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Community and Q&A

Best practice for insulation in exterior wall of bathroom

GBA Editor | Posted in Energy Efficiency and Durability on

I have a project here in Columbus Ohio where we are adding a bathroom in the attic space. So we are gutting everything to the stud. The existing house has aluminum siding over the old cedar clap boards, over 1x planks. Due to budget constraints, we are using fiberglass batt insulation in the walls. 2/3rds of the exterior wall will be covered with a cement backer board, the Schluter-KERDI waterproofing membrane and ceramic tile. The remainder of the wall will be Dens Armor Plus paperless drywall.

The intent was the kraft facing on the batt insulation was going to be our vapor retarder. However the installers removed the kraft facing in the bathroom exterior walls only. Stating the building inspector will not approve the kraft facing in the bathroom. Can anyone explain the inspectors logic and what would be a best practice for insulating an exterior bathroom wall in an existing home?

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  1. Riversong | | #1

    You're going to have to ask the inspector, or better yet the installers who are justifying their removal of the kraft paper by what they claim the inspector will not allow.

    The only possible logic in it is that kraft paper facing, like the paper on the drywall, is vulnerable to mold.

    But, since you asked about best practice, it's NOT fiberglass batts, which are the worst and most problematic insulation on the market - vulnerable to convection, condensation, mold, insects, rodents, and fire.

  2. Josh | | #2

    Yeah, I know. I don't recommend fiberglass batts and tried to convince the home owner otherwise. I had an alternate in for cellulose, but the budget did not allow for it.

    Would anyone recommend putting up a plastic vapor retarder in replace of the kraft face? Otherwise we are relaying on our latex paint to act as our only vapor retarder.

  3. Riversong | | #3

    As long as moisture is controlled with a bath exhaust fan, there should be no need for any more of a vapor retarder than latex vr primer. I haven't used poly except under slabs for 20 years.

  4. Danny Kelly | | #4

    If you run your AC during any part of the year, I would not install a poly vapor barrier on the inside of a wall assembly. I would also suggest putting the bath fan on a motion senser and timer or a humidistat to be sure it runs long enough.

  5. Riversong | | #5

    I've found dehumidistats to be unreliable, and timers work only if they're used. Energy Federation came up with a better idea: a single-pole switch that controls both light and fan and has a user programmed delay for the fan circuit. It can be used just for the fan so that the light can be switched separately.

  6. GBA Editor
    Martin Holladay | | #6

    The building inspector is ignorant. It could even be argued that the building inspector has it exactly backwards; if anything, the bathroom is likely to be the most humid room in the house, and is therefore more in need of a vapor retarder than any other room.

    However, as I have written before, vapor diffusion is rarely a cause for concern, so there's no need to fight this battle. You've just encountered one more building inspector displaying an incomplete understanding of building science.

  7. Josh | | #7

    It seams pretty typical in this area with the building inspectors. Anyways, I am going to recommend that the homeowner add a layer of 15# felt paper. This should help with the vapor diffusion and won't encourage mold growth. Plus it could be used as a drainage plane in the case of moisture getting behind the backer board.

  8. adkjac | | #8

    Bathrooms that I have demoed many times are water damaged everywhere... floors near toilets, walls behind tubs and tile. This is all before Schluter Kerdi and Ditra waterproofing products. Using waterproof products is IMO the best way to go now. There are tile backer boards now that are more waterproof than cement board but in my area no one yet stocks them. Another idea for those who want to build homes that may last for many lifetimes is to use water safe framing in water prone areas. Blue wood comes to mind along with wood made for decks such as pressure treated wood.

  9. GBA Editor
    Martin Holladay | | #9

    You're talking about water damage (damage due to liquid water penetrating through wall tile or floor tile). The purpose of kraft facing on insulation has nothing to do with liquid water penetration; it is there to address diffusion.

  10. Riversong | | #10

    Josh, #15 felt is vapor open (which is why it makes an excellent WRB), and will do little to limit vapor diffusion.

    And Martin, don't jump on the building inspector. It was the installers who CLAIMED that the inspector wouldn't allow kraft facing. That may be true or it may be a justification for what they did. We don't know that.

  11. Josh | | #11

    Actually we do. The home owner talked to the building inspector and verified they REQUIRE the facing to be removed in the bathroom only. So needless to say, I am not going to fight this battle. I am developing some seminars for homeowners, so this building inspector will get an invitation.

    And I Robert, I realize that the felt paper does little in terms of vapor diffusion. However something is better than nothing at all. But the final decision is we are not going to install the felt paper.

  12. Riversong | | #12


    The inspector may not need educating in this case. His reluctance to allow paper-faced batts may have the same basis as your choice to use paper-less gypsum board: paper is a good medium for mold growth.

    However, the installers were wrong in removing the paper facing from 14½" wide batts rather than using unfaced 15" friction fit batts.

    And your contention that "something is better than nothing" is illogical if that "something" is far more vapor permeable (6 perms dry, 31 perms wet) than the code-required 1 perm vapor retarder that I assume the inspector will require in the bathroom.

  13. GBA Editor
    Martin Holladay | | #13

    Robert and Josh,
    If anyone cared to challenge the building inspector, which I think would be pointless, this would be the question to ask: "Which code provision calls for the removal of kraft facing from insulation installed in bathroom walls?"

    There is no such code provision, and therefore the building inspector lacks the authority to insist on the kraft paper's removal.

  14. Riversong | | #14


    According to your own code expert (

    "The building official is allowed the latitude of defining the meaning of each provision of the code according to its intent and purpose...Section R104.1 [of the IRC] gives the building official most of the necessary authorization to interpret the code and make decisions in the field so long as he/she conforms to the "intent and purpose" of the code..."

  15. wjrobinson | | #15

    Martin... I hear you and thanks for chiming in on my post. Real bathrooms built and rebuilt.... is what I do. Tile placed as this poster states with Schluter is fantastic and is the vapor barrier where it is. Dense Armor for the rest of the enclosure painted with 3 coats of latex, primer plus two top coats will do a grea perfect job here in Lake George.

    As I state... and I am not forgetting your vapor chat... Bathroom water issues that I fix always entail the lower areas of wetted tile, all wetted joints in work (corners, intersection of tub and everything) and the floor below toilets and inside cabinets with the usual leaks there.

    Done enough bathroom rebuilds to know what I am talking about in 30 years.

  16. Roger | | #16

    "The building official is allowed the latitude of defining the meaning of each provision of the code according to its intent and purpose..."

    As is with any legal code, this is true only in reference to a specific building code. There has to be a code for an inspector to interpret, he can't just say anything he wants when it comes to "interpretation". In this case, there has to be a specific code regarding insulation in bathrooms, ideally with a reference to kraft-facing batts. If there is no code relating to this, an inspector cannot develop his own building practices within a community.

    There have been many cases where a contractor or a homeowner have won in court over what was deemed an arbitrary ruling by an inspector. Town administrations hate spending taxpayer money for such court cases, and they will do all they can to adjudicate before things get that far. If it is deemed that the inspector is being too arbitrary or subjective in his code rulings, towns will ask that he lighten up.

  17. Riversong | | #17

    There has to be a code for an inspector to interpret, he can't just say anything he wants when it comes to "interpretation".

    No? The Supreme Court is tasked with interpreting a very specific "code": the US Constitution. And yet that "strict constructionist" interpretation can lead to such absurdities as corporations are legal but immortal persons with more civil rights than flesh-and-blood persons.

    At least this inspector is on the right track: eliminating double vapor barriers and mold potential.

  18. tman | | #18

    Robert Riversong Is "RITE ON" When He Mentioned DOUBLE VAPOR BARRIERS
    DVB Are A Good Place To Trap Water That Will Never Dry Out

  19. GBA Editor
    Martin Holladay | | #19

    It sounds like you are claiming that when you install a Schluter-Kerdi waterproofing membrane (permeance = 0.75 perm), cement backerboard, and kraft facing (widely varying permeance, but let's call it 1 perm), there's a risk that water will be trapped between the two layers and the water will never dry out.

    So, I have several questions:
    1. Where is the water coming from? Is it liquid water passing through a defect in the Schluter-Kerdi membrane? (If so, the existence of the kraft facing is irrelevant -- you have a problem with or without the kraft.) Is it diffusing through the Schluter-Kerdi membrane? (Highly unlikely, but even if it did, it would easily dry to the exterior -- the kraft facing wouldn't inhibit the moisture diffusion much.)

    2. Why do you think that the kraft facing would inhibit drying to the exterior?

  20. Riversong | | #20

    Why do you think that the kraft facing would inhibit drying to the exterior?

    It's true that asphalt-coated kraft paper facing won't stop moisture diffusion dead in its tracks, like foil or poly, but its perm rating of 0.3-0.4 is a direct measurement of its ability to inhibit moisture diffusion.

    To suggest simplistically that some materials block water vapor and some don't is like saying that some insulations (bubble foil, e.g.) stop heat loss and others just slow it down. All materials inhibit water vapor diffusion, and its the rate of diffusion in the overall hygrothermal context (relative humidity and temperature over time, moisture vulnerability, moisture buffering capacity, as well as rate of moisture migration) that determines whether there could be a problem.

    It is an accepted rule of thumb in construction that double vapor diffusion barriers (or retarders) can be problematic and should be avoided.

    It would be helpful to know the inspector's rationale for his decision, but it seems to be a reasonable call.

  21. wjrobinson | | #21

    I am totally in the camp of one or less vapor barriers. Toughest areas to deal with number one is an indoor pool or hot tub. Second toughest is a daily use shower. Third is a room stuffed with plants that are watered with gallons of water per week. Maybe I skipped standard wet basement construction with no poly tight to concrete pour. Why do we insist on cellars where I live anyway when the ground is basically as wet as a swimming pool. We mostly can build dry basements here but when we are up against underground rivers of water... we should forget basements and build above such wet spots or not at all.

    Little off topic... but

    I say build it the way I posted, use automatic ventilation and all should be beautiful.

  22. wjrobinson | | #22

    Would be nice to be able to edit... Martin... please add edit capability. Please?

    My post just above... yaps on about tough areas of huge moisture loads... like pool enclosures... I am no pool enclosure expert, not even close... have read about properly engineered pool rooms and absolutely 100% insist anyone who builds such ... have it engineered. A friend had his pool building redone... looked nice inside... well... while taking it apart the fiberglass batt insulation came out fully saturated in water. The inside of all of the structure was wet... too wet for mold... too wet for rot even... IMO that's why it didn't fall down prior to the renovation that came about from an attached area that had a fire. Personally I will never attempt to build such a wet enclosure without engineering.

    Good luck with the bath... all should be fine... and no one may ever know how well water was handled anyway till it is ripped apart again some distant day in the future.

  23. GBA Editor
    Martin Holladay | | #23

    The types of rooms you list -- rooms with swimming pools, hot tubs, or growing plants -- are precisely the types of rooms that may need a more robust (not a less robust) interior vapor retarder or vapor barrier. In a cold climate, interior polyethylene may be the best solution in such rooms (as long as the wall assembly is able to dry to the exterior and the siding is not a "reservoir cladding").

  24. Dan | | #24

    Would any of you recommend sealing the grout in addition to a continuous shower pan? Thanks.

  25. Riversong | | #25

    It's always worth sealing the grout, both to prevent water absorption and to keep the grout clean.

    I would recommend siloxane or silane rather than a silicone sealer.

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