I'm planning to re-convert my garage (the previous owners did it, but not to regs, and I now need to sort it out). The lintels limit how much I can increase the floor height to insulate, so I'm having to go down the compensating route to calculate the whole heat loss from the space and increase performance of other components to balance out. There are two existing uPVC double-glazed windows that were fitted around 2008 : I don't have any info on them and there's nothing visible on / around the frame to identify them. Anyone got any suggestion of how I might find out / estimate their U-value, or what a worst-case scenario would have been for a new double-glazed window back then? Thanks!