A window is only as good as its installation. Here is the principle: weathertight outside and airtight inside, correct fastening and installation depth, joint tapes and the errors that cause thermal bridges, draughts and mould — the classic complaint cases.
Introduction
Installation of windows and external doors is one of those places where a little sloppy workmanship has big consequences. A skew or leaky installation creates draughts, cold bridges, condensation, and in the worst case mould behind the opening — and it is one of the most common complaints in construction. The element itself can be ever so good; it is the installation that determines the result.
Basic principle: tight outside, tight inside
A window joint must solve two tasks at once, and they pull in opposite directions. On the outside, the joint must keep driving rain and wind out, but at the same time allow any moisture to escape — it must be rain- and wind-proof but diffusion-open. On the inside, the joint must be completely airtight and vapour-retarding, so warm, moist indoor air is not pushed into the joint and condense inside the structure.
Fastening and installation depth
The window is fastened to the load-bearing frame/opening with screws or frame anchors through the frame — not through the seal alone. Number and position of fastening points follow the element size and the supplier's assembly instructions; typically fastening points are placed near the corners and distributed along the frame, and the hinge side gets extra fasteners because it bears the frame weight. Below the frame bearing blocks (pressure-resistant) are laid, so the window weight is carried down into the masonry, not hung from the screws.
- 01Set the window plumb and level, and wedge it up on support blocks under vertical frame members (at hinges and locks) before fixing.
- 02Check diagonal dimensions — a frame pulled out of square gives a frame that doesn't close tight or drags.
- 03Place the window at the correct depth in the masonry opening: normally at or near the insulation plane so the thermal bridge around the frame is minimised. The precise placement is shown in the project/detail.
- 04Leave an even installation joint all the way around (typically a couple of centimetres), so there is room for joint tape and movement — neither tight nor too wide.
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