I'm re-drywalling and re-insulating one of my bedrooms, so I've taken it down to the studs. For the ceiling, when I apply the vapour barrier, should I remove the old paper barrier that was installed when the house was built? Or is ok to leave it there?

I'm assuming it's fine to just put a six mill poly over it, but I'm having one of those "I'm not sure" moments. The other rooms I did, I removed the paper barrier, but that made it a challenge to keep the insulation between the ceiling joists while applying the new vapour barrier, so I would like to avoid that this time 'round

 

Before you put up the poly, just slit the paper to insure you have only one vapor barrier. - no need to remove it. It will hold the insulation in place.

Seal around the electric boxes to complete the barrier.

Dick

 

Thanks for the advice. Out of curiosity, why only one vapour barrier? Is that so moisture chan't get trapped between the poly and the paper?

 

6 mil polyethylene should be your last choice as a vapor retarder in walls and ceilings most everywhere but extremely cold northerly climates where vapor traveling through the building's thermal envelope should be stopped as much as possible.

For the rest of the continental US, 6 mil poly should only be used under concrete slabs and over masonry foundations.

Drywall manufacturers do not recommend using polyethylene vapor barriers under their products at all.

For ceilings, however, no vapour barrier is required if you have a ventilated attic.

For walls, no vapor barrier is required under the drywall provided your paint provides a perm rating of less than 1.

Most standard latex wall paints provide all the vapor barrier required by Code.

 

Ontario, Canada. Vapour barrier is required by code. Six mil poly

 

Ontario, Canada, vdr to 0.78 perm is required, but there is an ADDITIONAL requirement caused by the use of low-perm exterior sheathing, that causes the perm rating requirement to drop to 0.26 perm. This necessitates the use of poly in walls with OSB / plywood.

Ceilings don't have exterior sheathing, so paint is acceptable. This also applies to cathedral ceilings with vents shutes.

Similarly, in walls, OSB (0.77 perm) meets the requirement for a vapor retarder, and can be used AS THE VAPOR RETARDER so long as you sandwich it between insulation, say fiberglass on the inside and foam board on the outside. This also applies to unvented metal roofs -> exterior foam board + interior fiberglass.

There's a table in 9.25 that tells you how much exterior insulation you need in order to use the sheathing as the VDR.