[Greenbuilding] Airtight windows

[Bob klahn]

John,
I stand corrected.  I was only considering thermally driven flow and 
overlooked stack effect.   Mea culpa.
Bob
On 10/28/2012 10:46 PM, John Straube wrote:
> Hi Sacie
> I assume you have double-glazed low-e argon Pellas, with wood frames?
> If you have condensation problems it is very likely too high RH.  I am 
> curious about your comment that the RH is within the recommended range.
> This could be ture, but is that recommended range might be 20%RH or 
> 50%, since I have seen both "recommended" for the Kansas climate. Have 
> seen some significant condensation on low-e Argon filled windows in a 
> large multi-unit building about 1.5 hr drive from Kansas City which 
> was reported to always be bad at this time of year.
> Condensation like this tends to be worse early in the season as the 
> moisture from building materials is released.  In my airtight home 
> this delay is very noticeable (eg RH is stubbornly high until January 
> I find), and I have to ventilate aggressively during this time period 
> to keep my RH within safe levels.
> As rough guidelines, when it is 30F outside, I would target 30%RH and 
> when it is 10F, I would look for 20%RH. It takes a while to ventilate 
> down to those levels, but once you get there it tends to stay.
>
> PS I have triple glazed foam filled fiberglass framed windows. 
> Condensation can be made to occur on these rather high performance 
> units merely by letting the RH ride up to, say, 50to 55% during 30F 
> nights.  The condensation quantity is small but totally visible.
> PSS Bob's claim that windows leak outward in cold weather is not 
> true.  It is often true for the second storey of homes, but rarely 
> true for the 1st floor of 2 stroey homes and almost never true for 
> windows on the windward side during windy weather.
>
>
>
> Dr John Straube, P.Eng.
> www.BuildingScience.com
> On 12-10-28 1:20 PM, Sacie Lambertson wrote:
>> On Sun, Oct 28, 2012 at 10:36 AM, Bob klahn <Home-NRG at dnaco.net 
>> <mailto:Home-NRG at dnaco.net>> wrote:
>>
>>     Lance,
>>
>>     One guideline that has been implied but not stated in any reply
>>     I've read is that whatever approach you choose, the innermost
>>     pane must be the air-tightest to limit condensation.  The old
>>     "rule of thumb" was five times tighter than subsequent panes, if
>>     there is any leakage.  This allows any leakage (drying) to be to
>>     the outside.
>>
>> Re Bob's note on tight windows, we're now moving into the winter 
>> heating season.  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?  With the windows I 
>> have?  Or, are there windows out there that don't condensate in the 
>> winter?  Are there windows that don't leak cold air to the inside 
>> from the outside glass per Bob's dictum?  Or is this not a matter of 
>> tightness, but rather of the differences in temperature?
>>
>> thanks,  Sacie
>>
>>
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>
>
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