[Greenbuilding] Drying House in Humid Season

[nick pine]

"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