On 09/07/2012 09:20, ss wrote:
Just recently got `free` cavity wall & loft insulation and would like to
further improve the insulation and sound insulation in a couple of rooms.
In one bedroom there are 2 windows which although double glazed have
minimum spacing between the glass (approx cm) I also hear traffic noise
through them.
I was thinking of using some 10mm polycarbonate sheet in a frame to make
a third `pane` . I am using this method as it is something I could do
myself.

Why 10mm? Do you expect to need it to be bullet proof as well?

5mm polycarbonate sheet or perspex is pretty well behaved in frames.
I secondary glazed the remaining single glazed windows with it. Really
small windows you can get away with 3mm sheet. You have to think
carefully about any opening windows depending on how you do it.

Am I likely to gain much by way of heat loss and sound insulation or
would it not be worth the effort?

Good thick heavy curtains will prevent more heat loss than secondary
glazing when they are closed. Hard to predict how well it will perform
against traffic noise since it depends on what sort of noise and how it
is coupling to the room. Wider spacing and having the inner pane damped
rather than rigid and taught like a drum skin will help.

You will need a UV stabilised grade with the UV protection facing
outwards. Glass stops some UV but not enough to avoid degradation of
water clear plastics in windows.

--
Regards,
Martin Brown