[Greenbuilding] Humidity between multiple windows

Bob Klahn Home-NRG at dnaco.net
Sat May 14 15:57:13 PDT 2011


Nick,
I like the simplicity of your approach, provided you are allowed to 
drill the probe hole.  One questionable point is relying on the (total 
living space) -50 Pa with relation to outside air pressure as a measure 
of the window's leakage.  I'd feel more comfortable isolating the window 
unit.  It would be interesting to compare the results of the two 
approaches.

I'm not sure what would be an appropriate pressure differential but 10 
Pa sounds reasonable as a starting point; the ratio meets the old ROT.
At first thought, it doesn't seem that the direction of the pressure 
would be critical.  Have to chew on that a bit.


[For those not familiar with pressure testing conventions, the pressures 
are noted in pascals (Pa) and are usually in relation to another 
pressure zone.  Cavity (between windows) pressure as compared to the 
living space pressure.  The -50 Pa reading inside the living space is as 
compared to the outside air pressure.  Outside air pressure is, by 
definition, 0 Pa because it it the base reference point for both 
measurements. The inside pressure differential is typically created/ 
held with a blower door.]

Bob Klahn

On 5/11/2011 9:13 AM, nick pine wrote:
> 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.
>
> Nick
>
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