should I use xps or polyiso over plyw...

should I use xps or polyiso over plywood sheathing, under vinyl siding?

Posted in the Ask This Old House Forum


Grafton, MA

#1 Jan 24, 2013
I have a 35 year old wood frame 78 foot ranch currently sheathed w/ half inch plywood and masonite siding covered with vinyl siding. I suspect that the masonite has gathered moisture over the years, as the south side of my house resembles a wash board in the summertime when the sun is on it all day long. In the summer, the house is very hot, in the winter, the house is cold. After I insure that I have no sheathing issues with rot or carpenter ants, I plan on installing either XPS or polyISO rigid exterior insulation under my new vinyl siding. My interior walls are insulated with paper faced fiberglass and covered with full plaster. Should I also install a vapor barrier to keep rain out? If I do this will I be causing a condensation problem under the siding?

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