2.2.5 Envelope Load Considerations

Thermal blocks shall consist of spaces having similar envelope loads; for example, thermal loads from solar heat gains and conductive heat losses from roofs. In general, spaces close to the perimeter of the building should be in separate thermal blocks from interior spaces. The following guidance shall be applied in combining HVAC zones into thermal blocks:

• Exterior and interior spaces shall not be combined in the same thermal block, except as permitted below.
  
  Exception: Exterior spaces without fenestration or doors may be combined with interior spaces in the same thermal block.

• Exterior spaces having different glazed orientations shall not be combined in the same thermal block, except as permitted below.
  
  Exception: Exterior spaces having different glazed orientations but small effective apertures for solar heat gain (i.e., solar heat gain coefficient times fenestration area divided by zone floor area less than 10%) may be combined in a single thermal block.
  
  Exception: Exterior spaces having different glazed orientations but whose orientations differ by 45º or less may be combined in a single thermal block. This is not intended to prevent or discourage modeling of actual or anticipated corner zones or other actual HVAC zones which include fenestration of varying orientations in a single contiguous space.
• Spaces with envelope loads from floors and/or roofs shall only be combined within a single thermal block with spaces having similar loads from floors and/or roofs.
• Separate thermal blocks shall be created when fenestration area varies greatly. For example, a long perimeter corridor with small windows at one end, but all glass at the other should be split into two thermal blocks.