Insulation

Draughty floorboards after insulation

Warning Do not attempt any electrical or gas work yourself. Always use a qualified, MCS-certified, or Gas Safe registered professional for repairs.

What you’re seeing (symptoms)

You have recently installed floor insulation (either from below or above), but you can still feel a cold breeze or “draught” coming up between the gaps in your floorboards. The room feels chilly even when the heating is on.

What’s normal vs not normal

Normal:

  • A slight “coolness” when touching a floorboard directly if it isn’t carpeted.
  • Floors feeling significantly warmer than they did before insulation was installed.

Not normal:

  • Visible dust being blown up from between floorboards.
  • Being able to feel a distinct airflow through gaps when you place your hand near the floor.
  • Condensation forming on the surface of the floorboards in winter.

Likely causes (ranked)

  1. Missing draught seals: Insulation alone doesn’t stop air movement. If the gaps between the boards or the skirting boards weren’t sealed, air will still blow through.
  2. Gaps at the perimeter: The insulation doesn’t fit tightly against the walls, leaving a “chimney” effect at the edges of the room.
  3. Vents blocked: Underfloor air bricks have been blocked by the new insulation, causing dampness and air pressure issues.
  4. Loose-fill movement: If loose-fill insulation was used, it may have shifted, leaving uninsulated patches.
  5. Uninsulated trapdoors: The loft hatch or underfloor access hatch was not insulated or sealed.

Quick checks you can do safely

  • The “Candle” test: Carefully hold a candle or a piece of tissue paper near a gap in the floorboards. Does the flame or paper flicker?
  • Check the skirting: Most draughts come from the gap between the skirting board and the floor. Feel this area specifically.
  • Inspect from below: If you have access to the sub-floor (crawl space), look up at the insulation. Are there any visible gaps or “sags”?
  • Check the air bricks: Go outside and ensure your sub-floor vents (air bricks) are not blocked by the new insulation.

When to call an installer

  • If you find significant gaps in the insulation when looking from below.
  • If your floorboards feel “spongy” or you see mould, as this suggests the insulation is trapping moisture.
  • If you want to have a professional “airtightness” test performed.

Questions to ask your installer

  • “Did you use a breathable airtight membrane on the cold side of the insulation?”
  • “How are the perimeters (the edges where the floor meets the wall) sealed?”
  • “Is there sufficient ventilation in the sub-floor to prevent joist rot?”

Related NetZeroNow tools & guides

FAQs

Can I seal the gaps myself? Yes. You can use specialized floorboard gap fillers (like StopGap or DraughtEx) or a flexible wood filler. For skirting boards, a simple silicone or decorators caulk is often enough. Will my floor rot if I seal it? No, as long as the sub-floor (the space under the insulation) is still well-ventilated by external air bricks. You must never block those air bricks. Why didn’t insulation stop the draught? Insulation (like mineral wool or PIR board) is designed to stop heat transfer, not necessarily air flow. Air can move through or around most insulation types unless a dedicated airtightness layer is added. What is the best material for floor insulation? Rigid PIR boards or high-density mineral wool batts are common. For old suspended timber floors, breathable materials like sheep’s wool or wood fibre are often preferred to prevent moisture buildup. Is it worth insulating the floor? Yes. It can reduce heat loss in a room by up to 15% and, more importantly, makes the room feel much more comfortable by removing cold draughts at foot level.\n

Written by NetZeroNow | Last updated on 2026-03-02