Northern climate moisture barrier

[KC]

Nebraska house has 3mm plastic over the paper-faced insulation and behind the sheetrock in the area of the addition new-work (which itself is probably decades old).   No plastic behind the sheetrock where they just tore out the plaster and rocked over the lath (and the rock-wool/recycled paper or whatever was blown in at some point.  From seeing siding repairs going on about town, they're using TYVEK in front of the vinyl siding.  Don't if they did this on mine (aluminum siding).   Question is, I'm having a garage built which will be vinyl siding, and finished inside (by me).  IF they use the house wrap over the OSB sheathing in front of the vinyl, do I need to use plastic over the paper-faced insulation, or is that overkill?  Of worse, just asking to trap moisture?   For what it's worth, all the old blown-in I've removed in the house was dry, and when I had occasion to get into the paper-faced to add a receptacle, that was dry as well.

[R Clark]

In short, I would install a layer of poly between the studs and the finish interior wall.  That's to keep moisture from inside migrating to the cold exterior wall where it could condense.

I did that in my shop (I used OSB for the finish wall out there.)  Are you going to have plumbing in that outbuilding?  My shop building has no plumbing (no water source), and the propane heat is sealed combustion venting outside, so I don't think I have a great deal of moisture out there.  But, the poly went up in a couple of hours for the whole building since all the walls were open.

Edit to add: My insulation is unfaced fiberglass, so poly was more needed in my instance, I think. I'll be interested to read what others have to say.

[KC]

In short, I would install a layer of poly between the studs and the finish interior wall.  That's to keep moisture from inside migrating to the cold exterior wall where it could condense.

I did that in my shop (I used OSB for the finish wall out there.)  Are you going to have plumbing in that outbuilding?  My shop building has no plumbing (no water source), and the propane heat is sealed combustion venting outside, so I don't think I have a great deal of moisture out there.  But, the poly went up in a couple of hours for the whole building since all the walls were open.

Edit to add:  My insulation is unfaced fiberglass, so poly was more needed in my instance, I think.  I'll be interested to read what others have to say.

I don't think I'll run water to it.  Can't think of last time I wished for water in the shop.  I'll use one of the Farenheat electric wall heaters.  I've had those in the last two shops and they were adequate for Texas winters, so I'm hoping I can maintain maybe 50 degrees with good insulation in the dead of a Nebraska winter.

[R Clark]

I don't think I'll run water to it.  Can't think of last time I wished for water in the shop.  I'll use one of the Farenheat electric wall heaters.  I've had those in the last two shops and they were adequate for Texas winters, so I'm hoping I can maintain maybe 50 degrees with good insulation in the dead of a Nebraska winter.

How big is the garage going to be?  Two car?  Say about 25x25?  My shop calculated out as needing 66K BTU to warm it up 60 degrees over outside air temp with average insulation.  My shop is 25X44 with 12 foot ceilings; two 16' insulated overhead doors.  I got a furnace that cranks about 80K BTUs.

[Roly]

Nebraska house has 3mm plastic over the paper-faced insulation and behind the sheetrock in the area of the addition new-work (which itself is probably decades old).   No plastic behind the sheetrock where they just tore out the plaster and rocked over the lath (and the rock-wool/recycled paper or whatever was blown in at some point.  From seeing siding repairs going on about town, they're using TYVEK in front of the vinyl siding.  Don't if they did this on mine (aluminum siding).   Question is, I'm having a garage built which will be vinyl siding, and finished inside (by me).  IF they use the house wrap over the OSB sheathing in front of the vinyl, do I need to use plastic over the paper-faced insulation, or is that overkill?  Of worse, just asking to trap moisture?   For what it's worth, all the old blown-in I've removed in the house was dry, and when I had occasion to get into the paper-faced to add a receptacle, that was dry as well.

House wrap, TYVEK is a water barrier that allows moist vapor to pass but not liquid water,  so you will not be trapping moisture between the plastic vapor barrier on the inside and the Tyvek on the outside.    Roly

[Robert Adams]

In that climate you dry to the outside so you need a moisture barrier on the inside behind the drywall. On the outside you need a vapor permeable water barrier.

        Here in TX you either have your vapor barrier on the outside or none at all since we dry to the inside.

       The plastic goes on the side closest to the highest humidity source basically.

[srv52761]

You mention heating in winter. Are you planning on A/C in the summer?

[KC]

Garage will be 28' W x 24' deep.  16 x 24 of that will be 'shop' that can also house a vehicle when necessary.  No A/C.

[Robert Adams]

Garage will be 28' W x 24' deep.  16 x 24 of that will be 'shop' that can also house a vehicle when necessary.  No A/C.

          Oh how I would love to live somewhere that I didn't need AC for a shop....

[Turner52]

Wisconsin code. Tyvek on all detached buildings if heated. Not required if unheated. Interior insulation requires a vapor barrier. The paper backed is considered a vapor barrier. You do not want a double vapor barrier on the inside. I prefer unfaced insulation with a poly barrrier. Fewer seams and leaks that way. As has shown up in previous posts here the codes where you are might be different. Even if not having anything inspeted I would find out what they are and follow them.