Condensation Prevention and Control

What This Requirement Covers

Condensation occurs when moist air comes into contact with a cold surface, causing the moisture to condense into water. In buildings, condensation can cause mould growth, timber decay, damage to finishes, and health problems for occupants. Preventing condensation requires a combination of adequate ventilation (Part F), insulation (Part L), and moisture resistance (Part C).

Key Requirements

Surface Condensation

1. Occurs on cold surfaces such as windows, external walls, and thermal bridges
2. Prevented by maintaining surface temperatures above the dew point of the indoor air
3. Insulation raises internal surface temperatures (Part L)
4. Ventilation removes moist air and replaces it with drier outside air (Part F)
5. Heating raises air temperatures and reduces relative humidity

Interstitial Condensation

1. Occurs within the building fabric when moisture vapour passes through the construction and condenses on a cold layer
2. Prevented by:

- Placing a vapour control layer (VCL) on the warm side of the insulation to restrict moisture movement into the construction - Ensuring any moisture that does penetrate can be ventilated away (e.g., ventilated cavities, breathable membranes)

1. Condensation risk analysis should be carried out to BS EN ISO 13788 or BS 5250 for all new constructions

Cold Roofs and Warm Roofs

1. Cold roofs (insulation at ceiling level): The roof void must be ventilated to remove moisture. Minimum ventilation: equivalent to a continuous 10 mm gap at the eaves on opposite sides, plus a 5 mm gap at the ridge for roofs with a pitch of 15 degrees or more
2. Warm roofs (insulation at rafter level or above the deck): No ventilation of the roof void is required if a VCL is placed on the warm side of the insulation. A breather membrane must be used on the cold side
3. Hybrid roofs (insulation both at ceiling and rafter level): Must be carefully designed to avoid trapping moisture between insulation layers

Subfloor Ventilation

1. Suspended timber ground floors must have cross-ventilation beneath the floor to prevent moisture buildup
2. Minimum ventilation: 1500 mm² per metre run of external wall, or equivalent to a 150 mm x 150 mm airbrick every 1.5 metres
3. Airbricks must not be blocked or covered during landscaping or paving works

Practical Compliance Tips

1. The three-pronged approach to condensation prevention is: insulate, ventilate, heat - all three must work together
2. Ensure VCLs are continuous and sealed at joints; a gap in the VCL allows moisture to bypass it and condense within the construction
3. In bathrooms and kitchens, always extract moist air directly to outside, never into a roof void or adjacent room
4. Do not over-seal a building without providing adequate controlled ventilation; very airtight buildings need MVHR or continuous mechanical extract
5. Specify insulated lintels and cavity closers to eliminate cold bridges around windows where condensation commonly forms
6. In loft spaces, ensure insulation does not block eaves ventilation; use proprietary eaves ventilation trays
7. Advise occupants to use extract fans, open windows, and avoid drying clothes indoors on radiators