Roof Light & Heat Gain

Question

Hi again 
 I've created a 2-storey building with a roof and I want to determine the temperatures in the roof space with and without roof-lights. To do so, I've artificially created a room in the roof space (like an attic) so that I can use the Summer calculation to determine the temperatures when no HVAC is provided to the room. 
 After running the Summer calculation initially without the roof-lights, it calculated the temperature in the roof space to be about 36deg which was what I expected. Then, I added roof-lights using the Roof function and re-ran the Summer calculation. I expected the temperature to rise as there should be more heat gain with roof lights. However, the Summer calculation showed that the temperature dropped to 30deg. To make thing worse, when I allowed heat flow across the internal floor from the roof space to the room below in 'Select Surface>Internal>Adjacent Room', the temperature dropped further to 27deg. 
 Why is the temperature drop? As heat rises, I expect more heat crossing into the roof space (via the internal floor/ceiling) and increase the temperature. I also expect more temperature increase with roof lights as more sunlight is going into the roof space, hence more heat gain. 
 Thank you again for your help. 
 
 Regards 
 Kee

Duncan Brown · Accepted Answer

Hi Kee - Thanks for the projects. 
 
You need to consider that this is a 24 hour calculation. For a large part of the day, especially during the night, the outside temperature is colder than the room design temperature. This results in negative gains as heat flow goes from the room to the outside. This happens much faster in the model that has roof lights. If you examine the full results for the room in question you can see that the temperatures that the room starts from are much cooler in the model that contains roof lights. You can compare the results with the CIBSE Gain calculation to see the negative gains (losses) during the 24 hours period. 
 
regards,