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The building envelope serves as a barrier preventing heat transfer from inside to outside (during winter) and from outside to inside (during summer). But if you have substantial internal gains in your zone, increasing R-value of the building envelope will increase the amount of cooling required, because the heat produced in the zone can no longer exit through the envelope as easy. I was surprised when I first heard this, but it is entirely logical.
To check out if this is really the case:
- Save a copy of your file as something else,
- Remove all but the 1st scenario.
- Remove all lighting, motors, pools, processes and miscellaneous equipment and occupants from the zone. That should eliminate all internal gains.
- Make 2 scenarios: one with a lower R-value on windows (walls or whatever) and one with a higher R-value.
- Simulate and look at results.
Since you no longer have internal gains, cooling energy usage should decrease with increased envelope R-value. The increased R-value is keeping heat out of the zone.
Whereas, when you had substantial internal gains, cooling energy usage increased with increased envelope R-value. The increased R-value was keeping heat in the zone.
(A possible exception to this is a building with huge glass facades. If you think of the solar gains as internal gains, you can see why this could be an exception.)
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