Deal with condensation in a loft

Stop loft condensation by improving airflow, sealing draughts from below, and lagging cold surfaces.

LC-01Check it really is condensationLook for black mould speckles on rafters or the underside of the roof felt, damp patches on insulation, or frost/ice on nail heads in cold weather. This pattern points to condensation, not a leak. If you see a single damp patch that tracks back to a roof tile, chimney or valley, treat it as a leak instead and get the roof checked first.

LC-02Gather your gear before you go upYou'll be kneeling and crawling among fibreglass and dust, so wear a proper dust mask, gloves and old clothes. Take a head torch so both hands stay free, and a kneeling board or loft panel to spread your weight.

1 pack · FFP3 dust mask (pack of 3)

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1 pair · Work gloves

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1 each · Loft walkway/kneeling board

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LC-03Move around the loft safelyOnly put your weight on the joists or on boards laid across them – the plasterboard ceiling between joists will not hold you and you can fall straight through into the room below. Watch for cables running loose across the joists, especially near the hatch, and don't tug on anything you're not sure about.

LC-04Check the eaves vents are clearFind the eaves at the low edge of the roof, where the rafters meet the walls. There should be a continuous gap or vent strip letting outside air into the loft. Pull insulation back at least 50mm from this gap if it's blocking the airflow – a loft that can't breathe traps moisture and grows mould.

RULES

KNOW THE RULES

Building Regulations require roof voids to have permanent ventilation to prevent condensation damage. Don't permanently block eaves vents when adding insulation.

LC-05Top up the loft insulationCheck the depth of your existing mineral wool insulation – 270mm is the usual recommended depth. Add a second layer laid at right angles to the first if it's thinner than that, but keep it clear of the eaves gap you checked in the last step. Don't squash the insulation down, as compressing it reduces how well it works.

3 roll · Mineral wool loft insulation roll (270mm x 8m)

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LC-06Seal the gaps warm, moist air uses to get inFrom inside the loft, look down at the top of the ceiling below and find gaps where pipes, cables or the loft hatch pass through – warm bathroom and kitchen air sneaks up through these and condenses on the cold roof timbers above. Fill small gaps with sealant and larger ones around pipes with expanding foam.

1 can · Expanding foam filler (500ml)

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1 each · Sealant gun and mastic sealant

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LC-07Draught-seal and insulate the loft hatchStick foam draught excluder tape around the frame the hatch sits in, so it closes against a seal rather than leaving a gap. If the hatch itself is bare, tape or glue a piece of rigid insulation board to the loft-side face to stop heat and moisture escaping through it.

1 roll · Foam draught excluder tape (5m)

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1 each · Rigid insulation board, cut to hatch size

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LC-08Lag the cold water tank and pipesWrap any cold water tank in a purpose-made insulation jacket, and slide pipe lagging sleeves over exposed pipework. This stops warm loft air condensing on the cold metal or plastic surfaces. Leave the top of the tank slightly open to the air rather than sealing it completely, so the tank can still breathe.

1 each · Cold water tank insulation jacket

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4 each · Pipe lagging sleeves (2m lengths)

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LC-09Check extractor fans and vents actually go outsideTrace the ducting from bathroom and kitchen extractor fans and any tumble dryer vent. Each one should run all the way through the roof or a wall to open air – not just blow moist air loose into the loft. If a duct simply dumps into the loft space, it needs reconnecting or extending to the outside.

RULES

KNOW THE RULES

Building Regulations Part F requires extractor fans to discharge to the outside air, not into a loft space.

LC-10Add extra ventilation if the loft still can't breatheIf there are no eaves vents at all, or the loft still feels damp and stuffy after all this, extra soffit or roof tile vents may be needed. This means working on the roof covering, so unless you're confident and safely equipped, get a roofer to fit them.

RULES

4 each · Soffit or eaves vent

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KNOW THE RULES

Cutting into the roof covering to fit new vents is roof work best done or checked by a competent roofer, and any works affecting the structure should meet Building Regulations.

LC-11Recheck the loft after a few weeksGo back up in three or four weeks and look for fresh mould, condensation on the tank, or damp insulation. If it's dry, the job's worked. If damp keeps coming back, the cause may be elsewhere – a roof leak or rising damp – and needs a specialist look.

LC-12Clear up and dispose of waste properlyBag up any old, wet or mouldy insulation in plastic bags and seal them before putting them out – don't shake it loose in the loft or breathe the dust. Wash your tools, take down the kneeling board, and wash your hands and face thoroughly.

3 each · Heavy-duty rubble/bin bags

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