Damp, cavities, glazing and ventilation in the building fabric

Damp-proof course and membrane

A damp-proof course (DPC) is a horizontal barrier built into a wall, typically at least 150 mm above outside ground level. It stops moisture rising up through the wall by capillary action. Piling soil, rubble or anything else against the wall above DPC level "bridges" it, giving damp a way round the barrier and up into the wall above.

A damp-proof membrane (DPM) does the same job in a solid ground floor. It is a continuous barrier, often polythene, that stops ground moisture rising through the slab. It should link up with the wall's DPC, so the two barriers form one continuous line of protection.

Cavity walls

A cavity wall is built as two separate leaves, an outer and an inner skin, with a gap between them. The cavity resists rain penetration and, with insulation added, improves thermal performance. Debris or mortar droppings that fall and lodge in the cavity can bridge it and carry moisture from the wet outer leaf across to the dry inner leaf, so keep the cavity clear.

Where a pipe passes through an external wall, sleeve and seal the penetration. That protects the pipe, allows for movement and keeps out draughts, water and pests, so the wall stays weather resistant. For copper passing through masonry or a solid floor, the Copper Board's guidance is a sleeve, often another piece of copper tube, whose bore is at least 6 mm larger than the tube's outside diameter, or 25 mm larger for tubes over 80 mm. Keep the gap even all round and seal at least one end with mastic, so the tube can still move but fire, water, gas and vermin cannot follow it through.

Telling a cavity wall from a solid wall

Before drilling through an external wall for a pipe, flue or condensate, find out what kind of wall it is. The Energy Saving Trust's guide to wall types gives three clues:

• Age. A house built after the 1920s probably has cavity walls; older houses are more likely to have solid walls, usually of brick or stone.
• The brick pattern. In a cavity wall the bricks usually all run lengthways in an even pattern. In a solid wall some bricks are laid across the wall, so their small ends show in an alternating pattern.
• The thickness. Measured at an open door or window, a brick wall 260 to 350 mm thick is probably a cavity wall, and a narrower one probably solid. A thicker brick wall can be a newer facing over an older construction such as a timber frame, and stone walls tend to be solid whatever their thickness.

Render or cladding hides the pattern, and some walls are timber-framed, steel-framed or made of precast concrete. If you cannot tell, ask a building professional.

Safety glazing

Critical locations include glazing in or beside doors and glazing near floor level where someone could strike it. Suitable toughened or laminated glazing can reduce injury through its impact and breakage performance. England’s guidance also allows defined alternatives, including robust glazing, qualifying small panes or permanent protection. Where you are not sure the glass is suitable, flag it for assessment: a missing or unreadable mark does not tell you what glass is fitted.

Ventilation

Air changes per hour (ACH) measure ventilation in room volumes of outside air per hour. Divide the volume of outside air brought in each hour by the room volume. Air mixes, so it does not mean all the air in the room is replaced each hour.

Bathroom and kitchen extraction removes moisture and pollutants near their source and discharges them outdoors. The dwelling also needs its designed supply and transfer-air paths. Ventilation supports air quality and limits condensation; the combustion-air requirements of an open-flued appliance must be assessed separately.

Extract rates, fan position and door undercuts

For new homes in England, Approved Document F gives minimum intermittent extract rates by room:

• 15 l/s for a bathroom;
• 6 l/s for sanitary accommodation such as a separate WC;
• 30 l/s for a utility room;
• 30 l/s for a kitchen whose cooker hood extracts to outside, and 60 l/s without one.

A hood that only recirculates the air through filters does not ventilate the kitchen, which is why the rate extracted to outside doubles without an extracting hood. Fit an extract fan as high in the room as is practicable and no more than 400 mm below the ceiling; a cooker hood goes at the height its instructions give.

In a home ventilated by background ventilators and intermittent extract fans, a fan in a room with no openable window should have controls that keep it running for at least 15 minutes after the room is vacated. Any automatic control, such as a humidistat, needs a manual override.

So that air can move between rooms with the doors shut, internal doors are undercut to give the free area of a 10 mm undercut in a 760 mm door: 10 mm above the floor finish, or 20 mm above the floor surface if the finish is not yet laid. When a kitchen or bathroom in an existing home is refitted, its existing fans are kept or replaced. The 2026 edition of Approved Document F, which takes effect on 24 March 2027, keeps all of these figures.