re: an improved solar attic SOLAR.KnowledgePublications.com

re: an improved solar attic
29 dec 2005
http://www.cibse.org/pdfs/8cimbabi.pdf has an equation for the dynamic
metric u-value of a breathing wall, as corrected: 

ud = vrhoaca/(e^(vrhoacars)-1) w/m^2k, where

     v is the air velocity in meters per second,
     rhoa is air density, 1.2 kg/m^3,
     ca is the air's specific heat, 1000 j/(kg-k), and
     rs is the wall's static thermal resistance in m^2-k/w.

using v = 1/3600 (1 meter per hour :-), and rs = 5.7 m^2k/w (a us r32 wall), 
ud = 0.058 w/m^2, like a us r98 wall. a more typical v = 10 meters per hour
makes ud = 1.7x10^-8 w/m^2k, like a us wall with an r-value of 334 million :-)

but less air saves more energy, if outdoor air flows in through the wall with
no heat exchanger: with 0 c outdoors and 20 indoors and a square meter of r5.7
wall losing 20ud = 24000v/(e^(6840v)-1) watts and v m^3/s flowing in through
the wall requiring 1200v = 20ud watts of warming, v = ln(21)/6840 = 0.000445
m/s, ie 1.6 m/h is optimal, and the wall loses 0.534 w, with a 1m^2x20k/0.534w
= r37 effective metric r-value, ie us r213 :-) 

how does this compare with ashrae's 15 cfm per occupant fresh air standard?

nick