Window installation

A good window badly fitted loses most of what makes it good: draughts at the reveals, condensation, and mould in the corners. Window installation is not just putting a frame in a hole, but making a sealed joint with the wall that keeps water and cold air out for years.

Stages of fitting a window

• removing the old window, for a replacement, and clearing the opening;
• setting the frame plumb and level on packers and fixing it to the wall with anchors or frame fixings;
• filling the gap with foam and protecting it with tapes: a vapour barrier inside, a breathable membrane outside;
• hanging the sashes, adjusting the hardware and checking the seal all the way round;
• fitting the inside and outside sills, if they are part of the job.

Why the tapes matter so much

Expanding foam on its own is not moisture-proof. Water vapour gets into it from inside the flat, and rain from outside. After a few seasons the foam is soaked, stops insulating, and the area around the window starts getting cold. Tapes on both sides, a simple version of so-called warm installation, protect the foam and largely decide whether damp shows up on the reveals after winter.

What to prepare

• windows made to the size of the opening, with the maker’s installation instructions;
• clear space by the windows and covered furniture, because removing old windows is dusty;
• a decision on who finishes the reveals afterwards: plastering, skimming and painting are separate jobs;
• the number of windows, their sizes and the floor level in the job.

Common questions