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Thread: Do I put vapor barrier over bathroom insulation or not..??

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    DIY Senior Member lithnights's Avatar
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    Default Do I put vapor barrier over bathroom insulation or not..??

    I have gutted my bathroom and am installing R-13 kraft faced insulation between the exterior wall 2x4s.

    Question… do I need to staple a vapor barrier (4- or 6-mil polyethylene) over the insulation or not? I have heard conflicting answers online and looked for this question here on this forum but didn’t find a discussion.

    I know in certain areas of the country this is more important than in others. I live in Philadelphia, PA and my home is 45 years old.

    Any help would be appreciated…

    Thanks,

  2. #2
    Master Plumber Dunbar Plumbing's Avatar
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    An ounce of prevention is worth a pound of cure.

    Back in 1922 when I was young I was told that is the only to look at doing home repair projects.

    If you have a older home, the poly barrier is considered the same as the kraft-faced insulation, just tightens up any chance of air movement.

    There has been issues of walls "sweating" in regards to the poly barriers ontop of kraft faced but that was limited to super airtight homes, tyvek outside house wrapped and the fear of radon exposure in the home.

    If you have an older home, more than likely the house has enough openings that allows the house to breath which is considered better in some circumstances. < That statement right there alone has probably saved lives with homeowners with leaking heat exchangers in furnaces that are emitting Carbon Monoxide.
    Read what the end of this sentence means.

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    DIY Senior Member lithnights's Avatar
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    Quote Originally Posted by RUGGED
    An ounce of prevention is worth a pound of cure.

    Back in 1922 when I was young I was told that is the only to look at doing home repair projects.

    If you have a older home, the poly barrier is considered the same as the kraft-faced insulation, just tightens up any chance of air movement.

    There has been issues of walls "sweating" in regards to the poly barriers ontop of kraft faced but that was limited to super airtight homes, tyvek outside house wrapped and the fear of radon exposure in the home.

    If you have an older home, more than likely the house has enough openings that allows the house to breath which is considered better in some circumstances. < That statement right there alone has probably saved lives with homeowners with leaking heat exchangers in furnaces that are emitting Carbon Monoxide.
    So since my home is "older"... non-Tyvek, you would go without the vapor barrier?

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    Master Plumber Dunbar Plumbing's Avatar
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    No, I would install the barrier. That way the room contains moisture better, heat loss would slow down better. Make sure you install a vent fan in this room. Otherwise you will be dealing with mold growth.
    Read what the end of this sentence means.

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    DIY Senior Member lithnights's Avatar
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    Quote Originally Posted by RUGGED
    No, I would install the barrier. That way the room contains moisture better, heat loss would slow down better. Make sure you install a vent fan in this room. Otherwise you will be dealing with mold growth.
    OK, I misread what you stated about Tyvek. I will install 4 or 6 mil. The current bath does have a vent fan and I will be replacing it with a new one...

    Thanks much.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    The kraft paper (and tar on the back) is considered a vapor barrier. The polly is also a vapor barrier. It is not great to have two layers of vapor barrier by some. To accommodate that, you could cut some slashes in the kraft paper, then put up the polly. The problem is that you might get moisture trapped between the two layers from small imperfections in the installation. It could take a long time to dry out and mold could grow in that area until it does. Probably not a big deal unless this is in the shower area, then it becomes more important.
    Jim DeBruycker
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    DIY Senior Member lithnights's Avatar
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    Quote Originally Posted by jadnashua
    The kraft paper (and tar on the back) is considered a vapor barrier. The polly is also a vapor barrier. It is not great to have two layers of vapor barrier by some. To accommodate that, you could cut some slashes in the kraft paper, then put up the polly. The problem is that you might get moisture trapped between the two layers from small imperfections in the installation. It could take a long time to dry out and mold could grow in that area until it does. Probably not a big deal unless this is in the shower area, then it becomes more important.

    OK, so that's one vote for installing 4 mil poly, and one vote not so sure..

    Any other thoughts out there? I'm surprised I didn't see more opinions on this question seeing that mold is always such a big topic of concern with bathrooms...

    Thanks!

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Moisture that gets trapped in the wall is never good. Ideally, you prevent it from getting there. Thus, the vapor barrier between places where the vapor pressure is high to where it is low. In some parts of the country where a/c is the primary hvac function put the vapor barrier on the outside of the insulation. Most places where they use heating, the vapor barrier is on the inside of the insulation. So, you want one. Question is, if you have two, is it better or worse? Most people say it is worse. So, the stuff I've read is, especially if you are doing this in a shower area, to put some slashes in the kraft paper prior to putting up the new plastic sheeting. That way, you don't get moisture trapped between two places where it is not likely to get a chance to evaporate and dispurse quickly; in other words, you want to minimize the condensation, and if it does occur, give it a place to evaporate and dry things out. Note, if you use cbu in the tub area, you still need the vapor barrier - cbu is not waterproof, but it does not degrade in water.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Master Plumber Dunbar Plumbing's Avatar
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    That is why I mentioned a vent fan in the bathroom. If no bath fan you will definitely get mold growth in walls with no vapor barrier at all.

    Another mistake people make is in their choice of vent fans. Anything under 130 cfm's is usually a waste of time as it does not pull the moisture out of the room fast enough.
    Read what the end of this sentence means.

  10. #10
    DIY Senior Member lithnights's Avatar
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    Definitely installing a vent/fan. Not sure about the cfm.. I'll try to look for one at 130+. It's only a 7x5 room so I'm not sure how much is really needed for only 35 sq. ft. Would 130 be overkill for such small area?

    At this point, here is my plan..
    Install 4 mil poly behind the cementboard behind my shower tiles.
    Slash parts of the kraft paper and install 4 mil on my exterior wall then greenboard over it. (jadnashua, I have read about the double layer vapor barrier and the ill effects. I believe the slashing will alleviate that concern.)

    Sound like a plan?

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    Master Plumber Dunbar Plumbing's Avatar
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    That will work. I mention the larger fan because I've known/seen/dealt with the 60-85 cfm fans and basically they are just noise makers. They might fit the square footage to the bathroom to a tee but they are slow remove steam and smells out of a room.

    Remember, with a closed door in a bathroom the fan has a limited source for air, meaning that distance under the door and threshold will be where the fresh air pulls into the room to allow free movement of moist/damp air out of room. OR, the heat duct that is normally piped to the bathroom with no cold air return.

    Most times the price difference is minimal, you can find that the larger fans have more styles/lights/heaters and cater to less sones (loudness). I have a 140 cfm fan in a 18 by 13 master bath, centrally located and it still doesn't pull the steam/foul air out of the room. My next plan is to change the light in the shower to adding a second fan in the bathroom to help alleviate the heavy fog that builds in the room during showers. 2 showerheads are to blame I believe.
    Read what the end of this sentence means.

  12. #12
    DIY Senior Member lithnights's Avatar
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    Quote Originally Posted by RUGGED
    That will work. I mention the larger fan because I've known/seen/dealt with the 60-85 cfm fans and basically they are just noise makers. They might fit the square footage to the bathroom to a tee but they are slow remove steam and smells out of a room.

    Remember, with a closed door in a bathroom the fan has a limited source for air, meaning that distance under the door and threshold will be where the fresh air pulls into the room to allow free movement of moist/damp air out of room. OR, the heat duct that is normally piped to the bathroom with no cold air return.

    Most times the price difference is minimal, you can find that the larger fans have more styles/lights/heaters and cater to less sones (loudness). I have a 140 cfm fan in a 18 by 13 master bath, centrally located and it still doesn't pull the steam/foul air out of the room. My next plan is to change the light in the shower to adding a second fan in the bathroom to help alleviate the heavy fog that builds in the room during showers. 2 showerheads are to blame I believe.
    And I guess there's no harm (except for slight cost) for going bigger...

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    Master Plumber Dunbar Plumbing's Avatar
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    Nope. There is nothing worse than someone taking a crap, with the fan on and still 10 minutes later the room smells like you painted the walls with it. Most fans don't get used in older homes because when you turn it on it sounds like a car dragging a muffler down the road. I guess the importance of a working vent fan is all dependent on those who like steam baths and like the smell of their own crap. Those who take really hot showers and the steam soaks the walls and ceiling; it might look clean, but mold can start to grow and not be seen. For example when my jacuzzi is filled with 80 gallons of water it reaks of chlorine from the treated water. The smell is horrible. Even the fan cannot remove the odor fast enough.

    Just remember to "try before you buy" on these fans on the switchboards at the big box stores OR plumbing supply houses. You do not want anything that is extremely loud. What will end up happening is users will not turn it on because of that reason, thus defeating the purpose of having it to begin with.

    I like tieing the the fan to the light switch, no way it cannot be turned on in the room without it running.
    Last edited by Dunbar Plumbing; 03-21-2006 at 10:35 AM.
    Read what the end of this sentence means.

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    DIY Senior Member Hube's Avatar
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    Rugged; for a bathroom your size ,18 x 13,(8' high) you would need an 250 cfm fan to enable the reccomended 8 minimum air changes per hour for bathrooms.
    That 140 fan of your is only doing approx half the job.

    Lithnights; for a bathroom of 5 x 7 x 8' = 280 cu ft. a 50 cfm fan will give you approx 9 air changes per hour.

    Formula; L X W X H= cu ft divided by 60 (minutes), times 8 ach = minimum
    cfm of fan .

    And the key word for "quietness" in a fan is "sones" , the lower sones, the quieter the fan.
    Last edited by Hube; 03-21-2006 at 11:54 AM.

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    DIY Senior Member lithnights's Avatar
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    I have indeed checked out many of the vent/fans out there.. HD has a nice display to let you play with each one to get an idea of noise.

    The sones is something to think about but actually, some of the ones I've tested out were TOO QUIET. I don't want booming loud but I do want it to have some noise to it. Sometimes for privacy reasons, one would want the fan to cover up whatever noises they might be making while in the bathroom. Our downstairs bath doesn't have one now and, smell aside, I feel like putting one in just to get the room some "coverup noise" when someone is in there and uh.. not feeling so good.. I don't need to hear that.

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