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Hook
Bathroom Insulation
Hook

Hello,
I live in South West Michigan and redoing a bathroom with a exterior wall. My question is how to insulating that wall. There is foil faced foam board on the exterior which makes me question whether to install a vapor barrier on the inside, but being a bathroom it seems that I should, or cut out some of the foam board so that the cavity could breath with a vapor barrier on the inside?
Any thoughts would be appreciated
Thanks

keith3267
Re: Bathroom Insulation
keith3267

Look at the bottom and top of the foam boards to see if there is a gap at each end. If there is not, then put a small gap in each stud bay. Three 1/4" to 1/2" drilled holes at the top and bottom of each bay should be adequate. Do not make any holes in the middle. You want outside air to enter at the bottom and exit the top.

Note: the above advice is for a brick exterior.

If you have vinyl siding, you can do the same thing, just don't drill through the vinyl siding itself.

If you have some type of wood or stucco siding, it gets more difficult. If you have a conventional foundation, you can drill through the sill plate to the basement or crawl space for the bottom vent. If you have a slab, then you will need to use your imagination, and you will have to settle for less ventilation, maybe as little as one 1/4" hole per cavity.

For the top, if you have a soffit that extends below the top of the wall, you can drill straight through. If not, you may have to drill upward to the attic. The holes have to be as close to the exterior side as possible.

Then you absolutely use a vapor barrier on the interior side of the insulation. If you have a shower or tub/shower against this wall, and you are tiling this wall, you need to use a shower liner as the vapor barrier and it has to extend all the way under the mud bed (if a tiled floor shower) or lapping over the inside of the shower or tub base.

As an alternative, you can use a regular vapor barrier and then use tar paper inside that to shed any penetrating water back to the inside of the shower and drain. I recommend that you get a book on tiling shower stalls. I have one written by Michael Byrne that I like. There is a tile forum that is highly recommended by some of the regulars here but I can't remember the name. Maybe someone will post it here.

Hook
Re: Bathroom Insulation
Hook

This is kind of what I was thinking. I do have vinyl siding, so I will drill the holes at the top and bottom of each bay.
Do the same under the window? I have a tub surround that I will be installing and not tile. Should the vapor barrier wrap around the whole room, around the corners, or just to the corners.

keith3267
Re: Bathroom Insulation
keith3267

Don't worry about the windows, the are installed so that water sheds to the outside, but they are usually not to air tight. If you see any kind of a gap at the top or bottom, you wont need to drill any holes.

Since you are putting in a surround, you only need the vapor barrier on the external wall. If you use a faced fiberglass, staple the flanges over the ends of the studs so that the flanges overlap a little instead of pushing the fiberglass in and stapling the flanges to the side of the stud. If you have any gap in the fiberglass, you want it on the outside so that air coming in from the bottom will rise up along the inside of foam board. This will draw moisture out of the fiberglass and keep it nice and dry.

Hook
Re: Bathroom Insulation
Hook

Thanks for the help.
I will also be putting up a double layer of dry wall. The first will give the surround something to glue to (due to the plastic vapor barrier) and the second will go around the surround. Does this sound right?

keith3267
Re: Bathroom Insulation
keith3267

I don't understand. It sounds like you are putting a layer of drywall on the inside of the surround. Why would you do that? Define first layer, the one closest to the inside of the house or the one you put up first?

Edit: Have you considered using concrete backer board instead of sheetrock? At least use the concrete backer board where the faucet comes through the wall.

Hook
Re: Bathroom Insulation
Hook

The one that goes against the outside wall only, not the faucet side. How do you put the surround up against the vapor barrier? Glue on stapled up plastic I wouldn't think would be good. The surround is suppose to be glued to the studs then drywall up to it. The wall is only 8' long, so I thought I would put up a layer of drywall that would be behind the surround (for it to glue to) then the surround and then the finish drywall up to the surround. This would be only on one side opposite the faucet side.
Does this make sense or am I totally missing something.

keith3267
Re: Bathroom Insulation
keith3267

Sorry not making sense to me. The vapor barrier is stapled or glued to the studs. The drywall is nailed or screwed to the studs over the vapor barrier. It is not glued. The compression of the drywall to the studs pretty much seals the vapor barrier between them so the hole through the vapor barrier is not an issue. Staples are not an issue either for the same reason so I have never seen one glued in place.

The surround is glued to the drywall with a construction adhesive. So its stud, vapor barrier, drywall, surround. Usually you paint the drywall before installing the surround. Paint at least a few inches under the surround so you don't have to spend a lot of time edge painting around the surround after. That is really time consuming.

Hook
Re: Bathroom Insulation
Hook

The surround is suppose to go against the studs with glue, but I didn't think the glue against the plastic would be very good. So what I will have is stud, vapor barrier, drywall, surround. That way the surround will be glued to the drywall and not the studs.

Mastercarpentry
Re: Bathroom Insulation
Mastercarpentry

What does the surround manufacturer say? That's what you go with ;) These days most have websites and FAQ sections which usually cover things well with tech support emails for the rest.

In general terms, keith is correct- most are done that way and personally I've never used a surround intended to be glued to the studs as the unsupported span isn't stiff enough for good support for the surround walls. All I've used require either 1/2" drywall or a 1/2" plywood base structure as a minimum. YMMV!

Phil

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