If you have an unused open chimney, block it first. SAP, the calculation behind every Energy Performance Certificate, counts an open chimney as 80 cubic metres of air an hour and a permanently blocked one as 20. In our example house that difference is worth about £74 of gas between now and the end of March at the October 2026 price cap, against £19 for a chimney balloon. Draught strip on casement windows comes second. Doors and floorboards come a long way behind.
We worked every figure out on 1 October 2026 from the SAP 10.3 ventilation worksheet and the government's heating degree days for 1991 to 2020, priced at the Ofgem cap of 7.97p per kWh for gas and 26.32p for electricity. Product prices are Amazon UK listing prices read the same day. We have not fitted any of these products or measured a house, and the method is set out under the ranking.
The short version, 1 October 2026
- Blocking an unused open chimney is worth about £74 of gas this winter in our example house. A £19 chimney balloon has paid for itself by early December.
- Draught strip for ten casement windows costs £21.56 and is worth about £41 of gas this winter, so it is paid back by mid January.
- Sash windows need brush pile, which costs about twice as much for the same job. On gas it pays back early next winter.
- Doors come out near the bottom because SAP counts a door as one opening out of twelve, whatever the gap under it. Stripping two doors costs £18.37 and saves about £8 of gas a winter.
- Sealing a bare suspended timber ground floor costs about £241 in gap filler and saves about £25 of gas a winter. That takes ten winters.
- On direct electric heating every saving is about three times larger, and everything except the floor pays back before the end of March.
Prices and specs last checked by Look Into on 1 October 2026 · how we picked
Ranked by payback this winter
Every row prices one job in the same house: two storeys, semi-detached, solid walls, 200 cubic metres of heated space. It has an unused open chimney, ten openable windows, two outside doors and a suspended timber ground floor, and none of them is sealed. By SAP's arithmetic, air moving through that house, leaks and extract fans together, costs about £360 of gas between October and March.
| Rank | Fix, and what we priced | Cost | Saved this winter, gas heating | Saved this winter, electric heating | Back per £1 spent, gas | Paid back, gas | Paid back, electric |
|---|---|---|---|---|---|---|---|
| 1 |
Block an unused open chimney Chimney Balloon, medium, 14 x 24 inch | £19.00 | £74.39 | £221.10 | £3.92 | Early December 2026 | Early November 2026 |
| 2 |
Strip ten casement windows 4 rolls of Stormguard EPDM D-profile, 10m, sold for gaps of 3mm to 6mm | £21.56 | £41.33 | £122.83 | £1.92 | Mid January 2027 | Late November 2026 |
| 3 |
Brush pile on ten sash windows 12 rolls of brush pile, 5m, sold for gaps of 4mm to 6mm | £44.64 | £41.33 | £122.83 | £0.93 | Early next winter, around November 2027 | Late December 2026 |
| 4 |
Strip two outside doors 2 Stormguard gold brush strips, sold for gaps up to 25mm, plus 1 roll of the D-profile | £18.37 | £8.27 | £24.57 | £0.45 | Third winter, around December 2028 | Mid February 2027 |
| 5 |
Seal a suspended timber ground floor 7 rolls of floorboard gap filler, 40m, sold for gaps of 3mm to 7mm | £241.50 | £24.80 | £73.70 | £0.10 | Tenth winter, 2036 | Fourth winter, December 2029 |
The "back per £1" column is this winter's saving divided by the cost, so anything above £1 has paid for itself before April. Rows 2 and 3 are alternatives: a house has ten windows of one kind or the other, so count one of them. Do the chimney, the casement windows, the doors and the floor together and the saving is £148.15 of gas this winter, 64p less than those four rows added up, because SAP's formula flattens slightly once a house gets tighter.
Every strip and filler in the table states the size of gap it is made for, which is the test the listing audit further down this page applies. A wool Chimney Sheep, 9 by 16 inches, was £33.00 the same day and still pays back before the end of the winter on gas. We take no commission on this page and none of these names is an affiliate link.
One chimney, eighteen windows
SAP counts an open chimney as 80 cubic metres of air an hour and a permanently blocked one as 20, so blocking it removes 60. Draught stripping a window or a door cuts the whole house's leakage by 0.2 air changes an hour, shared out across all its openings. In our 200 cubic metre house with twelve openings, that is 3.3 cubic metres an hour for each one. Blocking the chimney does the work of stripping eighteen windows. We call this the eighteen-window chimney.
It also means that in a 200 cubic metre house the chimney on its own is worth as much as every window, both doors and the whole ground floor put together: £74.39 of gas this winter either way. Both come to 60 cubic metres an hour, and the table rows only add up to £74.40 because each one is rounded. In a bigger house the windows and floor pull ahead. In a smaller one the chimney does, because its 60 cubic metres do not shrink with the house.
The catch is that a balloon or a wool blocker is not what SAP means by a permanently blocked chimney. SAP has no line for a removable blocker, and its blocked figure allows for a vent of up to 30,000 square millimetres. We used it because it is the nearest line SAP has, so treat £74 as the most a blocker can save. Never light a fire with one in place. The balloon's listing tells you to leave its reminder card in the grate for that reason.
Why the doors come out near the bottom
SAP asks what share of a home's windows and doors are draught-proofed, and the version used for existing homes counts that share by number (RdSAP 10, section 8.1). A front door with a wide gap under it counts the same as a small window that shuts well. So two doors out of twelve openings are worth £8.27 of gas this winter in our house, and the brush strips and frame seal for both cost £18.37.
An equal share cannot see the size of the gap, so a door with a big gap under it is probably worth more than this and a well-fitted one less. We have no measurement that says how much, so we have not guessed. On direct electric heating the same two doors pay for themselves by mid February. A Stormguard letterbox brush and a keyhole cover were £5.99 each on 1 October, but SAP has no line for either, so neither has a figure here.
Sash windows cost more to seal
A sliding sash takes brush pile, which lets the sash slide past it; foam and rubber strips are made for windows that close against a frame. The cheapest brush pile we found that states its gap was a 5m strip for gaps of 4mm to 6mm at £3.72. A sash window needs about 6m of it, against about 3.6m of D-profile for a casement, so the same ten windows cost £44.64 to do. On gas that pays back early next winter. On direct electric heating it pays back by the end of December. A T-shaped seal sold for the side gaps of sash windows, 2mm to 7mm, was £6.99 for 3m.
Floorboards are the slow one
SAP adds 0.2 air changes an hour for an unsealed suspended timber floor and 0.1 for a sealed one. In our house that difference is worth £24.80 of gas this winter. The cost is the problem. Boards 150mm wide leave about 277 metres of gap across 41.5 square metres of ground floor, which is seven 40m rolls of gap filler at £34.50 each, £241.50 in all. On gas that takes ten winters to come back. On direct electric heating it takes four.
The filler we priced says "Potential savings up to 40 GBP per room per year". On SAP's arithmetic, sealing the boards of a 12 square metre room in our house is worth £7.17 of gas this winter, or £21.31 on direct electric heating. The listing does not say what house or energy price its figure assumes.
If only one room is bare boards, the job and the saving shrink together, so the payback is still counted in winters. Never block the air bricks in the outside wall below a suspended floor. They let air through the space under the boards and keep the timber dry. Seal the gaps from inside the room and leave the bricks clear.
Working it out for your own house
Each window or door you strip is worth about £4.13 of gas this winter in our example house, or £12.28 on direct electric heating. The chimney figure barely changes with the size of the house. The window and door figures depend on how much house each opening serves, and the floor figure on the volume above it, so a flat or a small terrace usually saves less on each.
If your windows are double glazed and close firmly, they probably count as draught-proofed already. RdSAP says that when an assessor cannot tell, double and triple glazing is taken as draught-proofed and single glazing is not. The window rows are for single glazing and for older frames whose seals have gone, sashes included.
The electric figures assume you pay the standard capped rate of 26.32p. Storage heaters running on a cheaper night rate save less. With a heat pump, divide the electric figure by its seasonal efficiency: at 2.97, the break-even point on our running cost index, a heat pump home saves the same as a gas one.
About 58% of each winter figure falls between January and March, and we have priced all of it at the October cap. Ofgem announces the January cap in late November. Whatever that does to the rate, it does to 58% of every figure on this page.
How this compares with the Energy Saving Trust
The Trust puts draught proofing windows, floors and doors at around £55 a year in Great Britain, and an unused chimney at around £45 (page updated 20 May 2026). Our SAP arithmetic gives £74.39 for the windows, doors and floor this winter, and the same £74.39 for the chimney. Ours is higher on both, by about a third on the first and about two thirds on the chimney.
Two differences push ours up. The Trust says it prices savings from an average of recent price cap periods, and its calculations page, updated 22 September 2026, uses 6.5p per kWh for gas against the 7.97p used here. It also cuts modelled savings with government in-use factors, to allow for homes that do not behave like the model. We apply no such cut. Read our figures as the most each fix is worth if it works the way SAP assumes.
The order does not move under any proportional cut. Take every saving here down to a third and the chimney balloon still pays for itself before the end of March, and casement strips before the end of next winter. Our home upgrade payback index uses a typical cost of £150 to £300 for £50 to £100 a year, about three years. The products above cost a fraction of that, which is what moves the chimney and the casement windows inside a single winter.
How we worked out the payback
Each fix changes one line of the SAP 10.3 ventilation worksheet. We ran the worksheet month by month for October to March with and without the fix, multiplied the change in heat loss by that month's heating degree days, and priced the heat. Savings build up in each month in step with its degree days and wind. Nothing is counted from April to September, which can only make the payback dates later.
| Input | Value used | Source |
|---|---|---|
| Gas unit rate | 7.97p per kWh | Ofgem price cap, 1 October to 31 December 2026, direct debit |
| Electricity unit rate | 26.32p per kWh | Same |
| Gas boiler efficiency | 90% | Energy Saving Trust figure for an A-rated boiler, used across this site. SAP's default for a condensing combi with no product data is 84%, which would raise every gas figure by 7%. |
| Chimney ventilation | Open 80 m³ an hour, permanently blocked 20 m³ an hour | SAP 10.3, Table 2.1 (BRE specification dated 12 June 2026) |
| Window and door leakage | 0.25 air changes an hour, less 0.2 times the share draught-proofed | SAP 10.3 worksheet, line (15) |
| Share draught-proofed | Counted by number of openable windows and doors | RdSAP 10 specification, section 8.1 (7 June 2026) |
| Suspended timber floor | 0.2 unsealed, 0.1 sealed | SAP 10.3 worksheet, line (12) |
| Rest of the house | Masonry 0.35, second storey 0.1, no draught lobby 0.05, two extract fans at 10 m³ an hour each | SAP 10.3 worksheet, lines (7a) to (13) |
| Shelter | One sheltered side, factor 0.925 | RdSAP 10, Table 5 default for a semi-detached house; SAP 10.3 line (20) |
| Wind | UK average monthly wind speed divided by 4: October 4.3, November 4.5, December 4.7, January 5.1, February 5.0, March 4.9 m/s | SAP 10.3, Table U2 and line (22a) |
| Effective air change rate | As calculated at 1 or above; 0.5 plus half its square below 1 | SAP 10.3 worksheet, line (24d) |
| Heat lost per air change | 0.33 W for each m³ for each degree | SAP 10.3 worksheet, line (38) |
| Heating degree days | 1,591 for October to March, base 15.5°C: October 143.9, November 238.0, December 320.3, January 328.3, February 289.3, March 271.1 | DESNZ Energy Trends table ET 7.1, 1991 to 2020 monthly means, September 2026 release |
| Example house | 200 m³ heated volume, 12 openings, 41.5 m² ground floor in 150mm boards | Our illustration, chosen for the sums, not a national average |
| Product prices | As in the ranking table | Amazon UK listing prices, 1 October 2026 |
Every figure on this page can be rebuilt from that table. For the chimney: 60 m³ an hour times the 0.925 shelter factor times 0.33 gives 18.315 W for each degree. Weighted by wind, October to March comes to 1,911 degree days, so the saving is 18.315 times 1,911 times 24 hours, divided by 1,000, which is 840 kWh of heat. Through a 90% boiler that is 933 kWh of gas, £74.39 at 7.97p, or £221.10 of electricity at 26.32p.
What 30 best-selling draught excluders say about themselves
We read 30 of the best-selling draught excluders on Amazon UK on 7 September 2026. Twenty-three are sold on keeping heat in. Eight say what size gap they actually seal. A basket of the ones that do say costs £28.76 and covers two external doors and the frames of a small house.
Draught proofing is the cheapest thing you can do to a cold house. The Energy Saving Trust puts the saving at around £55 a year in Great Britain for draught proofing windows, floors and doors, and around £250 to have it done professionally. At those numbers almost anything you buy pays for itself.
The problem is not the price. It is that most of these products do not tell you the one thing that decides whether they work.
The gap is the product
A draught excluder works by filling a gap. Fit foam that is thinner than the gap and the air still goes through. Fit foam that is thicker and the door stops shutting properly, so people peel it off in November. The gap the strip is built for is not a detail on the spec sheet. It is the product.
So we scored each of the 30 listings on one thing. Does the maker's own copy say what size gap this seals. We read only the title, the feature bullets, the product overview, the technical specification and the product description, so retailer blurb and review text are excluded.
| Counted | n | of | share |
|---|---|---|---|
| Sold on energy, heat loss or insulation in the maker's own copy | 23 | 30 | 77% |
| States the size of gap it seals | 8 | 30 | 27% |
| Sold on heat and does not state the gap | 17 | 23 | 74% |
| Puts any number on the benefit | 1 | 30 | 3% |
| States a saving in pounds | 0 | 30 | 0% |
Amazon UK, search for draught excluder sorted by popularity, first 30 distinct products, read 7 September 2026. All 30 returned usable text. Every count above was checked by hand, which changed two of them, see the method note at the end.
Seventeen of the 23 products sold on keeping your heat in never say what gap they fit. Call it the unmeasured seal. The claim that it saves heat is doing the work that a measurement should be doing.
The single listing that puts a number on it says "Reducing 30% Heat Loss". There is no test behind that figure on the page, no statement of what it was measured against, and no source. One number with nothing under it is not better than no number.
The eight that do state a gap are specific and useful. Two Stormguard brush strips say they seal gaps up to 25mm. Foam and D-profile tapes give ranges of 2mm to 6mm, 2.5mm to 6mm, 3mm to 5mm and 3mm to 6mm. One under-door seal says it compresses to fill gaps up to 5cm. That is all anybody needs, and most of the shelf does not print it.
What it costs to do a house, using only the products that tell you
If you buy only from the eight that state a gap, here is a basket for a small house with two external doors and a set of internal frames to seal. Prices are the Amazon UK listing prices on 7 September 2026.
| Item | Gap it states | Qty | Price |
|---|---|---|---|
| Stormguard gold door brush strip | up to 25mm under the door | 2 | £12.98 |
| D profile weather stripping, 20m | 2mm to 6mm | 1 | £11.99 |
| Foam sealing strip, 6m | 3mm to 6mm | 1 | £3.79 |
| Basket | £28.76 |
Prices read 7 September 2026 and they move. We take no commission on this page and nothing here is an affiliate link.
Against the Energy Saving Trust's £55 a year, £28.76 pays for itself in about six months. That comparison flatters the basket, because the Trust's figure covers windows, floors and doors and this basket does doors and frames, so it is doing part of the job rather than all of it.
The honest version is the one that survives being wrong. Even if this basket delivers only a third of the Trust's saving, it pays back inside a year and seven months, and it is 11.5% of the £250 the Trust quotes for having the work done for you. Draught proofing is one of the few things in a home where the arithmetic is not close.
How to measure your gap in a minute
- Under a door. Slide a coin under it. A 10p piece is about 2mm thick and a £1 coin about 3mm. If several stack in the gap, you need a brush strip rather than tape.
- Around a door or window frame. Close it on a strip of paper. If the paper pulls out with no resistance, there is a gap worth sealing.
- Choose the strip below your measurement rather than above it. Foam that is too thick stops the door latching, and a door you cannot shut gets the strip taken off within a week.
Draught proofing and condensation pull in opposite directions, so it is worth reading them together. Sealing a house tighter keeps the water vapour in as well as the heat, which is why our guide to stopping condensation on windows prices ventilation alongside everything else. Seal the gaps you did not choose, and keep the ventilation you did.
Chimneys are the other big one. The Trust puts draught proofing an unused chimney at around £45 a year on its own, which is most of the value of the whole rest of the job.
The catch
The Energy Saving Trust's £55 is a national figure for a typical home, updated on its site on 20 May 2026. Your house is not typical. A modern flat with two external walls has less to gain than a draughty Victorian terrace, and the same £28.76 of tape does far more in the second than the first.
Sealing a house up has a limit, and it is not a money limit. Homes need air changes, and a house with no ventilation gets damp, then mouldy. Never block an air brick, a trickle vent, or a vent serving a fuel-burning appliance. If you have a gas fire, a boiler with an open flue or a stove, the vents around it are part of the safety of the appliance and not a draught.
What this page cannot tell you
- Whether any of these products work. We have not fitted them, we have not run an air leakage test, and we do not claim to have.
- How long the adhesive lasts. None of the 30 listings gives a figure for that either.
- What you personally would save. That depends on your house, not on the product.
- Whether the one "30% heat loss" claim is true. We recorded that it is unsupported, not that it is wrong.
- What the prices are today. They were read on 7 September 2026 and Amazon repricing is constant.
- How close a balloon or a wool blocker comes to SAP's permanently blocked chimney. SAP has no line for a removable blocker, so we used the nearest one it has.
- What a letterbox brush or a keyhole cover saves. SAP has no line for either.
- Whether your doors leak more than your windows. SAP gives every window and door an equal share, whatever the gap.
- What the ranking prices will be next week. They were read once, on 1 October 2026.
Method and sources
- Sample. The first 30 distinct products returned by the Amazon UK search for draught excluder sorted by popularity, read 7 September 2026. All 30 returned usable maker copy and none were dropped.
- What was read. Product title, feature bullets, product overview, technical specification, detail bullets and product description. Reviews, editorial blurb and Amazon-generated blocks were excluded.
- Hand checks that changed the numbers. A first automated pass scored 9 listings as stating a gap size. Reading each by hand cut it to 8, because one gives only its own dimensions of 6mm by 3mm and tells you to measure your own gap, which is not the same as stating the gap it seals. The same first pass scored zero listings as putting a number on the benefit. Reading every mention of a pound sign or a percentage by hand found one, "Reducing 30% Heat Loss", which a rule looking for the word save had missed. Both corrections are in the direction that makes our own headline weaker, which is the point of doing them.
- Saving and cost figures. Energy Saving Trust draught proofing advice page, read 7 September 2026, page updated 20 May 2026.
- Prices. Amazon UK listing prices as displayed on 7 September 2026.
How we work is on our methodology page. For what running things costs at the current cap, see the running cost index, and for what changed on 1 October, the October 2026 price cap page. If new glazing is the bigger question, start with energy efficient windows.
Questions people ask
How much does draught proofing save?
The Energy Saving Trust puts it at around £55 a year in Great Britain for windows, floors and doors, and around £45 a year for draught proofing an unused chimney.
What size draught excluder do I need?
Measure the gap first, then pick a strip rated at or just below it. Foam thicker than the gap stops the door closing. Only 8 of the 30 best-selling listings we read state the gap they seal, so check before buying.
Is draught proofing worth it?
On the arithmetic, yes. A £28.76 basket of gap-rated products pays back in about six months against the Energy Saving Trust's £55 a year, and inside a year and seven months even if it delivers only a third of that.
Can you draught proof a house too much?
Yes. Homes need air changes or they get damp and then mouldy, and vents serving a fuel-burning appliance are part of its safety. Seal the gaps you did not choose and leave the ventilation you did.
Which draught proofing pays back fastest?
Blocking an unused open chimney. In our example house a £19 chimney balloon is worth about £74 of gas this winter at the October 2026 price cap, so it has paid for itself by early December. Draught strip on casement windows comes second, paid back by mid January.
Do chimney balloons save money?
On SAP's figures, yes. SAP counts an open chimney as 80 cubic metres of air an hour and a permanently blocked one as 20. At the October 2026 cap that difference is worth about £74 of gas over the winter in our example house, against £19 for the balloon. SAP has no line for a removable blocker, so treat that as the most it can save.
Is it worth draught proofing sash windows?
Yes, but more slowly than casement windows, because brush pile costs more per window. Ten sash windows cost £44.64 to do and save about £41 of gas this winter in our example house, so the job pays back early next winter, or by the end of December on direct electric heating.
Is floorboard draught proofing worth it?
Only slowly on gas. Sealing a bare suspended timber ground floor saves about £25 of gas a winter in our example house, and gap filler for 41.5 square metres of boards costs about £241, so it takes ten winters. On direct electric heating it takes four.