What thickness draught-excluder strip do I need?

That test exists because a seal works by compression. Guidance on foam strips for windows is blunt about the two failure modes: a strip that is too wide will hamper shutting the window, and one that is too narrow will have no effect at all². Thickness is therefore a fit question, not a performance rating, and the same strip depth can be right on one door and wrong on the next.

The type of strip matters as much as its depth. Brush strips are recommended for sash windows, where the bristles flex against a sliding section³, while foam, metal or plastic strips suit the frame of an exterior door, with brush strips along the bottom¹. Draught stripping for windows and doors also qualifies as an energy-saving material for a lower rate of VAT⁴.

Strip thickness at a glance: what the common sizes fit

There is no published table of draught-excluder strip thicknesses in the evidence, and no figure should be assumed. What the guidance gives instead is a method and a set of material choices, and those are what determine whether a strip of a given depth will work in a given gap.

The method is the 50mm test strip¹. A short offcut costs almost nothing and answers the question directly: place it in the gap, close the door or window, and judge the compression. The card test then confirms it, with a comfortable fit between strip and door meaning the seal is filling the gap without being crushed flat¹. This is why buying a single long length before testing is a mistake: the depth that works on the front door may be wrong on the back door, and wrong again on a loft hatch.

Material choice narrows the field before depth is considered. Foam, metal and plastic strips are described for door frames, with brush strips along the bottom of doors¹. Brush strips are also the type recommended for sash windows³. Foam strips are described for filling the gaps between a window and its frame². Door sweeps and brushes attach to the base of doors, and letter box brushes and chimney draught excluders cover the other common openings⁷.

For a household, the independence point is modest but real. A sealed door or window reduces the rate at which heated air leaves the home, which is the same mechanism that draught sealing is credited with in overheating guidance, where infiltration is reduced by draught sealing⁸. It does not remove the need for heating or ventilation, and it does not change the fuel a home runs on. It reduces waste at the point where the building meets the outside.

Why thickness matters: compression, gaps and the seal

A draught-excluder strip is a compressible gasket. It only blocks air if it is squeezed between two surfaces at the moment the door or window is closed, and that squeeze is what sets the usable thickness range for any given gap.

The two failure modes are documented. Too wide, and the strip hampers the window shutting². Too narrow, and it has no effect². Between those extremes the strip touches both faces, compresses slightly, and closes the path the air was using. The same logic applies to a door: an over-thick strip holds the door off its latch or stops it closing, while an under-thick strip leaves a visible line of daylight and a measurable draught.

Seals also degrade. Over time, seals deteriorate due to weather exposure and regular use, which is what produces draughts at a door or window that was previously sealed⁹. That is a maintenance point rather than a specification point: the strip that fitted correctly when installed may need renewing, and the replacement should be chosen by the same test rather than by matching the old pack.

Where draught-proofing is delivered as a funded measure, the standard is exacting. Under ECO, 100% draught-proofing must be achieved when delivering that measure type¹⁰. That is a completion requirement for the scheme, not a thickness figure, but it shows the intent: a partial seal is treated as a failed seal.

The limits are worth stating plainly. A strip cannot fix a warped door, a dropped hinge or a frame that has moved. It also cannot be used everywhere: extractor fans, trickle vents, wall vents, heater flues and underfloor grilles or airbricks should not be blocked or sealed, because they ventilate the home¹¹. Boiler flues, air bricks and window trickle vents must never be blocked, and over draught-proofing windows in kitchens and bathrooms should be avoided where moist air needs to escape and there is no mechanical ventilation such as an extractor fan¹.

Choosing by gap width: measure first, buy second

The gap decides the strip, and the gap is measured by testing rather than by guessing. The sequence is short: identify where the air is coming in, test a short length of strip in the gap, then buy the profile and depth that passed the test.

Start with the common locations. Draughts are found around external windows and doors, open chimneys, loft hatches, and in gaps around floorboards and skirting boards⁷. Each of those has a different gap geometry. A door frame gap is narrow and even down the sides and across the head. A threshold gap is wider and changes as the door passes over the floor covering. A loft hatch has a continuous gap around the frame, where strips of draught-excluder are fitted around the edges of the frame, with the hatch itself insulated with fibrous or solid board insulation¹. An unused chimney takes a different product entirely: chimney draught excluders such as chimney sheep or balloons, removed in summer to let air circulate, and never installed on a chimney that is in use¹.

The card test is the confirmation step. Close the door and slip a plastic card between the test strip and the door; it should be a comfortable fit¹. A card that will not pass means the strip is too thick for that gap. A card that falls through means the strip is too thin.

For windows, the same principle applies with a different profile. Self-adhesive foam strips fill the gaps between the window and the frame², and the size check is the one already described: too wide hampers shutting the window, too narrow has no effect². For sash windows, brush strips work better³, because the bristles flex against the moving section rather than resisting it.

Where a household is working through a whole house, it helps to treat the strip as one part of a wider sequence. The draught-proofing guide covers where to seal and where not to, and the airtightness and air leakage page explains how sealing relates to controlled ventilation. Strip thickness is the last decision in that chain, not the first.

Brush, foam, rubber or wiper: which strip suits which gap

The profile follows the gap, and the evidence points to a small number of pairings rather than a single best product.

Foam, metal and plastic strips are described for door frames, with brush strips along the bottom of doors¹. That split reflects the two gap types: an even compression gap at the sides and head, and a sweeping gap at the threshold where the door passes over the floor. Door sweeps and brushes attach to the bottom of doors, and letter box brushes cover the letter plate⁷. For interior doors, snake draught excluders reduce draughts, but only as a temporary measure¹.

For windows, self-adhesive foam strips fill the gaps between the window and the frame². For sash windows, brush strips work better³. The reason is mechanical: a sash slides vertically, so a rigid foam seal in the channel would resist movement, while bristles flex and still close the gap.

Chimneys take a dedicated product. Chimney draught excluders such as sheep or balloons are available from most DIY stores, should be removed in summer to let air circulate, and must not be installed on any chimney that is in use¹.

A wider list of methods shows the same logic applied across a home: draught-proofing strips for windows and door edges, brush or hinged-flap draught excluders at door bottoms, an inflatable chimney pillow for open chimneys, flexible silicone-based filler for floorboards, and foam strips for loft hatches¹². Insulating tape around door frames, a door brush, a keyhole cover, letterbox draughtproofing and a draught excluder are the door measures listed in draught-busting guidance¹³.

Where thicker is not better: doors, sashes and sticking

Thicker is not a safer choice. The evidence names the consequence directly: a foam strip that is too wide will hamper shutting the window². On a door, the same over-thickness shows up as a door that will not latch, a handle that needs force, or a frame that flexes when the door is pushed home.

The areas where sealing should not happen at all are as important as the thickness question. Extractor fans, trickle vents, wall vents, heater flues, and underfloor grilles or airbricks should not be blocked or sealed, because they play a role in ventilating the home¹¹. Boiler flues, air bricks and window trickle vents must never be blocked, and over draught-proofing windows in kitchens and bathrooms should be avoided where moist air needs to escape and there is no mechanical ventilation such as an extractor fan¹.

Condensation is the practical risk that follows over-sealing. Draughtbusters visits found that the most common jobs were fixing draught excluder to windows and doors, but the biggest issue was condensation¹⁴. That is the trade-off in one line: sealing reduces unwanted air movement and can raise indoor humidity if the ventilation routes are closed at the same time.

There is a wider lesson in how insulation thickness is specified elsewhere. For roof insulation, the thickness required varies depending on the material used and the manufacturer's specification¹⁵. For pipe insulation, the advice is to choose the thickest insulation that will fit around the pipe¹⁶. Both point the same way: thickness is set by the material and the space available, not by a universal number. Draught-excluder strip follows that pattern, with the added constraint that the door or window must still operate.

Where a home is being sealed as part of a wider retrofit, the condensation, damp and mould page sets out what to watch for, and the home ventilation page covers the systems that replace the air movement a seal removes.

Fitting the strip so it actually seals

A correctly sized strip still fails if it is fitted badly. The steps below follow the sequence the guidance implies: clean and prepare, test, cut, fix, then check the operation of the door or window.

1. Identify the draught path. Common locations are around external windows and doors, open chimneys, loft hatches, and gaps around floorboards and skirting boards⁷.
2. Test a short length of strip, about 50mm, in the gap with the door or window closed¹.
3. Confirm with the card test: a plastic card should be a comfortable fit between the test strip and the door¹.
4. Choose the profile for the gap: foam, metal or plastic for door frames, brush for door bottoms and sash windows¹.
5. Fit the strip around the frame, or along the bottom of the door for a brush or sweep¹.
6. For a loft hatch, fit strips of draught-excluder around the edges of the frame, and insulate the hatch itself with fibrous or solid board insulation¹.
7. Check the door or window still closes and latches without force, and that a sash still runs freely².

The VAT position is worth knowing before buying materials. Draught stripping for windows and doors qualifies as an energy-saving material⁵, and draught stripping, for example insulation fixed around windows and doors, qualifies for a lower rate of VAT⁴. The legislation lists draught stripping for windows and doors among the energy-saving materials⁶. The relief applies to the supply and installation as two separate supplies whose elements are not interdependent or connected¹⁷.

For households in Northern Ireland, draught-proofing appears among the measures available through NISEP individual measure schemes¹⁸. In the 2023/24 NISEP annual report, the predominant loft insulation measure was 100 to 300mm thickness, representing 72% of all loft insulation installations¹⁹, which shows how a funded programme specifies thickness by measure type rather than leaving it open.

Finally, the independence question. A sealed home depends less on the rate at which it loses heat, and that reduces the work the heating system has to do. It does not remove the gas connection, the electricity supply or the boiler, and it does not make a home self-sustaining. What it does is close the cheapest and most immediate leaks first, before any larger measure is considered. The low-cost and DIY insulation measures page places strip alongside the other small measures, and the insulation and energy independence page sets out what sealing can and cannot do for a household's independence.