Insulating heated bathroom in a unheated pole building.

goosegunner said:

Finishing a bathroom in my un-heated pole building.

6' X 7.5'
one wall adjoins boiler room
one wall adjoins insulated wall with storage tank
two walls to the interior of the building unheated for the foreseeable future

Do I need vapor barrier on the heated walls?

Unheated walls I will put vapor barrier.

Also thinking about putting up 1/2 foam board on ceiling before drywall and the outside unheated wall.

Think it is worth the effort to put up foam board?

Will foam board on outside walls trap moisture?

gg

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You only need vapour barrier on the exterior walls. If you use foam board install it tight fit in the stud bays then caulk the perimeter then no need for vapour barrier.The only time that rigid foam insulation would be acceptable on the exterior of a wall is if a vapour barrier is installed on the warm side and is within the first third of the overall insulation.There is rigid or semi-rigid insulation designed specifically for this purpose that should be readily available at local building centres. This may be rigid fibreglass or other material that has a vapour permeable membrane or sheathing attached to the exterior. This insulation may not have as high an R-value per inch as extruded foam, but will be much more breathable. I would think fiberglass batts would be a bit cheaper htough

You will need a bath fan, which will pull a negative pressure. If the light and fan come on with the same switch no vapor pressure will be possible.

Your plan to use Thermax on the interior wall is sound an presents a nearly perfect vapor barrier while mitigating thermal bridging of bare studs.

I have had good success with insulating like this... Fiberglass batt the stud bays, make sure it fits tight and isn't compressed anywhere. Then 1/2 foil faced foam board nailed to the studs, then foil tape the seams and silicone chaulk the perimeter edge. This gives you a thermal break from the studs as well as your vapor barrier on the warm side. Then Sheetrock over that. Super tight wall and good thermal break.

hondaracer2oo4 said:

I have had good success with insulating like this... Fiberglass batt the stud bays, make sure it fits tight and isn't compressed anywhere. Then 1/2 foil faced foam board nailed to the studs, then foil tape the seams and silicone chaulk the perimeter edge. This gives you a thermal break from the studs as well as your vapor barrier on the warm side. Then Sheetrock over that. Super tight wall and good thermal break.

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My whole house is done that way. 1.5 inch foil faced thermax nailed to inside of studs with sheetrock screwed to it. Use 2x4 s nailed flat over studs for door and window framing. Just need long drywall screws.

goosegunner said:

I had actually planned on putting the foam on the outside meaning cold side of the walls that border the unheated side of shed interior.
A lot of houses built in this area have foam on the exterior right under the siding.

Is there a problem of doing it that way?

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If you read the instructions on Thermax they ask for a corrugated insert be placed between the stud wall shoe and the Thermax to allow air to flow behind the foam in order to allow moisture to escape. When I read that I asked myself: Why should I put up high R insulation then let cold air flow behind it?
My last house which was a 3000 sq ft colonial was done with .75 inch thick Thermax and was very easy to heat. The house I'm in now is a 1100 sq ft ranch and I decided to use 1.5 inch Polyiso. Have done 1 room per year.
It isolates the framing from the drywall which reduces conductive heat loss and puts the vapor barrier where it belongs.

Nailed a 2x4 flat along the floor flat on the studs for a baseboard nailer and nailed a 2x4 frame around doors and windows to nail trim. You must extend the jams on your doors and windows.
On the electrical boxes I just rough wired the leads so I could pull them through a hole in the insulation board and drywall then inserted the electrical boxes with the ears that grab on to the back of the drywall. A buck or so at HD. You'll need to remove a bit of insulation to make room for the ears.