# Exterior Bathroom Wall Insulating



## dvaupel

and just to clarify, I'm talking about cutting the sheets into strips that fit between the studs. I've been somewhat confused by install instructions I've seen for this type of insulation, so needing clarification.


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## AGWhitehouse

dvaupel said:


> and just to clarify, I'm talking about cutting the sheets into strips that fit between the studs. I've been somewhat confused by install instructions I've seen for this type of insulation, so needing clarification.


This isn't a very good option as it is near impossible to cut the foam so that it fits in tight enough to be effective to its full potential. It would be ideal to leave a gap on all side and spray foam fill the perimeter to seal it in. But then that begs the question of just spray foaming the walls all together.

Unless you are using Polyiso or closed cell foam (2.0 lbs), XPS is relatively comparable in R-value to most batt. applications. (example: XPS is R-5 per inch, so a 2x4 wall will give R-17.5, while a 3-1/2" batt can give R-15. A minimal R gain for the cost difference.) The true benefits of spray foam is the air sealing capabilities. The true benefit of rigid foam is the thermal bridging capabilities.


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## dvaupel

I've been back and forth with the spray foam idea as well, but from what I've seen it would be more cost-effective to go with the sheets and then foam around the edges. And I've definitely considered batting but have mostly shied away because of potential for moisture problems....am I over thinking this?


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## AGWhitehouse

You should be treating the moisture drive at the wall's surface with some kind of vapor barrier. The insulation is secondary.


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## dvaupel

AGWhitehouse said:


> You should be treating the moisture drive at the wall's surface with some kind of vapor barrier. The insulation is secondary.


That's why I've been having so many questions about this...between the concrete board, Redguard, and the different insulation materials, I'm getting lost in understanding what will be a sufficient vapor barrier. It sounds like some batting insulations have the barrier on them and the same with foam....am I wrong on this?


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## oh'mike

If you are going to waterproof the face of the backer board in that area no additional vapor barrier should be used.


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## AGWhitehouse

oh'mike said:


> If you are going to waterproof the face of the backer board in that area no additional vapor barrier should be used.


I agree...

The redguard is considered a class II vapor retarder. Therefore the insulation doesn't need or want to handle vapor retarding. Un-faced batt type insulation with caulked joints for air sealing would allow for any moisture within the wall cavity to dry to the exterior. It will be cheaper on the wallet too.


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## oh'mike

AG--You do know your stuff---and can put to into words so much better than I do.


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## bubbler

dvaupel said:


> That's why I've been having so many questions about this...between the concrete board, Redguard, and the different insulation materials, I'm getting lost in understanding what will be a sufficient vapor barrier. It sounds like some batting insulations have the barrier on them and the same with foam....am I wrong on this?


My bathroom exterior wall--it's about 6' wide, 2' of that is tile and 4' is painted (with a window in the middle of the 4').

We pulled everything back to the studs & tore out the old R-1 fiberglass from 1950. 

I installed "faced" R15 FG batts w/ the face "out" (toward the room), the paper ear-flaps stapled to the studs and then taped the seams between batts w/ Tyvek tape. I used the window&door can of spray foam around the window and on a few particularly large gaps in the exterior sheathing where the felt was exposed.

I put up "half inch" hardi board for the 2' where tile would be and put up blueboard + skim coat of plaster for the other 4' (which was later primed and painted). Siliconed all seams of the hardi-backer on hardi-backer or hardi-backer on tub. Typical porcelain tile went up on the hardi-backer.

I also installed a generously sized bath vent directly above the shower with a ~2' duct run to the exterior, and put it on a motion activated switch w/ 30-min delay--someone goes in the bathroom, the fan starts and doesn't stop until 30 mins after they leave. Whatever energy cost penalty I have from over ventilating I feel I make up for in the moisture and negative-olfactory departments.


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## Gary in WA

Fiberglass batts give the least “bang for the buck” – the reason why they are the cheapest. http://www.diychatroom.com/f98/biggest-loser-fiberglass-insulation-90438/

I would use some 2” rigid foam board on the face of the studs, if enough clearances for other bathroom fixtures, then add d.p. cellulose or rock wool cavity insulation. At the very least, ½” f.b. or sill sealer on the studs. F.g. batt can lose up to 70% R-value when wet and more R-value because of non-lab installation (air-sealed on all six sides) due to convective air leaks from outside behind and around the batt, especially if you have older wood board sheathing? Minimum IRC (code) for your area requires R-19 wall insulation, anything less is like getting a ‘D’ grade on a school test; R-15 wouldn’t even be passing… http://findarticles.com/p/articles/mi_m0KWZ/is_3_6/ai_n8582994/
The R-15 is high density so interior c.loop. would not be there, though outside batt c.loops may, depends on installation. The cellulose would stop cavity air movement and redistribute any moisture from exterior leaks. 

Is there any foam board outside- under the siding? (The optimum solution- keeps the dew point on the sheathing high enough not to condense and thermally breaks the R-3 studs from the drywall as a heat sink). *Could you describe the wall assembly?*

Air-seal the plywood/OSB/boards at the exterior joint to studs, also the plates to the ceiling drywall, and plate to floor; with caulk or canned foam. Also any wiring/plumbing holes through either plate and the studs. Add rigid f.b. on the exterior wall top plate leaving room for your ventilation baffles.

You could stack the f.b.- with canned foam to air-seal, depends on the wall assembly…….

Gary


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## dvaupel

GBR in WA said:


> Fiberglass batts give the least “bang for the buck” – the reason why they are the cheapest. http://www.diychatroom.com/f98/biggest-loser-fiberglass-insulation-90438/
> *Could you describe the wall assembly?*


Here's the rundown....

House was built in the mid-50's, previous owner had siding done in the last 5ish years. There's 1/2" Green Guard foam underneath the siding, then I imagine the OSB/Ply. 2x4 studs for interior. I hesitate to put anything on top of the studs because of necessary fixture locations.


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## dvaupel

AGWhitehouse said:


> Un-faced batt type insulation with caulked joints for air sealing would allow for any moisture within the wall cavity to dry to the exterior.


Am I correct in assuming you're talking about the joints between sheets of concrete board, ceiling, tub, etc?


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## AGWhitehouse

The caulking was for air-sealing the wall cavity to ensure the best performance of the batt. insulation. Caulking at the stud and plate edges help to eliminate the air movements within the wall cavity that reduce the effectiveness of the insulation.

http://www.energystar.gov/ia/partners/publications/pubdocs/Seal_and_Insulate.pdf


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## dvaupel

what about a stone wool batt for this? With what I'm understanding so far, it seems like that (plus caulking, etc) is a happy medium in it all.


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## Gary in WA

Good to go.

ADA (airtight drywall approach): http://www.buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach/

Page #21: http://www.engr.psu.edu/phrc/training/understandingbarriers.pdf

Gary


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