[Greenbuilding] Humidity between multiple windows

nick pine nick at early.com
Wed May 11 06:13:22 PDT 2011

Bob Klahn <Home-NRG at dnaco.net> wrote:

>If you have access to a blower door, you could build a frame that would 
>just enclose the window, seal a cap on it with a hose barb for the pressure 
>gauge hose.  Pressurize the house, with relation to the outside air 
>pressure, to a benchmark...  Measure the pressure difference between the 
>house pressure and the pressure within your enclosure.

It seems simpler to drill a small hole in the storm window frame and push a 
manometer tube into the hole, then measure the pressure in the 
window-to-storm-window cavity. If it's 0 Pa outdoors and -10 Pa in the 
cavity and -50 Pa inside the house, we might say "the innermost window is 
five times tighter than the outer."

But is -10 Pa the right number above? How high can we make this number, and 
still avoid condensation on the inside of a 4'-tall U1 storm window over a 
U0.5 house window, if the house is 70 F at 50% RH and the outdoor temp is 30 
F with a 0.0025 humidity ratio on an average January day in Phila?

In a 2-story house, I guess this only applies to upstairs windows with 
outward air leakage, vs downstairs windows with inward air leakage.


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