There are a couple of comments that stick with me regarding XPS efficacy over the long haul:
1. A contractor uncovered foam insulation he had installed approximately 10-15 years ago on a house. He found gaps between the foam panels due to shrinkage over time.
2. Martin Holladay interviewed a representative from a major XPS manufacturer who said they were constantly reformulating their XPS products which could mean further unforeseen problems.
The questions that arise from these comments:
1. Assuming shrinkage of XPS can occur regardless of past or future formulations, will overlapping of 2 or more layers of foam board significantly reduce air leakage?
2. Will foil-backed tape (unless there is a better product available) be adequate to reduce leakage as the adhesive dries out and still hold in place as the XPS shrinks?
3. Will the pressure of foam boards on top of each other hold the tape with its residual adhesive in place?
4. Will the tape on the top layer of foam board be more susceptible to peeling off because the furring strips for the siding will be the only intermittent pressure on it?
5. Since gaps between foam panels can occur during installation, what high quality foam compatible calking brands(e.g., Geocel) will more successfully withstand shrinkage?
6. Mr. Holladay frequently mentions that housewrap is optional in this type of installation. Would the housewrap actually provide additional protection over the long term as panel, tape and calk shrinkage occurs so the wrap reduces the risk of water entering the housing envelope?
I’ll be off line for a few days and won’t be able to answer any questions.