[Greenbuilding] Drying House in Humid Season
nick pine
nick at early.com
Wed Nov 13 10:55:35 CST 2013
"conservation architect" <elitalking at rockbridge.net> wrote:
>This past year, I have added absolute humidity to my observations with the
>use of a psychometric chart using temperature and relative humidity. This
>also gives dew point. Absolute humidity and dew point are directly
>proportional...
I wouldn't say "directly," as in Tdp = kw, where k is a constant, altho air
with a higher absolute humidity has a higher dew point...
Vapor pressure P = RH/100e^(17.863-9621/(460+Tdp)) "Hg, at 100% RH, with the
dew point temp Tdp in degrees F, so Tdp = 9621/(17.863-ln(P))-460.
So 70 F air with a 50% RH has P = 0.374 "Hg and Tdp = 50.5 F and humidity
ratio w = 0.62198/(29.921/0.374-1) = 0.00788 pounds of water per pound of
dry air.
Doubling w makes P = 29.921/(0.62198/(2w)+1) = 0.739 "Hg and Tdp = 69.6 F,
not 111 F.
These formulas could be used with a spreadsheet to estimate the performance
of dehumidification by ventilation during dry spells in a normally wet
season, starting with measured hourly local weather data including dew
points for Stirling VA...
http://rredc.nrel.gov/solar/old_data/nsrdb/1961-1990/tmy2/State.html
"Ross Elliott" <relliott at homesol.ca> wrote:
>>http://draftexample.squarespace.com/storage/SHWG%20Nov.%205%202013%20Ross%20Elliott%20POST%20FINAL%20w%20transcript.pdf
> Re: page 78:
>
>> ... It's an 8000 SF weddings events facility with 15' ceilings, or
>> roughly like a 15,000 SF house with 8' ceilings. Last winter before the
>> final heating system was installed, this entire building was heated just
>> with a single 700 watt plug-in heater and was always comfortable.
Perhaps Ross can explain further. When I read this, it seems to me it was
saying that 700x3.412 = 2388 Btu/h can keep a house with about 19K ft^2 of
external surface ALWAYS comfortable, eg 60 F, even at night and after a few
cloudy days, with no other sources of heat, on an average -11.5 C (+ 11.3 F)
January day, with 60 = 11.3+2388Rv/19K, ie the external surfaces including
windows have a real average (60-11.3)19K/2388 = R30 US R-value, with no air
leakage at all.
To me that seems economically overinsulated, as in a typical German
Passivhaus, compared to a less expensive North American building with more
solar heat and less insulation and the same yearly fuel bill.
> It seems to me that this building could have been less expensive with less
> insulation and airtightness and more passive solar air heaters like Gary
> Reysa's
> http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htm
Nick
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