Double Hung Air Infiltration

Old 04-13-07, 02:45 PM
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Double Hung Air Infiltration

I have several new vinyl double hung windows on my house. With 1 of these windows, I am experiencing air infiltration during high wind loads (25-30 mph or more).

The installer was back to recheck the installation, and the window manufacturer rep upgraded the pile weatherstrip on the sides of the sashes to a thicker size (depth). The upgrade did make a difference, but I can still feel a cold draft along the edge where the meeting rail and frame meet under high wind loads.

This window is on the 2nd story of the house, and is the only window facing due west. The area west of my house is flat open land (at least 2-3 miles), so there is no wind break. When the wind is blowing west, this window gets hit hard and head-on.

I am in the process of planting evergreens for a windbreak, but this will still take several years to grow high enough to make any impact.

Any suggestions on what I can do, or is this just one of the drawbacks of owning a double hung window. I know that they are the "weakest" of all windows as far as air leaks, etc.
Old 04-13-07, 03:39 PM
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Casement or awning type windows have the best air infiltration figures. Some manufacturers offer better air infiltration protection than others. Those figures should be published and/or were posted on your windows at the time of installation. Do you know the manufacturer?? Installation variables could also affect your specific problems.
Old 04-13-07, 04:46 PM
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When wind travels up the sides of a double-hung window, we often refer to that as "channelling". And no, it has nothing to do with speaking to the dead.

Heavier weatherstrip is about the only thing that one can do, provided the masterframe of the window has been properly installed, and is not spread in the middle or bowed. Occasionally I'll encounter double hung sashes that are hourglassed- they have been glued to the glass that way in the factory. This is bad because the masterframe could be perfectly straight, and if the sash is bowed, it will increase air infiltration.

If you'd like to double check the installer and the service rep, you could tip the windows down and measure the window opening. If it's 24" wide at the top, it should be 24" in the middle and 24" at the bottom... not 24 1/16" or 24 1/8" in the middle. You can also check the sashes in the same manner. If they are 23 7/8" at the top and bottom, they should also be 23 7/8" in the middle. If they are 23 13/16" or 23 3/4" in the middle, then your sash is a bit hourglassed and you'll have some increased air infiltration because of that. If that's the case, you can demand new sashes, or they should at least deglaze / reglaze the windows so that they will not be hourglassed.

Other weak spots on double hung windows are on the ends of the interlock (where the upper sash and bottom sash come together). There really is no "fix" for that area because it goes back to the way the window is designed. (all windows will have some air infiltration).

Another problem area is the bottom left and right corners of the bottom sash where it contacts the sill. With welded sashes, it's difficult for manufacturers to completely weatherstrip 100% of the bottom rail and there is often a small gap at the corners. This gap will allow air to get in at the corners, bypass the weatherstrip, and race up the channels. (channeling) The factory fix for that is to install small foam "blocks" that are friction-fit into the channel. They are not completely effective at stopping air, they only block a portion of it and really only act to slow the air.

I've found that one thing that may help in this regard is to install thin 1/8" strips of foam to the bottom left and right corners of the sill. This usually helps to block up that problem area and will at least help to overcome that design flaw with welded sashes a little bit.

Keep in mind that when the wind is blowing it exerts a lot of pressure on your home, and it's impossible to keep that air pressure out completely. Windows also are tested and are supposed to meet certain minimum standards as far as how much air they are allowed to let in (calculated in ft/min @ "x" m.p.h. wind speed) However, you are right to expect your windows to function within the air infiltration tolerances that they advertise.

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