Siga Majrex

This is a homeowner forum discussion, not a news story, so there is no policy or rebate update here. Someone building a new house in a wet, humid part of southeast Alaska asked about a product called Siga Majrex, a membrane that lets moisture pass through in only one direction. The idea is to keep indoor moisture from getting into the wall while still letting the wall dry out. The builder already has 3 inches of foil-faced polyiso foam board on the outside of the walls, taped at the seams, plus standard fiberglass insulation in the wall cavities. Foil-faced foam already blocks moisture on its own, acting like a vapor barrier.

A building-science expert who replied pointed out that because the foil-faced foam already seals the outside so well, adding Majrex on the inside would not add much benefit — any similar "smart" vapor membrane, or even just painted drywall, would work fine in this case. The original poster agreed, noting that Majrex is not cheap, so he plans to leave it out of most rooms but may still add it in bathrooms, where extra moisture from showers is a bigger concern.

For a homeowner reading this, the general lesson is simple: vapor-control products like Majrex are useful for stopping moisture problems inside walls, but whether you need one depends on what other materials your walls already have. If your walls already include a foil-faced or foam vapor barrier, an added interior membrane is often unnecessary except in high-moisture rooms like bathrooms.