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Re: GSBN:More from Kelly about China
- To: GSBN GSBN@...
- Subject: Re: GSBN:More from Kelly about China
- From: Derek Roff derek@...
- Date: Tue, 25 Jun 2002 12:49:07 -0700
- Reply-to: "GSBN" GSBN@...
- Sender: "GSBN" GSBN@...
It is great to hear about the careful scientific data logging showing
the advantages of the SB houses in China. Can someone forward this
response to her?
-- Kelly, via Paul, told us:
They also observed that the relative humidity (RH) in
the straw-bale houses (SBH) is about 10% higher than in the brick
houses (RH in SB 61%, RH in BH 51%). The RH of the SBH is much
closer to the outdoor RH of 66.5%.
I'm postulating that the straw bale walls are more vapor permeable
than the brick walls and thus the interior RH follow the exterior
RH humidity more closely. It may also be that thicker plaster on
the straw bale walls (compared to brick walls) is acting as a
moisture sink and working to balance the RH between times of high
RH and low RH. Any other thoughts??
How can we explain the higher relative humidity (RH) in the strawbale
houses, compared to brick? Vapor permeability? I doubt it. Kelly
doesn't mention the average outdoor temperature in the RH example
above, but assuming it is pretty chilly, then the indoor air contains
a lot more water than the colder outdoor air, at a similar RH. Let's
remember how relative humidity works. RH shows the amount of humidity
in the air, relative to the maximum possible amount of humidity that
the air could hold at that same temperature. Air will hold much less
moisture at lower temperatures. If the outdoor air was at -5 degrees
C when they measured the relative humidity of 66.5%, then moving some
of that air indoors and warming it up to 20 degrees C will reduce its
relative humidity to about 10% (according to the RH calculator that I
found on the web). If it is -18 deg. C outdoors in the winter in the
places where Kelly is working, the RH of outdoor air, after being
heated to indoor temperatures, would become about 5%. Moisture won't
be flowing from outside to inside in the winter at the RH values that
I also doubt the "moisture sink" explanation. The plaster might
moderate humidity swings over a short time period, but plaster
couldn't store enough moisture to maintain a significantly higher RH
over the course of a winter. Assuming that the figures given were
average values over time, we have to explain how the SB house
maintained higher average humidity than the brick house.
My best guess is that the difference has to do with the heating
cycle, directly and indirectly. Not knowing the details of the
heating systems, we can't be sure of anything. If the heating system
exhausts air from the living space, as a fireplace or unsealed heater
does, then an equivalent amount of air must be drawn into the house
from outside, usually through open windows and/or small leaks and
holes. Assuming that the brick house and the SB house are equally
tight (resistant to air leakage), we could explain the higher
relative humidity of the SB house by the lower use of the heating
system. Here's my reasoning: If building a fire/running the heater
sends some living space air up the chimney, then each time you heat,
you will expel more inside air, and draw in more outside air. Since
the outside air has much less water in it, it will lower the humidity
of the inside air, as it warms up. The SB house requires less heat,
so it loses less of the humid indoor air out the chimney, and brings
in less of the drier outside air.
Another possibility is that the SB houses are a little bit tighter
than the brick houses. Although an unplastered bale wall would let
more air pass than a brick wall, it may be that the plastered bale
wall is a better air barrier than the brick wall. Maybe the
detailing at the wall-ceiling joint is better in the SB house. Or
perhaps there are other differences in construction that make the SB
house tighter. (It is also quite possible that the SB house is
leakier than the brick house). A tighter house will lose less humid
air, and therefore maintain a higher relative humidity. If Kelly
doesn't have good reason to suppose that the SB house is tighter,
then I would bet against this explanation.
Any other ideas?
Language Learning Center, Ortega Hall Rm 129, University of New Mexico
Albuquerque, NM 87131 505/277-7368 fax 505/277-3885