[Greenbuilding] Airtight windows

Peter Kidd peterkidd at shaw.ca
Sun Oct 28 21:37:53 CDT 2012


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).

I'd quibble with some other points you raise, but I can't pass on the above.

> 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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>
>
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