Hi Folks! Can you put a vapour barrier over top of existing drywall before new drywall is installed over top of the old drywall? There is existing vapour barrier behind the old drywall, but it is thin plastic and not taped or sealed.

This sounds like a renovation to an existing room. So as long as the walls you are referring to are exterior walls, there is no reason why you cannot install an additional vapour barrier membrane over the existing drywall before adding the new drywall layer. In fact, it’s a good idea because the old vapour barriers used in the 1960’s and 1970’s were thin (usually 2 mil, compared to 6 mil today) and poorly sealed. If you really want to go the distance, you can also install vapour boots behind your electrical outlets on all the exterior walls. Be sure to seal these vapour boots to the new vapour barrier with tuck tape or acoustic sealant. Or you can replace the old boxes with vapour tight, gasketted outlet boxes. This ensures that no moist air can seep into the exterior wall cavity through the wire openings in the receptacle boxes.

Another alternative to installing an additional vapour barrier is sealing the exterior wall(s) with vapour barrier paint. Most local paint stores can supply this product. Vapour barrier paints often have a perm rating of about 4, which is high enough to be very effective in preventing moisture leakage into the wall cavity. Since you will be painting these walls anyway, this product can save you the time and aggravation of installing the additional vapour barrier.