Condensation and ventilation in a modern dormer
Short answer
Modern dormers are airtight to keep the heat in. Without deliberate ventilation, the humidity from people, plants and showers has nowhere to go and condenses on the coldest surface — usually the bottom edge of the glazing or the frame corners. A trickle vent, a ventilation grille or a small mechanical extract solves it.
Why dormers condense
Two people sleeping in a room produce a litre of moisture overnight. A house with HR++ glazing and a tightly built dormer has no leakage path for that moisture. It accumulates, and in cold weather it lands on the coolest spot in the room.
How to stop it
- Trickle vents above the windows — passive, always-on slot ventilation.
- A dedicated wall vent if the dormer doubles as a bedroom.
- An extract fan if the dormer space includes a bathroom or laundry.
- Keep furniture 50 mm off cold walls so air can circulate behind it.
- Air the room for 10 minutes after waking — windows wide open is enough.
Ventilation options
The cheapest and most reliable is a trickle vent integrated into the window frame at order time. Retro-fit grilles in the wall or window are possible but more disruptive. A mechanical extract (MVHR or simple bathroom-style fan) is overkill for most bedroom dormers but right for wet rooms.