We have massive condenstion on both our bow windows.
Not in any of the bathrooms, not in the kitchen, not on any of the
original awning windows (dating back to the 60's). The living room
has two windows: one awning, one bow window. Condensation affects
ONLY the bow window.
I'm beginning to think that the problem could be due to a defective
installation - or to a poor-quality product. (We don't know who
installed them, or when. They were there when we bought the house.)
We have to replace them (the gyproc underneath is beginning to rot).
Can we have new bow windows put in, or would it be safer to go with
something that does not project out too much from the wall of the
I assume you have dual pane glass. Window glass construction of dual
pane is not made equaly by different manufacturers,.
Also you may not have Argon filled in your Bay.
Even new windows differ considerably co to co. Anderson condense less
- at higher humidiiy than Pella. [ I have both ] Windows are rated for
Condensing Factor, which can be found in an old consumer reports
article. Condensation is related to R value and construction. Tri pane-
Argon - with a modern plastic thermal break will condense the least and
have highest R value.
Use ratings for a new unit. There is also light tranmittance and solar
heat gain to consider depending if you need winter heat gain or your
exposure is N S E or west. There is alot of sience to new glass these
days, do your research you usualy get what you pay for
Not having seen the specifics of your situation, my guess is that the
condensation is due to the fact that since the glass of the bow unit
projects out from the inner wall surface more than a "normal" window
there is a pocket of relatively still air in the window well (ie the warm
air coming out of the register beneath the window doesn't get a
chance to affect the air against the glass) so the warmed, moist interior
air immediately adjacent to the cold glass lingers for long enough to
get cooled to the dew point, resulting in condensation.
The same thing happens to your car's windshield on cool, rainy days
in summer or every day in winter and you already know what one
solution is: increase the rate of air movement past the glass (
preferably with heated, dry air).
So for the bow window situation, a workable solution (short of
replacing the glass with superglass (ie triple-glazed, double low-e
films, warm-edge spacers, inert gas fill) would be to re-route the
warm air register that is currently on the floor beneath the window so
that it directs moving air past the surface of the glass, the bottom-most
portion of the glass being most critical.
~~~ * ~~~
Kanata, Ontario, Canada
(winnow the "chaff" from my edress in your reply)
On Tue, 17 Feb 2004 09:08:08 -0600, "PhotoMan"
What does a bow window have between sections of glass. Or is the glass
curved? To my thinking a bay window is a flat window extended out from the
wall. A bow window is a number of panes forming an arch.
A vertical frame. Mine are wood inside with aluminum exterior cladding.
They are just a series of flat windows joined at a slight angle to
create the arc.
If you have money to burn, one of the big window companies (Anderson,
Pella?) offers a corner window that has the glass bent 90 degrees so it
wraps around the corner of the building. Very cool look.
I've got a bow window with 6 verticle rows of 5 panes. All set in rather
small mullions and mintons or what ever they are called. The previous owner
had installed individual second panes made of plexi-glass. I replaced them
with 6 low-e framed panes covering the 6 rows. It works well, with virtually
no condensation between panes and none on the interior.
I've seen that corner window. As you say, cool. I don't know how long it
I really wanted to use it in the corner of my living room when I
replaced all my windows. Unfortunately I have a hip roof house so it
would have required cantilevering the headers on both walls. More
headache than I wanted to deal with. May not have even been possible
with the the 8ft wide bow adjacent to the corner in question. Maybe in
the next house (sigh).
The previous post re: heated airflow is one part of the problem. Large
windows dump heat.
Bow windows are typically casements, which should seal as well as the
awnings from the same window order. Exceptions would be 1)large sash with
insufficient draw locks (ie 6' sash with a single locking point) and 2)
Inadequately supported bow and bay combos often sag from the unit weight and
roof construction. This would affect the seal contact. To determine if
this is contributing, examine the side (flanking) sash for an out-of-square
condition relative to their frames. A 45 degree projection bay with large
flankers would be the most likely to exhibit this, a three unit bow with
small flankers the least likely.
Also, the head and seatboards of projecting windows should be insulated in
heating climates. Sounds obvious, but I've seen it omitted often. This
would create a zone of cooler air on the inside of the windows, contributing
If your comments about "seal contact" are intended to suggest that cold air
leaking in around the window sash are a cause of the condensation on the glass,
I'd have to disagree and say that the opposite is often the case.
On a window which is fogged up due to condensation, if there are small air leaks
present around the sash, one will usually see clear areas on the glass
immediately adjacent to the point(s) of leakage, but there will also likely be
puddles of water (or ice) below that area too, on the sill.
Even cooler air which is moving fast enough past the glass so as to not linger
long enough to reach the dew point, will not result in condensation.
If re-routing the warm air supply vent to immediately beneath the window as Rob
Tom suggests is not an option, a reactive solution would be to make shallow
sheet metal pans or troughs to collect condensate so that it can't wet moisture-
susceptible materials; the same principle as that of placing condensation
gutters beneath the edges of skylights.
If the condensate isn't too plentiful or there is someone around to empty the
accumluations regularly, then drywall J-trim (or similar) may suffice for the
I would say that it condenses as soon as the temperature drops to
single digits (Celcius, that is). Here in Ottawa, that usually means
October-November to March-April.
As suggested, it could be an insulation problem - the windows are
double-panes, with decorative muntins bars sandwiched in between them
- and muntins reportedly lower the R-value and condensation
resistance. The seals must be OK, since there is no condensation
between the panes - just inside the house.
It could also be a size problem: each bow window is 7 floor-to-ceiling
panes of glass.
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