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Old 06-21-2016, 04:15 PM   #1
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Vapour Barrier for Ceiling in Bathroom


I am currently renovating my bathroom and have run into some question about how to handle the Ceiling.

FIRST:
My father first suggested that I use BlueDow (blue foam board) on the ceiling under the joists to:

1. hold up the insulation Batts (Roxol 2x6 batts)
2. create a vapour barrier (once taped)
3. bump up the insulation value even more
Will this work?
I'm worried about screwing the drywall through 1.5" boards before finding the studs. I considered getting .5" foam-board. (which would also make installing the Pot lights easier) but i think i need 1.5" to make the .1 perm rating
Am I better off putting up 6 Mil plastic and just blowing in more insulation later?

SECOND:
I've read a few times now about actually having vapour be aloud to go through becasue the attic can draw it up and out of the house.... I really don't know what to think about that though.

THIRD:
Fire hazard. Is this something i should be concerned with?

Any help or pointing me in the right direction to get answers would be greatly appreciated.
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Old 06-21-2016, 06:46 PM   #2
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Re: Vapour Barrier for Ceiling in Bathroom


Is the roof vented? How is the area above the bathroom currently insulated?
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Old 06-21-2016, 07:20 PM   #3
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Re: Vapour Barrier for Ceiling in Bathroom


Just makes no since to me.
Foam board is not a vapor barrier, it's going to lower the ceiling.
Want a vapor barier then install 6 mil. plastic to the rafters from below, sheetrock holds up the insulation.
Need more insulation then add it from above the sheetrock.
In your area I'd want at least R-50.
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Old 06-22-2016, 02:25 PM   #4
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Re: Vapour Barrier for Ceiling in Bathroom


hi guys,

The attic does have venting. I will actually be replacing the shingles very shortly so there will be new vents too.

Insulation was blow in, but thats gone now as it was all tossed during the Gut.
Insulation will be 4x6 Roxul comfort Batts (the rest of the attic will still be the blow in stuff... and I would eventually add in more blow in.

As for Vapour Barrier... I think there should be one, but I just want to be sure.
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Old 06-22-2016, 06:22 PM   #5
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Re: Vapour Barrier for Ceiling in Bathroom


Check with code enforcement in you area as to what they they require exactly.

After that, air seal and insulate that area to avoid air movement as well airtight drywall approach: http://buildingscience.com/documents...ywall-approach

Get those can lights sealed up as well as the top plates and insulate up to an R-50 or so.
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Old 06-23-2016, 07:40 AM   #6
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Re: Vapour Barrier for Ceiling in Bathroom


Quote:
Originally Posted by Windows on Wash View Post
Check with code enforcement in you area as to what they they require exactly.

After that, air seal and insulate that area to avoid air movement as well airtight drywall approach: http://buildingscience.com/documents...ywall-approach

Get those can lights sealed up as well as the top plates and insulate up to an R-50 or so.
Thanks, that's a great idea! I over looked sealing and insulating the Light boxes.
I suppose building a foam box around them and taping that up would do the trick? then just covering that with insulation?

I've never heard of anyone using sealant like the link you have here behind drywall. Isn't putting in a 6Mil Vapour barrier enough?

I plan to 6 Mil the ceiling, and drape down 12" at each wall, as well as the 2 walls you see in the photo. (window wall, and partial Attic wall)
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Old 06-23-2016, 08:03 AM   #7
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Re: Vapour Barrier for Ceiling in Bathroom


Hi cd, although a vapor barrier like 6 mil plastic will perform some of the air sealing duties, air sealing and vapor protection are two different functions. Essentially, small amounts of air will get past that plastic and carry with it large amounts of moisture. Wood and drywall are great air barriers, except where the there are seams. seal those seams and you not only stop the air leakage, but stop most of the moisture issues.

Given your cold climate and this being a bathroom, a vapor barrier is still a worthwhile addition. Farther south they no longer advise the frequent use of plastic.

Air seal the walls and rim before the insulation that looks to already be in place .

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Old 06-23-2016, 09:39 AM   #8
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Re: Vapour Barrier for Ceiling in Bathroom


Thanks guys,

looks like the Pot light boxes are already sealed

Halo 4-in New Construction Airtight IC Recessed Light Housing


So that's good.

Do you recommend a certain product for sealing? It looks like Latex would be used around the window, and Adhesive around the wall frame.

Question:
If I am putting up 6mil on the studs... do i put the adhesive on the plastic? That wouldn't eat it would it?

@Bud ,
vapour barriers should be put on the warm side (in Canada, that is on the inside with the insulation on the out.
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Old 06-23-2016, 10:52 AM   #9
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Re: Vapour Barrier for Ceiling in Bathroom


cd,
"vapour barriers should be put on the warm side (in Canada, that is on the inside with the insulation on the out. " I realize that is the frequently quoted advice, but it fails to explain why. The why comes mostly from the past before they learned the negative sides of vapor barriers. Today, yes some areas of CA should still use them, but far more important to the issue is air sealing. Where diffusion through drywall may result in pints per year per 4x8 sheet a simple air leak can be gallons. The reading on this topic is extensive and any search for vapor barriers and GBA will take you into their collection. However, bottom line comes down to code requirements, even if they are not keeping up with the science.

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Old 06-24-2016, 03:43 PM   #10
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Re: Vapour Barrier for Ceiling in Bathroom


Bud,
Yes I agree, reading all the data on this has become quite the headache.

Suffice to say, it 'seems' like for me, best practice is to put a vapour barrier (6 mil) on the outside walls and ceiling. And that should be placed on the inside of the Beams.

That said though, if i am to Seal with caulking, it looks like I put it behind the plastic.
http://www.greenbuildingadvisor.com/...rtight-drywall

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