How to Keep Cool Air Trapped Inside: Draught-Proofing for a Cooler Home

Quick answer
Draught-proofing isn’t just a winter job — the same gaps that let warm air out in January let hot air in during a heatwave, and they undo the cool you’ve worked to create. Once you’ve cooled a room (by night ventilation, shading, or a fan or air conditioner), sealing it stops that cool being diluted by warm air seeping in through gaps around doors, windows, floorboards and unused vents. The one crucial caveat: don’t seal so completely that you can’t purge-ventilate at night or manage condensation. The aim is a room you can close up tight by day and open right up when it’s cooler — controllable, not permanently airtight.

This page is about the why and where of sealing a room for summer. For the specific cheap products that do it, see our draught excluders and window seals guide — this one covers the strategy they serve.

Why draughts work against you in summer too

Air moves from warm to cool and from high pressure to low. On a hot day the outside air is warmer than your carefully cooled room, so every gap becomes a route for that heat to seep in and mix — slowly erasing the difference you created overnight or with a fan or unit running.

It’s the same physics that loses you heat in winter, running in reverse. Which is why draught-proofing is a genuinely year-round improvement: warmer in winter, and easier to keep cool in summer.

Where the gaps actually are

Around doors — the gap under an internal door lets your cooled room bleed into the warmer rest of the house; an external door gap lets outside heat straight in.
Around windows — worn seals and ill-fitting sashes leak continuously.
Floorboards and skirting — gaps in suspended timber floors draw air up from underneath.
Unused chimneys and vents — an open flue is a constant air path.
Loft hatches, and service penetrations — where pipes and cables pass through walls and ceilings.

The single highest-value move for keeping one room cool is usually sealing the gap under its door, so the cool you’ve made stays in that room rather than diffusing through the house.

The crucial caveat: don’t over-seal

A home needs to breathe. The goal is controllable sealing, not a permanently airtight box. If you seal every gap and then can’t open up to purge-ventilate at night, you trap the day’s heat in. And year-round, homes need adequate ventilation to manage moisture and air quality — never block a vent that’s there for a reason (a boiler or fire needs combustion air; a bathroom or kitchen needs extract; trickle vents manage condensation). Draught-proof the uncontrolled leaks; keep the deliberate ventilation. Our condensation guide covers the moisture side.

Sealing only works with the right timing

Draught-proofing and ventilation timing are two halves of one method — sealing alone, done at the wrong time, can make a room hotter.

Overnight / early morning: open up fully when the outside air is cooler and flush the day’s heat out. A box fan in a window can pull cool air through faster.
Morning, before it warms up: close windows, close shading, and let your sealing hold the cool in.
Through the day: keep it shut and shaded. This is when draught-proofing earns its keep — every sealed gap is warm air that isn’t getting in.

Our window timing guide covers the daily rhythm, and our window and curtain guide covers the shading half.

What to do first

In rough order of effort-to-payback for keeping a room cool:

Seal the gap under the door of the room you’re cooling — biggest single win for keeping cool in one room.
Fix worn window seals so a closed window is actually closed.
Block an unused chimney (reversibly, and never one serving a working appliance).
Deal with floor and skirting gaps if you have a suspended timber floor.

The products for each — and which give the most cooling per pound — are in our draught excluders and window seals guide.

If you’re running a fan or air conditioner

Sealing matters most of all here. A portable air conditioner in a leaky room is fighting a constant inflow of warm air — and a single-hose unit makes it worse by putting the room under slight negative pressure, actively pulling outside air in through the gaps. Sealing the room (and the window kit) is often a bigger improvement than any setting on the unit. Our seal kits guide covers the window-kit side.

Frequently asked questions

Does draught-proofing really help in summer?

Yes — the same gaps that lose heat in winter let hot air in during a heatwave and dilute a cooled room. Sealing them keeps the cool you’ve made, provided you still open up to ventilate when it’s cooler outside.

Won’t sealing the house make it stuffy?

It can if you over-do it. Seal uncontrolled leaks, but keep deliberate ventilation — trickle vents, extract fans, and the ability to open windows fully at night. The aim is control, not an airtight box.

What’s the single most effective thing?

For keeping one room cool, sealing the gap under its door — it stops the cool air diffusing into the warmer rest of the house.

Should I block my air bricks and trickle vents?

No. Those are deliberate ventilation for air quality and damp control, and some feed combustion air to appliances. Draught-proof the accidental gaps, not the designed ones.