Today we will look at double glazing existing sash windows. Firstly before we go anywhere with our tools, we must use our tape measure. To double glaze an existing sash we require at least an inch and a half sash. For a beginner it might be worth starting on inch and three quarter thick sashes because the tolerances at 38mm are 2-3mm only.
If your sashes are 40mm + then remove them, cutting cords, and removing from frame. The old cast iron weights are no longer of use because we require heavier, more dense material to maintain full and smooth travel of the window when rebuilt.
Sashes on workbench, carefully remove the original glass. It pays to be careful and gentle here. Slowly break the glass up and throwing it away into a bin as you go. The last thing you want is to be working in a pile of broken glass! With glass removed we need to hack out the old putty. Putty will dull the edge on your router cutter too quickly and since a specialised rebate cutter costs nearly £100 it is important to look after it well.
So now the putty is neatly removed we are ready to start the process of double glazing our existing sashes.
Firstly take your half inch router, and the specialised rebate cutter. Then cut to a depth of 20mm. This will allow for a (4-6-4) double glazed units. Run your router around the inside edge to deepen the rebate. The corners will need neatly squaring up. Once cut back, with a quality silicone bed the new sealed unit into the old sash, and with a 6mm timber glazing bead finish the glazing process. It’s important to use a quality sealant here, we don’t want the double glazed unit breaking down.
Now you’ve double glazed the existing sashes it’s time to counterbalance and re-cord appropriately. Weigh out the sash and lead weight required. Then record with the correct balance. Rebuild the window and admire. Professionals will always install a draught proofing system with the double glazing as it is pointless in terms of energy efficiency to do one without the other and is part of any typical sash window refurbishment programme. After you’ve rebuilt the window then you will want to decorate the sash internally and externally. Make sure that prior to decoration there is a week of dry weather. This will ensure the moisture content in the timber is low enough that a quality paint like Dulux Weathershield will last well. Basic sand preparation will be fine, enough to key the original timber. No special requirements from decoration to produce a brilliant finish.
Here’s a little look inside a double glazed original sash window. As you can see the unit is hidden nicely and from straight on the window would certainly not appear double glazed which is exactly how it should be.
Perhaps this is a little beyond the level you’d like to go to. Why not give these double glazing existing sash windows experts a call and let them take care of the process for you. They’ll know all the pitfalls and all the tricks to make your windows secure, safe, warm, and draught free. Prices range from £500 upwards and include new window furniture, new timber beads, five year guarantee, and a reliable service from a market leader.