Relative humidity is not only relative, it is a foggy topic. Many believe that 50% RH is best for health and comfort, but data suggest otherwise. Dr Joe Lstiburek answers the question during a Building Science Fundamentals class based on solid research.
Andy asked the question: In some building science circles, I have seen a tendency where people want to force 50% indoor RH regardless of the season for personal comfort. Is this still poor practice, even if using a perfect wall design in a new building?
Well, there's no short answer. There's no correlation for 50% for comfort or for health. But if somebody wants 50%, we could make it work with the perfect wall, the perfect roof, and the perfect foundation.
Everyone uses 50% RH. Okay, hospitals are maintained at 50% relative humidity. Art galleries are maintained at 50% relative humidity. Data processing centers are maintained at 50% relative humidity. We try to reduce the relative humidity in indoor swimming pools to 50%. Those are the common places where we're talking about 50%.
I don't believe residential occupancy, office occupancy, and commercial office space occupancy is necessary. Nor do I believe it from a health perspective, based on the latest work, based on Stephanie Taylor's work. But hey, you know, if somebody says the art gallery's got to be at 50%, I get it.
50% RH came from hiding art in salt mines during WWII
You might have heard about where the 50% spec came from. During the Second World War, everybody knew that Hitler was going to take over Europe. What happened was Churchill said, "Look, if we give up state and land and whatever, that's just going anyway." The British had an enormous amount of artwork, and they knew that the war was coming. So in 1939, they put most of their important artwork—their paintings—in salt mines. And the salt mines were, believe it or not, at 50% relative humidity and 75°F. The artwork remained in the salt mines until 1948. From 1939 until 1948, the artwork was maintained under those constant conditions. The art historians were absolutely amazed and impressed, thinking, "Oh my God, this is the perfect condition for artwork." So that's where the 50% relative humidity at 75°F spec for artwork came from.
Research suggests 35% to 40% relative humidity is better
Believe it or not, the Smithsonian disagrees. The Smithsonian did different research and found that it could be as low as 35% or 40% as long as it was constant. So the Europeans have a different requirement for what's optimal for an art gallery than the Smithsonian.
What I'm trying to get at is that the Smithsonian is believable because they actually did research. The British experience was that they just happened to find a convenient place for the stuff to end up. So, what I'm saying is a lot of the stuff that we're asked to do is not based on research but is just what happened to be working under the time as a result of the circumstances that led us to have to live with that.
It's kind of funny because the Europeans won't let the Smithsonian show any of their artwork unless they change the relative humidity to 50% at 75°F. There are some art galleries where they have a European number and an American number. I've actually been involved in saying that this part has to be maintained at a higher relative humidity temperature than that part. So, there you go.