Polyiso vs. XPS: correcting a few common myths about R-value, LTTR, temperature, moisture, and UV

We recently received a note from a reader that raised important points—and also repeated a few oversimplifications that can mislead homeowners, builders, and DIYers. Insulation performance is real engineering: it depends on temperature, moisture exposure, and how R-value is measured and reported. There isn’t a single “always wins” foam. Here’s the reader’s message (quoted), followed by the most […]