[Greenbuilding] Adding glass pane to single pane wood windows ?

[Lance Hoffmeyer]

Hey all,

My house was build around 1955 and has single-pane wood windows.

When I first moved in I started adding storm windows to the outside.
They are very flimsy and a hassle on days like this when I want to
open windows because I have to go through a four step process:
open the wood window,  open storm screen, open storm window, close storm 
screen.

It occurred to me that I can have glass cut and get holes drilled in it
that would allow me to attach a second pane of glass to the existing
sash (frame) of the windows.

Attaching a second pane to the wood frame would give me almost 1 inch of 
space
between the original glass (1/8 inch) and the second pane.


I am now trying to decide what sort of glass to get.  the options I am 
considering are

1/8 or 1/4 inch

plain (probably only safe to use 1/4 inch, 1/8 inch may break too easily)
tempered (same cost 1/8 inch or 1/4 inch)
laminated (1/4 inch only)


I am looking for some ideas/help for advantages/disadvantages of these 
glass types and this
idea of adding a second pane of glass to the existing windows, in general.


I imagine R/U value doubles from 1/8 inch to 1/4 inch.
Don't know if there is any difference in R/U value window benefits for 
plain vs. tampered vs. laminated.
Since I will most likely use 1/4 inch glass anyway, I imagine plain 
glass is cheaper and what I would probably
purchase unless there is significant R/U value increase in using one of 
the other types.


TIA,

Lance