A.Paul Fisette, director of Building Materials and Wood Technology at the University of Massachusetts Amherst and a JLC contributing editor, responds: The short answer is no, it won’t cause a problem. But it’s worth noting that whoever installed the wrap there to begin with didn’t do the wall system much good, either. The primary function of correctly applied housewrap is to provide a water-resistant surface behind the siding, which is why all good housewraps share the ability to shed liquid water while allowing water vapor to pass through. In other words, it works something like a Gore-Tex jacket, which is breathable yet waterproof. (Interestingly enough, Gore-Tex developer Bill Gore once worked for DuPont — the manufacturer of Tyvek — as an engineer.) Encasing the plywood between the existing layer of housewrap and insulating foam isn’t a problem because the housewrap allows diffusion of vapor through and will not trap it at that plane.

The encased housewrap might have some limited value as an air barrier, but I’m not impressed with housewraps in that role because they’re not rigid and they’re rarely continuous. They tend to tear during or after installation and have thousands of air-leaking fasteners holding the fabric in place. Installing a continuous layer of insulating foam...

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