Wet Or Damp Loft Insulation

What's happening?

Insulation sits in the loft to keep heat in, and it should be dry. When it is wet, damp or dripping, one of two things is happening. Rain is getting in, through a slipped tile, failed flashing, torn underlay or a chimney, and the wet is in a patch under the entry point and worse after rain. Or damp air from the house is reaching a cold loft and condensing on cold surfaces, so the wet is widespread, worst in cold weather and often on the underside of the roof felt, the rafters and the top of the insulation. The common causes of condensation are a bathroom or kitchen fan venting into the loft, a hatch that does not seal, and eaves vents blocked by insulation pushed against the roof. Wet insulation insulates poorly, and wet timber rots, so it is worth finding which it is.

What usually causes it?

• A slipped, cracked or missing tile or slate, or failed flashing
• Torn, rotten or badly lapped roof underlay
• A chimney or valley letting rain in
• Warm, damp air from the house escaping through the loft hatch, light fittings or pipework
• A bathroom or kitchen extractor venting into the loft instead of outside
• Eaves vents blocked by insulation, leaving no air path to carry moisture out
• A leaking water tank, overflow or pipe in the loft

Is it dangerous?

Not immediately, but wet insulation and wet timber are a build-up problem. Moisture left in a loft encourages mould and rot in rafters and battens, and wet insulation weighs on the ceiling below and stops insulating. Switch off loft lighting and do not use it if insulation is dripping onto fittings.

Can I fix it myself?

Photograph the roof from the ground and the loft from the hatch. Note whether the wet is a patch or widespread, and whether it is worse after rain or in cold weather. From the hatch, look for insulation jammed against the roof at the eaves, and from the ground look for soffit vent slots that have been painted over. Check that no extractor pipe ends in the loft. Do not lay new insulation over damp: it hides the problem and holds moisture. Do not strip large amounts out before the cause is found. If you have to go into the loft, step only on joists or boards, never on the ceiling between them.

When should you call a roofer?

Call a roofer if the wet is a patch below a tile line, chimney or valley, or after rain. Call a roofer or loft insulation installer if it is widespread in cold weather and the eaves are blocked: they can fit eaves trays and vents so insulation does not cover the air path. Ask for the roof timbers to be checked for rot if the wet has lasted months.

What will the roofer actually do?

• Inspect the roof covering, underlay, flashings and chimney for entry points
• Check the loft for stains, wet timber and the line of the wet insulation
• Check the hatch, fittings, pipework and any extractor ducts for moisture escaping
• Clear the eaves and fit trays or vents so air can move through the roof void
• Repair the entry point and replace insulation that has been soaked
• Re-lay insulation with a clear air path at the eaves and the hatch sealed

Who fixes this

• Roof leak repair
• Roof ventilation installation
• Roof inspection

Questions people ask

Sources & References

1. Designing Buildings: condensation
2. GOV.UK: Approved Document F, ventilation
3. Property Care Association: condensation
4. GOV.UK: Approved Document L, conservation of fuel and power