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Posted by on July 9, 2005, 8:53 pm
>>Based on long term data logger use here, the times when ventilation
>>will work is from the middle of Otober to the middle of March.
>
>We've been through this before. NREL says Wilmington NC has a 63.4 F
>deep ground temp, with these long-term averages...
>
> Apr May Jun Jul Aug Sep Oct
>
>Avg daily max 74.0 80.8 85.4 88.5 87.6 85.2 76.9 F
Avg daily min 50.5 59.3 67.5 71.7 71.0 65.3 53.7 F
>humidity ratio 0.0080 0.0114 0.0147 0.0168 0.0167 0.0142 0.0099
>vapor pressure 0.380 0.539 0.691 0.787 0.782 0.668 0.469 "Hg
>dew point 56.7 60.6 67.7 71.5 71.3 66.7 56.7 F
Outdoor air that leaks into the basement in October through May won't
condense on the 63.4 F floor. It might in June through September...
If the house is AC'd to 80 F and 55% RH (the most economical corner of
ASHRAE-55 2004), with Td = (460+80)/(1-ln(0.55)(460+80)/9621) = 62.5 F
and we move air near the basement floor up into the house whenever its
RH rises to 60%, the house air that replaces it won't condense in the
basement. It will warm the basement floor while providing free coolth
for the house :-)
If the house is not air-conditioned, we have at least 5 low-energy
dehumidification/cooling strategies: 1) improve house airsealing,
2) fix other basement moisture sources, eg vent unvented clothes
dryers, remove firewood stored in the basement, fix gutter leaks
and improper grading, and paint floorslabs installed with no vapor
barriers, 3) dry out the basement concrete in wintertime by keeping
the RH low (this only requires a little warm air from the house),
so the concrete can absorb moisture from air leaks in summertime,
4) ventilate the basement with cool dry outdoor air in summertime,
on rare occasions when that's possible, and 5) circulate air between
the basement and the living space during the day with no ventilation
to the outdoors, when the living space requires cooling.
We (not you :-) might tweak these strategies with a TMY2 simulation.
Nick
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