Window Uglass and Uwindow

aymeric · Posts: 5

Dear all,

I have a problem being sure what properties my window actually have. Setting up the U value for the glass is not a problem but the parameter often asked for is Uw (including frame and dividers). But in DB, we can input frame and dividers properties. However, we do not have any output showing the overall window U-factor.
Now I have a very glazed appartment to simulate and of course the sensitivity of the results to the window performance is high!

Does somebody have any proper method to define a window being sure of what is the total window U-factor?

Thanks a lot!
Aymeric

AOG · Posts: 79

Aymeric,

I have not a good experience in this theme, so maybe this previous large topic could be of interest for you:

http://www.designbuilder.co.uk/component/option,com_forum/Itemid,25/page,viewtopic/t,479

I hope this helps

aymeric · Posts: 5

thanks AOG,

This is rich of information.
Though I am not sure what to conclude from what they say.
Indeed, I have tried both approaches:
1) I removed the frame and used window supplier data for U value and also modified SHGC to obtain similar solar gains
2)I used window5 to obtain equivalent Uwindow

I do not obtain the same results!
That means these methods are not equivalent right?

Somebody has an idea? It would be nice I clear that out because I won't trust my simulation results if I don't...

Thanks

bhandari · Posts: 78

Aymeric,
You are correct in saying that the two approaches (you mentioned) are not equivalent. Approach(2)includes frame around the window while in approach (1) you are not including the frame area - thus your glazing area is not the same in both the cases. Depending on the location, SHGC plays major role in the difference of results between two approaches.

In DesignBuilder window width (and height)corresponds to the glazed portion only. The frame is added around this glazed portion. If you would like to get similar results using approach (1) and (2), I would recommend using he window width/height the same as the glazed width/height in Winodw5 (say, total window height - 2xPFD). You can then add the frame width as PFD from W5. This way the results would be pretty close. For more accuracy, you may want to play with frame conductance and glass-edge-center conductance ratio (which you can get from window 5 E+ report). The latest DB version includes IGDB so you shouldbe able to construct the same glazing system in DB as you use in W5.

If you would like to do the simulations outside DB, then I would say approach 2 is more accurate. Even though E+ does not have an option of reporting overall window U factor, it takes into account the frame/edge into calculation fairly accurately. For dual glazing (two panes)options - you may want to consult "Importing windows from Window5" in E+ I/O reference (page 201-) for more inofrmtaion.

I hope it helps..

regards,

Mahabir

AJ Lester · Posts: 66 Location: Auckland

Hi Aymeric, Mahabir,

I do not have a lot of knowledge in this area, however in addition to the issue relating to frame areas (which makes a big difference sometimes!), a similar question was recently asked on the E+ forum relating to WINDOW5 calcs vs. E+ internal calcs.

Rob Hitchcock from LBNL had these comments on the issue:

bhandari · Posts: 78

Andrew,
Thanks for quoting Rob's response. I recently learned that there is a problem in the way E+ treats data from Winow5 generated report files. It affects the glazing system angular dependent data. E+ does not directly use the angular dependent data from the Window5 generated data file but internally calculates angular coefficients. I am not sure how much different the results would be but I agree with you until E+ team resolves this issue, we would assume that data entered through idf is more accurate, as Rob suggested.

Mahabir

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