The U Factor measures {HEAD ~ the rate} of {TAIL ~ heat transfer} or loss through a window as laid down by the National Fenestration Rating Council 
of
The U Factor measures { HEAD ~ the rate } of { TAIL ~ heat transfer } or loss through a window as laid down by the National Fenestration Rating Council
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH