Greeting all

Does anybody out there have any experience of having double glazing
added to origional sash and case frames. I live in a listed building
in a conservation area and already know that we don't have many
choices. If we want double glazing or any sort of additional heat
conservation other than draft proofing we would have get the origional
sash windows double glazed.

There is company based here in Edinburgh i think that can do this for
us although i'm skeptical about the claims on their website. They
propose that with low-emissivity glass they can reduce heat loss as
per below.


Slimlite Super Low Emmissivity

Construction 4mm Low E / 3.2mm Cavity, Gas / 3 or 4mm float = U
Value 1.5
Construction 4mm Low E / 4.8mm Cavity, Gas / 3 or 4mm float = U
Value 1.2
Construction 4mm Low E / 6.4mm Cavity, Gas / 3 or 4mm float = U
Value 1.0


My concern is that everything i read suggests that the cavity
between the glass will be so small using a traditional sash frame that
the double glazing can't make enough difference to my heating bills to
warrant getting it installed.

Can low-e glass really be that good?

If so why are conventional double glazed units made with such large
cavities?

I hope someone's got the answers.

Cheers in advance if you do.