Although they are still available, it would be difficult to find either aluminium or UPVC replacement windows which are intended for use with single pane glazing. Invariably, such windows are designed to be fitted with sealed units having a 6 or 12mm gap. It is common for companies specialising in these windows to operate a supply-and-fit service. However, most will also work on supply-only basis.
This means that they will provide you with all the component parts ready for you to install yourself. When doing your own fitting, the only measurements you need to give the supplier are the height and width of the opening into which the window is to be fitted. If the existing outer wood frame is in excellent condition particularly at the bottom of the iambs and along the sill, you can normally fit the new window in exactly the same place as the old one, with no trouble at all. If the outer frame is in poor condition and a new one is required, then the window supplier will be able to provide this as well. In this case, the only dimensions he needs are those of the opening in the wall. From these he will be able to calculate all the other sizes.
You will, of course, need to specify the style of window, the type of glass, whether or not the glass has to be leaded or fitted with a grille to be made non-standard in any other way. The sealed double glazing units fitted in replacement aluminium or UPVC windows are the same as those used in wooden framed windows butthe installation method is somewhat different.
The glazing unit is always fitted ‘dry’, rubber or PVC gaskets being used to provide a seal to the frame. No mastics or sealants are required at the glazing stage. There are two basic glazing methods in common use. One of them nvolves making up the frame around the glazing unit. Each frame section, complete with gaskets on either side, is pushed over the sealed unit and then the four corners are screwed together tightly to hold it in place. The other method is to make up the frame, which has an integral glazing rebate, insert the glazed unit and secure it in place with a ‘snap-in’ glazing bead.