Ground floor ventilation to insulated floor

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

Detail for the ventilation of an insulated suspended timber floor. Required where any existing air-bricks to the sub-floor void would be blocked by the installation of floor insulation. A periscopic vent is used to ensure a continued clear air-path to sub-floor, to ensure adequate ventilation.

Technical description

Section detail. Ventilation to perimeter of floor insulation. Existing air-bricks blocked.


Before start of works: Inspect any structural timbers, particularly those which enter external walls (e.g. floor joists); Inspect sub-floor, and timbers within for signs of damp and rot. Report immediately if any found. If possible record current condition of timbers and moisture content for future reference. Remove any organic matter from sub-floor and floor void. Ensure all existing air-bricks are unobstructed with clear ventilation path

Suggested Installation Sequence

  • Inspect floor void for signs of rot/damp and report immediately if found.
  • Carefully remove obstructions from below floor.
  • Seal between floor boards and fill gaps.
  • Friction fit insulation between between floor joists and fix in place.
  • Install webbing to hold insulation in place.

Reference Specification:

Please refer to Document A (structural safety) as reference for the condition of existing floor joists or members. Ensure existing joists meet requirements in Approved document A (structural) before starting works.

Refer to approved document C (resistance to contaminants and moisture) with reference to below ground works. Refer to approved document H (drainage and waste disposal), section H3, rainwater drainage in addition.

Document F(ventilation) should be referred to with reference to sub-floor ventilation guidance.

Documents L1b of Part L (conservation of fuel and power) should be referred when working on existing dwellings, with particular attention to sections 5 and 6 regarding material alterations. Document L2b should be referred to with existing buildings other than dwellings.

  • Air bricks in external walling to BS 493-1995, Class 1.
  • Gratings/ Ventilators in internal walling to BS 493, Class 2.
  • Pipe sleeves: Extend through full thickness of wall or floor, position accurately.
  • Generally: clearance around pipes: should be 20 mm (maximum) or diameter of service, whichever is the lesser.

Care should be taken to ensure air-tightness. Seal all service penetrations through existing wall. Lap and tape all membranes around edges of ventilation system.

All insulation to be continuous to avoid thermal bridging.

Parts list

  • Air Bricks And Wall Ventilators [FL011c]

    Plastics air brick. Cranked sleeve to suit and wall ventilator. Re-use existing air-bricks where possible.

    • Manufacturer: Contractor's Choice.
    • Colour: to match existing finishes
    • Work Size: As existing brick/blockwork.
    • Outer Face Position: Flush with wall face.
    • Spacing: Maximum 2m centres to provide 1500mm2 of clear opening per 1m run of external wall; unless otherwise agreed.

    Contractor's choice.

  • Skirtings and Wall/Floor Junctions [DP04]

    Draught-proof and seal all timber floors to ensure no air-leaks.

    • Location: All timber ground floors.
    • Floor: Draughtex neoprene strips or timber fillet/ mastic seal between skirting and floor.
    • Skirtings and Wall junctions: Seal all junctions and repair any cracks in plaster to ensure no air-leaks using standard decorators caulk.
  • Insulation Between Floor Joists [FL01]

    • Thickness 200mm.
    • Friction fit between joists to ensure air-tightness- cut batts approx 20mm wider than opening between joists.
    • Supported: Polyester webbing drawn underneath insulation at approx 600mm centres, fixed to joists.

    Unger-Diffutherm. Product Reference:Reference: UdiFLEX
    Supplied by: Back To Earth.
    Tel: 01392 861763

  • Single-sided air seal tape [DP09a]

    Tape for junctions critical to air-tightness.

    • Location: see detail drawings - generally critical junctions for air-tightness. Size: 60mm (Single sided)
    • Accessories: ORCON F sealant and TESCON PRIMER RP (for use on rough/old surfaces to assist adhesion)

    Back to Earth: Tel: 01392 861763.
    Product reference: TESCON VANA/ TESCON PROFIL: single sided tapes


  1. Admin - 2015-04-30 11:18:14 [Displayed version]

    DWG file uploaded. General re-formatting.

  2. Admin - 2015-03-30 11:07:00

    Preview image and pdf detail file updated.

  3. Admin - 2015-03-26 12:50:20

    Parts list updated. General re-formatting.

  4. Admin - 2015-03-17 13:53:16

    Text formatted to lists where required and supplier links added.

  5. Admin - 2015-03-17 13:52:45

    Text formatted to lists where required and supplier links added.

  6. Admin - 2015-03-17 13:51:49

    Text formatted to lists where required and supplier links added.

  7. Admin - 2015-03-17 12:07:50

    Technical description updated.

  8. Admin - 2015-03-17 12:06:32

    Technical description updated.

  9. Admin - 2015-03-17 11:09:11

    Introduction updated. Technical description revised and updated with BS information and approved docs. Parts list updated with codes and additional parts added to specification.

  10. Admin - 2015-03-04 16:41:25

    First Version

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