Window u-values and SHGC

dancole · Posts: 9

Andy,
The majority of window suppliers (and national window performance ratings) give U-value and SHGC figures for the overall window system. We would like to use those figures to model windows. My question is, if we turn off window frames in the openings tab at building level, and create glazing components with U-value and SHGC provided, is this a reasonable way of modelling the overall window performance?
Thanks
Jamie

Andy Tindale · Posts: 2432 Location: Stroud

Jamie,

It sounds like a reasonable approach.

We do have plans to allow Windows systems to be defined according to their SHGC and U-values in a future version.

Regards,

Andy

bhandari · Posts: 120

Jamie,
Just to add - the U factor, SHGC supplied by window manufacturer and supplier include the contribution from frame as well. Depending on frame material, width (PFD), glazing system used and overall window size, the frame contribution could be as high as 20%. You can also use WINDOW5 (a free program aavailable from Lawrence Berkeley national lab : the link is a not allowed here but you can Google it), to get approximate contribution of frame, if additional information about frame is not supplied by the supplier. In the future version of DB you can directly import windows from WINDOW5.

In you case, if you adopt the approach you mention here (exclude frame), I would recommend increasing glazing system SHGC by 10-15%, especially if you have large glazing area and your building is cooling dominated climate.

regards,

Mahabir

dancole · Posts: 9

Hi,
Thanks both for your response. As the values given by the rating organisation are for window system frame etc included) i'm not sure why seperate data for frame contribution (and increasing SHGC accordingly) might be required in this approach (Mahabir's comment).
Thanks again for your help.
Jamie

bhandari · Posts: 120

Jamie,

Because when you assign the glazing system only (as you proposed) and ignore the frame, DB (or E+)will consider the whole window (dimensions provided by user) as glazing system.

To explain it little more: If you only create glazing system and assign the value given by supplier to it, you are actually increasing the glazing area (by ignoring the frame area) and at the same time assigning the lower SHGC value (the whole window SHGC value provided by supplier will be lower than glazing only SHGC due to framing around window)to glazing system.

I hope it helps..

Mahabir

SPlamp · Posts: 31 Location: Omaha

Dear Mr. Mahabir Bhandari,

I'm currently trying to simplify my assumptions for modeling a curtain wall glazing for mid rise commercial buildings.

I highly recommend the connection of DesignBuilder and Window5. I can see from working with Window5, that it allows much more detail in modeling window surfaces in which the information is also actually used in EnergyPlus - but this is rather tedious using the idf-editor (basically identifying window surfaces once exported as idf and replace them with the information from Window5).

I currently see no way of specifying a window surface with a different divider section from a typical frame section in DesignBuilder - which would be accomplished by Window5. Also, I appreciate Window5's extensive database of glazing, frames, dividers, etc.

However, I also recognize a drawback in all of that, which is that the reports generated by Window5 are rather stringent and DesignBuilder may loose its ability in modeling odd shapes of windows. Also, the time burden will significantly increase, since each window would have to be created in Window5 and imported into DesignBuilder and applied to the selected surface.

The major advantage I could see is a more realistic and detailed window model for larger buildings reflecting a significant contribution of the window surfaces to the overall heating/cooling energy requirement.

So essentially, the import of Window5 detailed reports should be optional, but definately included in future versions (DBv2?).

Regards,

Sandro

bhandari · Posts: 120

Sandro,
I use THERM and WINDOW on a regular basis. I agree with you on how important it is to use detailed Frame and center of glass modeling, especially in curtain wall constructions. Dividers also make difference, especially on SHGC, which can have a large impact on cooling loads and consumption. We have a plan to implement option of reading E+ report from WINDOW5 in DesignBuilder. I think it would be included in version 1.X..

Best regards,

Mahabir

Andy Tindale · Posts: 2432 Location: Stroud

Just to confirm that we do plan to include WINDOW5 reports as an option for Window definition in DesignBuilder version 1.4 which is scheduled for release August this year.

Andy

Steve Potter · Posts: 572 Location: newcastle

It's probably worth pointing out to less experienced users following this thread that although it may appear from the context that it is possible to specify window performance through DB in the form of U-value and SHGC data, this is not actually the case.

The U-value and SHGC data which is displayed on the 'Calculated' tab of the 'Glazing Data' dialogue box although currently editable is actually calculated from the thermophysical material properties together with dimensions specified on the 'Layers' tab. It is the data from the 'Layers' tab that is used for the E+ calculations and not the derived U-value/SHGC data. The calculated data is not intended to be editable and this is in fact a bug that will be corrected for version 1.3.

Steve

jamie · Posts: 26

Steve,

"It's probably worth pointing out to less experienced users following this thread that although it may appear from the context that it is possible to specify window performance through DB in the form of U-value and SHGC data, this is not actually the case. "

Sadly this is exactly what has been assumed and what i (wrongly) understood from the responses in this thread.

This sounds like it invalidates the approach mentioned above in which we define panes, then glazing systems with user defined U-value and SHGC? If so how do we use the values given by local Window Energy Rating authority (for whole window). Also, i have never seen the values in the calculated tab change for different panes etc. In that case, how are we to know what values E+ is actually using??

Cheers
Jamie

Steve Potter · Posts: 572 Location: newcastle

Jamie,

A couple of modifications have now been made to DB in order to prevent editing of calculated data and this data is now automatically updated after glazing construction data has been modified. An update incorporating these modifications is now available for download.

Also, we are planning to implement a mechanism to enable glazing data to be specified in the form of U-value/SHGC data in version 1.4 which will be available later this summer.

Steve

arl · Posts: 3

Hi to all,

I am working with a retail warehouse, with 7% of the roof surface occupied by skylights.

I made a lot of simulations already to evaluate the possible improvements in a building.

In all of these simulations, the reference building I am using has to comply with the new Spanish Building Code, so I need to introduce my own values to comply with it.

The wall glazing is ok (I used DB database).

The spanish code demands low roof U value (average U value of roof material and U value of glazing), so I had to play with the U value of the skylights and the insulation on the roof.

Since I had a specific model of skylight (with U=2,9 and SHGC=0,7) I had to introduce these values manually and then change them also manually.

IF I UNDERSTOOD CORRECTLY BEFORE, DB DID NOT SIMULATE WHAT I DID!!!. so I have the following situation. [/color]javascript:emoticon('

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Crying or Very sad

1.- My reference building is not correct, so I do not know I am simulating a building which complies with the Spanish Building Code.

2.- The rest of the improvements I made, are not correct, since they were simulated based on the reference one.

3.- I changed manually the U=2,9 value to U=2,0, and it affected to the glazing heat lost value (in 19% but not to the rest of the values.

Please, could you assist me in what changes I can made to:

1.- Get the right reference simulation.
2.- To assume my results of the other 7 simulations I did in a fast way. (out of the evaluation of glass value).

PLEASE, I WAS SUPPOSED TO PRESENT THIS VALUES ON MONDAY!!!! so, I would really appreciate your rapid support.

Thank a lot for everything,

Asier

Andy Tindale · Posts: 2432 Location: Stroud

Asier,

You should still be able to set up a glazing type having approximately the required specific U-value and SHGC but you will need to do by 'trial and error' (i.e. in the Glazing dialog test various glazing configurations until you get the right summary data).

I hope it helps,

Andy

arl · Posts: 3

Hi Andy;

Only problem is that Calculated values do not change in the screen when I change the thickness of the window layers.

So, do they change the values despite I do not see them changing?

Asier

bhandari · Posts: 120

Asier,
I presume thickness of the window layers means thickness of individual glass panes. Changing glass pane thickness will not change the U factor significantly. That may be the reason you do not see the change in values.

Mahabir

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