Window data interpretation
I am hoping for some help interpreting window test data. I am looking at the two UPVC lines from Intus: Eforte and Arcade. I was sold on the Eforte line, which is clearly marketed as the better of the two options and is accordingly ~20% more expensive. When the price of Intus doors stretched my budget, I took a closer look at the ratings. What I found has thrown me off. With Intus’ mixture of AAMA and ASTM ratings and the breakout of various window types in the Eforte line, I am not entirely sure I’m comparing apples to apples and I’m hoping someone can set me straight. The data I am finding is as follows:
The Eforte has a better Uf with .167 to Arcade’s .211. After that…Arcade seems to win out.
Arcade AI <.01 Eforte AI .05 – .1 Arcade WP – no leak at 15.04psf Eforte WP 9.0psf – 12.11psf Arcade DP 70psf plus passed overload test at 105psf Eforte DP 50psf-70psf Is the Arcade really a tighter window with as high or higher a design pressure? Am I just not understanding how much weight I should be placing on the U value variable? Is a .044 Uf reduction indicative of a higher quality window, even with all other variables pointing the other way? What am I missing?
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Jim,
By "AI," I assume that you mean "air infiltration"? You don't provide any units, so your numbers are confusing. I found an online document (see below) for the Intus Eforte inswing casement with an Air Infiltration rate result of 0.5 L/s/m2 (0.10 cfm/ft2) and a Design Pressure of 70 psf.
Without delving more deeply into the test results, I can certainly report that engineers can make window frames that are strong and able to resist wind and have low infiltration rates, and can fill those frames with any type of glazing they want -- glazing with a low U-factor or glazing with a high U-factor.
There is no technical reason to expect that strong frames or low air infiltration rates have anything to do with glazing U-factor.
Jim- I can't help with the technical questions at all. But my Intus Eforte windows are great and the doors are insanely great and well worth the very high cost.
Martin,
Yes, I was referring to air infiltration – sorry for the lack of clarity.
The Arcade windows are listed as having an infiltration rate of <.01cfm/ft2 while the Eforte are listed at .1cfm/ft2 for almost all windows except the tilt and turn, which are rated at .05cfm/ft2. Part of the difficulty I am having is that it appears different tests were used (ASTM 283 for the Arcade, AAMA/WDMA/CSA101/I.S.2/A440-08 for Eforte). Things are also compounded by the fact that the Eforte line is broken out into window type, while only one score is provided for the Arcade windows. The test results are all under the “Description” tabs for each window line on the Intus website.
I understand that the frame does not determine the U value of the glazing, but I had been interpreting Ug as a glazing U value and Uf as a U value of the entire window assembly, frame included – is this incorrect?
My interpretation is that the Eforte has a lower Ug from deeper glazing and more argon, and a lower Uf from better spacers and more thermal breaks in the frame. So far I’m on board. Where I get a little shaky is that it looks to me like the Eforte frame lets more air in, and may not be quite as robust as the cheaper alternative.
Again – this is just the way I’m reading things, and I’m wondering if I’m looking at it from the wrong angle (or just flat out not understanding what I’m looking at…it’s happened before).
Stephen, that’s good to hear. I went and looked at the Eforte in person earlier this week, and I have no doubt I’d be extremely happy with them.
Copying and pasting from http://www.luxal.co.uk/aluminium-windows-whats-the-difference-between-uf-and-ug-values/
Uf – refers to the U Value of the window frame itself without glass.
Ug – refers to the U Value of the glass itself.
Uw – refers to the combined U Value of the frame and the glass combined.
Uf and Ug can be quoted for the series; but the way they get combined depends on the size. So in the end you care about Uw, but you often only get that value with a quote on a particular size.
Thanks for the info Charlie. I vaguely remember seeing Uw referenced somewhere now that you post that.
I guess in the end what it comes down to is that I expected all the ratings of the more expensive windows to be better. Obviously, I don't completely understand all the ratings and how one relates to the other - so I am left wondering how much the improved U value is worth, not in terms of plugging it in for some certification, but in the real world with the wind blowing on it. Intus says it's worth 15% - 20% of the value of the windows, and that may be the case.
I was hoping that maybe I misunderstand the ratings, and when the windows undergo tests with identical protocol, the Eforte scores better. I have read that the AAMA and ASTM air infiltration and water penetration tests were identical, and see that the AAMA frequently references ASTM protocol in "Air and Water Leakage Testing of Installed Windows and Doors." This left me hoping someone could explain how the scores of the Eforte line represent a better window - an explanation which may be over my head.
It may come down to staring real hard at showroom samples and deciding if one feels better than the other.
Dusting off my math skills and checking calculations with examples on the Internet, I calculate the following. For 0.1 cfm air leakage per square foot of rough opening at design pressure for a 2,500 sq.ft. home with 12.5% glazing (0.125 x sq.ft. of floor area) plus door area yields about 40 cfm maximum air leakage when the wind is howling outside. 40 cfm sounds like a lot as this is on par with the natural air leakage of the 2,500 sq.ft. home with 1.0ACH50 in still air (not at design pressure!). But I wouldn't worry about this number. The windspeed required to get to a design pressure of 70 lb/sq.ft. is basically a hurricane. (See the graph on this web page http://chicagowindowexpert.com/windowtags/wind/ ). Pressures on windows due to stack effect are much much smaller: on the order of 0.25 lb/ sq.ft. in a very cold climate three story home. On a normal cold climate winter day, expect less than 0.2 cfm air leakage through the the windows in this example. You will get more air leakage through the air sealing surrounding the windows.
Bill - thanks for giving this some thought. I do not believe these air infiltration and water penetration ratings are determined by tests that put anywhere near design pressure loads on the windows. The maximum pressure that would be tested at is, I believe, 15psf. It looks like lower pressures may also be used in some cases.
I agree that the windows rated at .1cfm are more than tight enough - my issue is paying more for them than a window rated at <.01cfm.
The link you provided is interesting. Based on this chart, it takes about a 60mph wind to hit the 9psf at which the Eforte fixed windows are rated for water penetration. Does this mean that rain driven into the window by a 60mph wind would cause leakage?
Good point Jim - I made the assumption that the air leakage test is at the design pressure. An erroneous assumption on my part. Sorry about that. I find on the web "a new product sample is subjected to constant air pressure to simulate 25 mph winds." If that's the universal test pressure, then windows at 0.1 cfm/sq.ft. are not so impressive. 25 mph winds correspond to a pressure of 1.6 lb/sq.ft. This is much more concerning - the 40 cfm total leakage, with 25 mph winds, through 400 sq.ft. of windows and doors now reduces only a factor of 3 to worst-case cold climate no-wind conditions, about 13 cfm. That is not huge, but it's not something I would ignore considering how hard people work at air sealing.
In this view, the 0.01 cfm/sq.ft. rated windows are critically better than 0.1 cfm/sq.ft. rated windows. The measured value that Martin found for the Eforte windows of 0.03 cfm/sq.ft. strikes a seemingly acceptable middle ground.