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Vapor Barrier Question


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I'm remodeling a bathroom. I have it gutted down to the studs and will be putting in a new tub and will tile the walls. The wall at the foot of the tub is an exterior wall and there will be fiberglass insulation with kraft paper toward the inside. The wall along the side and head of tub are interior walls.

My question is what if anything should I put behind the cement board that the tiles will be placed on?

Thanks

Mark

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David's right, but personally, I wouldn't do any VR or waterproofing outside of the tile layer.

I've never torn out a bathroom with an outside wall vapor retarder that didn't have mold or crap somewhere in the assembly. My choice is to let it breath either way.

OK, I'm ready for the barrage of disagreement. Have at it.

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Thanks for the input everyone!

Kurt - The house was built in 1959. The covering is metal (AL). I'll have to take a look at the sheating next time I'm over there, but in this picture it looks like a manufactured wood product.

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. . . My question is what if anything should I put behind the cement board that the tiles will be placed on?

I don't believe that it's necessary to put anything there other than the paper that's on the insulation.

There are three possible sources of moisture that can cause trouble in this wall: vapor drive, air-transported, and leaks.

The amount of moisture that moves through a tiled wall via vapor drive is trivial.

If you build the wall properly, there should be no air-transported moisture.

If the tiled wall that you install leaks liquid water, then you'll have a big problem that the vapor barrier won't solve anyway. It might just delay your discovery of the problem.

Personally, I wouldn't use RedGard on a wall like this. There's nothing wrong with the product, it's just unnecessary.

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Thanks for the input everyone!

Kurt - The house was built in 1959. The covering is metal (AL). I'll have to take a look at the sheating next time I'm over there, but in this picture it looks like a manufactured wood product.

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Looks like celotex sheathing.

You know, given the age of the house, and just for fun and experience of doing it, you should consider doing mudset walls.

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Yeah, celotex. I actually like celotex. It breathes.

I'd just use straight batts. If paper (easier to install), I'd slit it with a razor knife so it could breath.

Mud set wall tile is a bear. I haven't done one ever since durock came out, i.e., 30+ years.

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In the July 2011 Journal of Light Construction there is an article titled "An Energy Retrofit in Stages."

In this project, all of the exterior sheathing was removed. Over most of the house they put up new sheathing and blew in cellulose. But at the exterior wall of the bathroom they used spray foam.

If you're at all concerned (and you must be at least a little because you posted the question), I would suggest you do the same and use spray foam instead of fiberglass.

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He could get a two part kit for a couple hundred bucks and spray the whole room from inside, but it's just one room. It won't make a difference.

Carefully placed kraft faced fiberglass batts are going to be far more efficient than what was originally there and won't cause any funny vapor issues.

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Nice discussion and I appreciate the tips. This was a HUD foreclosure I picked up a few weeks ago. I'm going to either rent it or sell it, or rent it and then sell it. Regardless - I want to do things the correct way, I take pride in my work - even it it will be hidden behind a wall ansd no one will ever know.

I'll pass on doing the "mud set wall" but thanks for the suggestion. I'll have to build character another way.

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He could get a two part kit for a couple hundred bucks and spray the whole room from inside, but it's just one room. It won't make a difference.

Tom, I'm not sure what your point is. But I'm not suggesting using foam because it would improve the energy efficiency of the house. I'm suggesting its use because it's a great vapor barrier.

To quote from the JLC article:

"To eliminate any possibility that high indoor vapor pressure in the bathroom could drive moisture into the framing cavities, we completely filled the stud cavities of that short section of exterior wall with spray foam."

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Steve,

You referenced a pretty intensive retro fit that addressed the entire envelope. Mark ripped out a bathroom. I don't know the scope of his entire project but there are economies of scale at work with spray foam. One could hire out the drywall and the fiberglass insulation for the cost of a spray foam kit, and in my market none of the foam companies will spray a job that small.

There's also the pesky problem of turning a small bathroom into a styrofoam cooler while the rest of the house is left to breathe. I don't particularly like that idea.

If the bath is the only room that's to be gutted, and he really wants foam insulation, then I say to put it back together without any insulation and then hit the whole house with something like Retro-Foam or Applegate R Foam.

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Steve,

You referenced a pretty intensive retro fit that addressed the entire envelope. Mark ripped out a bathroom.

Even though the JLC article details an intensive retrofit job, the basic principle is exactly the same as Mark's situation. I'm just taking one small idea from a larger project.

I don't know the scope of his entire project but there are economies of scale at work with spray foam. One could hire out the drywall and the fiberglass insulation for the cost of a spray foam kit, and in my market none of the foam companies will spray a job that small.

Agreed that economies of scale may be a problem here. This will probably need to be a do-it-yourself job. That's OK. I'm sure Mark's a handy guy.

There's also the pesky problem of turning a small bathroom into a styrofoam cooler while the rest of the house is left to breathe. I don't particularly like that idea.

Why? What mechanism are you proposing that might make this problematic? The author of the JLC article seems to think it's a great idea.

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There's also the pesky problem of turning a small bathroom into a styrofoam cooler while the rest of the house is left to breathe. I don't particularly like that idea.

Why? What mechanism are you proposing that might make this problematic? The author of the JLC article seems to think it's a great idea.

You are proposing creating an environment with a volatile RH in a confined space, and in a potential rental property no less where the standard of care is questionable.

I propose a little experiment to see what would happen without an automated means of moisture control: remove the contents of your fridge, unplug it, then promptly close the door. By this time Friday, that inside of that fridge will be a furry moldy mess.

It will undoubtedly take longer in the bathroom in question, but it will happen. Vapor drive and frequent periods of high RH will seek out any imperfections in the foam shell and concentrate there. If the whole building envelope is foamed there will be far more volume to stabilize the RH and lessen the effect.

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You are proposing creating an environment with a volatile RH in a confined space, and in a potential rental property no less where the standard of care is questionable.

I propose a little experiment to see what would happen without an automated means of moisture control: remove the contents of your fridge, unplug it, then promptly close the door. By this time Friday, that inside of that fridge will be a furry moldy mess.

It will undoubtedly take longer in the bathroom in question, but it will happen. Vapor drive and frequent periods of high RH will seek out any imperfections in the foam shell and concentrate there. If the whole building envelope is foamed there will be far more volume to stabilize the RH and lessen the effect.

The problem is that your experiment doesn't come anywhere close to modelling the physical situation that I'm proposing. I'm not suggesting foaming all six surfaces of the bathroom (4 walls, ceiling, floor), let alone installing a magnetically sealed airtight door. I'm just suggesting foaming the exterior wall where the vapor drive from warm and wet to cold and dry is the highest, and where cold sheathing could cause condensation and all its problems.

Jim Olson, the author of the JLC article, thinks it's a good idea and I agree.

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Steve,

I don't dislike spray foam, but there are limits to what can be done with two part insulation in retrofits. It is exceedingly difficult to apply it properly in confined spaces and partially gutted rooms. If it's not applied properly, you can't expect it to live up to it's performance potential.

If Mark is going to gut the whole house (inside or out) spray foam is a very viable option, if not it should be limited to spot treatments for air sealing. It's prohibitively expensive and potentially problematic otherwise.

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