Published on May 7th, 2013 | by Henri


Insulating walls of a 1950s ranch

Q.    We have a 1951 vintage ranch in northern NJ. The house now has good modern Andersen windows. But I’ve found that the original exterior walls literally are hollow — no insulation of any kind whatsoever. Adding insulation to these empty walls is my latest project.

The wall construction from the inside out is: Sheetrock, air gap (2-inch by 4-inch stud space), chipboard with black tar-like surface, wooden clapboards, plus 1987 foam board, approx 3/4″ thick with a silver surface facing outside and vinyl siding.

Because of the finished exterior vinyl siding, I’m inclined to add  insulation by boring through the Sheetrock with a hole saw, blowing  in cellulose insulation, then patching, spackling and painting. I  know it will be messy this way. Fortunately, I only have 3 rooms to do.

However, in mentioning this plan outline to a coworker, he was concerned about my lack of a vapor barrier. He was of the opinion that the cellulose could become a humidity sponge that would then create an ideal environment to foster the development of mold. Your thoughts?

Also, any suggested alternatives to blow-in cellulose, such as spray-in expanding foam, would be appreciated.

A.    First — a slight digression — although “Sheetrock” is used generically, it is the trade mark of USG and should be capitalized. (I have made that change in your question.)

You will be taking a slight risk blowing in cellulose as you now have a vapor retarder in the aluminum foil of the rigid insulation added in 1987 — on the wrong side of the walls.

You can retrofit a vapor retarder on the inside walls, but you’ll never be sure that it is foolproof. This can be done by painting the inside of the exterior walls with B-I-N topped with a finish paint of the color of your choice, but there will be interruptions in the coverage at the intersection of the interior wall with these outside walls, and where there is trim, electrical boxes, etc.

A safer alternative is to have closed-cell polyurethane sprayed in the stud cavities. It is an expensive process, but it will reduce your energy bills substantially while increasing your comfort. You should recover the capital cost over the years if you stay in the house long enough. Or,  if you move, the house should be easier to sell.

One word of caution: Insist on closed-cell polyurethane. Do not accept open-cell polyurethane such as Icynene; it absorbs moisture and should never be used if there is a vapor retarder on the cold side of the walls in moderate to cold climates.

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