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Peter,<br>
<br>
I don't intend to start an argument but have to rise in defense of
my comments.<br>
There are three driving forces involved in leakage across the
window, or any other building surface, 1) air pressure, 2) thermal
pressure and 3) vapor pressure. All flows are created by a
differential in one or more of these. Both heat and moisture ride
on air movement. <br>
I rarely see simple explanations, or fixes, to this sort of problem
but physics rules all of them.<br>
<br>
Bob Klahn<br>
<div class="moz-cite-prefix">On 10/28/2012 10:37 PM, Peter Kidd
wrote:<br>
</div>
<blockquote cite="mid:508DEC01.6090108@shaw.ca" type="cite">
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<div class="moz-cite-prefix">Air flow through the window is not
the same as heat flow and in NO way will it always be in to out,
hot to cold. Within one window assembly air may even flow in two
directions, typically in such a case, in at the bottom and out
at the top. Your simple case is incomplete, but in any case
pressures do drive air flow, but not temperature difference
(although pressures can be established because of temperature
but that is not at all the same thing).<br>
<br>
I'd quibble with some other points you raise, but I can't pass
on the above.<br>
<br>
</div>
<blockquote cite="mid:508DD521.8060007@dnaco.net" type="cite"> Two
points, in response to two of your posts (bearing in mind that
I am speaking on the assumption of a cold climate). <br>
<br>
In a cold winter, without extreme exterior humidity and wind -
in coastal Alaska, for example - the flow through any leaks in
the window assembly will be from warmer to colder. The greater
the temperature differential, the greater the pressure driving
the leakage. So, "cold' air should not be leaking into the
space between panes because the air between panes will be
somewhere between the interior air temp, at the inside pane, and
the air temperature against the outside face of the outer pane.
Therefore, the air/heat flow will be from inside to out.<br>
<br>
Of course, that is the simple model. Solar loading and wind
will have their effects. Of course, the ideal is to have all
seams/joints in a window unit hermetically sealed, but that's a
hard thing to achieve and sustain over time. <br>
<br>
Adding appropriately sealed layers to the window units will
change the temperature of the air film under whatever conditions
are producing condensation, but any window can condense, given
the "perfect storm" of conditions. Look to the
RH/temperature/lifestyle conundrum for each "problem" window.<br>
<br>
The solution is to determine the RH/temperature combination that
allows the problem, then locate the moisture source(s) and
change them or the actions/assumptions which are causing the
elevated RH.<br>
<br>
As Mr. Holladay points out, there are several ways to calculate
a given window's condensation likelihood ranges. Unfortunately,
these have to assume a standard set of conditions, which may not
match those at you home, for any give window or any given time.
For all of the complex factors involved, whether or not you will
have a condensation problem on any given window finally comes
down to the surface temperature of the window (which as he
points out varies for different areas of the window) and the
dewpoint of the film of air actually in contact with that window
surface.<br>
<br>
Recommendations and RH guideline for house temperatures are
useful as starting points but, if you have a condensation
problem, the two approaches are either to experiment or to have
an accurate RH gauge to measure the room air near the window at
any given time and to get an accurate measurement of the
window's surface temperature. You will need some form of
psychrometric chart to look up the dewpoint for that temperature
and RH. Someone will need to chart those factors over the time
the problem occurs, then go looking for the excess moisture
source(s).<br>
<br>
The chart can take a bit of practice to understand but there are
"slide rule' forms of the psychromatic chart, to more easily
find dewpoint. Bachrach makes a sling psychromometer which has
a slide rule built into the case, but it can be a pain to use.
Electronic psychrometers give dewpoint as an automatic function,
but are significantly more expensive.<br>
<br>
Reading the air film temperature or RH is difficult to measure;
better to use the room air near it and add a cushion. Trial and
error should show you a workable adjustment factor.<br>
<br>
Reading the window surface temperature can also be tricky. I
have the best luck with a quick response contact thermometer.
Remember the edge of the glass at the frame is the most likely
spot for condensation to start. (IR thermometers do not read
accurately on glass, unless they can be adjusted for the
reflectivity, only possible on some expensive models. You could
use a "black body" target to get around the reflectivity, but
that is probably beyond what you need to get into. <br>
<br>
I hope this is helpful.<br>
Bob Klahn<br>
<div class="moz-cite-prefix">On 10/28/2012 6:30 PM, Sacie
Lambertson wrote:<br>
</div>
<blockquote
cite="mid:CAM6L0t4GLPeOyYVJuDWCCvG9w9CS8LKAob67om8X4_x7ZdUjFQ@mail.gmail.com"
type="cite"><font><font face="verdana,sans-serif">thanks <font>for
the suggestion Cor<font>wyn but we have about 50 windows
in this house, all of them 9 year old Pellas (the only
manufacturer who would make my custom sized windows <font>within
my time frame--a long story here)</font>. I can't
possibly imagine building storms for the inside of
these.<br>
<br>
<font>Our indoor humidity is within the <font>suggested
range.</font></font><br>
<br>
<font>S</font><br>
</font></font></font></font><br>
<div class="gmail_quote">On Sun, Oct 28, 2012 at 5:24 PM,
Corwyn <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:corwyn@midcoast.com" target="_blank">corwyn@midcoast.com</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div class="im">On 10/28/2012 1:20 PM, Sacie Lambertson
wrote:<br>
Our windows are starting to condensate at the base on
the<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
inside. Many of you have suggested this is the
penalty of winter.<br>
Isn't there anything I can do about it?<br>
</blockquote>
<br>
</div>
See my previous suggestion for interior storm windows.<br>
<br>
Here's the basics of when you will get condensation. When
the dewpoint temperature of the inside air is a the same
as (or greater than) the temperature of the inside surface
of the windows (or anything else in your house), that
surface will condense water. So in order to avoid
condensation you need to either lower the dewpoint of the
air inside, by reducing the humidity, or increase the
temperature of the inside surface of the windows.<br>
<br>
Whether the humidity is too high is a matter of
perspective and some controversy. Get a humidity gauge
and measure what yours is. Generally somewhere in the 40%
- 60% range is recommended, but balance is between being
too dry (and the dry throat and colds that that produces)
and being too wet and the mold and mildew (and the
allergies that aggravates), so decide for yourself. That
said, lowering the humidity will reduce the condensation
(and vice versa). One of the things we generally see is
that as people increase the air tightness of their house
they need to go from worrying about too low humidity to
worrying about too high humidity. At that point, you are
probably in the place where you will need to have
supplemental ventilation (for fresh air reasons), and an
HRV will take care of the humidity issue.<br>
<br>
The other way is to raise the temperature of the glass.
Traditionally this was done by putting the central
heating output right under the windows so that hot air
washed over the surface warming it up. Now, the solution
is generally to increase the insulation value of the
windows. The interior storm windows are a cheap way to do
this, adding about R-2 to a window for about $1.50 per
square foot (if you make them yourself), I make them for
around $4.00 per square foot. Raising the temperature of
the glass will also increase comfort levels (even at the
same air temperature.<br>
<br>
Thank You Kindly,<br>
<br>
Corwyn<span class="HOEnZb"><font color="#888888"><br>
<br>
<br>
-- <br>
Topher Belknap<br>
Green Fret Consulting<br>
Kermit didn't know the half of it...<br>
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<a moz-do-not-send="true"
href="tel:%28207%29%20882-7652" value="+12078827652"
target="_blank">(207) 882-7652</a></font></span>
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