[Greenbuilding] Airtight windows

Sun Oct 28 20:00:17 CDT 2012

Sacie,
Two points, in response to two of your posts  (bearing in mind that I am 
speaking on the assumption of a cold climate).

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.

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.

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.

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.

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.

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).

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.

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.

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.

I hope this is helpful.
Bob Klahn
On 10/28/2012 6:30 PM, Sacie Lambertson wrote:
> thanks for the suggestion Corwyn 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 within my time frame--a long story 
> here).  I can't possibly imagine building storms for the inside of these.
>
> Our indoor humidity is within the suggested range.
>
> S
>
> On Sun, Oct 28, 2012 at 5:24 PM, Corwyn <corwyn at midcoast.com 
> <mailto:corwyn at midcoast.com>> wrote:
>
>     On 10/28/2012 1:20 PM, Sacie Lambertson wrote:
>     Our windows are starting to condensate at the base on the
>
>         inside.  Many of you have suggested this is the penalty of winter.
>         Isn't there anything I can do about it?
>
>
>     See my previous suggestion for interior storm windows.
>
>     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.
>
>     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.
>
>     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.
>
>     Thank You Kindly,
>
>     Corwyn
>
>
>     -- 
>     Topher Belknap
>     Green Fret Consulting
>     Kermit didn't know the half of it...
>     http://www.greenfret.com/
>     topher at greenfret.com <mailto:topher at greenfret.com>
>     (207) 882-7652 <tel:%28207%29%20882-7652>
>
>
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