Hello:

I am remodeling a bathroom in the northeast U.S.A. On the outside wall I have placed
Kraft paper R21 glass. Is it o.k. to use greenboard over it or regular whiteboard? My concern is the vapor issue in the bath getting trapped between the greenboard and the kraft paper leading to problems. Thank you for your thoughts!

Shannon

I would cover the studs with 6 mil visqueen and then greenboard. Greenboard is fine for most non-wet bath applications. For wet areas use Hardiebacker, Durorock, Wonderboard or Shluter Kerdi.

Shannon 2000 Just on a side note don't have any water lines that outside wall for freezing problems.

Go with Bal 1 advice and no worries then. 100% correct.

Another side note is get a good bathroom exhaust fan and it will seriously cut down the moisture in that bathroom. Not one that just spins and does Little to move air.

No it should not get wet there. The get "wet" is from to quick of thermal changes.

Paper backed insulation is not a high end vapor barrier. Many Goes its fine. But In many parts of Canada the fiberglass insulation doesn't come with a paper vapor barrier. Because its not a 100% vapor barrier. They are Required to use a 6 mil poly simply because of the extremes in temps diffs.

The shower area will get hotter then a regular room quick. The added vapor barrier will stop any penetration/transfer of airflow that could create more moisture. Added barrier will slightly Increase the (R) factor thus completely stopping cold air getting past the paper and hitting hot air. "Mini Canada"

This is why I stated earlier that Bal1 was dead on here.

Hello:

It has been recommended that in a outside bathroom wall that I can use kraft paper insulation (R21) and then plastic under greenboard. I live in upstate New York. My question is will vapor get trapped between the plastic and kraft paper leading to problems?

Shannon2000

You mean fibreglass insulation with the kraft paper vapour barrier ~and~ plastic vapour barrier too?

No - this is not correct in any room including the bathroom. Choose one or other. Never both. Generally when you have easy access to the inside, you would choose plastic vapour barrier and un-faced fibreglass insulation

I see you asked the same question in your other thread. Are you tiling around the bathtub? If so, use a cement board rather than greenboard for tiling, and use greenboard wherever you are just painting as ballengerb recommended.

Rtw travel... This is a pickup from the ops re posting a new thread form yesterday and a bit misleading that way.

The OPs "double vapor barrier" is on the Paper side of the kraft paper which won't trap any moisture in the actual insul, which if that was on the opposite side of the kraft paper then the "Double" barrier you are talking about is wrong and you are completely correct.

Kraft paper is a partial vapor barrier on the get go. This is for a bathroom only. If we go with 100% go with a double vapor barrier then when we paint a bathroom with a primes/SEALER that in itself is also a vapor barrier and the kraft next which technically is a double vapor barrier.


No it should not get wet there. The get "wet" is from a quick of thermals thus trapping that moisture in because of having a vapor barrier on opposite sides of insul and the insul gets wet.

Paper backed insulation is not a high end vapor barrier. Many Goes its fine. But In many parts of Canada the fiberglass insulation doesn't come with a paper vapor barrier. Because the paper is not considered a 100% vapor barrier. They are Required to use a 6 mil poly simply because of the extremes in temps diffs.

The shower area will get hotter then a regular room quick. The added vapor barrier will stop any penetration/transfer of airflow that could create more moisture. That added barrier will slightly Increase the (R) factor simply because of completely blocking the walls cold air flow to the warm side that's getting past the paper and hitting hot air. "Mini Canada"

This is why I stated earlier that Bal1 was dead on here.

I agree, you'll be fine. Are you painting or tiling this outside wall? Is it a wet area or just a bath wall?

Actually, in Canada the unfaced insulation is very common. I don't think we've ever bought faced insulation for any of our houses except for a reno where we wanted to insulate the attic properly and could not put a plastic VB underneath.

Building Inspectors here would require either the removal of the paper, or cutting it in shreads if you are putting plastic over top. Actually, I would put money on them not allowing kraft paper on a wall that you can get easy access to - as you say plastic is much better.

I do recall one or two inspectors who wanted me to split the kraft paper before I installed my visqueen. I always like to pull their chain and ask if water can get through 6 mil visqueen why would paper matter. LOL this just ticked them off so I stopped it.

Did the old rip/split also to appease inspectors still do it to have fun... bathrooms are rain forest and different rules apply there.

Seriously, shannon, I answered this the first time you asked about it. No plastic behind greenboard. You can use felt or kraft paper, both with a perm rating above 1, but not poly sheeting or anything else with a perm rating below 1.

Now if I recall from your first of many times you've repeated this question, you are covering this with some kind of solid surface tub/shower surround??

From the USG Installation Handbook...


Water Resistant Panel Limitations

1. Adherence to recommendations concerning sealing exposed edges, painting, tile adhesives, framing and installation is necessary for satisfactory performance.
2. Not recommended for ceilings with framing spacing greater than 12″ o.c. for 1/2″ board or 16″ o.c. for 5/8″ board, or for single-layer resilient attachment where tile is to be applied or in remodeling unless applied directly to studs.
3. Panels that would normally receive an impervious finish, such as ceramic tile, should not be installed over a vapor retarder nor on a wall acting as a vapor retarder.


The anonymity of allows anyone to be an "expert" and many people in the trades take what they think to be right as gospel. 2+2 won't make 5, even if I've been saying it's true for all my life. I hope this puts to rest your questioning and gives you confidence as you continue your project.

BLJACK:

Thank you for your thoughts, however if you read all the responses you will see others do not agree. Some say kraft paper and poly together are o.k. behind greenboard. Some say use one or the other. Some say use regular sheetrock with vapor barrier otherwise you will have two vapor barrier, if greenboard is used. I have e-mailed manufactures and they do not offer an opinion. Local code officer is unsure. I have just been looking for a consensus and all views are welcome.

Shannon 2000

however if you read all the responses you will see others do not agree.

I know there is disagreement. I had this discussion with the technical director of the tile council of north america a few years back. Roofing felt or Auqabar behind GB is OK, you can use a moisture retarder, not a vapor barrier behind it. The previous quote is from the manufacturer's installation guidelines. Anything not part of the manufacuter's specifications is an example of claiming 2+2=5. USG (Sheetrock brand name manufacturer) had a technical bulletin on their site a few years back, too. Specified unfaced only. There site is not as user friendly as it was before. Slow, too, almost enough to make you think there server is dial up. Instead of emailing, just call there tech support number.