Where to put vapor barrier in relation to interior sheathing
martinenv
| Posted in Energy, Heating & Insulation on
I have a 100 yr old house in Seattle that has 1×5 clear fir T&G on the exterior. There is no plywood or boards under the the T&G. I am remodeling two rooms and plan to put plywood on the interior side of studs to strength the exterior walls for earthquakes. I will cover the ply with sheetrock and finish. My question is: what insulation should I use and should I apply a vapor barrier? What type of vapor barrier and where should the vapor barrier go; inside or outside of the ply?
Doug
Replies
Are you
insulating walls that are currently un-insulated? If so, I would probably blow them full of cellulose before adding the plywood. Assuming your walls are not currently insulated, and that there's no felt or other weather-resistant barrier (WRB) installed under the siding, you're taking a small risk by insulating, but the reality is that older homes without a WRB are retrofit-insulated with cellulose all the time. If there is currently water infiltrating into the walls from the outside, the presence of the insulation will slow the drying and accelerate any degradation of the framing. You will probably be able to tell if you have any problem areas once you open the walls to remodel.
As for a vapor barrier on the inside, you do not need one. Just install the plywood and sheetrock. It's not nearly cold enough in this area to require an interior-side vapor barrier. You do want to try to air-seal your plywood and drywall, and you do want to control interior humidity, but no VB.
I think most plywood might be considered a vapor retarder (i.e. it has a perm rating of less than 1.0). But to answer your question, it wouldn't make much difference if you put one under or over the plywood.