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How to insulate and finish basement when there are signs of efflorescence?

J2D2 | Posted in General Questions on


I am re-finishing/insulating my basement in a 1930 home in Zone 4 (Pacific NW). I am planning to do it as follows from foundation to interior which meets local building code: Basement Concrete Foundation Wall > 1”XPS Rigid Board(Pink Owens Corning) > 2×4 Studs against the XPS > ROXUL insulation in stud cavities > Sealed Poly Vapour Barrier > Drywall.

There is evidence of white efflorescence on the basement walls indicating that there may be or has been in the past moisture seepage. There have been no flooding events. Being a 1930 house, the foundation is not waterproofed.

When i pulled out some walls from a previous renovation from the 90’s there was poly vapour barrier against concrete foundation wall, 2×4 studs with some loose foam board, another layer of poly vapour barrier and then drywall. The foundation wall was damp but there was no mold or rot.

I have a few question i am hoping someone can help with:

1. Given the efflorescence is it required to clean off and seal with waterproofing on the interior (Xypex or Drylock)? Excavating and waterproofing exterior is not an option. Old framing is still up so i would need to work around that.

2. Or can i just install XPS board directly against the wall? Or could that lead to mold?

3. When installing XPS does it require a seal on top and bottom edges using foam can spray seal? (Joints will be taped) Should it be airtight or is it ok/better to let some air flow in behind XPS board to let wall dry out?

Any feedback greatly appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, when you assemble your new wall, you don't want to install any interior polyethylene. Interior polyethylene can trap moisture and make things worse. For more information on this issue, see How to Insulate a Basement Wall.

    Q. "Given the efflorescence is it required to clean off and seal with waterproofing on the interior (Xypex or Drylock)?"

    A. The efflorescence is evidence of evaporation. If there is no evidence of liquid water entry, you probably don't have to worry. That said, cleaning the wall and applying Xypex or Drylock can't hurt, and might be worth considering if you can afford it and if you want a belt-and-suspenders approach.

    Q. "Can i just install XPS board directly against the wall? Or could that lead to mold?"

    A. Green builders try to avoid the use of XPS, because XPS is manufactured with a blowing agent that has a high global warming potential, so either EPS or polyiso is a better choice. (For more information on this issue, see Choosing Rigid Foam.) Attaching rigid foam directly to the wall won't lead to mold. The only worry is possible water entry, but if you're fairly confident that water entry isn't an issue, or if you have decided to install Xypex or Drylock, you can proceed with the rigid foam.

    Q."When installing XPS, does it require a seal on the top and bottom edges using canned spray foam to seal? (Joints will be taped.)"

    A. Yes. Absolutely.

    Q. "Should it be airtight?"

    A. Yes.

    Q. "Or is it OK/better to let some air flow in behind XPS board to let wall dry out?"

    A. No.

  2. J2D2 | | #2

    Thank you Martin,

    Really appreciate you taking the time to respond. Unfortunately, i am required to use Poly to be to code so not sure what i can do there.

  3. GBA Editor
    Martin Holladay | | #3

    Interior polyethylene is not required by code in any location in the U.S.

    Who told you it is?

  4. Randy_Williams | | #4

    Hi Martin, I copy and pasted this directly from the Minnesota Department of Labor and Industries web site, 2015 Minnesota Building Code.

    CHAPTER 11 [RE]
    Provisions for energy, energy conservation, or references to the
    International Energy Conservation Code are deleted and replaced with
    Minnesota Rules, Chapters, 1322 and 1323, Minnesota Energy Code.

    R402.1.1.7Fiberglass batt interior insulation.
    Fiberglass batt insulation shall comply with the following:
    1. 1.The above-grade exposed foundation wall height shall not exceed 1.5 feet (457 mm).
    2. 2.The top and bottom plates shall be air sealed to the foundation wall surface and the basement floor.
    3. 3.A vapor retarder and air barrier shall be applied to the warm in winter side of the wall with a permeance not greater than 1.0 in accordance with ASTM E 96 procedure A and a permeance not less than 0.3 in accordance with ASTM E 96 procedure B meeting the following requirements:
    1. a.the vapor and air barrier shall be sealed to the framing with construction adhesive or equivalent at the top and bottom plates and where the adjacent wall is insulated;
    2. b.the vapor and air barrier shall be sealed around utility boxes and other penetrations; and
    3. c.all seams in the vapor and air barrier shall be overlapped at least 6 inches (152 mm) and sealed with compatible sealing tape or equivalent.

    I believe only poly will satisfy the requirement of .3 perm on the warm side of the wall. I don't agree with it, and I would like to get it changed.

    Jason-this code only applies to Minnesota, I would follow Martin's advice, but when in question, ask your building inspector, they are going to have the last say.

  5. GBA Editor
    Martin Holladay | | #5

    First of all, Jason is talking about a below-grade wall. The code provisions you quote only apply to above-grade walls. I know that Minnesota has strange requirements for basement wall insulation, but even in Minnesota, there is no requirement for polyethylene in a below-grade wall.

    Concerning the Minnesota code for above-grade walls: The code's reference to the ASTM E 96 procedure B is unusual. Almost all codes are based only on ASTM E 96 procedure A. (For more information on this topic, see All About Vapor Diffusion.) That said, note that the permeance specified for the ASTM E 96 procedure B in the Minnesota code is a minimum permeance, not a maximum permeance. That means that a material with a higher vapor permeance meets this requirement. It's not about poly.

    Concerning the other requirement -- the requirement for a vapor retarder with a maximum permeance of at least 1.0 according to the ASTM E 96 procedure A -- Vapor retarder paint meets this requirement. So does kraft facing on a fiberglass batt.

  6. GBA Editor
    Martin Holladay | | #6

    I will take this opportunity to indulge in a rant.

    In the past, I have often criticized the opaqueness of most building codes. They aren't written in clear English. This is a problem, because the intended readers -- builders and architects -- can't implement the codes if the meaning is unclear. Technical writing is not rocket science; editors exist, and a code authority could, in theory, hire an editor experienced at technical writing to help with code clarity. But code authorities don't do that.

    Here is a prime example of bad code writing. It is written in such a way -- mixing up maximum permeance requirements per one ASTM test with minimum permeance requirements per another ASTM test -- that educated readers of Green Building Advisor are left scratching our heads. Imagine the average builder confronted by these requirements. How many will understand?

  7. Randy_Williams | | #7

    Sorry, I read the interior poly comment as all exterior walls, not just below grade. No wonder Minnesota builders are so confused. I think a lot of the building officials are confused too. I've only blower door tested one house that did not use either poly or spray foam for the air barrier. The builders I've talked to about the issue said their inspector would only accept poly. If I understand your explanation correctly, we have an argument based on which test method we want to argue with. I'm a fan of the air tight drywall, but the vapor barrier paint would also be required if using the fluffy insulation.

  8. Randy_Williams | | #8

    Well said Martin!

  9. GBA Editor
    Martin Holladay | | #9

    I may be wrong, but the way I read the code, interior poly is illegal in Minnesota.

    I've never seen a wet-cup test result (a Procedure B test result) for poly, but the usual Procedure A test shows a permeance of 0.06 to 0.08 perm. If the results are similar for the Procedure B test -- and frankly, I can't think of any reason why they wouldn't be, because poly isn't a "smart" retarder with variable permeance -- then the vapor permeance of polyethylene is too low to meet the requirements of the Minnesota building code. Only a smart vapor retarder would comply.

  10. GBA Editor
    Martin Holladay | | #10

    There are lots of weird things about this code requirement. Clearly, the code forbids the use of OSB as an interior air barrier and vapor retarder, for two reasons: OSB has a dry cup permeance of about 2 or 3 perms, which exceeds the 1 perm maximum for the dry cup test; and it's impossible to overlap the seams of OSB 6 inches as required by the code.

    But when one reads a clumsy code like this, one can't help wondering: Did the code writers intend to forbid the use of OSB as an interior vapor retarder? Or is the code simply badly written?

  11. Jon_R | | #11

    Membrain meets the requirement of = .3 wet. So does vapor retarder paint on drywall, although it doesn't have "seams" that can be overlapped.

    Reducing evaporation from basement walls known to get wet (as yours have been) is a high risk for water on the floor. Mold too, although your air sealing might be good enough to prevent significant odor problems.

    Sounds like you can avoid some code issues by using only EPS (no rock wool).

  12. Expert Member
    Dana Dorsett | | #12

    Minnesota has long had some very odd-ball requirements for foundation insulation straying far from the IRC, and even some variance with specifications for above grade walls (2x6/R19 framed walls still code legal in southern MN.) But none of MN's code perversions show up in WA or OR building code.

    In B.C. codes are based on Canada's National Building Code (NBC), but the NBC definition of "vapour barrier" is < 60 ng/(Pa•s•m²), which is just over 1 US perm which is about 57 ng/(Pa•s•m²).

    With sufficiently low permeance foam board ( < 60 ng/(Pa•s•m²) ) of sufficent R-valve on the exterior of the fiber insulation there is no requirement for an interior side vapour barrier in the NBC either, though many inspectors need to be educated on that.

    In Canadian climates with 4999 HDD (celcius) and fewer as long as the "Minimum Ratio of Total Thermal Resistance Outboard of Material's Inner Surface to Total Thermal Resistance Inboard of Material's Inner Surface" is at least 0.20, you can skip the interior vapour barrier as long as the temperature at side of the foam facing the fiber insulation isn't prone to dwelling below the dew point of the interior are.

    With R5 foam exterior to R15 rock wool you have a ratio of R5/R15= 0.33. At a soil temperature of +5C (typical for western BC) and an interior temperature of 20C and an R20 stackup, the interior facing surface of the foam below grade will be about 8-9C, which is above the typical indoor dew points.

    For the above-grade portions you may have to make the seasonal-averages argument, or install a smart vapor retarder. Certainteed publishes this short-sheet spec to make it easier on Canadian code inspectors:

    This issue regarding the NBC comes up from time to time on this forum, eg:

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