Find the draughts first, then spend money on insulation

Heat leaves your house two ways. It conducts through solid surfaces — walls, roof, floor, glass — and it rides out on moving air through gaps. Almost everyone spends money on the first problem while ignoring the second, because insulation feels like a proper job and squirting sealant behind a skirting board feels like nothing.

The air leaks are usually cheaper to fix, faster to fix, and you'll feel the difference the same evening. Start there. Then insulate.

Do the survey before you buy anything

Pick a cold, windy day. That's not fussiness — wind is what drives air through your house, and on a still day you'll miss half of it.

Turn off the heating. Shut all the internal doors and windows. Then walk the house slowly with the back of your hand out, palm side up, moving it along skirting boards, around window frames, along door edges, over sockets and switches on external walls, and around anything that passes through a wall or ceiling. The back of your hand is more sensitive than the palm. You'll feel cold air movement you'd never see.

For the gaps you suspect but can't feel, light a stick of incense and hold it a few inches from the suspect spot. The smoke will pull sideways or get shredded if air's moving. Keep it away from curtains and anything loose, and don't do this with a candle — open flame plus dry fabric plus a draught is a genuinely bad combination.

Write down every location on your phone as you find it. You'll forget half of them by the time you're at the merchant.

The places it's almost always coming from

Certain spots turn up in nearly every house:

• The loft hatch. Often a thin plywood board with no seal and no insulation on top. It's a hole in the warmest part of the house, and warm air goes up.
• Under and around the front and back doors. Including the letterbox and any cat flap.
• The gap between skirting board and floor, especially on suspended timber floors where there's a ventilated void underneath.
• Service penetrations. Where a waste pipe, boiler flue, extractor duct or cable comes through a wall or ceiling, there's usually a ragged hole with a cover plate loosely over it.
• The under-stairs cupboard, if it backs onto anything unheated.
• An unused chimney. A working flue pulls air out of the room constantly. That's what it's designed to do.
• Old extractor fans with no backdraught shutter. You can feel wind blowing in through the bathroom fan on a gusty night.
• Loose sash windows and tired seals on casements and patio doors.

Fix in order of cost per degree

Draught-proofing strip for doors and windows, a brush seal for the door bottom, a proper insulated letterbox cover, a chimney balloon or draught excluder for the dead flue, decorator's caulk for the skirting gaps, expanding foam or mortar for service holes. All of it is inexpensive, all of it is a Saturday, and you don't need a trade.

Two rules while you're at it. Use caulk on gaps you might want to open again and foam on gaps you won't, because cured foam is a nightmare to reverse. And don't foam around a boiler flue or any hot flue pipe — that needs the correct fire-rated sealant, and if you're unsure what's a flue and what's a duct, stop and ask a heating engineer.

Then go up into the loft.

Loft insulation is where the big money hides

Warm air rises, so the ceiling is the highest-value surface in the house. Current UK guidance is around 270mm of mineral wool at the ceiling level, which is a lot deeper than most homes built or topped up decades ago. Check what the current recommendation is where you live, then go and measure yours with a ruler pushed down to the joist.

If you're topping up, run the new rolls at right angles across the joists, over the top. Don't compress it. Mineral wool insulates because of trapped air, so squashing it under stored boxes or a raised floor ruins it. If you need loft storage, build a raised deck on battens so there's a clear gap above the wool.

Three things people get wrong up there:

• Blocking the eaves. Leave a clear air path at the edge where the roof meets the wall. Roofs need to breathe or you'll get condensation on the underside of the felt and rot in the timbers.
• Insulating under the cold water tank. Don't. It needs warmth from below or it'll freeze. Insulate the sides and lid, and lag the pipes.
• Forgetting the hatch. Fit a compression seal and glue a piece of insulation board to the back of the board. It's a ten-minute job with a real effect.

Wear a mask, long sleeves and gloves. Mineral wool fibres itch for days. Take a board to kneel on and a decent light, and know where your joists are before you put weight anywhere.

The bit nobody wants to hear about ventilation

A sealed house is a damp house. Every shower, kettle, wet coat and breathing human puts moisture into the air, and that moisture has to leave. If you close every gap and don't replace the airflow, you get condensation on cold surfaces, then black mold in the corners of bedrooms and behind wardrobes.

So draught-proof the uncontrolled gaps, and keep the controlled ventilation working. Don't block trickle vents in window frames. Don't seal air bricks. Don't cover the extractor in the bathroom or kitchen — instead replace a noisy one so you'll actually use it.

The safety line is absolute where combustion is involved. A gas fire, a solid fuel stove, an open flue or an older boiler in a cupboard may rely on an air vent you're tempted to block. Blocking it can cause incomplete combustion and carbon monoxide. Never seal a vent serving an appliance. In the UK, that conversation is for a Gas Safe registered engineer; elsewhere, a licensed heating contractor. And put a CO alarm in every room with a fuel-burning appliance, this week, whatever else you do.

When to stop and call someone

Cavity wall insulation. Needs a survey, the right wall type, and a guarantee. Done wrong or in an exposed, weather-beaten wall, it can bridge damp across to the inside. Not a DIY job.

Solid wall insulation, internal or external. Changes how the wall handles moisture. Get a professional design, especially on older solid-brick or stone properties.

Spray foam on the underside of rafters. Tread very carefully. It has caused real problems with surveyors and lenders, and it can hide rot you can no longer inspect. Ask your own surveyor before anyone brings a hose into your loft.

Older homes and unknown materials. If you're cutting into textured coatings, old boarding around a boiler, or mystery insulation in a pre-1990s property, there's a chance of asbestos. Don't sand it, don't break it, get it identified.

Anything touching gas, a flue or your consumer unit belongs to a qualified trade. Everything else on this page is yours.

Do the walk-around first. Your hand is a better diagnostic tool than a quote from a salesman who hasn't been in your loft.