Cutting The glass Rebates in the sash timbers

I'm going to fitting double glazed units in the windows, these will be 4-16-4 to meet Building control U-Value specifications. This means the units will be 24 thick.

I need a huge rebate compared to traditional sashes, this where the design detail differences start to come in.

I'll be fitting the units using glazing tape either side (2mm) thick, so the rebates I've cut are 28mm wide. I might end up adjusting this if I need some extra compression on the seals.

From research I found that the glass has to be supported on spacers to allow an airgap all round. This ensures the edge seal of the units won't fail prematurely due to moisture attacking them. Its also a good idea to hide the edges behind the up stand of the rebate to shade them from direct sunlight. I've cut the rebates 20mm deep.

I cut the rebates first with the table saw, leaving about 1mm to do with the router to the finished dimensions. I then used a 45 degree bevel bit with a roller to add the bevel. The bevel is to simulate putty hold the glass in, keeping the look of the window as traditional as possible.

And in case you were wondering the sashes will be internally glazed.

There's a flat at the top of the bevel that will take a bead of silicon as an extra seal.