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Community and Q&A

Insulating Below-Grade Walls

SebfromEd | Posted in Energy Efficiency and Durability on


I know there is a lot of information on this website, and elsewhere, but for some reason I cannot get a definite answer on this. I live in Edmonton, Alberta Canada and I am in the process of finishing my basement, and decided to remove all the insulation from the rim joists and frost wall (it was fibreglass with polyethylene vapor barrier). The rim joist were so poorly sealed, and the poly had rips and tears everywhere. My joists are TJI’s and are very difficult to seal.

I install Halo Interra,, it is a XPS infused with graphite, and comes with a built in vapor barrier. It is currently installed throughout my walls, taped or foamed in between boards. I have also installed it in the rim joists and tied it into the walls. I have decided to replace my existing steel stud framing with wood stud framing, and the existing fibreglass with 6″ mineral wool (R22). My question around all this is once this is all completed, do I need a vapor barrier, vapor retarder, or just air tight drywall? I am concerned about putting a vapor barrier / retarder and having moisture trapped between the two barriers. I am also concerned that if I do not put the barrier in, I might get condensation build-up on the foam board, either on the walls or the rim joists or both.

Any advice would be greatly appreciated. I was looking at Siga Majrex as a smart vapor retarder, but want to make sure that it is required before investing all that money. Also, is it worth changing the steel stud framing to wood framing? I read that steel stud framing can cause a lot of thermal bridging reducing your R-value drastically. Also, I was considering installed 1″ of foam board below the wood bottom plate (if I go this route), to reduce thermal bridging.

Thank you,


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  1. user-2310254 | | #1

    Hi Sebastian,

    How thick is your XPS? Martin's basement article ( recommends R-15.

  2. SebfromEd | | #2


    My foam board is 2” thick, R-10. Since I went thinner could I not use mineral wool to increase my R value?

  3. user-2310254 | | #3

    Hi Sebastian,

    I think there are steps you can take to make the assembly "safer," but I'd like to see what one of GBA's expert members might advise.

  4. GBA Editor
    Kiley Jacques | | #4

    Hi Sebastian,

    Regarding your question about replacing steel framing with wood, you might find this article of interest.

  5. Expert Member
    Dana Dorsett | | #5

    Averaging a bit more than 5000HDD (celcius) Edmonton's climate is the warm edge of IECC zone 7:

    Per the IRC, to skip the interior vapor retarder with R10 foam on the above grade portion of the wall would require dropping back to 2x4/R15, which will mitigate against moisture accumulation at the upper part of the foam where the foundation is at or near grade. See R702.7.1:

    If the wall is fully below grade, with absolutely NO above grade exposure on the exterior, or if the above-grade portion of the foundation is insulated with another R5+ on the exterior latex paint on air tight gypsum would be sufficient even with the 2x6/R22.

    Alternatively there would be very little risk simply using "vapor barrier" latex primer, which at about a 0.5 US perm would meet the NBC code definition of "vapour barrier", but still has sufficient drying capacity toward the interior that it's would not become a moisture trap (the way 6mil poly WOULD be a moisture trap). There are also much cheaper not quite as vapor tight smart vapor retarders such as 2-mil nylon (Certainteed MemBrain), which also meets NBC code as a vapour barrier in a dry-cup test, but quickly becomes much more open when the entrained air in the rock wool becomes high enough to support mold growth (much more open than half-perm latex.)

    Ultimately it's a matter of what your code inspectors would allow. Starting with a continuous R10 foam on the exterior, dropping back to 2x4/R15 from the 2x6/R22 isn't really much of a thermal performance hit but makes it much easier to argue that it's moisture safe even with only 5 perm (standard latex) paint as the interior side vapor retarder.

    An inch of rigid foam under the bottom plates are fine, and would serve as a capillary break as well as a thermal break. Summertime outdoor dew points in Edmonton are still above the slab temperatures most of the time, so it doesn't really need the thermal break to keep mold from forming in summer, but some sort of capillary break to keep ground moisture from wicking is always a good idea. Inexpensive sill gaskets are usually good enough, but it doesn't take a lot of foam board to go one better.

    1. user-2310254 | | #6

      Hi Dana,

      Welcome back! It looks like Sebastion has already installed the R-10 XPS on the interior. Does that change your advice?

    2. SebfromEd | | #7


      I have already install the 2” foam board. I talked with the building safety officer and they don’t even inspect the insulation here. It is really up to me what to do. He said he didn’t think I needed a Vapor barrier. He recommends I ensure there is a 1” air gap between the foam board and mineral wool, and use drywall as the Vapor retarder to allow any moisture to come back into the house. Also, my basement is about 6 feet below grade and 3 feet above grade (if you include the1 foot ceiling space for the joists).

    3. SebfromEd | | #8

      Hi Dana,

      I was just wondering if you could provide some more background how using more insulation causes condensation issues? I would prefer to use as much insulation as possible as we do live in quite a cold climate, and I would like my basement to be as warm as possible.

      Would it be acceptable to use R22 mineral wool with Certainteed MemBrain to reduce my change of having condensation issues?

      Thank you,


      1. Expert Member
        Dana Dorsett | | #9

        >"I was just wondering if you could provide some more background how using more insulation causes condensation issues? "

        The dew point of comfortable & human-health 20C, 30% - 35% relative humidity air is roughly +2C - +4C. Mineral wool is air permeable, so parts of the wall (the upper part, above grade) where the interface where the mineral wool and foam can average ess than 4C risk accumulating liquid moisture on the surface of the foam. The greater the R-value of the mineral wool, the colder the average temp of the surface of the foam. Conversely with a lower-R mineral wool the surface is incrementally warmer. (With NO interior mineral wool the surface of the foam stays pretty close to the average room temperature, well above the dew point temperature of the room air.)

        With a vapor retarder the rate of accumulation can be controlled, but a true vapor barrier would risk trapping moisture- it becomes a seasonal balancing act. The vapor retardency of standard interior latex paint runs 3-5 perms, which would be relatively moisture safe with R15 rock wool, R10 foam at your wintertime outdoor temperature averages, but is potentially too vapor open to control moisture accumulation to acceptable levels with R22 fiber on the interior.

        With 2 mil nylon (MemBrain) the vapor retardency of vapor retarder is less than 1 perm when the entrained air in the mineral wool is dry, but becomes >5 perms at humidity levels high enough to support mold. When the surface of the foam is cold and condensing out the moisture of entrained air the nylon vapor retarder dramatically reduces the rate of moisture diffusion from the room out toward the foam. When the foam warms up in spring the moisture on the foam is released into the entrained air raising the vapor permeance of the nylon, allowing the assembly to dry toward the interior at a rate much higher than it was accumulating moisture during colder weather. Other "smart" vapor retarders operate in a similar fashion, albeit with different baseline permeance numbers. MemBrain is one of the more vapor open versions, but is cheap compared to some others, and good enough for your R22 fiber + R10 foam assembly in your climate. But it would not really be needed for an R15 +R10 stackup.

  6. SebfromEd | | #10

    Hi Dana,

    Thank you so much for the information.

    I still do not know what is the best path forward. The problem with using a smart vapor barrier would be my rim joists. I have TJI joists and they are very difficult to seal with vapor barrier, so I would want to stay away from that if possible (see pictures above). If I reduce my batt R value from R22 to R15, will it make the walls cold? Also for the rim joists, I reused the R22 fibreglass insulation that was installed already. Would this need to also be replace with R15?

    Sorry, I am just looking for the easiest option right now.

    Thank you,


  7. Expert Member
    RICHARD EVANS | | #11


    You wrote: "If I reduce my batt R value from R22 to R15, will it make the walls cold"

    By lowering the R value on the warm side of your air barrier, you warm up the inside of that wall so that dewpoint is no longer an issue. This is definitely the safest option now.

    Also, when you factor in framing fractions, it's really a comparison between R 16 vs R 12.5. An extra R-3.5 isn't going to make much difference, especially in a basement with low Delta T. I would guess it would add up to around 300-500 btus/hr... But this depends on a lot.

    Also, basement don't suffer from the kind of thermally driven vapor boyancy that higher walls and roofs experience. And, because they are lower to the ground, the outer surfaces may be a bit warmer compared to higher parts of the house to due decreased radiation.

    Even if you did use the thicker insulation, you might be okay. The lower Delta Ts of a basement wall might change the ratios some. Your above ground walls might be in Zone 7, but your basement walls might effectively be in Zone 4.

    In my basement, I have a good 16" of dense packed fiberglass between my floor joists. (See photo) I too have 2" of XPS but on the exterior of my Rim Board. There is no vapor retarder covering the fiberglass. Just insulweb. I've been meaning to down there and cover it with an air barrier but haven't got around to it.

    Out of curiosity, last April, I stuck my hand in there to feel if the Rim Board was wet. Physics tells us it should have been soaked. But it was as dry as a bone.

    I am in Zone 6 but at elevation (8400 HDD). We've hit -30F since I've lived here. Our basement is uninhabited. We also have a heat pump hot water heater that dehumidifies the space some. The basement is within our thermal envelope but is unheated. Also, the fiberglass is packed into there at 2# density. At 16", this is a lot of material in the way so it may be serving as a kind of air barrier. Together, this may explain why my Rim Board isn't rotting.

  8. JohnJones171 | | #12

    Hi Seb-I'll offer my opinion as I am in a similar situation to you, but I am not an expert-do not act on my opinion without the advice of a senior member with more experience from GBA.

    For your rim joist-if easy is what you want and you want to play it safe, use the R15 fluffy instead of R22-this should avoid condensation at your foam/fluffy barrier as Dana suggests. I personally would use Roxul if you need to replace the fiberglass; I tend to avoid that in basements wherever I can, above or below grade. I believe the Roxul is actually R14.

    Alternatively, but slightly more work, is to add another 2 inches of your Halo GPS if your rim joist depth allows (assuming you have some leftover kicking around). Your fiber should be fine after that as your rigid is easily thick enough that condensation should not form, even given the weird weather that Edmonton sometimes gets. As an fyi, Halo Interra I believe is graphite infused EPS, not XPS but doesn't matter for your assembly as the film on the GPS acts as a vapor barrier.

    The potential kink to your whole plan is your inspector. Canadian code is notoriously old school-they may still insist on a vapor barrier, despite current building science. In that case, Membrain should suffice.

    Again, hopefully a senior member will either confirm or rebuke what I say, as we are in similar situation. Any advice given here is helping me as well.

  9. SebfromEd | | #13


    I have decided to go with the 5.5" thick mineral wool insulation, and install a new smart vapor retarder (Membrain). I have a question about running the vapor barrier up to the rim joists. I am thinking about using 9' or 10' high vapor barrier to contain the walls and the rim joists in one sheet, instead of cutting separate sheets for each rim joist (which is way harder and more difficult to seal). Is there any issues with the area of conditioned space from the upper floor possible causing condensation issues in the rim joists? I will paste an image to illustrate more clearly what I am talking about.

    I have attached the picture.

    Thanks for all your feedback!


  10. SebfromEd | | #14


    Could any one give me feedback on my question above? Is it acceptable to vapour barrier from the bottom plate to the top of the floor joist all in one shot?

    I submitted a sketch. Is there risk of the air from upstairs producing condensation in the rim joist?

    Thank you,


    1. Expert Member
      Dana Dorsett | | #15

      It's perfectly acceptable to place the MemBrain as drawn, but probably pretty difficult to make it air tight where it comes up between joists. Do the best you can!

    2. Expert Member
      BILL WICHERS | | #16

      In the past I've used urethane caulk like glue and pressed the membrain into a bead of caulk to make a seal. I then use a staple gun with the staple placed through the membrain on the caulk bead to hold things together until the caulk cures. It's a bit messy, but it works. I use a 3M flashing tape to reinforce cut corners to keep the membrain from tearing as I install it.

      I highly recommend using a pneumatic staple gun if you do this, it will make things MUCH easier.


  11. SebfromEd | | #17

    I think I would prefer to use tape instead of caulking to seal. I have a pneumatic staple gun so that helps!

    So just to confirm there is no risk of condensation in that space? Won’t conditioned air from upstairs fill that void?

    1. creativedestruction | | #18

      No such thing as "no risk". What others are saying above are good approaches. Focus on airtightness and keep your basement DRY. If a bath is in the basement plan, of course put in a good exhaust fan and use it religiously. Vapor-laden air rises, so condensation would come from basement moisture sources.

      Less mineral wool is safer from a moisture perspective -- more insulation means less energy exchange and thus less drying potential. The Membrain means less drying still. It can work but seal it as well as you can.

  12. SebfromEd | | #19

    If I install 2x4 R-15 instead of the 2x6 R22 mineral wool, will it make a difference in temperature and comfort in the basement? I would like to keep the basement comfortable and of course dry. I am still not certain if I should use a vapor barrier to avoid trapping moisture behind the walls.

    1. creativedestruction | | #20

      It would not make any difference in comfort, no. The difference in heat loss for R15 vs R22 mineral wool on top of your foam is maybe a half-dozen more furnace cycles per heating season (i didn't do a calc, don't hold me to that ;). To me that's a reasonable trade for a safer wall that has better drying potential in zone 7.

      The foam is your vapor control, so long as it stays warmer than dew point. Refer to Dana's points on this.

  13. johngfc | | #21

    Have others have had Rick's experience ( #10):
    "Out of curiosity, last April, I stuck my hand in there to feel if the Rim Board was wet. Physics tells us it should have been soaked. But it was as dry as a bone."

    Our builder says he has routinely insulated basements on the inside with 6" fiberglass batts, no foam in or outside, with no problems. This is CZ5B and 6B, about 12" of rain per year, another 12" precip as snow, with building sites 5,000' to 8,000' elevation. RH is very low summer and winter, soils are usually dry to moist; a bit wetter from April to June. I'm wondering if these conditions allow those of us in the west and SW to be far less concerned about moisture in basements. I've lived here for 40 years, and I suspect few, if any, basements I'm familiar with were built to current standards. In this climate, I haven't seen the moisture problems that are common in the humid east or NW. I grew up in Maryland in a house with a damp, leaky, and musty basement, and it's just really different in the arid west.

    Bottom line: Should I trust my builder - with substantial cost savings - or should I insist on external foam or Roxul, at a much increased cost?


  14. SebfromEd | | #22

    Hi All,

    I was talking with a friend who is a carpenter, and he had some concerns with the foam board being underneath the bottom plate, stating that he is not sure how strong it would be. I think putting the foam board underneath the bottom plate would definitely help with thermal bridging (plus I could use untreated lumber, and it is cheaper), but I want to confirm with other peoples experiences how it turned out. I was thinking to use 2" thick EPS (Graphite Infused EPS), Halo Subterra (, which has a compressive strength of 16 psi. I would use at least 5" long nails to get through the bottom plate, foam board and into concrete slab.

    What are people's experience with this? Also, could the nails cause thermal bridging to the slab? I have decided to go with 2x6 mineral wool insulation and a vapor retarder (comments above said I could use 2x6 insulation with a vapor retarder).

    I am trying to get the best thermal performance, air tightness and resistance to moisture issues / mold I possible can in a retrofit.

    Thank you,


  15. SebfromEd | | #23

    Does anyone have any input please?

    Thanks in advance.

    1. creativedestruction | | #25

      Are you planning to insulate the floor slab? If not, there's no value in trying to create a thermal 'break' under your furring wall, and even if you do add foam on top of the floor slab, without a solid sill plate you could get drywall cracks from foam compression.

  16. user-2310254 | | #24


    You are not building load-bearing walls, correct?

  17. SebfromEd | | #26

    It is all non-structural, just supporting drywall electrical, plumbing, maybe some shelving and cabinets.

  18. SebfromEd | | #27

    Hi All,

    I left this conversation unsure what to do. I have a few options but I’m not sure what is the best option.

    1. 3.5” miner wool batts, no vapour barrier

    2. 3.5” mineral wool batts, membrain smart vapour barrier, from the slab to the top of the rim joists (see attached picture)

    3. 5.5” mineral wool batts, membrain smart vapour barrier, from the slab to the top of the rim joists (see attached picture)

    What is the best option? From what I’m understanding, the extra 2” of mineral wool insulation is not worth the risk of condensation. I am trying to go energy efficient, but if you guys are saying it won’t make much of a difference it might make sense to use 3.5” of insulation.

    Also I do have concern about the way the vapour barrier will have some interior air inside of it due to it being below the main floor. What is the best way to install vapour barrier (if I go this route) on the rim joists and how do I tie it into the vapour barrier on the studs?

    Additionally, if I run without vapour barrier do I need to complete the air tight drywall approach? Will I need to use gaskets? Any advice on a path forward is greatly appreciated.

    Thank you,


    1. creativedestruction | | #28

      You've already done such a nice job with the foam that what you do from here is icing on the cake, honestly. That is your air barrier, and an awesome thermal break. Fussing with the membrane is hardly worth it; it'll never be as airtight as what you have already. If you really want higher thermal resistance and super low risk of moisture accumulation, add another layer of foam, any kind you like, then furr it out and be done.

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