[Greenbuilding] Humidity between multiple windows

John Straube jfstraube at gmail.com
Tue May 10 06:38:33 PDT 2011

Hi David
I think I can attach photos so I tried.
The size of the holes on the old windows as based on leaky interiors.  The exterior, as has been said, must be leakier than the inside, 5:1 being a swag.
If we apply storms to existing windows, we should of course first seal the heck out of the interior window.  Thus the exterior vents holes can be a lot smaller than the typical 1" diameter holes I have seen.  And the one in this photo is actually closed because they did not need to add the venting.  The adjustable flap David spoke of is on the inside here.
We have, but hate to, leave the windows in when doing a deep energy insulation upgrade, not because the windows cant be made decent R-value and airtightness, but because of the risk of rot due to existing windows leaking. The real risk with leaving windows in place if you insulate is that a rain leaky window that used to not damage the wall below when uninsulated, can become a serious problem after you retrofit.
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On 2011-05-10, at 12:58 AM, David M Delaney wrote:

> On 5/9/2011 6:09 PM, nick pine wrote:
>> Bob Klahn <Home-NRG at dnaco.net> wrote:
>>> The old rule of thumb for minimizing / avoiding condensation between windows (e.g. between prime and storm windows) has been to be sure that
>> the innermost window was five times tighter than the outer.
>> How would you measure that?
>> I might be sure the inner window is as airtight as possible and drill one 1/8" weephole in the bottom of the storm window frame, and drill another if there's any condensation during the first winter, and so on.
> --------------------------------------------------------
> I have a childhood memory about ventilation holes in storm windows.  My parents  lived in Winnipeg in three houses with storm windows from 1942 to 1975. At that time you could count on at least two winter months in Winnipeg  during which the average daily temperature was well below 0 F.  I remember that the storm windows had three ventilation holes in the bottom of the frame of each storm window. They were about an inch in diameter, and were positioned on approximately 4 inch centers in the bottom center of the frame.  A narrow wooden flap fastened on the interior side of the storm window frame by a single screw in one end was positioned so that it could be adjusted to cover any fraction of the total area of the three holes.
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Dr John Straube, P.Eng.
Associate Professor
University of Waterloo
Dept of Civil Eng. & School of Architecture

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