How do I fit trickle vents to an existing window?

Trickle vents provide the background ventilation a sealed modern house needs and does not otherwise get. Retrofitting them is straightforward joinery, but where the slot goes matters enormously on a uPVC frame.

Safety

What you'll need

• Trickle vents sized in equivalent area
• A drill with a 10mm bit
• A jigsaw with a fine blade, or a slot cutter
• A file
• A tape measure and square
• Silicone sealant

Step by step

1. Work out how much you need.Trickle vents are rated in equivalent area, in square millimetres. Approved Document F in England sets figures per room, and the other UK nations set their own; 5,000mm² per habitable room is a commonly used background figure, more for kitchens and bathrooms.
2. Decide where the vent goes.Through the head of the outer frame is the usual position. On a sash it goes through the top rail of the opening light. Either way it must be above the glass and above the drainage chamber.
3. Find the steel reinforcement.uPVC frames have a galvanised steel section inside the main chamber. A magnet finds it. Cutting into it is possible with the right blade but should be avoided; on the head rail there is often none.
4. Check what is behind on the outside.The vent has to emerge into open air, not into a cavity closer, a lintel or a rendered reveal. Look from outside before drilling.
5. Mark the slot accurately.Use the vent's template or the vent body itself. Mark the ends and use a square — a slot that is not parallel to the frame shows badly once the cover is on.
6. Drill the ends first.A 10mm bit at each end of the slot, then a line of overlapping holes between them. This is far more controlled than plunging a jigsaw into a frame.
7. Join and file.Cut between the holes with a fine jigsaw blade or a slot cutter, then file the slot straight and clean. Both halves of the vent have to sit flat.
8. Drill through from the inside face outward.So any breakout happens on the outside face where the external cover hides it, rather than on the visible internal face.
9. Fit the internal and external parts.The internal part is the controllable vent, the external is the hood or grille with an insect mesh. Bed both on a thin bead of silicone.
10. Seal without blocking the airway.Sealant around the vent body, not across the slot. It is easy to seal the vent shut with too much silicone.
11. Check it opens and closes.The internal flap should move freely. Test it before you tidy up.

Tips

• Over-frame vents fit above the frame in the head of the reveal and avoid cutting the window at all. Worth considering where the frame is reinforced or under warranty.
• Acoustic trickle vents cost more and cut noise substantially. They are the answer on a road-facing bedroom where people otherwise close the vent permanently.
• Fitting a vent to the outer frame rather than the opening sash means it still works when the window is locked shut, which is the whole point.

Common mistakes

• Cutting the slot into the drainage chamber — The lower part of a uPVC frame carries water out through the drain slots. A vent slot cut into that chamber lets rain straight into the room and cannot be sealed without blocking the drainage.
• Fitting a vent that is too small for the room — Equivalent area, not physical size, is what counts, and the two differ substantially. An undersized vent gives the appearance of ventilation without delivering the air change the room needs.

If it doesn't work

Questions people ask