Our house had PVC windows almost throughout that were ill fitting and really didn't suit the property.  These have been replaced with new Building Regulation-standard double-glazed sash windows.  The front ground floor bay was the only remaining original sash window and it was in need of some care and attention.

Replacing it wholesale was going to be very expensive and as new sash boxes need to be thicker to hold the larger cavity in a new double glazed window, a lot of changes would have been needed.

We also wanted to retain some of the original features and it seemed a shame to take out wooden sashes to replace them with new wooden sashes.

The final issue was that beneath the windows were the original wooden sash shutters which we through it would be nice to renovate.

This did however mean that the brickwork under the windows would be a 'cold bridge', so the plan was to externally insulate the bay bricks - around 3 square meters.

It was soon discovered during the restoration that the wooden shutters were rotten at the bottom - a consequence of the earth out the front being raised at some point - we'll be dropping the height of this areas and putting in a French drain of sorts.

We have therefore opted to remove the shutters, insulate on the inside and replace the shutter wooden casing.

For the rest of the bay we have opted to have them refurbished with thin double glazing fitted into the original sash windows.  Thin double glazing is pretty amazing stuff as some of these photos hopefully show.

Our window frames are pretty thin so we have had to opt for the thinnest option - 3mm glass, 3mm cavity with inert gas and 3mm glass on the inside. Because of the thin cavity and thin glass, the overall u-value will not be as low as Building Regulations, but it will not be far off.

The process involved:

1. stripping 140 years of paint
   
2. repairing any timber joints and damaged areas
   
3. removing the old glass
   
4. installing new thin double glazing
   
5. routing-out for brush draughtproofing system
   
6. applying putty
   
7. repairing the glazing bars
   
8. priming
   
9. reweighting, cording and hanging the sashes
   

The brush draughtproofing system is pretty inconspicuous unless you really look hard. It's hidden either between the sash frames where they meet at the meeting rail, at the top and bottom and behind the staff beads. You have to look pretty close in the photos below to see them.

In terms of pure cost effectiveness upgrading or replacing the glass in windows doesn't often make pure financial sense - draughtproofing often does but usually on if done DIY.  However there are lots of other reasons in addition to cost savings for having windows upgraded:

• Radiative losses can be reduced improving thermal comfort.  Or more simply, this bay will now not be an uncomfortable place to read a book on a winters day.  Radiative loses are what causes a whole side of your body to go achingly cold when sat next to a single glazed window for a while.
  
• Uncomfortable draughts. Air movement greater than 1m per second is recognised as a draught and of course cause heat losses, but they are also pretty uncomfortable.  Draughts will be reduced both by the brush system but also by reducing convective draughts from cold air dropping down the inside of the windows.
  
• Condensation. Anyone with single glazing knows the downside of cold nights.  As the glass temperature drops lots of the moisture in the inside air condenses and by the morning there is a puddle on the frame.  This can be worse with draughtproofed houses and insulated walls.  The draughtproofing can lead to higher humidity levels indoors if controlled ventilation is not installed, and the wall insulation raises the inside temperature of the walls so they don't absorb as much of the water varpour.  These windows should now be free of condensation.
  
• Noise.  These windows are on the street side of the house and will now be a lot more sound-proof.
  
• Security. The windows will now be a lot more secure.
  

It's quite a specialist job so we employed Ron Bowie of Alexander Restoration. We are thoroughly please with the service and end product. Ron provided a detailed item by item breakdown in his estimate. In actual fact the final price ended up slightly less as some of the contingencies in the estimate weren't needed.  He even left the inside and outside areas tidier than he found them.