Window calculations vs Values calculated following BS EN 673

[Michael Donn]

Hi folks

As a long time user of Window, I have made a habit for 2 decades to teach students how to calculate appropriate U-Values (U-Factors in your terms) for glazing to enable them to understand the value of types of frame material, effect of size of window and layers+gases+coatings. This worked well when the New Zealand practice was to have a standard 1800x1500 window with a centre mullion. However, the practice here is moving to a process of publishing 'typical' R-values (R-factors to you as they include the air layers) for a suite if typical size windows. This has created a problem. I can no longer replicate believable R-values from Window. The documentation (https://www.buildmagazine.org.nz/assets/PDF/Build-198-38-Design-Right-Rules-For-Windows-H1-Compliance.pdf) lists the British Standard 673 as the process followed. Having wasted NZ$300+ on said standard, I suspect that this is because that is what is referenced by the software package now used. As the Building Code requires a minimum R of 0.46 - this is now crucial. I am hoping to replicate their Ïmproved Spacer"as Class 3 in Window, and I have tried to represent the dimensions of the frames more accurately, but double-glazed with thermally broken aluminium frames are WAY different than the code published values: Any and All thoughts welcomed. Extensive queries here have not provided convincing evidence.

Thanks

[Michael Donn]

Essentially, to achieve the values the code publishes as low-E, Argon-filled, double-glazing, with high-performance spacers, (R-0.46), with Window I can only achieve close to this with triple glazing and Class 3 spacers…

I cannot reconcile these numbers, and asking code developers has not helped - cross reference to the BS EN 673 just takes me to a bunch of formulae about how to do the calculations…

M

[D. Charlie Curcija]

Hi Michael,

I was able to replicate R=0.46 for Generic T/B Al and double low-e Argon glazing. Ug is roughly compliant with the table that you enclosed (I am getting Ug=1.35, while table lists Ug=1.3 for Low-E3). I used a class 3 spacer in my calcs. I have used a 1.2 x 1.5 m NFRC fixed size window. If I use 1.5 x 1.8 m with mullion, I also get R=0.46.

Charlie

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D. Charlie Curcija, Ph.D.                              Tel: (510) 495-2602

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[Michael Donn]

Kia ora tatou

Thanks Charlie.

I set this aside for a while, but I am returning to it out of concern to understand what I might be doing wrong, prior to making a public submission on the Current setup of the Building Code Energy Performance requirements (28 Feb deadline).

I have been through the window frame options typically available in New Zealand that must be behind this number (see re-upload of Building Code Window R-values below)

:

These R-values have been calculated for a suite of "representative" house window sizes - and averaged and the range of sizes available is not published yet: the calculations were apparently done using Flixo (https://www.flixo.com/products/flixo-pro/ ) to estimate the psi value of the frame plus the glass conductance to enable a combined R-value to be estimated.

I still have no idea how an analyst looking at a curtain wall or highly glazed non-residential building is supposed to complete the  required analyses with these data. There is no accompanying VT or SHGC data, just this table of R-values. I should note that I believe these latter data are also necessary in residential performance calculations as well!

Last year when I started to get radically different data than  the published data, and when I was trying to determine what to do about the "Improved" spacers, I looked more carefully at the Window Frames in Window. This is what I came up with. I was keen to get the Pfd areas correct to ensure that, within the same "hole in the wall" in EnergyPlus we were modelling how much solar gain might actually get through, dependent on the solid vs transparent AND the VT or SHGC for the glass.

The Building (Energy) code proposed change is that no-one  will be able to state they have followed a list of complying component R-values to prove compliance. A heat loss calculation or a  simulation will be necessary. Getting the right input values just became a whole lot more critical.

m  

[D. Charlie Curcija]

Hi Michael,

I am not sure that I understand what the issue is. In my original reply I calculated the R-value for one of the windows and got an equivalent/same answer (R=0.46). To help me understand what is the problem that you are reporting, can we focus on one of the products for which you are getting vastly different answers and send me WINDOW mdb in attachment. That way i can focus on the product that shows discrepancy.

BTW, pfd (projected frame dimension), as defined in North American standards (and our software tools) is the length of the frame projected onto the plane of glazing. Frame starts at the intersection of Adiabatic BC and ends at the sightline. Sightline may sometimes be on the glass on the indoor (room) side, since sightline is defined as a "highest" opaque point on the indoor or outdoor side of the window, so sometimes the frame projects more on the outdoor side, creating an end of frame on the indoor side on the glazing itself. As a point of definition, the surface(s) where the window would be in touch with an insulated mask wall in a hot box is where the Adiabatic surface is applied in THERM. 

Charlie 

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