How to spec high performance windows?
So I’ve done quite a bit of reading on windows. I’m finishing up the details of a high performance home build in Ottawa, Ontario (Zone 6a/7a equivalent), and need to get my window performance nailed down before I can have a Manual J completed.
I’m a little bit confused to how window ratings and/or energy ratings are relevant? Here’s why. I want to keep my indoor climate around 21-22C (70-72F) at roughly 45% RH in the winter, and cold nights in January/February regularly hit -25C (-13F) with nights to -30C (-22F) and even colder not uncommon. My main goal with the windows will be to not have condensation forming on them, hopefully ever. That includes the whole window; glass, frame, everything.
Whole window R and U values don’t really tell you much since the glass package could be outstanding in order to “cover-up” a substandard frame. The scenario playing out as the glass remaining clear of condensation but the frame being soaking wet. Since many frames are uninsulated this seems like a real weak point for these windows.
Are there any tips/advice to specifying a window that will meet this requirement? After speaking with a couple local window sales outfits their sales staff are limited to the spec sheet, which doesn’t come close to answering my questions. When I start pushing for more detailed information their eyes just glaze over… which tells me most of what I need to know about their products I think.
Before speaking directly with a few window companies I’d like to know whether specifying a window to a specific climate is reasonable. Thanks!
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Replies
Lance,
I suggest that you start by reading this article: Rating Windows for Condensation Resistance.
Excellent article, Martin! Unfortunately, for me anyway, it answers some of my questions with more questions. But at least I can see my concerns are valid.
At a very minimum, the AAMA website has an online calculator still available that looks at your specific environment:
http://www.aamanet.org/pages/crf-tool
The NFRC site has no info that I can find, save a mention of the voluntary Condensation Rating on the label. No mention of the scale or how to use it, that I can see anyway.
It at least gives me a metric I can ask for when speaking with window companies. Hopefully they will be willing to share the information (I'd be surprised if their Engineers hadn't at least tested internally).
As an experiment this fall, I covered our home's windows with that shrink-fit clear film to see if the condensation resistance was improved measurably. To my surprise it worked very well, however the frames of the windows would gather condensation at the humidity levels I kept, between 35-40% RH. We have builder-grade Jeld-Wen windows that were installed in 2004, a double pane IGU of unknown specs.
The other reason for my concern is, we're looking at about a 15" thick wall assembly with the windows located near the center. Being 7-8" recessed, airflow will be reduced and this will further exacerbate the condensation issue. I'd like to ensure my targets are met, but I'm also pretty sure that a full-on Passivehouse window is out of my budget.
So has the Condensation Resistance rating idea improved any in the 5 years since you wrote that article? It seems like a huge issue with our building industry is being overlooked here...
Lance,
This is an issue where overthinking is a danger.
My biggest piece of advice: your goal of 45% indoor relative humidity when it is below zero Fahrenheit outdoors is unreasonably high, unless (a) you or a family member has unusual medical issues, or (b) you are the curator of a harpsichord museum.
Lance, for what it's worth, we are in Maine. Our walls are 12 1/2" thick, with windows in the middle of the wall. We have Intus triple pane passive haus certified windows. Over two winters, we have never had a drop of condensation on the glass or frames.
There are more and more good window choices available. This month's Fine Homebuilding Houses issue has a story about a Federal Farmhouse with triple pane Kohltech casements that look like double hung. You might check Kohltech out. I believe they are made in Canada.
Koltech Windows
P.O. Box 131
Debert, Nova Scotia B0M 1G0
Canada
902-662-3100
Fax: 902-662-2154
Stephen/Martin, thanks for the information on Koltech, I will definitely be contacting them. Are there any other Canadian made windows that you can think of off the top of your heads? I'm already in contact with Inline and Fibertec, as well as Internorm just to see what Europe has to offer. I'm assuming they will be beyond the budget though!
Martin, as for my requirement, it's a soft spec. I would like to stay as close as is practical to 45% RH without going too crazy on window price. My experiment with my builder grade windows + shrink film was very encouraging as I was able to keep RH in the 35-40% range, and only the bedroom windows showed excessive condensation after being slept in with the blinds closed. We didn't get any -30C nights this season, but many below -20C. Even our bathroom window stayed low on condensation after showers once I removed the insect screen to increase airflow.
Granted, most of our windows have floor ducts below them and I kept the fan circulating air, which surely helped. But these are truly unremarkable windows from a performance standpoint, so I have high hopes that a decent triple pane unit will outperform these by a long shot. I've got one chance to get it right, so Id' like to get as close as possible.
Your help is invaluable as I have yet to get anyone from a window store to speak at this level. Hopefully soon as I develop some conversation with the companies directly... one salesman I spoke with at a local store looked at me real funny when I mentioned "passive solar gain". Yup, probably not buying from him!
Lance,
Q. "Are there any other Canadian made windows that you can think of off the top of your heads?"
A. Here are some:
Accurate Dorwin
1535 Seel Ave.
Winnipeg, MB R3T 1C6
Canada
888-982-4640
Fax: 204-982-8383
http://www.accuratedorwin.com
Duxton Windows
10 Higgins Street
Winnipeg, Manitoba R3B 0A1
Canada
204-339-6456
Fax: 204-334-1800
http://www.duxtonwindows.com
Entreprises Marchand Portes et Fenêtres
165 Ave. Lamarche
Ville Sainte-Catherine, Québec JOL 1EO
Canada
450-632-1694
514-365-2905
Fax: 514-365-3704
http://www.entreprisesmarchand.com
Thanks again Martin!