Chris,
We had a look at this recently. For a curtainwall system with panels
captured on two sides by a non-thermally broken aluminium mullion and
structurally glazed (butt joint) on the other two showed a
theoretical overall vision panel of 2.1 W/m2 oK for a system with a
COG U-Value of 1.7 (divide by 5.678 for IP) according to the
consultant using Therm and Window 5.2. The high values in the ASHRAE
handbook (2.6-2.8) I believe reflect a larger frame area ...
G