The example you provided, childhood asthma on the increase--
>I would not single out building tightness as the main factor but I would
>say it depends on the issue - the rise in chilhood asthma for example
>has been linked to moisture born contaminants resulting from either air
>tight buildings or improperly vented buildings depending on how you want
>to look at it.
--is a favorite topic--I have asthma that started when I was a kid, long
before buildings started getting tight or full of VOCs (they were still
built out of logs back then!).
Two pollutants that can cause asthma are molds and dust mite fecal matter.
(The more I learn about mold, the higher up my list of scary pollutants it
rises.) When we look at buildings--houses and schools, in the case of
kids--what are the characteristics of their construction that would promote
the growth of these two asthma triggers? How does building tightness affect
these pollutants?
Both mold and dust mites thrive in conditions of high humidity--but, that
really means, localized humidity. In the case of dust mites, they thrive in
mattresses and pillows where humans provide the elevated humidity every
night, and they live in carpets and pads that may be installed over poorly
insulated and/or leaky floors over unconditioned spaces where cooler air
and higher relative humidity can operate somewhat independently of room RH
to create micro-climates. Mold likes high humidity and condensation that
occurs in places like bathrooms, kitchens, basements, or at cool
uninsulated framing components--but these locations aren't necessarily part
of a tight envelope, in fact, it's often just the opposite.
For example, for many years Oregon code allowed floors over crawl spaces to
be insulated by installing batts along the inside of the foundation walls
instead of under the floor. Since the code also required foundation vents,
this meant the floors were essentially uninsulated. Adding to the air
leakage was the fact that floors were built post-and-beam, with 2x
tongue-and-groove subfloors. Every T&G joint was an air leak that
transported air right up under the underlayment. As a result, floors were
often cool. Mold and dust mites are common in carpet and pad laid on these
floors. I think part of the reason is that RH in the cellular structure of
cool carpet and pad could be higher than RH in the open and warmer room
air. The solution is to insulate under the floor, and tighten up the air
leaks, to warm up the interior surface.
Another common example of mold growth in our climate is at the top plate
along exterior walls. The code required ceiling insulation and ceiling
ventilation, but didn't pay much attention to how they were installed. As a
result, outdoor air often blew back the insulation along eaves, or got
under the batts in cathedral ceilings, because the bird blocks were loosely
installed. This air could cool the framing enough for mold to grow. "Loose"
construction was the cause of the problem, and tightening up is the way to
keep the air where it's needed for structural ventilation, but out of the
ceiling insulation.
I appreciate that indoor RH contributes to these problems--your point about
ventilation is very well taken; it will certainly help control mold and
dust mite growth (in our climate, anyway) by keeping down the RH. I think
ventilation works best in tight envelopes--a couple of days ago I was
helping Jack Bartholemew pressure test the Habitat for Humanity Earth Smart
house, and we found two of the biggest leaks were in the bathroom. That
meant the bath fan might draw outside air in through the bathroom framing,
instead of through the window vents. When we had sealed up the leaks, the
fan did depressurize the house and pull air in at the vents.
In the case of schools, it's my impression that asthma incidence increased
when schools started installing carpets. School administrators thought of
carpet as creating a warmer, more user-friendly environment, reducing
noise, and so on; but in fact carpets have been a reservoir for a lot of
different pollutants. Carpet laid on poorly insulated frame or
slab-on-grade floors has the same potential for dust mites and mold as in
houses. Carpets may be simultaneous with tighter building envelopes--but,
they would be a problem even in a leaky envelope.
Schools also economized on heating and cooling costs as fuel prices went up
by recirculating indoor air; which, combined with inadequate maintenance of
ducts and equipment, ensured that humidity, standing water and mold and
bacteria growth would occur in HVAC systems.
Our most progressive private school in Portland has just completed a master
plan that simply doesn't mention indoor air quality. I don't know why. If a
school does decide to think about preventing asthma, I hope they look at
delivered air flows, air temperature, relative humidity, interior surface
temperatures, air filtration, floor coverings, and moisture sources like
concrete foundations. My guess is, to reduce asthma by controlling such
variables will require making the building envelope tighter rather than
looser.
You are right in saying that controlled air flow can be a key--one example
is Joe Lstiburek's vented rain wall, similar I gather to exterior wall
design of tall buildings subject to high wind and driven rain--where an air
space in back of the siding acts as a pressure break and a water drain.
This wall would be tight on the interior, and loose on the exterior.
"Letting the building breathe" does make sense for structural cavities, and
the mechanisms are usually passive--so maybe that's why builders want
interior moisture control to be passive, and seize on looseness as a
cure-all?
Mike
O'Brien & Associates
Environmental Building Consultants
Portland General Electric Earth Smart program
obrien@hevanet.com
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