Draught proofing is the cheapest and fastest way to prepare a home for a heat pump. Sealing uncontrolled air leaks means less cold air for the heating system to warm, which matters more with a heat pump than a gas boiler because a heat pump runs at around half the temperature.
The catch is that you cannot simply seal everything. Air still needs to move through a home, particularly in kitchens and bathrooms, or you trade draughts for damp. This guide covers where the draughts are, how to find them, how to fix them, and how to keep the house healthy while you do it.
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The heat pump ready series A nine-part guide to preparing your home, produced with low-carbon homes architect Colin Usher. |
If large volumes of cold air move through your home, your heating has to warm all of it. Slow that movement down and you cut the demand at source — for a fraction of the cost of insulation, glazing or a new heating system.
Research published in the International Journal of Building Pathology and Adaptation found that ventilating a home properly and eliminating draughts can reduce a home's energy needs by up to 62%. That is a striking figure for work that often costs a few hundred pounds.
Colin Usher, the low-carbon homes architect advising on this series, puts it plainly:
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"The house is likely to be extremely draughty — airtightness alone may make enough difference!" " Colin Usher, Retired architect and low energy house designer |
Homes built before the 1990s were designed around a completely different heating system: the open fire, which needed to draw large volumes of air through the building to work. Windows and doors were fitted before the technology existed to weather-seal them properly. The draughts were not a defect — they were the design.
Homes built after the 1990s generally have no chimney, but were built before Building Regulations required meaningful airtightness. You will still find leaks, usually at junctions and around service penetrations rather than at obvious openings.
A formal blower door test is expensive. You can find the major leaks yourself in an afternoon with an incense stick.
Work through these locations in each room:
Under the windowsill is one of the most common findings, because installers frequently fail to seal beneath a replacement window. As Colin's wife Jenny asks of any existing installation:
"When the installers put double glazing in, did they seal underneath the windowsill or leave a great big hole?"
Jenny Usher
A chimney is designed to pull air out of your house. In a home with a heat pump, that is simply a hole in the roof.
Do not seal it completely. A fully blocked chimney traps moisture, which leads to condensation and damp in the stack. You need something that stops the airflow while letting moisture pass.
There are three options, in ascending order of cost:
"Out of preference, I just take a good-sized paper bag, fill it with scrunched up newspaper and push it up the chimney. This allows small amounts of air and moisture to get past and keeps the chimney dry."
Colin Usher
Whatever you use, leave it slightly visible so nobody lights a fire underneath it.
The smoke test will show you which of two problems you have.
If smoke is drawn under the windowsill, the gap beneath the frame was never sealed. Sealant applied by a competent builder fixes this cheaply.
If smoke is drawn through the frame itself, the draught seals have perished. A glazing repair company can often replace the seals or the sealed unit while keeping the frame. If the frames are past saving, a FENSA-approved installer can replace the window.
Internal doors rarely need sealing, with one exception: any door leading to a garage, unheated porch or other semi-outdoor space should be treated like an external door.
These are two deliberate holes in your external doors, and both show up strongly on a smoke test.
For letterboxes, ask whether you still need one. Colin favours the European approach of a secure box mounted outside the door, noting that these "can have the added advantage of a parcel shelf for your online shopping parcels."
For cat flaps, the fix is a better flap rather than no flap:
"You can get electronic Passivhaus quality cat flaps. If not, I think the most important thing is to have one that is related to a microchip. These hold themselves closed and won't blow open."
Colin Usher
A microchip-operated flap also stops the neighbourhood cats letting themselves in for dinner.
Downstairs floorboards in homes built between roughly the 1850s and 1950s sit over a ventilated void, and air moves up between the boards and at the skirting.
The simplest fix is a good quality foil or vapour-barrier underlay beneath carpet or laminate, taped at the edges and junctions. It seals the draught and adds a little insulation at the same time. Our guide to floor and loft insulation covers the more substantial options.
Do not block the air bricks that ventilate the void. They are there to keep the joists dry, and sealing them causes rot.
If you want to keep a wood-burning stove alongside a heat pump, the combustion air and flue need to be properly designed. Where an open fire draws air from the whole house, a well-specified stove should be sealed from it:
Set up this way, the stove takes its air from outside, burns, and expels the exhaust without affecting the airtightness of the rest of the home.
Air still needs to move through your home — just far more slowly, and in controlled places rather than wherever the building happens to leak.
Moisture from cooking, washing and breathing has to leave, which is why extractor fans in kitchens and bathrooms matter. Fresh air has to come in. Without both, you get condensation, cold spots and black mould, whose spores cause real problems for anyone with asthma or another lung condition.
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Never block these up
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Once the draughts are dealt with, Colin rates positive input ventilation (PIV) as one of the best-value improvements available. A fan, usually in the loft, draws outside air through a filter and pushes it into the home, gently raising the internal air pressure so stale, damp air is pushed out through whatever gaps remain.
A basic unit starts at around £400, with fully installed systems typically £700 to £1,200. Running costs are only a few pounds a year.
Jenny Usher describes it as the first thing they recommend to anyone with a damp problem:
"People come to us saying that they've got black spots in their house — mould spots. And the first thing we say, before they do anything else, is put positive input ventilation in the house."
Jenny Usher
One case shows why draught-proofing and ventilation have to be considered together:
"The people had lived there for 20 years, and then we had done double glazing in the front windows, and then we got round to replacing double glazing in the back. Suddenly, they started getting mouldy black spots in certain cupboards. We put positive input ventilation in, and the mould went. The damp air in the house was pushed out by the positive input, out of the remaining gaps, and taking the damp air with it. And that solved the problem."
Jenny Usher
The lesson is worth stating directly: tightening a home without addressing ventilation can create damp problems that did not exist before. The draughts were doing a job badly. Something has to do it well instead.
MVHR is a step up from PIV in both capability and cost. It works like this:
In hot weather it also helps. A well-insulated home can hold unwanted heat, particularly upstairs, and many systems include a summer bypass that stops recovering heat when the outside air is cooler.
MVHR really suits a home that has already been made properly airtight. In a leaky house, much of the recovered heat escapes anyway. We cover costs and payback in our guide to energy-saving technologies.
Airtightness is measured by pressurising a building with a fan — a blower door test — and measuring how much air escapes.
New-build homes must meet a maximum leakage rate under Building Regulations Part L, which was tightened in 2021. The Passivhaus standard, the international benchmark for low-energy building, requires no more than 0.6 air changes per hour at 50 pascals — several times tighter than Building Regulations require.
Testing is not required for refurbishments or extensions. But if you are having professional draught-proofing done, you can ask for a test as part of the work, and the result can support an updated EPC if you are improving the home to a high standard.
Most draught-proofing is straightforward DIY: door and window seals, sealing gaps at skirting boards, insulating the loft hatch, fitting a chimney draught excluder.
Call someone in if the property is old, if the smoke test reveals leaks in many places at once, or if you have damp as well as draughts. A professional assessment and remedial work starts at around £250, and there are three advantages worth paying for:
Savings depend on the age and condition of your home, but draught proofing has one of the best cost-to-benefit ratios of any energy efficiency measure. Research published in the International Journal of Building Pathology and Adaptation found that proper ventilation and draught elimination can cut a home's energy needs by up to 62%.
Yes. Homes need fresh air to remove moisture, pollutants and stale air. Good draught proofing eliminates unwanted leaks while maintaining controlled ventilation through extractor fans and purpose-designed openings. Sealing a home without addressing ventilation can cause condensation and mould that were not there before.
Yes, and more so than with a gas boiler. Heat pumps run at lower temperatures and work best when heat loss is minimised. Because draught proofing is cheap relative to insulation or glazing, it is usually the first thing to do and often makes the biggest single difference.
Use a lit incense stick on a windy day with internal doors closed, and watch where the smoke is drawn. Check windows, external doors, chimneys, suspended timber floors, loft hatches and points where pipes pass through walls. Under the windowsill is a very common finding.
No. Sealing a chimney entirely traps moisture and causes condensation problems in the stack. Use a chimney draught excluder, a stuffed paper bag, or a terracotta cap, all of which reduce airflow while still allowing moisture to escape.
Much of it, yes. Fitting seals around doors and windows, sealing gaps at skirting boards, insulating the loft hatch and blocking an unused chimney are all suitable DIY jobs. Older properties with leaks in many places, or homes with existing damp, benefit from a professional assessment.
Not on its own, and done badly it can make mould worse. What prevents mould is the right balance between airtightness and ventilation. Seal the uncontrolled leaks, then make sure moisture has a controlled route out through extractor fans or a ventilation system.
Usually not. Internal doors do not generally need sealing unless they lead to an unheated space such as a garage, integral store or enclosed porch, in which case treat them as external doors.
Draught proofing is the cheapest step in preparing a home for a heat pump, and often the most effective per pound spent. Find the leaks with an incense stick, deal with the chimney, seal beneath the windows, and rethink the letterbox and cat flap.
But airtightness must never come at the expense of ventilation. Kitchens, bathrooms and occupied rooms need a steady supply of fresh air to manage moisture and keep the air healthy. Seal the leaks you do not want, then give the moisture a controlled way out.
Get that balance right and you have a home that is warmer, healthier, cheaper to heat, and considerably better suited to a heat pump than it was before.
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Next in this series 4. Floor and Loft Insulation: Reducing Heat Loss Before Installing a Heat Pump — how much insulation is enough, why more is not always better, and what to check before you start. Previously: 2. Assessing your home for heat pumps |
Produced with Colin Usher, low-carbon homes architect. Last updated: 3 August 2026.
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