Re: GBlist: Dustmites

Loren Abraham (loren@cstone.net)
Thu, 27 Feb 1997 11:20:43 -0500

Mark wrote:

> I'm curious what RH you are aiming for in the winter. My own
experience in
> homes I have either built or designed is that in winter, because
the homes are
> quite airtight, they need ventilation or they reach excessive RH
(which I
> define as much greater than 40% - certainly I like to see it
below 50%, because
> of the dust mite issue.)
>
> --- Loren wrote:
> As to the RH, I think it is just as much a problem to have too
> little as too much. We have to use a humidifier in winter to
> maintain an adequate RH or he has problems breathing.
> --- end of quoted material ---
>
> The homes that need a humidifier are those which are either
leaky or just so
> large in ft2 per occupant that it's hard to keep RH up. I've
designed the
> envelope on one home that has two occupants and is 4500 ft2 and
still keeps
> comfortable RH levels. I would think that in the milder climate
of VA
> (compared to mine in northern NE) it would be less likely that a
home would
> need a humidifier because of higher absolute RH in the outdoor
air.

This is a common misperception. Outside RH is not an indicator of
indoor RH. E.g., as temperature decreases RH increases so that at
20 deg. F, a common January temp. here in VA, it is common to have
outside RH of 75% or more. As one moves indoors however and the
temperature increases to 68 deg. F, the RH plummets to only
15-20%. This is not enough moisture for comfort as it dries out
air passages, skin, eyes, etc.

You can see this readily by observing moisture content in dried
wood left outdoors in the winter. The high relative humidity
drives large amounts of moisture into the wood (up to 50% moisture
content or more), which when brought inside a heated environment
without moisture controls, rapidly loses moisture to the desirable
20+/-5%, and consequently bows and twists. People go through a
similar change as they move from outdoor high RH to indoor low RH,
but instead of bowing and twisting, they simply become stressed.

To answer your question, I agree that 35-40% RH is comfortable
for most people (there is some evidence that dustmites are
comfortable in this range as well) and if you have a fairly tight
home, normal activitiy (showers, cooking, perspiration, etc)
maintains a
comfortable RH level in winter. However in most homes which are
relatively leaky, i.e., .3-.7 CFM psf/hr or more, one must augment
this RH through some additional humidification. This is even more
of a problem for homes with forced air heat than for those with
radiant hot water or steam heat.
>
> BTW - Terry asked quite a while ago about the existence of
studies showing
> adverse health effects at low RH. I believe no one replied. Any
comments
> about acceptable RH range for human health?
>
> Marc

See Hal Levin's message from this thread dated: Wed, 26 Feb 1997
12:08:18 -0800
Thanks Hal.

Loren
_____________________________________
Loren E. Abraham, AIA, IDSA
President - DTI/ERG
912 St Charles Avenue, Charlottesville, VA 22901
Vc: 804 296 4197 Pg: 804 963 1511 Fax: 804 971 3729
© 1997 Daybreak Technologies Inc. / Environmental Research Group

__________________________________________________________________
This greenbuilding dialogue is sponsored by Oikos (www.oikos.com)
and Environmental Building News (www.ebuild.com). For instructions
send e-mail to greenbuilding-request@crest.org.
__________________________________________________________________