[Greenbuilding] Humidity between multiple windows

Bob Klahn Home-NRG at dnaco.net
Mon May 9 19:22:03 PDT 2011

We always just guesstimated the apparent gaps and watched for moisture, 
on the assumption that a bit too much was good insurance against 
moisture damage.

Here's how I'd approach testing  to compare the leakage:

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 (which could be 50 pascals, 25 pascals or 
whatever you choose.  Measure the pressure difference between the house 
pressure and the pressure within your enclosure.

Move the enclosure fixture to the outside and seal it again to the 
window.  Pressurize the house to the same benchmark difference with the 
exterior and measure the differential between the air inside the fixture 
and the outside air.  If you have a long enough hose, also compare it to 
the living space pressure.

You might also compare the pressure differential between the enclosed 
window (both inside and out) and the living space pressure.

I haven't had an occasion to run such a test but that's how I would 
approach testing for a difference in the leakage of each layer of 
window.  (Some time back, I bought a rig (used) for pressure testing 
windows for leakage.  I haven't had occasion to try it out, but it is 
set up to work this way.  I gather it was provided by a manufacturer as 
a sales tool; to demonstrate the superior seal of their product.  To 
give an idea of its vintage, it is set up with a slant tube manometer to 
measure the pressures.)

Then again, your trial and error technique sounds equally practical - 
and a lot easier.  You might start with two, or more holes; then plug 
them slowly until you notice condensation and remove one plug.  That 
might reduce the risk of moisture damage from unnoticed condensation.

Bob Klahn

On 5/9/2011 6:09 PM, nick pine wrote:
> Bob Klahn <Home-NRG at dnaco.net> wrote:
>> The old rule of thumb for minimizing / avoiding condensation between 
>> windows (e.g. between prime and storm windows) has been to be sure that
> the innermost window was five times tighter than the outer.
> How would you measure that?
> I might be sure the inner window is as airtight as possible and drill 
> one 1/8" weephole in the bottom of the storm window frame, and drill 
> another if there's any condensation during the first winter, and so on.
> Nick
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