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Learning, Sharing, and Teaching => Ask a Mustachian => Topic started by: scottydog on February 03, 2015, 08:24:34 AM

Title: Need help understanding whether our new windows are sealed properly
Post by: scottydog on February 03, 2015, 08:24:34 AM
Last summer we replaced the windows in our eat-in area off the kitchen.  Our old windows were cheaply made and extremely drafty, and we replaced them with high quality windows in the hopes that we would no longer need to install plastic film in the winter.  Even with the new windows, our electric baseboard heater can't keep up when it drops down to -18 C outside (0 Fahrenheit).

Our new windows have double panes on the outside and single panes on the inside, with a screen in between; they're the 2163 triple glazing assembly on the website: (

Here's a picture of the assembly.  The open slits go through the bottom three layers, in the vertical section A-A, where it slopes down and to the left.  My guess is they're there to allow water to drain outside.

I'm confused about two things: First, underneath each outer window there are open slits (2 on one side; 1 on the other).  Do you know why these slits are there?  When I open the inner windows, there is a clear draft coming from each slit.  Is it just common knowledge that these should be blocked during the winter?  Second, this morning I inserted some foam to block the slits and there is still an obvious draft when I open the inner window.  Does this indicate that the windows weren't sealed properly?  We had these done professionally and the contractor who installed them seemed very professional.

I had assumed that the outer windows, being double-paned, should provide a reasonable seal on their own.  Maybe I have unrealistic expectations, and I'm hoping some of you might be able to help me understand it better.  Thanks in advance for your help!
Title: Re: Need help understanding whether our new windows are sealed properly
Post by: PEIslander on February 03, 2015, 09:15:07 AM
I believe the small slits you refer to are "weep holes" intended to let any water (infiltration from the outside or condensation from the inside) drain out. They are important and should not be plugged up or otherwise sealed. The window as designed is intended to have all sliding sashes (inner single glazed one & outer dbl. glazed one) closed when you don't want ventilation. With just the outer double glazed slider closed you will, as you have observed, get air infiltration from those weep holes. The weeps become inconsequential when all sliders are closed because for infiltration to occur there needs to be a pressure difference. With all sliders closed there is no significant pressure difference between the outside environment and the space between the inner & outer sliders. In other words, the weeps act as pressure equalizers -- they not only let any water out but actually help keep outside water from getting in. This aspect of the design is what the manufacturer calls a "rainscreen design". I hope that stuff is making sense...

The issue of interior draftiness or condensation in the winter are other matters. You indicate the baseboard heaters can't keep up. Draftiness at a window is related to how cold the surface of the interior glass pane is relative to the room's air temperature. In Montreal you can get some damned cold winter conditions so even with the triple glazing it is likely the surface temperature of the glass will be quite cold. No doubt that glass on your triple glazed window is warmer than a standard double glazed window. The surface temperature can be raised, with a corresponding reduction in drafts, if the baseboard heater can get the heat up to the window. The effectiveness is often hampered by the design & fit of any curtains or draperies. Sometimes a curtain prevents an adequate amount of heat from reaching the window.

If condensation is an issue there are things you can do. It is caused by moisture in the room's air condensing to liquid water on the relatively cold surface of the glass. In general this will be reduced if all sliders on the widow are closed. If you have a problem with condensation (and frost when it is really cold) you need to think about reducing the moisture level in your home. Ideally you will have a kitchen range hood that vents to the exterior. If you do then use it when cooking! Similarly always turn on bathroom exhaust fans when having a bath or shower. If your home has a HRV (heat recovery ventilator) make sure it is on. (It is amazing how many people turn them off in the winter because they don't understand how important ventilation is). If you don't have a HRV and you have condensation problems, consider getting one installed. They are code required in all new home construction in Canada. A dehumidifier can also be used to reduce interior humidity but reducing much of the moisture at its source (kitchen & bath exhaust fans) is a better first step. There are other potential causes of high humidity in a home but it is probably too much for me to get into that here.
Title: Re: Need help understanding whether our new windows are sealed properly
Post by: QajakBoy on February 03, 2015, 10:34:31 AM
I'm confused too.  Weep holes are normal for storm windows. 
But I would have expected the single panes in your window assembly to be equivalent to storm windows and that the single panes would be toward the exterior and the double panes on the interior side.   Your description and the diagram both indicate that the double panes are on the exterior.  Maybe this setup is better for noise abatement rather than thermal energy efficiency.  I think that sliding windows may have a bad reputation as far as thermal energy efficiency to begin with though.
Title: Re: Need help understanding whether our new windows are sealed properly
Post by: PEIslander on February 03, 2015, 11:08:02 AM
The design of the window suggests it was manufactured for the replacement window market. Windows of this type are not used commonly in new home construction. That isn't say there is anything wrong with the design although thermal performance of a wood or vinyl window would typically be better. Note that the construction is in essence an aluminum double glazed slider separated by a thermal break from a aluminum framed single glazed slider. In a market where thermal breaks aren't important a window like the outer part of the unit could be installed on its own. In such a system the weep would likely only be in the outer-most track. Such a window would have poor performance on infiltration tests and would not be suitable in our Canadian climate. Adding the inner single-glazed design with the thermal frame break improves the performance considerably and allows the window to pass the infiltration tests.

Could the window have been designed with the single-glazed portion on the exterior? Yes, probably. That it wasn't is not a weakness of the design.
Title: Re: Need help understanding whether our new windows are sealed properly
Post by: scottydog on February 03, 2015, 02:52:00 PM
Thanks for the feedback!  It makes more sense now.  We considered casement windows but chose these because the casement windows would have opened at a very awkward height in our backyard.

We haven't had any condensation issues on these windows, possibly because we do use our range hood and bathroom fans as you suggest, PEIslander.  One thing that surprised me was that there was still infiltration even after I covered the weep holes, but after posting I noticed that the website says, the inner sash perimeter is equipped with fin-type weatherstripping to prevent air and vapour leaks, which implies that the outer sash perimeter was intentionally designed and manufactured without weatherstripping.

I'll uncover the weep holes and ponder whether to install some window insulation film.  It's at least as good as our previous windows plus the film; it's just not as cozy as we'd like.  There's also an exterior door in the same room, so I'll look into sealing that better rather than focusing on the windows.

Our place is lower-level condo in a quad-plex that's 90 years old, and we pretty much always have incoming drafts from both ends of our place.  I suspect we're losing heat upwards.  There aren't any obvious paths for the air to take, but I haven't searched intensively because it seems like a big, open-ended problem with small returns and we're not certain we'll stay here for life.  When it's cold enough outside, we even get cold air coming down the chimney through our wood stove/insert at the opposite end of the place.  One day I may swap that stove out for a natural gas insert that takes its combustion air from outside, but that's a whole other kettle of fish.

Thanks again for your insights!