Condensation in the loft: what causes it, how to stop it, and when spray foam is part of it

What causes condensation in a loft

Historic England puts the physics simply: "All air contains some water vapour, but warm air can hold more water vapour than cold air." Source: Historic England

Government guidance on damp and mould describes where that vapour comes from: "Moisture from normal occupant activities, such as showering, drying laundry and cooking, builds up in the air around a home if it cannot be removed." Source: GOV.UK

When that warm, moist air rises into the loft and cools to its dew point — the temperature at which it can no longer hold its water — the excess comes out as liquid on the coldest surface it meets. Signs to look for:

• Droplets on the underside of the felt or membrane
• Damp or darkened rafters and purlins, often worst near the eaves
• Black mould spots on timber or on the underside of boarding
• Rusted nail tips coming through the roof boarding or battens
• Damp patches on top of the ceiling insulation
• A musty smell when the hatch is opened

Five checks to do yourself before paying anyone

1. 1

   1. The loft hatch

   A hatch that does not seal lets warm, moist air straight into the loft. Draught-strip it so it closes tight.

2. 2

   2. Gaps in the ceiling

   Historic England's guidance: "All cracks and holes, particularly around pipes and cables where they pass through the ceilings, should be sealed." Look at pipe runs, cable drops and light fittings from inside the loft.

3. 3

   3. Extract fans

   Follow the duct from the bathroom and kitchen fans. It must run all the way outside. A fan that discharges into the loft, or a duct that has come apart, moves the moisture into the roof rather than out of the house.

4. 4

   4. The eaves

   In a loft insulated at ceiling level, "Ventilation is provided to the roof void in cold roofs by open eaves or by vents in gable walls," and Historic England advises the insulation is "kept free at the eaves to allow for ventilation." If insulation has been pushed into the eaves, pull it back.

5. 5

   5. Moisture at source

   Lids on pans, extract fans on while showering and for a while after, and laundry dried outside or with ventilation. The government's damp and mould guidance: "If a dwelling is inadequately ventilated, moisture in the air cannot escape the building and is therefore more likely to cause condensation."

When the loft has spray foam

The RICS consumer guide on spray foam states: "Spray foam creates a warm roof void. It seals the gaps to prevent draughts and retain heat but will also seal in moisture unless adequate precautions are taken." Source: RICS

In 2024 the Health and Safety Executive, as the Building Safety Regulator, published modelling of moisture risk for spray foam retrofitted to timber sloped roofs in homes. The project was started by the Department for Levelling Up, Housing and Communities; the report states its conclusions are the authors' own and do not necessarily reflect government policy. Its findings: Source: HSE report on GOV.UK

• "The modelling indicates that risks are low when an open cell (moisture permeable) insulant is applied in accordance with the prescriptive roof constructions."
• Sprayed onto a high-resistance (non-breathable) underlay: "a higher moisture risk (compared to LR underlay), particularly for open cell SFI, as vapour diffuses through the insulation to the colder side."
• Sprayed directly onto the tiles: "The application of spray foam insulation directly onto the tiles results in the highest risk for both insulation options."
• On adding ventilation afterwards: "A high rate of ventilation within the loft void would reduce mould and condensation risks but would almost entirely undermine the thermal benefit of the installation such that the original purpose of insulating would be lost."
• The thresholds it uses: timber is at risk of decay above 20% moisture content by mass, and mould growth can occur when relative humidity on a surface stays above 80% for an extended period.

The government's position, as reported by the House of Commons Library, is that "where spray foam insulation has been installed in line with the relevant standards, it should not require removal or prevent homeowners obtaining a mortgage." Source: House of Commons Library

What an inspection actually measures

The Building Regulations require a roof to protect the building from the harmful effects of "interstitial and surface condensation" (requirement C2). Approved Document C, the official guidance on that requirement, points to BS 5250 for avoiding condensation in roofs; the current edition is BS 5250:2021, the British Standard code of practice on managing moisture in buildings. Source: GOV.UK — Approved Document C · BSI — BS 5250:2021

An inspection turns that into readings rather than opinion:

• Air temperature, relative humidity, dew point and wet bulb at the loft apex, recorded together with a psychrometer
• Surface temperatures of the felt, timbers and any foam with an infrared thermometer — a surface at or below the dew point of the air around it is where condensation forms
• Timber moisture content with a moisture meter, read against the 20% threshold used in the HSE research
• Where foam is present: its type, what it was sprayed onto, and whether any timber can still be seen and tested

One honest limit: a single visit records one set of conditions. Condensation is seasonal and weather-driven, and readings taken on a mild, breezy afternoon can understate a problem that shows on cold, still mornings. The report states the conditions on the day, and where staining and timber moisture readings tell a longer story than the air readings, it says so.

When you don't need us

• Staining that appears after rain, in one area, is more likely a roof leak than condensation — a roofer is the right first call.
• A loft with no spray foam where the five checks above clear the problem may not need an inspection at all.
• An inspection is worth it when timber stays damp, mould is spreading, spray foam is present, or you need independent evidence of the loft's condition for a sale, purchase or claim.

For a loft without spray foam, the loft condition report covers the readings above. Where foam is present, the standard spray foam inspection adds the foam assessment. Both are £295 fixed, carried out by Jan — DEA qualified (ABBE Level 3), PAS 2035 training completed. Inspections are carried out in line with PCA guidance. Jan has inspected 1,500+ lofts. No removal arm and no referral fees.

Sources

• 1. Historic England — Energy Efficiency and Historic Buildings: Insulating pitched roofs at ceiling level (cold roofs) (historicengland.org.uk)
• 2. GOV.UK — Understanding and addressing the health risks of damp and mould in the home (gov.uk)
• 3. RICS — Spray foam insulation: consumer guide (rics.org)
• 4. Health and Safety Executive — Spray foam insulation applied to timber sloped roofs in dwellings, 2024 (assets.publishing.service.gov.uk)
• 5. House of Commons Library — Spray foam insulation and mortgages (commonslibrary.parliament.uk)
• 6. GOV.UK — Approved Document C: site preparation and resistance to contaminants and moisture (gov.uk)
• 7. BSI — BS 5250:2021 Management of moisture in buildings. Code of practice (bsigroup.com)