Window Installation: the Joint Nobody Sees

The window is the only envelope component that is bought with a certified performance and then installed by somebody else, often in a hurry and almost always without a drawing. Hence the gap everyone knows and nobody measures: excellent windows badly fitted, whistling in the wind and growing mould on the reveal. One principle holds it all together, and it takes a minute to learn: the joint must be tighter inside than outside. Inside you stop the vapour pushing from the warm room towards the cold; outside you let out whatever got in anyway, and stop only liquid water. Swapping the two tapes — by far the most common mistake — turns the joint into a trap: vapour enters, cannot leave, condenses in the filling and rots it from within. The second principle is that fixing is mechanical and foam is not a fixing. Foam fills and insulates, it does not carry: the weight of the window and the wind load must be discharged into the masonry through anchors or straps, at a regular spacing and closer at the corners, where the frame works hardest. The sub-frame is not a luxury: it is what separates shell from finish. Fitted while the wall is still open, it gives a square reference, allows building around it without touching the window and — thirty years later — lets the window be replaced without demolishing the reveal. Qualified installation is covered by standards, whose logic is the design of the joint and the qualification of whoever executes it; but the substance stays a craft matter: a square opening, a continuous bearing plane, two tapes the right way round.

It is the only thing to remember, and on its own it explains nearly every pathology of the window node. Warm room air holds far more vapour than outside air, and that vapour pushes towards the cold. If the joint is open on the inside, vapour enters the filling; if it is closed on the outside, it can never leave. It condenses inside the joint, where there is neither ventilation nor sun, and the filling stays wet for years. Close it on the inside and let it breathe on the outside, and the vapour does not get in at all, while the little that slips through defects finds its way out. It is the same logic as external insulation and a ventilated roof: stop vapour on the warm side, let it dry on the cold side.

The sub-frame looks like an added cost and is the first thing cut when the quote is squeezed. In fact it decouples two trades that otherwise obstruct each other: with it in place the mason closes the opening, plasters and finishes without waiting for the window, and the installer arrives at an already verified square reference. Without it, the window must be fitted before the finishes and then sits on site for weeks, badly protected, at risk of knocks and of dirt in the hardware rebates. The real bill, though, arrives at end of life: replacing a window built directly into the masonry means demolishing the reveal, and therefore redoing plaster, sill and paintwork throughout the room. With a sub-frame, the old frame comes off and the new one goes on the same day.