Dear all,
I’d like to ask a question about the assembly U-value calculation according to NFRC norms. In most of our projects, the window manufacturers provide us with the window thermal properties calculated according to EN 673 and ISO 10077 norms. However, we have to use the thermal properties calculated according to NFRC norms in the energy model. We use Window software to calculate the assembly U-value and other data. We have no problem with the glazing part. However, we are not sure about to what extent we should model the frame? Do we have to generate a sophisticated model in Therm? We cannot obtain the detailed info from the manufacturers to model the frame in Therm. Or would it be enough to create a generic frame (with the U-value, edge correlation, material absorbtance and dimension data)?
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Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
September 4, 2015 - 3:40 am
You are definately on the right track.
What you are doing is reverse engineering a physical model from rated value inputs.
I've put some guidance here: http://www.designbuilder-big.com/index.php/2013-02-25-13-40-45/2013-02-2...
Don't be perfect. Just be good enough.
PS
Using Therm is actually also not that hard, once you get into it, but in my opinion seldomly required. If the reviewer is convinced that your model is energetically equivalent and will behave appropriately, you're okay.
MURAT DOĞRU
GREEN BUILDING EXPERTECOBUILD
1 thumbs up
September 4, 2015 - 9:07 am
Thank you very much Jean.
That guide is of great help. We will apply the steps in the guide to convert EU Uf value to the American Uf value to be used in the assembly U-value calculation. Thanks..
Also I want to ask another question related to Window software again.
For a custom size casement which differs considerably in size from NFRC-defined casement in Window 7.3, which window type should we select? Custom single vision? or Casement single?
Which is more predominant in the assembly U-value calculation- the dimensions or the window type?
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5867 thumbs up
September 4, 2015 - 3:10 pm
The EU standards with conversions should suffice for most situations.
You should also be able to model the whole assembly U-value in the modeling software. Having the rated values is a great reality check against what the modeling results indicate. You do need to be careful as it can be easy to do this incorrectly in many modeling software. A really good description and product literature can sometimes help the reviewer understand the situation.
The U-values in the model should accurately reflect the window product used. In the US market the most common commercial windows are metal framed with a thermal break (a very small one) and double-pane low-e glazing. The center-of-glass U-value is usually about 0.28 or so. The whole assembly U-value is typically between 0.4 and 0.5. Unless the frames are far more robust than the norm if we see a claim of an assembly U-value of 0.3 to 0.39 it will get questioned because it is outside the norm and has not been explained. So double check the assembly U-values against the norm to make sure your modeling software is giving you a reasonable result. Too many modelers do not really know the norm and just accept what the software says as accurate.
Window type would be the more predominant in my experience. In general you do not have to determine this in separate software most of the time.
Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
September 7, 2015 - 4:16 am
I agree with Marcus "The U-values in the model should accurately reflect the window product used." The Window software will accurately calculate the NFRC U-Value Rating (because it uses the algorithms and rating conditions required by the NFRC) for the 'as built' window assembly. To my knowledge, only the glass part Ug rating is deturmined from a standard size...So I would tend towards using the custom size options especially regarding the total assembly calculations.