Thanks Brian. In instances like that, is there a specific % that is “too high”, in your opinion?
What RH is too high? IMO, any time the RH is above 90% inside the CS (not outside the home). If you have 80% or higher humidity outside the home, if there are open CS vents that allow that air to enter the CS, then the humidity inside the CS is probably near or at the dew point.
You do understand that relative humidity is temperature dependent? Warm, humid air that enters the CS will start to cool. As it cools the RH increases and eventually reaches the dew point. At that point it condenses on building material surfaces. Mold, mildew, and fungal growth need moist conditions plus relatively warm temperatures to grow.
3 Likes
I think Michael gave as good as an explanation as any. But I think the rule of thumb is about 55-65%. Vented crawlspaces I look for other conditions. Elevated humidity alone really does not mean much. It will just be part of a longer narrative (if I include it all). Same with basements, I need a collaborating concern to mention it. Some people recommend dehumidifiers in any subterranean space (that’s not vented).
I would only take readings in those locations if I were doing a mold/moisture inspection on a property. Not during a typcal home inspection.
1 Like