Ideal indoor relative humidity for building and occupants?
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Realizing that this is a worn out topic, but wondering if anyone out there has a target indoor humidity level that is both safe in terms of moisture issues for the wall system and comfortable for building occupants.
Twice this week, as a builder, I’ve spoken with people who complained that their high performance homes are too dry in winter, causing sore throats and dry sinuses, to the point of attempting to meddle with their HRVs; dialing them back or turning them off.
The home I grew up in in Northern VT, had a similar problem, but for different reasons; an an old, very leaky farmhouse that had so many air changes in winter, exchanging warm moist air from inside with cold dry air from outside, that we woke with mild sore throats, dry sinuses and sometime headaches.
To combat this, we left a big pot of water on top of the furnace, which was probably okay for a leaky old home, but never a good idea for a tight modern home.
Is it possible the high performance home owners I heard from are over-ventilating?
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Just spent the first winter in our central/northern NH home: tight construction and Panasonic bath fan ventilation w/ passive air inlets. Small house, 2 people, ran one of the fans on 30CFM continuous. 25-35% was our target winter relative humidity to be safe with our double stud wall.
I kept a few of the cheap Acurite relative humidity monitors in various places. This is from memory, but we we're generally between 25-45% during the winter depending on weather. Sometimes close to 20% on super dry days. I always felt comfortable health wise.
I would have these people try a few of the monitors in various places so they have a rough idea of actual relative humidity levels. Maybe they are over ventilating if levels are consistently below 25%? Or just not drinking enough water.
Zehnder HRV systems, perhaps? Zehnder designs use a higher air exchange rate than most people would consider necessary. They now tend to recommend ERV instead of HRV to avoid excessive winter dryness, but dialing it back is more energy efficient.
Generally speaking 25-50% is a good range for winter humidity.
Dylan,
Q. "Is it possible the high performance home owners I heard from are over-ventilating?"
A. Of course. Our houses exist to make us comfortable. If the occupants are uncomfortable because the interior RH is too low, then they should adjust their ventilation equipment so that it runs less often. That will raise the interior RH, which is what they want.
Of course, if they smoke tobacco indoors, they may want a high ventilation rate. But that's another story entirely.
According to data collected by Nest thermostats and shared by Michael Blasnik, Vermont homes have the lowest average indoor RH in February of any state in the U.S. The mean RH in Vermont living rooms in February is 22%. (That doesn't mean that 22% is a good goal, of course -- 25% or more would be a little better.) In Florida, it is 45% RH.
From a human-health & comfort point of view the widely accepted medical standard (as well as the US EPA) suggests 30-50% RH as the Goldilocks zone. (Some have narrowed the recommended standard to 35-45%.) Above 50% dust mite populations take off exponentially, and above 60% skin fungus/yeast/mold risks go up, and above 65% mold growth soars on all sorts of mediums. Below 40% airborne virus susceptibility & bacterial sinus rates start to climb, and below 30% it climbs rapidly.
http://www.ncbi.nlm.nih.gov/pubmed/3709462
http://www.epa.gov/mold/prevention.html
http://www.mprnews.org/story/2013/01/11/health/flu-humidity
https://www.lewrockwell.com/2012/03/joseph-mercola/the-hidden-perp-behind-sinus-infections/
From purely a comfort point of view ASHRAE suggests 25-65% RH.
What's healthiest for your HOUSE may differ from that, depending on how it was built. In general, keeping the house 30-35% RH in winter and under 50% RH in summer may be the best compromise.
It's rare for air conditioned houses to have problems when the interior RH is no higher than 60%. Keeping the RH to 50% isn't always necessary.
Ten percent of U.S. homes have indoor RH during the summer of 55% or more.
The 50% RH isn't a comfort issue, and only a mild mold/fungus issue. But it's an important limit for asthmatics & others sensitive to dust mites (a very common allergen, and asthma trigger.)
Thanks for the responses, everyone.
Brian: Although they're not my projects, I'm intrigued enough to follow these cases a little closer and do as you suggest; convince them to monitor the actual RH a little closer throughout the year, as well as investigate the ventilation rate prescribed for these homes.
Charlie: Not sure what they're using yet but I'm following up on it.
Martin: Good point. A ventilation system or prescribed rate of ventilation that makes people uncomfortable in their own home, needs to a second look. Something basic is amiss there. Always nice to hear a new obscure Vermont record: ", Vermont homes have the lowest average indoor RH in February of any state in the U.S." So proud....
Dana: 50% seems high, but it is likely that or worse during summer months, particularly the last two here in the Northeast Kingdom. 35% seems a touch high for a sustained indoor RH in winter months for building health, but I see what you mean. Somewhere in that range is a sweet spot. Thanks for the info.