Insulation to suspended floor, accessible from below.

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

Detail for insulation to a suspended timber floor, accessible from below, to increase thermal efficiency and reduce heat loss.

Target Values U-Value : 0.2W/m2K

Technical description

Section detail. Floor insulation between joists with air-tightness works. Ground floor- suspended. No ceiling required.

Suggested Installation Sequence:

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

  1. Check floor void for signs of rot and damp report immediately if found.
  2. Remove below floor obstructions take particular care to remove organic matter eg. timber pieces.
  3. Seal joists/gaps between floor boards from above or below. Ensure all gaps in floorboards are filled.
  4. From below: ensure gap between floor and skirting is sealed with tape/membrane.
  5. Fit insulation butts between joists.
  6. Install rigid board insulation.
  7. Plaster finish to underside of rigid insulation board.

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.

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.


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.

Any alterations to drainage and other fixtures, or work to prepare the substrate, to be agreed with contract administrator prior to work being undertaken. Prior to start on site inspect the form and condition of the structural substrate; institute repairs and/ or additional works necessary to make the substrate suitable to receive EWI system.

Parts list

  • Woodfibre Insulation Between Floor Joists [FL01a]

    Woodfibre Insulation.

    • Thickness 200mm.
    • Installation: Friction fit between joists to ensure air-tightness- cut batts approx 20mm wider than opening between joists.
    • Support: 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

  • Sealing to: Floorboards, Skirtings, Wall/Floor Junctions. [DP02]

    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.
  • Polyester webbing to hold insulation [FL01a]

    Webbing drawn underneath insulation at approx 600mm centres, fixed to joists.

    Bowmer Bond: Tel: 01335 342244
    Product Reference: Polyester Webbing


  1. Admin - 2015-04-30 11:07:40 [Displayed version]

    General reformatting. DWG file uploaded.

  2. Admin - 2015-03-30 11:13:01

    Preview image and pdf detail file updated.

  3. Admin - 2015-03-30 10:59:22

    Air seal tape removed, code added to webbing in parts list

  4. Admin - 2015-03-26 13:52:16

    General re-formatting. Introduction and Technical description expanded. Parts list updated with suppliers lists.

  5. Admin - 2015-03-17 10:23:12

    Version 3. Parts list updated and errors removed.

  6. Admin - 2015-03-16 17:03:27

    Introduction and technical description updated. Parts list updated with codes.

  7. Admin - 2015-03-04 15:20:28

    First Version

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