Loft insulation touching roof felt
What you’re seeing (symptoms)
The edges of your loft (near the eaves) have insulation packed tightly against the roof felt or tiles. You may notice dampness on the underside of the felt, water dripping onto the insulation, or a musty smell in the loft.
What’s normal vs not normal
Normal:
- Insulation covering the entire ceiling area to a depth of 270mm-300mm.
- A small amount of daylight visible at the very edges of the loft (this is your eaves ventilation).
Not normal:
- Insulation pushed all the way into the corners so that no air can enter from the outside.
- Water droplets (condensation) forming on the roof felt above the insulation.
- Black mould growing on the wooden rafters near the eaves.
Likely causes (ranked)
- Over-insulating: Trying to be “extra thorough” by stuffing insulation into every corner, accidentally blocking the airflow.
- Lack of eaves baffles: The installer didn’t use “rafter trays” or baffles to keep the insulation away from the roof’s edge.
- Wind-wash: Strong winds have blown loose-fill insulation into the eaves, blocking the vents.
- Storage boarding: Pushing insulation into the eaves to make more room for storage boards in the centre of the loft.
- Inadequate ventilation: The roof doesn’t have enough tile vents or ridge vents to compensate for the reduced airflow at the eaves.
Quick checks you can do safely
- Visual inspection: Go into the loft (stay on the joists!) and look at the edges. Is there a gap between the top of the insulation and the roof felt?
- The “Daylight” test: Switch off your torch. Can you see any daylight coming in from the eaves? If not, they are likely blocked.
- Feel the wood: Gently touch the rafters near the eaves. Are they damp or “tacky” to the touch?
- Check the felt: If the felt is wet, try to pull the insulation back by about 50mm-100mm to allow air to circulate.
When to call an installer
- If you cannot reach the eaves safely to pull back the insulation.
- If you find significant mould growth on the rafters or roof timbers.
- If you want to install “lap vents” or “eaves trays” to permanently fix the ventilation issue.
Questions to ask your installer
- “Can you install rafter trays (baffles) to ensure a permanent 50mm air gap at the eaves?”
- “Do I need additional tile vents to improve the cross-ventilation in the loft?”
- “Are the roof timbers showing any signs of ‘wet rot’ due to the condensation?”
Related NetZeroNow tools & guides
- Loft insulation guide
- Condensation and mould
- U-value calculator
- Draught proofing
- Maintenance schedules
- Choosing an installer
- Grants and funding
- Energy assessment guide
FAQs
Why does it need a gap? Your loft needs a constant flow of fresh air to carry away any moisture that drifts up from the house. If you block the gap at the eaves, that moisture gets trapped, turns into water on the cold felt, and rots your roof. How big should the gap be? A minimum of 50mm is recommended between the top of the insulation and the underside of the roof covering (felt or tiles). What are lap vents? They are small plastic wedges you can insert between the overlaps in your roof felt to allow more air into the loft space. They are a cheap and effective DIY fix. Is wet insulation ruined? If it’s just a little damp on the surface, it will dry out once the ventilation is fixed. If it’s soaking wet and “clumped,” it has lost its insulating properties and should be replaced. Should I remove the old insulation? Usually, no. You can just “top up” old insulation, as long as it’s dry and you don’t block the eaves vents in the process.\n
Written by NetZeroNow | Last updated on 2026-03-02