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Q&A Spotlight

Bathroom Walls, Mold, Vapor Barriers, and Building Codes–Where’s the Love?

Is a building inspector justified in insisting the kraft paper facing be removed from fiberglass batt insulation because of mold potential, or will that actually cause moisture problems?

A building inspector has ordered the removal of the kraft paper facing on fiberglass batt insulation before it can be installed in the exterior walls of a bathroom. Will that increase the risk of moisture damage in the walls?
Image Credit: Fine Homebuilding magazine

Josh, a builder in Columbus, Ohio, has been hired to add a bathroom in the attic of an existing house. Although he had hoped to use cellulose insulation in exterior walls, the homeowner’s budget allowed fiberglass batts. Josh was counting on the kraft paper facing on the insulation to serve as a vapor retarder, but to his surprise the building inspector insists the paper be removed before the insulation is installed.

What gives? And will the inspector’s decision increase the risk of moisture problems in the bathroom, surely one of the most humid rooms in the house?

The Q&A discussion actually provided some support for the misunderstood inspector, as well as a look at materials and building techniques that will keep moisture problems at bay.

Will water vapor rot the walls?

Between tub and shower, sinks and toilet, a bathroom has a high potential for water damage, not only from leaks of liquid water but also from water vapor that can collect inside exterior walls and condense. Vapor retarders are there to slow the diffusion of moisture, so the building inspector is off base, argues senior editor Martin Holladay.

Even so, Holladay adds, vapor diffusion is rarely as big a problem as we might think. If the inspector’s ruling underscores an “incomplete understanding of building science,” it’s probably not going to have a serious practical effect. This is one battle that’s probably not worth fighting.

Robert Riversong isn’t so sure the inspector is wrong. Paper facings on conventional gypsum drywall provides food for mold. Riversong notes that the tiled portion of the wall will include a Schluter-Kerdi waterproofing membrane, so removing the kraft paper may have a double benefit: reducing the chances for trapped moisture by having one rather than two vapor retarders,…

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  1. homedesign | | #1

    Back Venting Shower Walls
    Thank you Scott,
    Another nice Blog/Article

    I live in a hot/mixed humid climate
    Most new homes here have brick veneer cladding
    Most of us know better than to use a vapor barrier on the interior.

    There has been some discussion about "back venting" cabinets and mirrors adjacent to exterior walls.
    How about Shower Walls adjacent to exterior walls?

  2. wjrobinson | | #2

    Or go another direction
    Carl Mezoff may be onto another build that works.

    Sheet foam under bath wall board.

    AC cools just the interior items and leaves inner wall space above dew point.

    I would like to see this discussed.

    Q&A topic?

  3. GBA Editor
    Martin Holladay | | #3

    Interior or exterior foam?
    Either exterior or interior rigid foam will solve the problem of inward solar vapor drive.

    The topic has been debated for decades, but I think it has been fairly definitively resolved in favor of exterior foam, which has the following advantages:

    1. It doesn't take up interior square footage.
    2. It addresses thermal bridging at rim joists.
    3. It easily addresses partition intersections.

    With all these advantages, it's hard to make a strong argument in favor of interior rigid foam.

  4. Brian | | #4

    nobody can be more strict than the code...period
    An inspector or addendum to any code cannot be more strict than the current code being's taht plain and simple. There must a vapor retarder of some sort where walls are insulated in bathroom installations. But I do somewhat agree that kraft-faced insulation is not a good insulation for around showers and the like. Mold will feast off of any kind of paper if it gets wet and stays wet. Furthermore, I have demolished a bathroom where greenboard drywall with moldicide has been completely covered in mold which is not supposed to happen. That being said...there is still absolutly no way to be 110% perfect in moisture proofing a bathroom and preventing mold. Even if you use plastic vaper retarder over insulation and then drywall over it you can still trap moisture between the layers because that is where condensation starts when the temp on the interior side is greater than the temp in the wall cavity (opposite sides of that vapor retarder). I feel that there should be an airspace of atleast 1 inch between these two assemblies to allow airflow the dry out and/or remove any moisture that may become trapped. The code needs further review in my opinion and more expertise and experience needs to be applied in the thought process for future codes and code enforcement.

  5. RR Whitbeck | | #5

    Bathroom Mold Prevention
    From reading the comments, so far, the one that seems to have the most validity, to me, and possibly the greater ability in achieving the goal, of preventing mold, is Brian's - adding an air space between the interior and the exterior walls, of moist rooms. It may reduce interior floor space but it would be a better choice than living with unknown mold than remodeling to remove mold if it is suspected to have developed. Also, Mr. John Brooks' suggestion to back venting the walls. It would provide air to dissipate moisture. Make note, I do not advocate using these solutions alone. They need to be combined with the Schluter and DensArmor products to prevent moisture penetration. I, also, like the use of a programmable fan delay, for it's energy conservation and the human error ability. Remember KISS [Keep It Simple Stupid].

    One question for Mr. Martin Holladay, GBA Advisor, what are you referring to as interior and exterior rigid foam? Are you referring to the extruded rigid foam insulation's used in place of batt insulation? If so, then which would you recommend using - ESP, XPS or POLYISOCYANURATE? What do you think of using spray on foam insulation? Closed or open cell?

  6. RR Whitbeck | | #6

    Question on Interior or Exterior Foam?
    For Mr. Martin Holladay, GBA Advisor, what are you referring to as interior and exterior rigid foam? Are you referring to the extruded rigid foam insulation's used in place of batt insulation? If so, then which would you recommend using - ESP, XPS or POLYISOCYANURATE? What do you think of using spray on foam insulation? Closed or open cell?

  7. GBA Editor
    Martin Holladay | | #7

    Response to R.R. Whitbeck
    Q. What are you referring to as interior and exterior rigid foam? Are you referring to the extruded rigid foam insulations used in place of batt insulation?

    A. Yes -- either in place of or in addition to whatever insulation is installed between the studs.

    Q. Which would you recommend using - EPS, XPS or polyisocyanurate?

    A, Any of the three will work. Green builders currently advocate using polyiso, which has the lowest global warming potential of the three types of rigid foam.

    Q. What do you think of using spray-on foam insulation?

    A. It is a useful product that makes sense for certain applications.

    Q. Closed or open cell?

    A. Closed-cell foam has a higher R-value per inch. Closed-cell foam is a more effective vapor retarder, and therefore installations using closed-cell foam experience fewer problems with diffusion and condensation. In every application, closed-cell foam is the better product. However, closed-cell foam costs more.

  8. RR Whitbeck | | #8

    Rigid or Spray Insulation - use a Vapor Barrier or not?
    For Mr. Holladay, GBA Advisor. I am in favor of eliminating kraft-faced batt insulation or cellulose, at exterior wet walls, and substituting rigid or spray insulation for them. I am unsure of the need to use a Vapor Barrier [VB] then.

    Q. What is your recommendation, regarding a VB. Should a one be used when using either a rigid foam insulation or a closed cell spray foam insulation?

  9. GBA Editor
    Martin Holladay | | #9

    Response to RR Whitbeck
    RR Whitbeck,
    Four of the products you mention -- EPS, XPS, polyiso, and closed-cell spray foam -- are vapor retarders, to varying degrees. When using any of these products, no further vapor-retarding strategy is necessary.

    Open-cell spray foam, however, is fairly vapor permeable. In a cold climate, an interior vapor retarder is recommended when using open-cell spray foam. Since open-cell spray foam is vapor permeable, all of the usual precautions need to be taken to ensure than inward solar vapor drive does not cause problems.

  10. Constitutional, Chris | | #10

    Dictatorial Mandates of the Building Inspectors
    Where do these self aggrandized Beareaucrats, obtain the "Liscense to Dictate Dogma of How to Build your home?

    A Recent attempt to "DUPE" Americans by; FRAUDs of, Ownerous terms related to the "BUILDING PERMIT" / Liscense:: Permission to do that which is colored as ILLEGAL, have been THRUST on unsuspecting Trusting, Americans by: stealthily devised, tacit operations, of what are but COLORED as LAW, but in fact, are applied by ADMINISTRATIVE acts or Statutes. To EXTORT , PRIVATE PROPERTY( the Fruits of Your Labors) under a Guise of Public Safety, to apply SPECIOUS FINES ans TAXES to protect you from yourself.

    When you CONTRACT, with the Entity, by asking for a PERMIT-LISCENCE=Permission, to improve your "PRIVATE LAND" by "VOLUNTEERING" ! (YOU ASKED FOR THE PERMIT) from the COUNTY or Entity, you may NOT have NOTICED the small , NON threatening, allusion to the OWNEROUS TERMS of the CONTRACT / Building permit, Which CONSCRIPT you, to OPERATING and abiding by the International Building CODE, which EMPOWERS the AGENTS of the BUILDING Department to: "ENTER YOUR PRIVATE PROPERTY" and CONDUCT a "WARRANTLESS SEARCH" and or SEIZURE of your "CASTLE" {for YOUR safety} by your voluntary surrender of your IVth4th) & Vth(5th) amendment protections of the Constitution as provided by the Inspired by GOD, Founders of this Great NATION.

    Your, Elected Officials, guided by: Masonic Lawyers, Judges ,and TERRORCRATIC Enforcement Agents or Actors, under: mere COLORS of what are but ,TERMED LAW, have by SECRET Oaths and obligations to their Caanannite Crafts, have CONSPIRED against you to DESTROY your FREEDOM, Liberty, Rights of Private Property and The COUNTRY to install but" ONE MORE PLANK" of the "COMMUNIST MANIFESTO" : gain control of all Private Property! Do Your Homework and DO NOT elect LAWYERS, or career beaureacrats! If the building inspector told you to: JUMP of a Cliff! will YOU? May God Bless and PROTECT America from the Satanic influence of the Cursed of Caannaan, AHMEN

  11. Randell | | #11

    Get a grip, Chris
    Chris, I recommend looking up the nearest chapter of Paranoids Anonymous.

    Local governments most certainly have the legal authority to regulate building practices, and to enforce such regulations whether you agree with them or not. If you don't agree with them, either a) move, b) deal with getting fined and other enforcement actions, or c) use the political process to get the rules and regulations changed.

  12. Harry | | #12

    back to the orginal question...Was the inspector justified?
    Having worked for a number of years as both a code official and a contractor (not at the same time) I would like to point out something. The "code" is the minimum standard that you as a contractor can build to, but it is the maximum standard the code official can enforce or cause you to build to.

    It wasn't mentioned, but what type of gypsum wall board was being used? I would suspect that a water resistant wall board (green board) was to be used and the International codes (as well as the old UBC) do not allow this to be installed over a vapor retarder (2006 IBC & IRC and 2009 IBC Editions) or a Class I or II vapor retarder (2009 IRC) in tub or shower compartments. How do you get vapor retarder and still use the green board? We have allowed a PVA primer rated at one dry cup perm rating to be applied on the face of the water resistant wall board as being code equivalent.

    If the code issue was the water resistant wall board being installed over a vapor retarder, then Josh should have just asked the inspector for a clarification. No matter how busy we are, I always encourage my inspectors to take the time to explain the code requirement if there is a question. This approach saves everyone time in the long run. The contractor will now know how to apply the code requirement in different situations which helps him pass the inspections and that reduces the inspector 's workload; a win-win situation for everyone. So if you have a question regarding one of the inspector's corrections; ask for a clarification, but ask in somewhat polite manner for the best results.

  13. Anonymous | | #13

    VB in Bathrooms
    Is there an issue with interior walls having Fiberglass between studs and VB's. I have never specified a VB on interior walls with FG Insul. but the moisture would move through the wall and would it not be trapped by the FG acting like a sponge?

  14. TC Feick | | #14

    Dens Armor Plus may be the wrong choice.
    The inspector was right in his application of the code, (IRC R702.3.8), as long as he considered Dens Armor Plus conforming to ASTM C 1178. GP does not say this product complies anywhere in the technical literature, and further, this from their technical manual: DensArmor Plus panels can be used as a tile backerboard in dry areas or areas with limited moisture contact such as areas
    adjacent to sinks and toilets, bathroom ceilings and areas above tile in residential shower areas. In wet areas where 2006 IBC
    and IRC codes have been adopted, Georgia-Pacifi c Gypsum recommends the use of DensShield® Tile Backer, which incorporates
    a built-in moisture barrier in wet areas. The WPM over DensArmor Plus likely is a great system, but GP will not stand behind any failure. The larger question is well detailed; how well do codes reflect sound building science? Not always well, but don't kill the messenger. The inspector was right on.

  15. TC Feick | | #15

    Clarification for DensArmor Plus
    Testing in accordance of ASTM C 1396 and ASTM C 1658 are what GP reference for DensArmor Plus. The former puts it squarely under the jurisdiction of IRC '09 R702.3.8.

  16. Carl Mezoff | | #16

    The Code and VBs in bathroom
    As noted by TC Feick, above, Section R702.4.2, and R702.4.3 (section numbers in my '03 IRC) prohibit application of "Water-resistant gypsum backing board" "over a vapor retarder in in a shower or bathtub compartment." So, the official was probably within his competence to request removal of the Kraft paper. (Of course, the code does not explain itself and say what theory this prohibition is based on.)

    However, I agree with Martin Holiday that at the end of the day, it makes little difference whether the Kraft is there or not, because the Kraft paper has such a high permeability that it scarcely qualifies as a retarder. The ASHRAE handbook lists the Kraft paper permeability as about 30 times greater than that of 6 mil polyethylene (wet-cup test). Plus, to a tiny water vapor molecule, the fish mouths, tears, and staple holes normally seen in the Kraft paper look like the grand canyon and offer ample "end runs" around any limited resistance to vapor flow the undamaged sheet may have.

  17. no red tag please | | #17

    Building inspectors
    Most inspectors in my region are clueless. Most have never worked as a Contractor, or tradesman. When they show up and tell me something completely wrong, I am nice so I do not get red tagged then call the head guy and he clears it up. I feel inspectors are so insecure they need to find something wrong, even though they are wrong. It's a power trip.They should hire retired builders, not kids who did two years at community college.

  18. Anonymous | | #18

    Moisture and vb's simultaneously
    Can I use a vapor b. (plastic 4mil) over the insulation stapled to the 2x4's, then the cement board (Durock) and then apply a water proof membrane over the cb, then thin ct cemnt, then tile, then grout.
    Do you recommend this process? My question is, can I use a vapor b, and a moisture membrane simultaneously on the same wall? Thank you in advance for your help.
    Rick R.

  19. GBA Editor
    Martin Holladay | | #19

    Response to Rick R.
    Rick R.,
    We need much more information to answer your question.

    1. What is your climate? Where are you located?

    2. What type of insulation are you using between your 2x4s?

    3. What kind of exterior sheathing are you using? Is there any exterior rigid foam?

    1. EliH | | #21

      Hi Martin, I hope you're doing well! I was wondering if you or anyone else might be able to help answer a couple questions that I posted below. I thought maybe the notifications aren't on anymore, so I thought posting directly on your last post might notify you. If this isn't allowed, please disregard. Thanks!

  20. EliH | | #20

    I know this is a very old thread, but if anyone is still paying attention, I could use some advice. I'm building a tiny house on a trailer in Atlanta, GA and am getting close to installing the interior walls of the bathroom. My wall insulation is 1" continuous exterior foam and then kraft faced fiberglass batts between the 2x4 framing which I used based on my understanding of articles and comments from this site. I have a glue-up tub wall surround that I was planning on installing over hardie backer. I'm using pine tongue and groove interior paneling in the rest of the house, but I'm wondering if I should use something that can resist vapor more in the bathroom. I could do drywall, but was thinking I'd run into cracking issues whenever the house is moved. I could put plastic sheeting up before the tongue and groove, but that seems just as likely to create moisture problems as solve them. I'm also curious what to do with the bathroom ceiling because directly above it is the loft floor which will hold a mattress. There are slats built into the floor of the loft for under the mattress and a 3.5" gap between the slats and the ceiling of the bathroom. I'm considering some thin, painted, finish-grade plywood for the ceiling and could use it for the walls as well. I do have active ventilation in the form of a bathroom fan that will be installed in the shower area and a timer to leave it on for 30-60 minutes after each shower. In summary, my questions are as follows:
    1. Will hardie backer work well behind the glue up shower and if so, do I need anything between it and the studs?
    2. What wall material/vapor barrier would you recommend for the rest of the bathroom?
    3. What ceiling material/vapor barrier would you recommend?

    Thank you for your help,

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