11. Interstitial Condensation

Interstitial condensation commonly occurs when warm, moist air moves through elements of the building fabric, typically, when warm internal air moves towards the colder parts of external walls or roofs. If the temperature drops sufficiently, condensation will occur on surfaces within the insulation layer.

Condensation may well be forming in an unseen part of the construction, for instance above a ceiling or behind a wall lining, so the potential damage may go unnoticed for some time. Some modern building materials, lightweight boards and composite timber joists, may be vulnerable but even solid roof and wall timbers can be damaged and rot if they are regularly exposed to moisture from condensation.

Concerns regarding the risk of moisture accumulation and the consequences for occupants have prompted a review of the guidance in a new version of BS 5250, Management of Moisture in Buildings, revised in July 2021. Advice is also given in the guidance of Approved Document Part L.

Certain constructions are more vulnerable than others, for instance when the insulation is placed between roof rafters in a ‘cold’ roof construction. This is because of the difficulty of forming and maintaining an effective vapour control layer below the insulation and of providing sufficient ventilation above the insulation.

Any indication of condensation post-construction should be investigated and although there will also be the possibility of condensation occurring as the home ‘dries out’ it should not necessarily be assumed that this is the likely cause.

Things that can go wrong:

• Mould and mildew growth;
• Corrosion or decay of building fabric;
• Poor performance of insulation and reduced thermal performance of other elements;

Future-proofing recommendations:

• DESIGN:  Identify all risk areas and follow good practice guidance;
• INSTALL: Install membranes and ventilation voids carefully; do not alter position of insulation in relation to structure and vapour barriers;
• COMMISSION: Record the construction carefully – condensation often forms in hidden areas;
• MAINTAIN: Ensure the ventilation strategy for the home is well explained to the homeowner.

Find below relevant NHBC reports for further reading:

NHBC NF68 - Home Comforts

NHBC RR16 Moisture in new homes - a guide for occupants