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Window U-factor improvements - increase window heat gain?

asked 2023-10-12 08:25:18 -0500

jassociate's avatar

updated 2023-10-12 09:22:29 -0500


For a large office prototype model, 20 stories, 38% fenestration ratio, climate zone 5A. I've modelled two scenarios one with a Window U-value of 4.24 and the other with U-value of 2.04 - holding SHGC constant at 0.65 (I know this wouldn't normally be the case but I wanted to isolate for U).

The heating load dramatically decreases, however cooling load marginally increases, and there is actually more heat gain on the windows in the winter months.

Is there a logical basis for this?

Using EnergyPlus (V23) through OpenStudio

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@jassociate what simulation tool are you using? Please mention it in the title or body of your post, as well as add a tag so that others can provide better help.

Aaron Boranian's avatar Aaron Boranian  ( 2023-10-12 09:07:19 -0500 )edit

Using Energy Plus through Open Studio, exporting report "envelope_and_internal_load_breakdown_report"

jassociate's avatar jassociate  ( 2023-10-12 09:23:07 -0500 )edit

Similar question. Are you seeing an increase in cooling during Spring/Autumn (cooling off-season) as well? Any differences between North/South, East/West? Glazing U-factor improvements can trigger a slight increase in cooling, namely during off-seasons, by hindering the glazing's ability to passively cool outwards (e.g. after sunset). Yet as you noted, the benefits far outweigh the downside. However, this can also be linked to solar incidence, e.g. South glazing.

Denis Bourgeois's avatar Denis Bourgeois  ( 2023-10-12 11:41:44 -0500 )edit

I ran another simulation, and compared to the similar question.

Holding SHGC constant , improving U-value I get the following

U- Value {4.24, 2.04, 1.70} Beam + Diffuse Radiation (J) {7.07 x10^12, 7.27 x10^12,7.27 x10^12}

This is consistent with how Tsol is calculated: .

However, when I look at the total Surface Window Heat Gain it varies more:

Window Heat Gain {1.85 X10^6,1.94 X10^6,1.98 X10^6} or by % change: % increase { 0%, 5%, 8%}.

jassociate's avatar jassociate  ( 2023-10-13 13:20:40 -0500 )edit

For U-value from 4.24 to 2.04 - Increases in cooling seen throughout the entire year besides July, from 4.24 to 1.70 - increases throughout the entire year.

Increases the most significant in December/January. I will try and look at cooling hours.

jassociate's avatar jassociate  ( 2023-10-13 13:33:59 -0500 )edit

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answered 2023-10-13 05:06:34 -0500

Petros Dalavouras's avatar

As Denis mentioned, incident solar radiation can increase the temperature of your building. Add to that, interior loads (lights, people, equipment) and you will notice that your building probably needs cooling even at times when the outside temperature is lower than your cooling thermostat. Check the hours of the year that your building needs cooling and the outside temperature is lower than your cooling thermostat and compare it with the hours that your building needs cooling and the outside temperature is above your cooling thermostat. Include also the temperature difference (OA/Cooling thermostat) for each hour in the 2 scenarios and that should be your answer.

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Asked: 2023-10-12 08:25:18 -0500

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Last updated: Oct 13 '23