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Re: GSBN:More from Kelly about China

Congratulations, Kelly,

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 were measured.

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?

Derelict Rough

Derek Roff
Language Learning Center, Ortega Hall Rm 129, University of New Mexico
Albuquerque, NM 87131  505/277-7368 fax 505/277-3885
Internet: derek@...