Draught-Proofing Guide: Windows, Doors, Floors and Part L Air Permeability

Quick Answer: Draught-proofing reduces uncontrolled air infiltration through windows, doors, floors, chimneys, service penetrations and ceiling-to-wall junctions. The Energy Saving Trust estimates £45–£60/year savings for a typical home from comprehensive draught-proofing. Approved Document L 2021 (Volume 1) sets target air permeability of 8 m³/(h·m²) at 50 Pa for new dwellings and 10 m³/(h·m²) for replacement dwellings; existing dwellings have no statutory target but quality renovations should aim for under 10. Background ventilation under Approved Document F must not be blocked — draught-proof, then ventilate to spec.

Summary

Draught-proofing is one of the few energy-efficiency measures that pays back in under 3 years (often under 12 months) without specialist installation or major disruption. It addresses uncontrolled air infiltration — the cold air movement through gaps around windows, under doors, between floorboards, around skirting, through chimneys and via electrical penetrations. Uncontrolled infiltration is wasted heat; it serves no ventilation purpose because the air enters at unpredictable rates and locations, often where occupants are sitting (window seats, by exterior doors) — causing the felt-cold sensation that no amount of additional radiator output can solve.

Draught-proofing differs from insulation (which slows conductive heat loss through fabric) and controlled ventilation (which replaces stale air with fresh, deliberately). All three are needed for a comfortable, low-energy home: insulation reduces fabric loss, draught-proofing eliminates uncontrolled loss, and controlled ventilation handles indoor air quality. A common mistake is to fit lots of insulation and then assume "the house is sealed" without verifying air permeability or installing background ventilation — which causes condensation, mould and poor indoor air quality.

This article covers the main draught-proofing zones (windows, doors, floors, chimneys, services), how to coordinate with Approved Document F ventilation, and the Approved Document L air permeability target framework. The complementary draught proofing focuses specifically on window/door product selection — this guide is broader.

Key Facts

• Energy Saving Trust savings estimate — £45–£60/year for comprehensive draught-proofing in a typical UK 3-bed home (2024 figure; rises with energy prices)
• Approved Document L Volume 1 (2022, new dwellings) — target air permeability ≤ 8 m³/(h·m²) at 50 Pa
• Approved Document L Volume 1 (2022, replacement dwellings) — target ≤ 10 m³/(h·m²) at 50 Pa
• Approved Document L Volume 1 (extensions, retrofit) — no statutory test required for extensions ≤25m² unless wholly new envelope; quality target ≤ 12
• Blower door test (Part L compliance test) — pressurises the dwelling to 50 Pa and measures the air leakage at that pressure; required on new dwellings
• Air permeability calculation — total air leakage (m³/h) divided by total envelope area (m²) at 50 Pa
• Typical UK dwelling air permeability — pre-1970s solid wall homes: 12–18 m³/(h·m²); 1980s–2000s: 8–14; post-2010: 6–10
• Passivhaus standard — ≤ 0.6 air changes per hour at 50 Pa (n50); roughly equivalent to ≤ 1.0 m³/(h·m²) at 50 Pa
• AECB Building Standard — ≤ 3 m³/(h·m²) at 50 Pa
• Approved Document F (2021) trickle vents — required on replacement glazing; minimum free area per Table 1.6 (5,000 mm² for habitable rooms typical)
• Common leakage paths (by area) — eaves to wall junctions (largest typically); floor-to-wall junctions; chimney; service penetrations; loft hatch; suspended ground floor; sash windows; letterboxes
• Smoke pencil / smoke pen — diagnostic tool for finding leaks under negative pressure (extract fans on, doors/windows closed); £20–£40
• Thermal imaging camera — diagnostic tool for finding cold spots indicating leakage paths; £200–£800 for trade-grade
• Self-adhesive compression seal (P or D profile) — typical lifetime 5–8 years for foam, 10–15 years for EPDM/silicone
• Brush pile draught strip — typical lifetime 8–15 years; for sliding components only (sash windows, letterboxes, sliding doors)
• Open-flued combustion appliances — open-flue gas fires, solid fuel stoves and open coal fires require air supply; excessive draught-proofing without ventilation can cause flue gas spillage and CO risk

Quick Reference Table

Detailed Guidance

Find the leaks first

Before fitting any draught-proofing products, identify where the air is leaking. Three methods:

1. Smoke pencil under negative pressure. Close all external doors and windows. Switch on all extract fans (kitchen, bathroom, utility) and the cooker hood. The internal pressure drops by 5–20 Pa, driving outside air in through any leakage path. Walk the dwelling with a smoke pencil; the smoke drift shows you the leakage point precisely. Most effective method for spot-leaks.

2. Thermal imaging camera on a cold day. Inside-outside temperature differential drives air infiltration. With heating on, the camera shows colder bands at leakage points: around windows, along skirtings, at eaves junctions, around loft hatches. Best for whole-house overview.

3. Hand test. With the same negative pressure created by extract fans, run the back of your hand along window perimeters, skirting lines and floor edges. Cold air movement is obvious. Cheap but slower; misses high-level leaks.

For a renovation project, do the diagnostic survey before specifying. Most homes have 2–4 dominant leakage paths that account for 60–80% of total air infiltration — fix those first.

Windows

Casement windows (timber or uPVC):

• Compression P-strip or D-strip self-adhesive seal in the rebate of the opening sash; the seal compresses when the window closes
• Strip width and depth: 9mm × 4mm typical for most casement rebates
• Replace existing degraded seal — don't lay new seal over the old one
• Test by closing the sash on a strip of paper; should grip lightly along the entire perimeter

Sash windows (timber, traditional):

• Meeting rail brush seal — between the meeting rails where the two sashes meet, 5mm brush pile
• Staff bead and parting bead V-strip seals — V-section spring-fold strips on the inner faces of the staff bead and parting bead
• Pulley wheel gap — typically left as service ventilation, otherwise small foam plug
• Box frame to wall sealant — silicone or acrylic sealant on the external joint between box frame and surrounding masonry

A full sash window draught-proofing service costs £180–£350 per window installed by a specialist — significant outlay but transformative for comfort. Pays back in 5–10 years on energy savings alone, less if you value comfort.

Old single-glazed windows under conservation:

• Secondary glazing inside the original sash is the most effective draught-proofing for a sashed listed window
• Slim VIG (vacuum insulating glass) retrofits give modern U-values without affecting the external sightline
• Always coordinate with the conservation officer

Doors

External doors (front, back, side):

• Perimeter compression seal — P-profile or bulb-profile self-adhesive seal in the rebate of the doorframe; closes when door shuts
• Threshold seal — aluminium or brass threshold strip with rubber or brush flap; closes against the bottom of the door
• Door bottom seal — spring-loaded brush or rubber seal that drops when the door is closed and lifts when it opens; can be retrofitted to existing doors
• Letterbox seal — brush pile curtain behind the flap; magnetic or sprung internal cover
• Keyhole escutcheon — spring-loaded cover that closes the keyhole

Internal doors:

Don't draught-proof internal doors — they're part of the deliberate ventilation strategy of the house. A 10mm gap under each internal door allows the kitchen-extract-driven negative pressure to draw replacement air from less-used rooms, providing ventilation throughout.

Floors

Suspended timber ground floors:

The largest single leakage path in many older homes. Cold air from the sub-floor void enters through gaps between floorboards, where they meet the skirting, and through service penetrations.

Solutions:

• Polymer floor sealant (Stop-Gap, Silvert Acoustic Polymer) injected into the gaps between floorboards while they're closed; flexible and accommodates seasonal movement
• Underfloor insulation with airtight membrane on top (covered in airtightness)
• Skirting-to-floor junction sealant — flexible caulk along the joint
• Service penetration sealing — fit grommets or expanding foam around pipes coming up through the floor

A heavy-traffic kitchen floor with gaps that flex underfoot may need to come up and be re-laid with tighter joints; sealant on a flexing floor will eventually crack.

Solid floors: generally non-leaky. Check the perimeter joint where the floor meets the wall for gaps; seal with flexible mastic.

Chimneys

A disused chimney is a major leakage path — a typical 9" × 9" flue at room temperature draws warm air up and out through the cold flue, even when the fire is off.

Disused chimney:

• Chimney balloon — inflatable bladder fitted into the throat of the flue; £20–£30; reduces leakage by 80–90%
• Chimney sheep (sheep's wool stuffer) — alternative to balloon; same idea
• Cap or cowl at the top — typically a stainless steel cap with a small ventilation slot to allow residual moisture out

Operating chimney (gas or solid fuel): do not block. Living flame gas fires and open coal/wood fires need the flue free at all times for safe combustion. If the fire is removed and the chimney decommissioned, fit a cap to the top with a small ventilation slot.

Loft hatch and service penetrations

Loft hatch:

• Compression seal around the perimeter where the lid closes against the trim
• Insulation board (50mm PIR or 100mm mineral wool) on the underside of the hatch
• Latches to compress the seal properly

Service penetrations (pipes, cables through walls/floors):

• Acrylic frame sealant around pipework where it passes through the ceiling, floor or external wall
• Expanding PU foam for larger gaps; cut flush with a knife after cure; do not use near combustion appliance flues
• Fire-rated sealant for penetrations through fire-rated walls (separating walls in flats, between dwellings)
• Air-tight grommets for cables passing through airtightness membranes in new build

Approved Document F coordination

Approved Document F (2021) requires every dwelling to have controlled ventilation. The strategies (Systems 1–4):

• System 1 — Background and intermittent extract — trickle vents in windows + extract fans in kitchen/bathroom on demand
• System 2 — Passive stack ventilation — vertical ducts from kitchen/bathroom to ridge; gravity-driven
• System 3 — Continuous mechanical extract (MEV) — single or multi-point continuous extract
• System 4 — Continuous mechanical supply and extract with heat recovery (MVHR) — fully ducted, supply and extract balanced, heat exchanger

Draught-proofing without retaining background ventilation is dangerous because:

• Condensation rises (moist air can't escape)
• Mould grows
• CO2 levels rise (occupant fatigue, sleep disruption)
• Open-flue combustion appliances can spill flue gas back into the room

When draught-proofing an older home:

1. Determine the current ventilation strategy (intentional or accidental)
2. Plan replacement ventilation matching Approved Document F System 1, 3 or 4
3. Draught-proof comprehensively
4. Verify with a blower door test or smoke pencil that you have the air leakage you intended (not too high, not too low)
5. Install the planned ventilation

For new dwelling extensions, Approved Document L requires the extension to meet the dwelling's air permeability target — if the existing house was 12 m³/(h·m²) and the extension is built to 4 m³/(h·m²), the combined dwelling permeability is dominated by the worse part.

Open-flued appliances — CO risk

Critical safety issue: open-flue gas fires, open coal/wood fires, and older open-flue boilers require continuous combustion air supply during operation. Excessive draught-proofing without an air supply can cause:

• Flue gas reversal — the chimney draught reverses because internal pressure drops; combustion products (CO, CO2) enter the room
• Incomplete combustion — soot, smoke, and lethal CO levels
• Pilot light extinction or burner instability on gas fires

Rules:

• Open-flue gas appliances >7 kW require dedicated air vent — typically a 5,000 mm² air brick or hit-and-miss vent in the room
• Open-flue gas appliances <7 kW require dedicated air vent if dwelling air permeability is <5 m³/(h·m²) at 50 Pa
• Solid fuel stoves with multi-fuel/wood-burning units require dedicated combustion air supply per the appliance manufacturer's spec — typically a 50mm direct-air supply pipe from outside

When you draught-proof a dwelling with an open-flue appliance, always verify the appliance's air supply is adequate post-works. The combination of comprehensive draught-proofing + open-flue gas fire + no dedicated air supply is a CO incident waiting to happen. CO detectors are non-negotiable in any room with a fuel-burning appliance.

When to do a blower door test

Approved Document L Volume 1 requires a blower door test on all new dwellings for Part L compliance (the air permeability target is verified by test). For a renovation, a blower door test isn't required by regulation, but:

• Deep retrofit projects — useful pre-works and post-works to verify the project achieved the intended air tightness
• Customer wants quantified improvement — a £350–£500 test before and after shows the measurable performance benefit
• Investigation of poor heating performance — a high permeability reading explains why a well-insulated house feels draughty

Typical test cost: £350–£600 single test for a domestic dwelling.

Frequently Asked Questions

Will draught-proofing cause condensation problems?

Only if you don't replace the leaked air with controlled ventilation. Uncontrolled draughts are doing two things: heat loss (bad) and moisture removal (incidentally useful). When you seal the leaks, you must also provide controlled ventilation (trickle vents, MEV, MVHR). A house properly draught-proofed AND properly ventilated has lower condensation than an under-ventilated leaky house, because the controlled ventilation removes moisture more reliably than random infiltration.

Should I keep the trickle vents on my new windows open?

Yes — they're there for a reason. Approved Document F requires trickle vents on window replacements to provide background ventilation. Closing them defeats the purpose and increases condensation risk. If a customer asks to "switch them off" — explain that the vent is a Building Regulations requirement and closing it transfers the heat-loss to mould remediation costs.

Is foam strip really worth fitting?

Cheap foam compression strip is worth fitting on infrequently-opened windows; it lasts 3–5 years before degrading. For doors and windows opened daily, specify EPDM or silicone seal (10–15 year life). The labour to replace cheap strip every few years exceeds the material savings versus quality seal.

What's the difference between draught-proofing and airtightness?

Conceptually the same — both are about reducing uncontrolled air infiltration. "Draught-proofing" is a retrofit / domestic vernacular term; "airtightness" is the Approved Document L / Passivhaus / new-build engineering term. The products and techniques overlap heavily.

Can I draught-proof a house with an open coal fire?

Yes, but carefully. Maintain the dedicated combustion air supply to the fireplace (a minimum 5,000 mm² air brick is typically required); do not block the chimney while the fire is in use; and check the CO detector reading after works. If the customer plans to stop using the fire, fit a chimney cap (with small ventilation slot) and a chimney balloon below; reinstate the air brick to standard background ventilation purpose.

How much can I expect to save on energy bills?

EST estimate £45–£60/year on a typical 3-bed home for comprehensive draught-proofing alone — but this assumes 2022 energy prices. At 2026 prices the saving is approximately £70–£100/year. The bigger comfort gain is the elimination of cold spots near windows and external doors, which often allows the customer to set the thermostat 1–2°C lower without comfort loss — saving another £40–£60/year. Total realistic saving: £100–£160/year on a typical home.

Regulations & Standards

• Approved Document L Volume 1 (2022) — Conservation of fuel and power in new dwellings (air permeability target 8 m³/(h·m²))

• Approved Document L Volume 1 (2022) — Conservation of fuel and power in existing dwellings (replacement target 10 m³/(h·m²))

• Approved Document F (2021) — Ventilation, all dwellings (background ventilation requirement)

• Approved Document J (2022) — Combustion appliances and fuel storage systems (air supply for combustion appliances)

• BS EN ISO 9972:2015 — Thermal performance of buildings — determination of air permeability of buildings — fan pressurisation method (blower door test)

• BS 5925:1991 — Code of practice for ventilation principles and designing for natural ventilation

• BS 5250:2021 — Management of moisture in buildings

• AECB Building Standard / Passivhaus standard — non-statutory but reference for ambitious draught-proofing targets

• Gas Safety (Installation and Use) Regulations 1998 — Reg 26 and 27 on flueing and ventilation for gas appliances

• Energy Saving Trust draught-proofing guide — savings estimates and product overview

• Approved Document L 2021 edition (Volumes 1 and 2) — air permeability targets

• Approved Document F 2021 edition — ventilation requirements

• Approved Document J 2010 edition (amended 2022) — combustion air supply

• BRE IP 1/15 — air infiltration in dwellings — research and best practice

• Sustainable Energy Authority of Ireland — airtightness guide — useful technical detail (similar UK context)

• draught proofing — detailed window and door product selection (complementary)

• airtightness — Part L target framework and membrane installation

• part f ventilation — controlled ventilation strategy (System 1/2/3/4)

• part l energy — wider Part L compliance including U-values

• condensation vs leak diagnosis — diagnosis of condensation issues post-draught-proofing

• chimney pot and cowl types — chimney capping for disused flues

• parge coat — parge coat for masonry airtightness in new build