The standard advice for a cold, expensive house is to replace the windows. It is also close to the worst place to start — high cost, long payback, and rarely the largest source of loss.
The largest source is usually air moving where it should not: through gaps around pipes, gaps at the top of walls, an unsealed attic hatch, and a hundred small penetrations nobody thinks about. Sealing those costs very little and returns a lot. The problem is finding them, which is what an energy audit is for.
What changed in 2026
- Thermal cameras got cheap. Phone-attachable thermal imagers dropped to a price where a homeowner or a local tool library can reasonably have one, which makes DIY diagnosis far more effective.
- Smart meters made measurement easier. Half-hourly consumption data lets you see what individual changes actually did, rather than comparing bills across different weather.
- Heat pump adoption raised the stakes. A well-sealed house lets you specify a smaller, cheaper heat pump — so sealing first changes the equipment you need, not just the running cost.
- Incentive programmes kept shifting. Rebates for audits and improvements change frequently and vary by region; check current local programmes before paying full price.
Do the free version first
An hour of your own time finds most of the obvious problems.
Read your own consumption data. If you have a smart meter, look at overnight usage when everything should be off. A high baseline means something is drawing power constantly. Compare winter against summer to separate heating from everything else.
Walk the perimeter on a windy day. Hold the back of your hand near window frames, door edges, letterboxes, and any point where a pipe or cable enters the building. You will feel the significant leaks directly.
Look up. The attic hatch is frequently the single largest hole in a house — an uninsulated, unsealed panel opening straight into a cold void. Check whether insulation is compressed, missing in patches, or has been pushed aside by someone who went up there.
Find the penetrations. Under sinks, behind the boiler, around extract fans, where cables come through. Each is a hole someone drilled and did not seal.
Check the obvious behavioural things. Thermostat schedule, water heater temperature, whether rooms nobody uses are being heated.
Write down what you find rather than fixing as you go. You want the full list before deciding what is worth doing.
When to pay for a professional audit
The one thing a professional has that you do not is a blower door test. A fan in an exterior doorway depressurises the house and measures how much air leaks in, giving you a number rather than an impression — and, with a thermal camera running, showing exactly where.
That measurement is what makes a paid audit worth it. Without it you are guessing at leakage; with it you know whether your house is unusually leaky and which specific gaps dominate.
It is worth paying when your bills are high with no obvious explanation, when the house is older or has been extended, when you are about to spend real money on insulation or heating equipment, or when rooms are uncomfortable in ways you cannot account for.
It is not worth paying when the problems are already obvious and you have not fixed them yet. An audit telling you the attic hatch leaks is not useful if you already knew.
Check for subsidised or free audits through local energy programmes before booking a private one.
Where the returns actually are
| Measure |
Cost |
Return |
Priority |
| Air sealing gaps and penetrations |
Very low |
High |
First |
| Attic hatch sealing and insulating |
Very low |
High |
First |
| Attic insulation top-up |
Low–moderate |
High |
Second |
| Pipe and tank insulation |
Very low |
Moderate |
Second |
| Draught-proofing doors and windows |
Low |
Moderate |
Second |
| Floor insulation |
Moderate |
Moderate |
Later |
| Wall insulation |
High |
Moderate–high |
Later |
| Window replacement |
Very high |
Low |
Rarely first |
The pattern is consistent across most housing: the cheapest measures return the most, because air movement carries away more heat than conduction through reasonably-insulated surfaces.
Windows are the perennial trap. They are visible, salespeople push them, and they feel like the problem because you can feel cold near them — often from draughts around the frame rather than through the glass. Draught-proofing an existing window costs a fraction of replacing it and captures much of the benefit. Replace windows when they have failed or you are renovating anyway, not as an energy measure on its own.
Order matters for equipment too: seal and insulate before sizing new heating equipment, or you will buy a larger, more expensive system than the improved house needs. That applies to heat pump water heaters and to space heating alike.
Common mistakes
- Starting with windows. Highest cost, lowest return.
- Insulating without sealing. Insulation slows conduction; it does not stop air moving through it.
- Blocking ventilation. Sealing a house too tightly without adequate ventilation causes condensation and damp. Seal deliberate gaps, keep intentional ventilation.
- Buying equipment before improving the envelope. You over-specify and overpay.
- Comparing bills across different weather. Use consumption data, not cost, and account for temperature.
- Doing everything at once. Change one thing at a time or you cannot tell what worked.
- Ignoring the thermostat schedule. The cheapest change of all, and frequently the largest single one.
FAQ
How much can I realistically save?
Enormously variable — a leaky older house can see a large reduction from cheap measures, while a well-built modern one has little left to capture. That variance is exactly why measuring first beats copying someone else's improvement list.
Is a thermal camera worth buying?
For one audit, borrow or rent. Phone attachments are affordable enough to own if you will use it repeatedly. Use it on a day with a large indoor-outdoor temperature difference, or it shows nothing useful.
Do smart thermostats save money?
They can, mostly by improving scheduling rather than through any inherent efficiency. If your heating already runs on a well-tuned schedule, the gain is small. See the best smart thermostats.
Should I replace an old but working boiler or furnace?
Usually not for efficiency alone until it fails or is close to it. Plan the replacement in advance so you choose deliberately rather than in an emergency in January — and do the envelope work first so you can size it correctly.
Where to go next
For the water heating side, read the heat pump water heater guide. For scheduling improvements that cost nothing structural, the best smart thermostats, and if you are adding an EV to your household load, EV charging at home.