• When installing new 50mm thickness windows with double glazing, the sash 'box', the frame the window sits in, also has to be replaced. This is straightforward.

• Double glazed units use Mighton sash balances instead of weights and pulleys to hold them in place. Each pulley can hold up to 40kg (90lb)... about double the weight of a Victorian terrace double-glazed living room window... and because the sash box doesn't hold weights it can be reinforced to take the extra weight of the glass with no risk of bowing. Sills are traditional sapele (African Mohogany) hardwood and resistent to rot.

• An Aquamac™ strip is routed into the window professionally to ensure no air or water passes between the window and the bead. The same is applied where the two windows meet, and at the top and bottom. The parting bead of the box is tucked into the window itself... thus eliminating airgaps between itself and the two sashes. This means that there are two of them.

• The glass is building regulation 20mm thick, made up from two sheets of 6mm glass with an 8mm gap. It is set into the frame with tape by our FENSA registered glazers.