[Greenbuilding] Relative humidity in heated houses

[John Straube]

As far as I know, the studies referenced in the Sterling paper are not the problem, the translation of the results in those references studies from glove boxes to buildings, and single data points to linear lines.  The beautiful graphic is very compelling, and has received the most  criticism.  And the takeway of "the best RH is between 40 and 60%" is dangerously simplified and not correct. I dont know the Oregon study John, and would be glad to hear more of this one.

A lot of knowledge is not written down in widely available papers. Most of what I know on this topic comes from attending ASHRAE conferences, and IAQ meetings over the last almost 20 years not the paper reference below, or several more that I have.  The reason for my "scorn" is that this is a very common question, Sterlings drawing is widely quoted, yet people in the business know this is not a good basis of design.  I have seen many people with serious health impacts because of too high a humidity, and never seen low humidity actually connected to health problems (lots of people claim this, but people are not hospitalized, you cant measure any IgE or similar in blood, etc etc ).  My experience is shared by every forsensic building scientist I have spoken to.  Really low RH (like <20 or so) regularily cause discomfort, although good proportions of  people are unable to identify if the air is dry or normal between 15 and 25% (based on student surveys). 

The only more recent paper I have on my laptop (other than the aforementioned ASHRAE Standard 55, 62, and Handbook of Fundamentals 2009) at the moment is which has some nice relevant quotes:
Baughman, A, and Arens A,"Indoor Humidity and Health: Part 1: Literature Review of HEalth Effects of Humidity-Influenced Indoor Pollutants" in ASHRAE Transactions 1996, Vol. 102, Part 1
"A paper’by Sterling et al. (1985)also addressesthe topic humidityandhealthin buildingsandis the onlycited reference pertinent to humidity in ASHRASEtandard 62-1989. This paper includes a figure that has received wide circulation within the HVAC engineering profession. It graphically depicts humidity impact zones using bars that decrease in width, suggestinga decreasein the effect for eachof the eight environ- mentalhealth factors addressed. Thesebars convergefor all of the eight categories into a nat~’owrecommende"doptimum" zone between 40%and 60%RH;both low- and high-humidity effects are addressed. There is clearly a need for’ such a summary because it has appeared in numerousjournals and conferences(Arundelet al. 1986,1992).It is alsoarrestingly drawnandeasyto grasp, whichaddsto its appeal. ....For the purpose of setting humidity standards, the figure is clearly inadequate."
Who writes the paper is also important to me. Arens is a researcher with a long record of good work that has borne the test of time.

On the cold or influenza front: colds are spread in water droplets from sneezes and coughs . Drier air causes these droplets to evaporate more quickly, and the virus therefore desiccates more quickly and loses viability more quickly. To have this effect noticeable it helps to have low RH.  Apparently viral and bacteria viability does not vary much above 50%RH (does not get worse that is).  The Sahara desert is very dry, and yet people dont have a lot of colds there.  They have lots of colds when packed together indoors where they exposed to each other.  The problem is, essentially none of the viability studies of virus and bacteria that consider RH were conducted in buildings.  This changes how the air actually moves from host to site, other factors that affect viability, etc. Most importantly, it ignores that in cold climates, an air RH of 50% means an RH of 80%+ at most windows, in corners, etc.   Some disease agents are not affected by RH, and some, in the lab, seem counter intuitively to like drier air.  So even here the evidence is less than equivocal.



On 2010-10-27, at 3:11 PM, JOHN SALMEN wrote:

> I was just going to reference that article as well. It is from the 80's but
> I wasn't aware it was 'widely debunked'?? Are the 24 studies it references
> also debunked? I have an article in front of me 2008 that references it as
> well as the humidity/influenza studies. And what about the stuff from Oregon
> a couple of years ago that retested data and did establish low absolute
> humidity as the control.
> 

Dr John Straube, P.Eng.
Building Science Corporation
Somerville MA     Waterloo ON
www.buildingscience.com