Insulation Installing siding

Published on February 1st, 2013 | by Henri


Improving insulation while replacing windows and siding

Q.    I am planning on replacing the windows and siding on my 45-year-old ranch home. It is built of standard 2×4 wall construction, fiberglass insulation and plywood sheathing. I plan on removing the existing wood shingles and replacing them with vinyl siding. At the same time, I would like to increase the R-factor of the walls by adding a layer of 1-inch thick foil-faced polyisocyanurate rigid insulation by Dow (Super Tuff-R — 6-1/2 R-value) under the vinyl siding. As this is a do-it-yourself project, the labor cost is not a factor.

Would this likely cause condensation problems within the walls? Should I tape the seams between the rigid insulation boards? Should I apply Tyvek housewrap under or over the insulation boards? The house has forced air heat and air-conditioning.

A.    You have a great plan! This should really increase the efficiency of your house and make it a lot more comfortable. You didn’t mention it but I assume you are also going to add insulation to the attic, or you have already done so, to obtain an R-factor of 36 or more. This will complete the job.

It is quite possible that the dew point may be reached within the walls unless you have an effective vapor retarder protecting the fiberglass insulation. It depends on the size of your house, the number of people living in it, your habits and the relative humidity in the house in winter.

If you can ascertain that the vapor retarder consists of a plastic film (it is possible in a 45-year-old house), you should be OK. But if it consists of Kraft paper integral to the fiberglass, I would be concerned. In that case, caulking all joints between trim and wall finishes with a paintable caulk, and painting the walls with B-I-N and your choice of finish coat or two coats of a low-perm paint should provide you with some security. Also use closed-cell gaskets behind all your electric outlets and switches.

Yes, tape the seams between the insulation boards but skip the Tyvek as the foil-faced insulation and its taped joints will give you the needed protection against any wind-driven water penetration.
Good luck with your project; it sounds exciting.

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