Impact of sensible heat gain component and cooling load calculation

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I'm comparing different energy retrofit in house. I used in my model ideal load air system, and the modification in the roof have the biggest impact in the cooling demand. However, when I compare the annual-building-heat-gain-components, the biggest proportion is because of the window...and not the opaque surfaces. There is any documentation you can recommend me to understand this difference? There is any output variable of cooling loads...or heat gain ... To understand which part of the building have biggest impact in the cooling demand?