[Greenbuilding] Relative humidity in heated houses

[Michael O'Brien]

Hi, John--

See ASHRAE Transactions 1985, Volume 91, Part 1B, pp. 611-622 for a summary of general health effects of low and high humidities. It includes a widely-used chart.

Best,

Mike O'Brien 

On Oct 27, 2010, at 11:29 AM, John Straube wrote:

> Corwyn, can you please share with us the references to the health studies of lower RH exacerbating colds?
> I have never seen this evidence.  Ranges like "40-60" or "35-55" are bandied about, and there is graphs floating everywhere show that low RH is good for one thing or another (the most famous one is by Sterling, and has been widely debunked), but I, and others, have never seen the research although we have looked pretty hard.  ASHRAE does not, with all of its resources, have anything to offer to show this.  Currently I believe they say there is no lower limit to RH (because there is no data supporting that), but recommend over 25% (because that feels right, but also has no data).
> As far as I know, "low RH" starts to be noticeable to people, and possibly connected to health, when it is really low, like under 20%RH, perhaps even 10 or 15% and I surmise it is these types of "low" RHs that are a problem for people.   Eyes and noses and upper throat do evaporate lots of moisture at these low RH levels, and so people begin to feel itchy eyes, scratchy throats.    
> 
> I regularly run into MDs who suggest people with respiratory problems add humidifiers to their homes (and I get involved because of the disasterous results on the home) and challenge them for the data that supports this recommendation. Never ever got it.
> I do know that people with respiratory illness were often sent to sanatoriums in places like Arizona and New Mexico, which have very low RH's.  But that is anecdotal.
> 
> On 2010-10-27, at 1:56 PM, Corwyn wrote:
>>> 
>> 
>> What constitutes high relative humidity is subject more to preference and other conditions than a hard number.  Lower humidity exacerbates colds while higher encourages more mold.  So, whichever of these you are more susceptible to will drive you opinion on humidity.  The best I could find in the medical literature is somewhere between 40% and 60% is most healthy (and that is probably for average persons).  Additionally, a tight house with a weakness in the insulation envelope somewhere (say, bad windows) will be more prone to condensation while one without such a weakness will allow a higher humidity with no problems.
>> 
> 
> 
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