secondary glazing

[greenandaway]

Help please! 
I have replaced the windows in a splayed bay in a Victorian flat with double glazed, low e, gas filled units, but they still feel very cold, and there still seems to be some amount of heat loss, particularly when temperatures drop towards freezing.
I'm now considering installing secondary glazing (which would actually be tertiary) with low E glass and am having to decided whether to have aluminium (with its poor conductivity) UPVC, or wood.  Can anyone advise on which to choose, whether glass is better than clear plastic, or what other issues I should consider?
Thank you
Peter Lang

[Chris Herring]

Peter are you sure cold is not air leakage (draughts)?  In a Victorian house is is likely that your windows now have a better U value than your walls.  If you replaced single glazed windows with U value around 5 W/m2K with good double glazed U value around  1.5 W/m2K, and properly sealed then this should have made a big difference.  Tertiary glazing will probably not have a big effect, maybe 0.1 or 0.2 on the U value at most. Have you considered heat loss through the roof of the bay, which may not be insulated at all?

[Peter Wilshaw]

What you have to take into account with bays also is
1.Is it of the same construction as the rest of the building.
2.How much window area is there as a proportion of the bay.
3.What is the construction of the ceiling/roof above the bay.
All of these will determine the effectiveness of the double glazing installation and could give the affect that they are not doing their job. I find draught proofing and taking extra insulation measures in bay areas is often the best remedy!