 to eliminate thermal bridging, 6mil poly air barrier, cellulose blow in insulation (what method to use?), 7/16 exterior OSB sheathing, (or is there a better alternative?) exterior foam with taped joints, suggesting this product but what thickness to use for foam? , James Hardie siding

I was also trying to determine your installation recommendation for windows, I plan to use triple pane units with glazing spec’s like cardinal glass has on their LoE180 on 2 surfaces, with argon in both spaces. These will be European units and I am awaiting a few quote details to finalize what brand I am going with. Most builders in this area only know to install nailing flange windows that they pound up and spray foam the inside and done. I thought I saw references to an install guide Martin Holliday had online for installing high efficient windows into foam sheeted walls but can’t locate it….

So virtually my question is really two questions:

With my 2×8 wall thickness framing constraint in mind, what in your (GBA) opinion is my “best” wall construction method and with what materials?

Do you have a window installation best practice guide you can share or have the link?

Thank you!


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  1. iLikeDirt | | #1

    > he says they use the fiberglass batts as the fiberglass has better air flow at the same R value.

    That's rather worrying. Your contractor seems to believe that air blowing though the insulation is desirable. Considering that he's planned on installing an interior plastic air & vapor barrier, where does he think the air's going to come from?

    Probably the simplest approach in your situation is to frame with 2x4s with smaller fiberglass batts between them, and then four inches of exterior rigid foam boards over the OSB. The cost savings going from 2x8s to 2x4s and omitting the plastic air & vapor barrier may be significant enough that this alternative is barely more expensive. In your climate, you really should go with a ventilated rainscreen between the siding and the Tyvek. If your outer layer of rigid foam has a foil facer, you get a radiant barrier for free with that gap, which will help with cooling loads and reduce the likelihood that you'll need air conditioning.

  2. DickRussell | | #2

    Ok, Chad, I'll bite first, although others will have much to say on this.

    First, I wouldn't say you are constrained to a 2x8 wall plate because of the existing foundation. You could easily go with a double frame construction, with roof load bearing on the outer wall, and go R40 (12" cavity). Or you could stay with 2x6 studs and add the appropriate R value in exterior foam. If you do a double wall design, use plywood, not OSB on the exterior, and install the siding over 1/4" or 3/8" furring strips nailed through the housewrap into the studs, to provide a top and bottom vented air gap between siding and housewrap. Details of this are found easily elsewhere on this site or others.

    If your contractor thinks FG batts are better at providing air flow within the wall, fire him and go with someone who knows his stuff. OK, it's probably too late to switch, so you need to educate him, which means you first need to educate yourself by reading voraciously on this site and on There should be, ideally, absolutely no air movement within the wall cavity. That destroys the insulation value of the insulation. Any suggestion that "the house has to breathe" is absolutely wrong; it doesn't work as a strategy either for providing fresh air to the occupants or for avoiding moisture accumulation problems within the structure. The wall ought to be as absolutely tight against air infiltration as possible, and fresh air then must be provided by mechanical means for occupants and to keep interior humidity down, even in winter. This typically is done by a heat recovery ventilator, in heating climates.

    While in your location you probably can get away with having a sheet of polyethylene on the inside as an air barrier, in general it isn't a good idea. Of course, you do want to keep interior air from communicating with the inside of the wall cavity. With a well-detailed air barrier on the inside of a double wall, in the form of the Airtight Drywall Approach and a vapor retarder latex paint or by using a "smart vapor retarder," such as MemBrain (Certainteed) in place of the poly, you provide drying to the interior at times when you need it, while really limiting diffusion of interior humidity into the cavity in winter.

    I'll defer to others to suggest other things or add to what I've said. There are multiple ways to get to a highly insulated and very comfortable house, without breaking the bank. Remember, it isn't rocket science, but it is building science.

  3. Expert Member

    You have a builder and are part way through the construction, so it's probably worth coming up with a solution that doesn't markedly change either the types of materials or techniques your builder is familiar with.

    I think your suggestion of a double stud wall on a common 2"x8" plate makes sense. If you go this route you shouldn't add exterior foam, both because it precludes the wall drying to the outside, and because I doubt your contractor is familiar with the unconventional flashing and detailing it involves.

    More useful would be upgrading the sheathing from OSB to plywood, and strapping underneath the Hardi siding to allow for increased drying potential.

  4. user-4524083 | | #4

    Chad - I agree with both Nate's and Dick's assessments. 2X8's alone have too much wood watering down the insulation. Foam should never be used on a double or thick wall because the sheathing gets too cold with little of the home's heat escaping to keep the sheathing warm. So then you need even more foam to keep the sheathing warm. There is an article on this site by Martin that talks about how much foam would be needed. If you want to stay with the 8" then the foam ( or Roxul "boards") over a 2X4 or preferably a 2X6 wall filled with R-23 Roxul would work well. You'll want at least 1/2 the insulation R-value outside the sheathing, though if you go with Roxul outside the sheathing, it could be less than 1/2. I don't have these numbers easily accessible, but they would be easy to find on this site. The reason the Roxul outside could be less than 50% of the R-value is that it allows drying to the outside when/if the sheathing takes on moisture,whereas the foam will not allow any drying to the outside - so you have to make sure there is enough foam that the sheathing stays warm and condensation from moisture inside the heated space does not condense on the cold sheathing. I'm sorry you are in the middle of this project without having made these decisions earlier, but that's your situation, and there's still time to figure it all out. A nice thing about the outside insulation is that it does not need a foundation under it, but can be cantilevered off the stud wall, so these change can still be made. I wish you the best of luck. It's unfortunate, but most building contractor's don't know enough about insulation and moisture issues.

  5. user-4524083 | | #5

    Chad - Malcolm's comment was being written as mine was. He offers good advise. If you go that route, I'd strongly consider the Membrain air barrier inside ( as Dick suggested), and plywood with a rain screen as Malcolm suggests.It's going to be potentially difficult to educate your contractor. The double wall is the most straight forward. Good luck!

  6. Dana1 | | #6

    Best thermal performance "easier to build" wall in 7.25" (2x8 dimension) + 1/2" structural sheathing:

    *interior air

    *half-inch wallboard detailed as an air barrier

    *optional "smart" vapor retarder (NOT 6 mil polyethyene)

    *2x4 wall with R15 rock wool batts

    *half-inch wood sheathing

    *3" of Type -II (1.5lb per cubic foot nominal density) EPS (~R12.6), implemented as two 1.5" thick layers (seams taped), staggering the seams of the two layers

    *housewrap (This could be between the foam & sheathing if windows are mounted "innie", with the flashing lapped to the housewrap)

    *1x4 strapping through-screwed to the studs 24" o.c. with 5" pancake head timber screws.

    *standard siding materials

    The R12+ foam is more than sufficient for wintertime dew point control at the half-inch wood sheathing. If insulating the exterior side of the foundation, align the planes of the above-grade foam with the foundation foam. Otherwise, align the plane of the wood sheathing with the edge of the foundation, and extend the wall foam at least 8" below the top of the foundation. (If the foundation wasn't already poured you could build in a step to accommodate the foam.)

    Caulk the framing to the sheathing in every stud bay, and use an EPDM sill gasket, or lay a bead of expanding foam under the sill plate, between the sill-plate & foamy sill gasket for better air tightness.

    This will outperform a staggered double-stud wall, and is simpler to build. For more money you could use graphite loaded EPS for a ~7% or higher thermal performance on the foam layer. You could get a higher labeled-R out of polyiso or XPS, but polyiso would underperform spec in this stackup, and XPS loses R-value over time at it's blowing agent escapes (doing environmental damage in the process.) You could also use 3" rigid rock wool instead of EPS, but it's more expensive and requires more tweaking to get the walls flat.

    Your location is equivalent to US climate zone 7, which needs only R10 on the exterior of the sheathing to be able to skip the usual 6-mil polyethylene vapor barrier, per the IRC:

    With R12+ you'll have plenty of margin. If the code inspectors want to press the point, use 2-mil nylon as the vapor barrier (eg: Certainteed MemBrain ), which meets Canadian code definitions for vapor barrier when the proximate air is dry (as it would be in the winter), but becomes vapor open if humidity builds up in the cavities for any reason, which would allow it to dry at reasonable rates. It's a "smart" vapor retarder.

  7. Expert Member

    I think we need to distinguish between easy to build by someone familiar with both options, and a builder used to how he always does things in Saskatchewan. A double stud wall on common plates is not much to get your head around, but exterior foam and all the attendant details of flashing, window and door installation is a completely different animal. I would imagine the cost implications of going with foam are quite high too.

  8. nvman | | #8

    Chad, you probably know this but offset the 2x4's in each wall by 8 inches so as to reduce thermal bridging.

  9. GBA Editor
    Martin Holladay | | #9

    I'll echo what's been said.

    Your builder's comment -- "he says they use the fiberglass batts as the fiberglass has better air flow" -- is worrisome in the extreme. Educate, educate, educate -- and aim for airtightness.

    I tend to side with Nate G and Dana Dorsett -- if you can convince your builder to go for a 2x4 wall with exterior rigid foam, you'll get better performance than using 2x8 plates. If you are forced to use 2x8 plates, then I agree with Malcolm -- offset two rows of 2x4s to interrupt thermal bridging.

    You should read this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    Here is a link to the article about installing windows: Installing Windows In a Foam-Sheathed Wall.

  10. Dana1 | | #10

    If double-studwall on a 2x8 bottom plate, use 2x4s as the exterior structural studs, and use 2x3s for the non-structural interior side studwall wall. Finger jointed 2x3s are straighter than milled lumber, and can be ordered in non-structural grades to make it cheaper. Install a layer of 1" or 1.25" foil-faced polyisocyanurate on the interior side of the 2x4 studs before installing the 2x3s, and skip the interior side vapor barrier- the foil facer is the vapor barrier. Tape the seams with a temperature rated foil tape, and seal the top & bottom edges to the framing with can-foam. Use 2x4 top plates on the structural studwall, thermally broken by the polyiso separating it from the 2x3 studwall. Be sure to air seal the 2x8 bottom plate with a bead of can-foam or polyurethane construction adhesive if installing it over a subfloor.

    Insulate the 2x3 studwall with compressed R13 batts (unfaced or kraft faced), or compressed 3" sound-abatement batts (rock wool or fiberglass.) R15 rock wool or fiberglass goes in the exterior studs. To promote exterior side drying capacity, build in a 1/4"-3/4" rainscreen under the siding with either ripped down 1/4-1/2" OSB/ply ring shank nailed 12" o.c. at every stud or 1x furring 16-24" o.c. which to mount the siding.

  11. Saskatchewanite | | #11

    Reading all of the above, i am wondering now about for simplicity and a good wall

    inside air
    1/2" drywall
    1x4 strapping
    2 overlapped layers of 1.5" rigid foam, seams taped
    2x4 structural wall insulated with R15 rock wool
    7/16 OSB (tape the seams pointless with the foam on the inside taped??)
    strapping?? what for rainscreen? we get very little rain here and lots of sun, i don't think that i have ever heard of a wall rot issue and its usually wood or hardie planking directly to exterior sheathing, comments?
    hardie plank

    Top and bottom plate 2x4 with foam extending to the floor
    outie windows in 2x4 window bucks in the structural wall, interior drywall returns to window, foam extends past window buck for width of framing to reducing thermal bridging, drywall return sits flush on foam
    door bucks 2x8, so a little thermal bridging but solid and so trade off in favor of solid and simple


    Thanks for this wonderful resource and intelligent people willing to help!


  12. GBA Editor
    Martin Holladay | | #12

    Remodelers sometimes install rigid foam on the interior side of walls, but only because they have to. (If the siding is already installed, homeowners usually don't want to remove the siding to install exterior foam.)

    If I understand correctly, you haven't built your walls yet -- so nothing is preventing you from putting the rigid foam in the right location: the exterior side of the wall (where it belongs), not the interior side of the wall.

    There are many reasons why exterior rigid foam is better than interior rigid foam. Here are a few of them:

    1. Exterior rigid foam does a better job of insulating rim joists and partition intersections than interior rigid foam.

    2. Exterior walls have fewer penetrations than interior walls -- on interior walls, there are lots of electrical boxes that complicate the air sealing work.

    3. Exterior rigid foam keeps the exterior sheathing warm and dry, limiting the possibility of sheathing rot. If the rigid foam is on the inside, the sheathing will be cold in winter (and therefore damp).

  13. Saskatchewanite | | #13

    OK yes your points are excellent, i think we are back to this

    inside air
    1/2" drywall (will read more on ADA,likely will be attempted ADA with some imperfections....)
    2x4 structural wall insulated with R15 rock wool or similar
    7/16 OSB (tape the seams ?? with the two layers of taped foam??)
    2 overlapped layers of 2" rigid foil faced foam, seams taped
    1x4 strapping
    hardie plank

    Outie windows in 2x8 window bucks in the structural wall, interior drywall returns to window on foam
    door bucks 2x8, so a little thermal bridging but solid and so trade off in favor of solid and simple

    I think the compromise here would be doing 2x8 window bucks to keep it simple for my builder as they would be able to do their window install detail as normal and their siding and trim detail as normal, my water plane would be the face of the exterior foam at the rain screen

    Foam extending past entire rim joist and overlapping ICF structural basement walls, caulked/sealed to ICF foam (Whats the recommended amount to overlap the ICF for sealing purposes? anyone have a theory? For the buried part of the ICF the overlap will be about 2' for aesthetic reasons but what to do at the walkout? It's not a large sq ft so the cost likely wouldn't be to prohibitive to extend the foam all the way to the foundation, and could strap the ICF wall at window and door bucks that are already installed in the ICF to fur it the same amount with lumber, making those window wells extra deep, which I think a deep window ledge made by an outie is nice actually )


  14. Saskatchewanite | | #14

    ok the wall is as listed here:
    inside air
    1/2" drywall (going to be attempted ADA with some imperfections....)
    2x4 structural wall insulated with R15 rock wool or similar
    7/16 OSB
    2 overlapped layers of 2" rigid foil faced foam, seams taped
    1x4 strapping
    hardie plank

    the inspector wants 6mil poly between the drywall and the studs :( i am in Saskatchewan, can I convince him otherwise and meet "code", sorry i couldn't find the answer to this myself.

    should i tape the osb, what detail would you use at the top plate, the roof is engineered web trusses, i saw another discussion relating to doing OSB strips slightly wider than the top plate on the top of the top plate and then installing the trusses and OSB installed to the trusses and sealed to these strips? is that the best way to integrate my walls with my roof for air barrier?

    i believe in this construction the taped osb/rigid foam is my air barrier, with the tyvek assisting on the exterior of the foam

    Note that the foam is faced with polypropylene laminate and the manufacturer states: the foam permeance of 1.7 ng/Pa-s-m2 and therefore, with taped joints, no additional vapor barrier is required (A vapor barrier must have a vapor permeance less than 60 ng/Pa-s-m2). Does that assist with the code question listed above, I believe my inspector to be confusing the age old vapor barrier / air barrier terminology.

    The windows are going to be installed instead of 2x8 i mentioned above, in framed 3/4" plywood that is secured into the framed 2x4 opening and is the width to extend to the exterior of the foam, flush with the face of the 1x4 strapping

    appreciate the excellent help on this forum, my builder has become very interested and is right along with me in developing a better wall which has been great.

  15. GBA Editor
    Martin Holladay | | #15

    Q. "The inspector wants 6 mil poly between the drywall and the studs. (I am in Saskatchewan.) Can I convince him otherwise and meet code?"

    A. It's certainly worth a try. Many Canadian builders have successfully convinced their local inspectors that interior polyethylene makes no sense on walls with adequately thick layers of exterior rigid foam. Other builders have tried persuasion and failed, because some building inspectors are blind to building science. So the answer depends on your diplomatic skills, your familiarity with the technical issues, and the level of pigheadedness of your local inspector.

    If your diplomacy fails, you can install MemBrain (a "smart" vapor retarder) on the interior side of your walls instead of poly. This approach sometimes satisfies Canadian building inspectors. In this type of wall, the MemBrain is unnecessary but harmless.

  16. Dana1 | | #16

    Definitely tape or otherwise air-seal the OSB layer.

    The Tyvek only goes on the exterior of the foam if the windows are mounted "outie", with the glass roughly co-planar with the siding. If the glass is co-planar with the OSB, use the crinkled type Tyvek, installed between the foam and OSB, with the window flashing properly lapped to the Tyvek.

    In your stackup list foil faced foam is indicated, but in the rest of the post polypropylene facers are indicated. Which is it?

    Independent of facer type, 6 mil poly would make the assembly less resilient to bulk water intrusions, ESPECIALLY if you don't have sufficeint R-value on the exterior for dew point control (and I believe you're marginal there.)

    What type of foam is it? To be consistent with the IRC, you would need a minimum of R10 for dew point control at the sheathing. You are in a zone 7 type of climate:

    If the exterior foam is rated less than R10, the MemBrain or other smart vapor retarder is indeed necessary, even though you'd probably be able to get by without it if you used cellulose instead of rock wool in the cavities.

  17. GBA Editor
    Martin Holladay | | #17

    Q. "I saw another discussion relating to doing OSB strips slightly wider than the top plate on the top of the top plate and then installing the trusses and OSB installed to the trusses and sealed to these strips. Is that the best way to integrate my walls with my roof for air barrier?"

    A. It's a good way. Remember that you need raised-heel trusses (and remember that your truss company may not realize how high your heels need to be unless you tell them the thickness of your insulation and the depth of your ventilation channel). Remember that your OSB needs to be the same thickness as the drywall installed on your ceiling.

  18. GBA Editor
    Martin Holladay | | #18

    You wrote, "I believe in this construction the taped OSB/rigid foam is my air barrier, with the Tyvek assisting on the exterior of the foam."

    You've almost got it right, but not quite. If you tape the OSB, then the taped OSB is your air barrier. The rigid foam layer will assist in providing some additional airtightness, especially if the foam seams are taped.

    However, the Tyvek can't really be expected to contribute to the airtightness of this wall assembly. The Tyvek is a water-resistive barrier (WRB), not an air barrier. For more information on WRBs, see All About Water-Resistive Barriers.

  19. Expert Member
    MALCOLM TAYLOR | | #19

    To be fair your building inspector probably isn't trying to be obstructive, he is simply doing his job enforcing the building code. It requires "(a) vapour barrier ... installed sufficiently close to the warm side of the assembly to prevent condensation at design conditions".
    He is asking for poly because that has been, and still is, what is used as a vapour barrier in most Canadian houses. The code does however allow for a variety of other materials to be used, including sheet goods and coatings like paint on the drywall.
    As Martin said, with exterior foam you don't want any vapour barrier on the interior, and an argument could be made to the inspector that the foam "prevents condensation at design conditions" thus satisfying the code. If not you will probably need to include a variable perm membrane where the poly would have been.

  20. Expert Member
    Dana Dorsett | | #20

    If you need to sell the inspector on using variable permeance 2-mil nylon in lieu of 6-mil poly, the short sheet specifications can be found here:

    "(a) vapour barrier ... installed sufficiently close to the warm side of the assembly to prevent condensation at design conditions"

    It's not clear from the language of the code whether the condensation being prevented is condensation at the vapour barrier, or whether the intent is to prevent condensation at the sheathing. In the event that the vapour barrier is the facer on foam exterior to the sheathing it becomes a much more severe restriction, since it's at the outdoor design condition rather than the seasonal average condition, and would require substantially more exterior insulation. Putting the vapour barrier on the exterior side of the moisture susceptible sheathing increases risk of rot, etc. since wintertime moisture drives are much higher than exterior moisture drives over a Saskatchewan winter.

  21. Irishjake | | #21


    I would recommend a different approach, one which I'm using too. The REMOTE strategy. There is a GBA blog currently discussing it and even better, Rob Myers, (the blogster) writes eloquently about it, and is very insightful.
    The Cold Climate Housing Research Center (CCHRC) has extensive info available on it, (as they developed it). They have produced excellent manuals and videos about it and lots of other cold climate issues. The videos and manuals walk you through it's construction step-by-step, and benefits.
    Check out these links for that info:

    The CCHRC has found after extensive studies that you should have a ratio of insulation in your area that is 2/3 exterior to 1/3 interior or R-20 outside and R-10 inside, with the vapor barrier on the outside of the sheathing. Their website, along with the staff there (whom are also very helpful), may also help you with any issues you might have with your inspectors, seeing as how they are building in the same environment as you.

    Good luck!

  22. Irishjake | | #22


    A couple other items -

    Have you actually started putting up walls (above the first floor deck)?

    Have you actually tried nailing windows to the edge of plywood? In a drawing it seems adequate, but I think you'll find your builder will be very frustrated, and you as the homeowner will be extremely disappointed. 2x material is your best longterm material (yes you take a thermal break hit, but your windows are strong too). Plywood just splits out and doesn't give a large enough face for the window flange to attach too. If you go with window clips, the plywood may be sufficient though...

    I can tell you that I have a couple folks helping me who have NEVER lifted a hammer and find the process of the REMOTE system extremely straight forward and easy to wrap their heads around. Detailing flashing is made easy, and straight forward by liquid applied flashings and WRB systems. We are using Prosoco R-Guard on my house. Your builder sounds like he is eager to learn newer're luck for this.

    The current IRC/IBC requirements for insulation ratios are setting up homes to fail. It is on the right path, but by following the advice and ratios outlined by the CCHRC you will not have issues going forward. The ratio for exterior to interior insulation just isn't high enough in the IRC/IBC.

  23. user-4072556 | | #23

    I am hoping to build a new house in southeastern Manitoba. I had originally gotten a price to have my main floor exterior walls built using ICF's. After much reading I am very intrigued by some of these new ideas for high performance wall types. After reading your last two posts and taking your advice I went on the CCHRC website and did a bunch of reading. I have no problem believing that the more insulation on the exterior of the building the better but from all the reading on the website it seemed like all of their tests have been done in Alaska in areas that have HDD of 7000 or more. My understanding is that Winnipeg has HDD closer to 6000. How much should the HDD variable play(if at all) in the amount of exterior insulation? I am also wondering if there is a strong preference to EPS vs XPS, in layered exterior insulation, due to its higher permeability?

  24. Irishjake | | #24


    I'm not nearly as eloquent and versed as some here, so bare with me.

    ICF's make a great wall, both above and below. (You can always add additional layers of insulation too to one side or the other to increase the whole wall r-value...and it isn't hard either!)

    By knowing your heat loss, based on the makeup of your wall, you can "play" with different variables, to compare and contrast the cost, benefits, ROI, GWP, etc., etc., etc., of different designs. (I did this quite literally hundreds and hundreds of times!) You do this by knowing design temp, and HDD.

    HDD will help you to determine your heat loss. You should be able to find out a close approximation to what your heating degree days are - I used Wunderground, and my local forecasting stations. I used three different weather reporting stations nearby me to verify, and averaged all three. (They are all within a handful of miles to my house).

    My opinion, is that it is more economical to use EPS, and when it's reclaimed/recycled - that makes it even more affordable, and environmentally friendly. All my EPS was diverted from a densifier (where it gets crushed, squeezed and compressed into bundles and then sent to China for other uses).

    There are LOTS of "discussions" on this, but here is why I used it - Affordability, availability, ease of use, it is reclaimed, EPS has a higher perms than XPS, and holds water more easily, but also sheds any water it absorbs, far easier than XPS. So if there is a bulk water intrusion, and I've got to get rid of water, the EPS will let go of it. When you have 5.5"+ of rigid foam on the exterior of the house, the perm rating is very low anyways...

    If money didn't matter, I'd have used rockwool, but that would have been $40-50,000 more expensive, than the reclaimed EPS I bought! Yes, that was four zeros!

    Remember it's not just about being vapor impermeable or vapor permeable, it's knowing which you have and controlling it to dry one way or the other without impeding it too much to cause damage.

    Try to think about the whole wall as a system, not just as individual parts. By doing this, you can study how each part affects the whole system. One wrong part, in the wrong place though can screw a whole wall up.

    I took away from the long term studies that CCHRC did on walls is that vital to get your insulation ratio right, manage indoor RH, and do really great air sealing.

  25. GBA Editor
    Martin Holladay | | #25

    Q. "How much should the HDD variable play (if at all) in the amount of exterior insulation?"

    A. The colder the climate, the thicker the exterior rigid foam needs to be to keep the sheathing above the dew point in the winter. For more information on this issue, see these two articles:

    Calculating the Minimum Thickness of Rigid Foam Sheathing

    Combining Exterior Rigid Foam With Fluffy Insulation

    The two articles listed above assume that you will be installing some fluffy insulation between your studs. If you are going with the classic PERSIST approach -- putting all of the insulation on the exterior side of the sheathing, and leaving the stud bays empty -- then the amount of rigid foam you need is dictated by your local building code.

    For more information on PERSIST, see Getting Insulation Out of Your Walls and Ceilings.

    Q. "I am also wondering if there is a strong preference to EPS vs XPS, in layered exterior insulation, due to its higher permeability?"

    A. No. The vapor permeance of the foam layer is irrelevant, because this type of wall is designed to dry to the interior. The reason that EPS is preferred over XPS is because EPS is more environmentally friendly. XPS is manufactured with a blowing agent that has a high global warming potential.

  26. user-4072556 | | #26


    What is your opinion of using ICF for above grade residential walls. I realize it is not the cheapest option but there is something about the consistency of the water, air, vapour and thermal control layers that has always intrigued me. I would be using a Logix block which has 2 ¾" of EPS both inside and out. I realize the R-25 the company gives their product is probably too generous. I have tried to find information regarding this wall system and have not been able to find much. It does not seem very common for above grade application which makes me wonder if people know something that I don't regarding its performance in cold weather climates.

  27. GBA Editor
    Martin Holladay | | #27

    The main reasons that ICFs are rarely used for above-grade walls are: (1) ICF walls are expensive, and (2) most ICF walls have a low R-value.

    In the case of the ICF wall you cited, the R-value of the concrete is negligible, so the R-value of the wall is about R-22. There are cheaper ways to build an R-22 wall.

    Remodelers hate ICF walls.

    In their favor, ICF walls are relatively airtight and are an excellent choice in high-wind regions (areas that see frequent tornadoes or hurricanes).

    For more information on cost-effective walls, see How to Design a Wall.

  28. user-4072556 | | #28

    I do have one more wondering. With the IRC allowing R-10 of exterior insulation over a insulated 2x4 wall and R-15 exterior insulation over a insulated 2x6 wall both with no interior vapour barrier would I be building a risky wall in Winnipeg, Manitoba. My plan is to spend more money on my wall system then what is currently the standard here but I want to make sure that I am spending that money on the right wall. If in you opinion these exterior insulation values are inadequate what would you recommend for the area?

  29. GBA Editor
    Martin Holladay | | #29

    The IRC (which as far as I know does not apply in Canada) does not require the R-values for rigid foam that you quote ("R-10 of exterior insulation over a insulated 2x4 wall and R-15 exterior insulation over a insulated 2x6 wall") in all climate zones. These minimum R-values for rigid foam apply only to Climate Zones 7 and 8.

    Those minimum R-values do apply, however, for your location (Winnipeg).

    Q. "If I follow these recommendations, would I be building a risky wall in Winnipeg, Manitoba?"

    A. No. These recommendations aren't risky. That's why I made them. For more information, I urge you to read my article, Calculating the Minimum Thickness of Rigid Foam Sheathing.

    Here is a link to a climate zone map for Canada: Climate Zone Map Including Canada.

  30. Dana1 | | #30

    Note, a 2x4/R15 + R10c.i. wall runs about R20-R21 "whole-wall", which is comparable in steady-state performance to a minimalist ICF, but without the thermal mass benefit it will measurably (if only slightly) underperform the ICF, even in Winnepeg.

    A 2x6/R23 + R15 c.i. wall comes in at about R30 whole-wall and will outperform a minimalist ICF in almost all climates.

    If you run into a "code inspector problem" building without interior vapor barriers, install 2-mil nylon (eg Certainteed MemBrain) on the interior side rather than 4 or 6 mil polyethylene. It meets NBC code specifications for a vapour barrier when the proximate air is dry (as it is in winter), but becomes vapour-open when the humidity is high, and thus can't create a moisture trap.

    Building the foundation walls with ICF can often make sense from a time-is-money perspective. If the above grade walls are built with exterior foam approach, place the studwall plate such that the ICF foam and framed wall foam are co-planar, providing a continous thermal break at the foundation sill.

  31. user-4072556 | | #31

    I recently read the article, "Doubling Down—How Come Double Vapor Barriers Work?" by Joseph W. Lstiburek. In the article Joe says that we have been building walls with interior vapour barriers and exterior foam for years and have not seen major problems.

    Joe states, "The interior poly is never “perfect” but it is pretty, pretty darn good. Note that the exterior foam is never “perfect”. It is pretty darn good, but not as good as the interior poly. And even better, the foam has a thermal resistance. Not much moisture condenses and the stuff that condenses dries outward."

    I am wondering if anyone has thoughts on this.

  32. user-4072556 | | #32

    I also have a question regarding R-value of Rigid Insulation. Looking at the Halo brand (as mentioned by Chad earlier in this post) the specs say the R-value for 1.5" thickness is 6.8 at 75 degrees and 7.2 at 40 degrees. Can someone explain why the R-value changes with temperature.

  33. GBA Editor
    Martin Holladay | | #33

    Here is a link to an article that answers your question: A Bold Attempt to Slay R-Value.

    In that article, I wrote:

    "Building scientists have known for years that the rate of heat flow through insulation materials varies at different temperatures. Many researchers have tested insulation materials at a variety of mean temperatures, and their results have long been published. At this summer’s conference, Schumacher summarized a few well-known facts. “If you measure the R-value of an R-13 fiberglass batt, you’ll get different results at different outdoor temperatures,” said Schumacher. “If the outdoor temperature rises, the R-value goes down. If the outdoor temperature drops, the R-value rises. Why? Because as you move to a higher temperature, you get more radiation happening, and therefore a lower R-value. But at lower temperatures, there is less conduction, less convection, and less radiation — and therefore a higher R-value. On the other hand, polyiso doesn’t perform as well at low temperatures. That’s because the trapped blowing-agent gases start to condense at cold temperatures.”"

    Note that R-value has a legal definition in the U.S. -- so we can't really talk about "R-value at a mean temperature of 40 degrees F." R-value is always reported at a mean temperature of 75 degrees F.

  34. GBA Editor
    Martin Holladay | | #34

    Response to James Barkman (Comment #31):

    Q. "Joe says that we have been building walls with interior vapour barriers and exterior foam for years and have not seen major problems. ...I am wondering if anyone has thoughts on this."

    A. Joe's article is consistent with what I've been saying on GBA for at least the past 4 years. For example, see my answer to the question in this thread from this Q&A thread from January 2012.

  35. user-4072556 | | #35

    If using a 2x6 wall that is insulated with cellulose, has 6 mil polyethylene on the interior and 2" of styrofoam on the outside would you tape both the OSB and styrofoam joints or just one or neither? If I am reading Joe's article correct he says these various layers are being done pretty darn good but not perfect. Is taping either of these layers making them too close to perfect?

  36. user-1072251 | | #36

    Exterior foam + interior poly is a good recipe for a mold farm. Allow the wall to dry to the inside or the outside.

  37. user-4072556 | | #37

    I have come to a final design for my house in Winnipeg, Manitoba, Climate Zone 7. I am planning a 2x4 wall with dense pack cellulose insulation and 3.5" of Halo brand Exterra EPS on the exterior. The EPS at 3.5" is given a 15.8 R-value(Link Below). I have had a discussion with the local building inspector and they came back to me with the suggestion of putting poly between the OSB and first layer of Exterior Rigid insulation. The inspector does not like the permeability of the Halo Exterra EPS. In the footnotes of Table 2, on the website linked below, it says the permeability is based on 1" of the foam. I am wondering if the inspector over looked that. They are asking for my thoughts on their suggestion. I don't really like the idea but do not have a strong argument for why I do not like it besides it is not needed and would likely be difficult to seal in that location. Would there be possibility of vapour being trapped between the poly and the OSB? Would the poly in this location have a detrimental effect to the function of the wall? Seems if they insist on poly somewhere I should just plan to install MemBrain on the interior of the wall which has been suggested by Martin in previous posts regarding discussions with inspectors. Any help on this would be much appreciated.

  38. Saskatchewanite | | #38

    FYI you can order the Exterra un-perforated which eliminates the permability. Just wondering, if you are going to the work of the foam, what made you decide on 3.5"? just curious i guess...
    Our build is underway and the dobule overlapped 2" for a total of 4" is no issues with 6" Dekfast screws its a solid assembly. I would say for the extra 2.5R with the 4" of foam that your cost is going to be very minimal as all the other costs of installation labor and materials will be identical, unless i am missing something....

    also, be sure and plan for the foam at adjoining walls, for window openings and doors, we just about had an issue of not enough room for a door as the allowance for the foam and 1x4 was't made, the rough opening had to be redone and there was room, but would have been better if had planned ahead with that.

    also be sure and have your window plan 100% as you have to consider the rough opening being larger for lining the RO with plywood etc

    James, you can email me at [email protected] be glad to have a discussion with you being in our neck of the woods so to speak!

  39. GBA Editor
    Martin Holladay | | #39

    You are correct that MemBrain on the interior side of your dense-packed cellulose would be a better solution than the suggestion that you install polyethylene on the exterior side of your OSB.

    Your building inspector apparently doesn't understand that your exterior EPS will keep your OSB warm and dry, so there is no chance of condensation or moisture accumulation. You don't need a vapor barrier under these circumstances -- why is the building inspector worried about condensation if everything is warm?

    Perhaps you can appeal to your building inspector's supervisor, and suggest the use of MemBrain.

    If you really are painted into a corner, and end up installing polyethylene on the exterior side of the OSB, your assembly will probably work -- it just won't be as resilient as it would be without the poly. If poly is installed there, you end up with a wall that resembles the PERSIST approach. For more information on PERSIST, see Getting Insulation Out of Your Walls and Ceilings.

  40. user-4072556 | | #40

    Thank you both for the help. Much appreciated.

  41. Expert Member
    Dana Dorsett | | #41

    Take a look at section in the NBC. It's on p39 (PDF pagination)

    Section 1 of reads:


    1) Sheet and panel-type materials described in Article shall be installed

    a) on the warm face of the assembly (see also Article,

    b) at a location where the ratio between the total thermal resistance of all
    materials outboard of its innermost impermeable surface and the total
    thermal resistance of all materials inboard of that surface is not less than
    that required by Table, or

    c) outboard of an air space that is vented to the outdoors.


    The header for Table reads:

    "Minimum Ratio of Total Thermal Resistance Outboard of Material's Inner Surface to Total
    Thermal Resistance Inboard of Material's Inner Surface"

    That means if the exterior foam were the innermost impermeable surface it is ALLOWED to be vapour impermeable if the ratio minimums are met, not that it's REQUIRED to be impermeable.

    But it's also the wrong layer to be addressing. The innermost impermeable surface in your stackup is the OSB, which meets the NBC definition of vapour barrier when dry (but not when damp), not the foam.

    If you look at the table, a ratio of 0.75 is good for even 12,000 HDD climates. At 3.5" dense pack cellulose comes in at around R13, so with R15.8 EPS on the exterior of R13 cavity fill, you have a ratio greater than 1.0- it's BETTER just fine, even in a 12,000 HDD climate (colder than Whitehorse or Churchill).

    The greater the permeance of the exterior foam the greater the drying capacity toward the exterior. But if the foam is 1.78 perms max @ 1" means it's no more than 0.5 perms (and thus classified as a vapour barrier by the NBC) at 3.5". But that just doesn't matter- the OSB is less than 1 perm, and still the innermost impermeable layer, and it has PLENTY of exterior to interior R-ratio, which means the foam's permeance matters not a bit. If the inspector demands exterior side poly it'll still work, but it's the wrong thing to do.

    If this is all way over the head of the inspector, installing the MemBrain on the interior side would make it the inner-most impermeable layer, without impeding drying capacity toward the interior. It doesn't hurt, but it doesn't really help either.

  42. Alphadog69 | | #42

    I as well used this same design back in 2002.I had formed my own foundation using a form design we used on bridges.I used five eighths T&G plywood,2x4 uprights,2x4 whalers to form and reused it all in the construction I only had to purchase the 2x8 plates and 2x8 for cripples.I covered the plywood with plastic to keep concrete from adhering for reuse.You will be very happy as every dime in insulation pays off big time in both cold and heat down the road.I also strapped the ceiling with 2x4's as I had plenty,thus not breaking the vapour barrier with drywall screws.I highly recommend this construction.

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