Window U-factor issues

asked 2022-01-20 10:53:15 -0600

Jacob's avatar

I am working to model window improvements in a multifamily building tower. I have confirmed that the only difference between the models is the change of U-0.38 to U-0.20 windows, with a substantial window area on the tower.

When the change to U-0.20 windows is made cooling energy goes up which is unexpected. Has anyone experienced this issue? The models can be found here:

edit retag flag offensive close merge delete


The model is created in the Sketchup plugin and modified in OpenStudio 2.9

Jacob's avatar Jacob  ( 2022-01-20 10:54:27 -0600 )edit

You may want to look into summer cooling (i.e. outdoor T > indoor T) vs shoulder season cooling (i.e. outdoor air < indoor T). I've come across cases where envelope improvements trigger increased cooling in shoulder seasons (excess solar loads ... you mentioned "substantial window area"). In a nutshell, such improvements may hinder the building shell's free cooling potential in the shoulder seasons. Just a pointer ...

Denis Bourgeois's avatar Denis Bourgeois  ( 2022-01-20 14:33:16 -0600 )edit

One thing you may want to look at is not only reducing the U (insulating value) but also choosing the right SHGC for the climate. When there is a lot of cooling a low-solar-gain SHGC (0.25 or less) should be used. This will help to reduce cooling loads. But in a cold climate, it may increase heating due to reducing passive gain.

KerryHarp's avatar KerryHarp  ( 2022-01-23 04:57:03 -0600 )edit

If you are using the SimpleGlazingSystem model in EnergyPlus, you might review the engineering documentation for it. It takes U/SHGC and automatically constructs a detailed window that matches those inputs. So, for example, changing just the U-factor can result in different angular properties between the two simulations among other things.

shorowit's avatar shorowit  ( 2022-01-24 14:11:15 -0600 )edit

Thanks for the thoughts on that. Additional info: this is in climate zone 7.

1) For SHGC being the same in both models, the only scenerio I can think of is that if high internal loads had more trouble leaking out of the envelope on a cool morning the cooling could run more during the day?

2) Yes SHGC is a seperate issue to consider, but bieng in climate zone 7 reduces the benifits of low SHGC are reduced as we have heating dominated annual usage. That makes it all the more suprising that a cooling increase could come even close to countering the heating savings.

Jacob's avatar Jacob  ( 2022-01-28 14:08:05 -0600 )edit