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Re: GBlist: Air Vented Windows

In a message dated 10/13/97 3:33:37 PM, you wrote:

<<Regarding the 'best' window thread, I am not familiar with air vented
windows and would appreciate more information; several of you have said you
were going to look into these some more, but please let us know what you
have about them at this time.
                                        thanks, Sacie Lambertson>>


An air vented window is a window with two or more layers that has air between
the panes and has some small air leaks at the edges.  So for example, a
single glazed window with a storm window is an air vented window.  Pella
makes air vented and sealed edge windows.  The air vented Pellas have little
holes leading from the air gap between two layers of glass to the outside.
 There used to be, and may well still be a Canadian window manufacturer who
made air vented windows designed to provide make-up air for exhaust only
ventilation in heating climates which preheated the incoming air with the
energy moving out through the window by conduction and radiation (an elegant
idea called the dynamic window by the Canadians and the Solar staircase by
Norm Saunders).

There a re a few important things about air vented windows.

1)  Sometimes they fog up.

2) Sometimes stuff gets between the layers (particles and arthropods mostly)

1)  Fogging happens when the dewpoint is reached inside the window.  This
happens for a few different reasons.  First, if warm air enters the gap
between the panes moving from the warm air mass towards the cool air mass,
the air is cooling as it passes through the window, increasing the likelyhood
of condensation.  The solutions: plan the air pressure relationship between
the inside of your house and the outside so that air moves from the cold air
mass towards the warmer air mass, drying out the window interior and air vent
the window to the colder side.  What this comes down to in practice is:

	for heating climates - depressurize the house (with all due respect to
backdrafting, spillage and other running negative issues) and air vent to the
outside (e.g tight prime window, leaky storm) and you won't have chronic
condensation between the panes.  (You can see that observation of condesation
on windows can tell you a lot about whether the prime window or the storm is
leaky or tight)

	for cooling climates - pressurize air conditioned houses and air vent to the
inside (e.g. leaky prime, tight storm).  Truthfully, I have no field
experience with air vented windows in a primarily cooling climate.  The
buildings I've worked on in the south were mostly single glazed windows, in
the Gulf coast, often those louvered jobs, and sometimes sealed edge.  Any
Gulf coasters care to join in?

Another reason that condensation occurs in the gap is if the window and storm
are leaky enough for moisture to enter because of air pressure or partial
water vapor pressure differences , but tight enough so moisture that migrates
in during the day has trouble getting back out at night when the glazing
cools off.  Then the dewpoint is reached and the window fogs.  Pella had a
lot of problems with this because the air vent hole size they used says "put
larvae and host here" to some of the mud-dauber species of wasp.  I found
that changing the hole size changes that, and so did Pella.  (Wasps have a
single gangleon for a brain, so it is kinda hard for them to change their act
(course I have a big brain and suffer from the same problem)).  You can also
just go clean the hole out with a nail when the window fogs.

2)  Lastly, dirst, dust and bugs between the panes.  Guess the holes are too
big. Keep the vents small, or covered with filter fabric, and make it easy to
open the gap and clean them is my best advice.

The question I'm intersted in now is comparing the life cycle energy and
environmental costs of sealed edge windows to those of air vented windows.  I
do not actually have an idea of whether there is a clear path or not, but the
first order stuff is close enough that I want to explore it. Hope this helps.
 - Terry
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