Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
I believe that the ground is cooling down your model. In E+ (and OS) if you don't enter ground temperature into the model the default values might cool down the building.
In the case of ground coupled small to medium house, it might be that the default ground model is cooling down the house. You can start by changing the Site:GroundTemperature:BuildingSurface to the year average outside air temp.
If you further read in the above link to the Input Output Reference you will find that it is better to use the Slab or Basement program to calculate ground temperatures.
I would recommend on using the foundation Kiva you can read more about that here.
Depending very much on the site weather (dry and cloud less) night sky temperatures can also result in heat loss enough to bring the zone temperature below outdoor air temperature, but only in special cases with a weak roof, and this is not likely to draw the zone ~8C below the air temperature.
