Floor insulation to cover joists [Version ID #635]

Author details

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Detail summary

Detail for insulation to an internal floor with shallow or thin joists, with rigid insulation beneath to cover joists and improve airtightness.

Target U-value U-value: 0.2W/m2k.

Technical description

Section detail. Floor insulation for suspended timber floor. Insulation between and covering joists. Accessible from below.

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

  • Rigid woodfibre insulation fixed across joist. [FL01d]

    • Thickness: to create total 200mm insulation thickness under floorboards/floor finish
    • Thermal conductivity: 004W/m2K.
    • GWP: 0.

    Back to Earth: Tel: 01392 861763.
    Web: http://www.backtoearth.co.uk/
    Product reference: Reference: Unger-Diffutherm insulation board Unger-DiffuthermNF, or udiUnger-Diffutherm L (reveal board).

  • Woodfibre Insulation Between Floor Joists [FL01d]

    Flexible woodfibre insulation.

    • Thickness: to full depth of joists.
    • Thermal conductivity: .004W/m2K.
    • GWP: 0.
    • installation: Friction fit between joists to ensure air-tightness (batts cut approx 20mm wider than opening between joists.)

    Unger-Diffutherm. Product Reference:Reference: UdiFLEX
    Supplied by: Back To Earth.
    Tel: 01392 861763
    Web: http://www.backtoearth.co.uk/

  • 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.
  • Single-sided air seal tape [DP09a]

    Tape for joints between rigid insulation boards below joists.

    • 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.
    Web: http://www.backtoearth.co.uk/
    Product reference: TESCON VANA/ TESCON PROFIL:single sided tapes


  1. Admin - 2015-04-30 13:26:33

    Introduction/summary updated to include references to undercroft/cellar as per title.

  2. Admin - 2015-04-30 11:13:07 [Displayed version]

    Reformatting changes. DWG file uploaded.

  3. Admin - 2015-03-30 11:10:07

    Preview image and pdf detail file updated.

  4. Admin - 2015-03-30 10:53:22

    DP09a amended for use on "joints of Rigid insulation board". Plasterboard part removed.

  5. Admin - 2015-03-26 13:51:36

    Removed reference to Approved document H.

  6. Admin - 2015-03-26 13:26:49

    General re-formatting. Introduction revised. Technical description updated with approved docs and additional notes.Parts list updated with supplier links.

  7. Admin - 2015-03-17 10:29:21

    Introduction updated, Technical description expanded. Parts list revised with relevant codes.

  8. Admin - 2015-03-04 15:54:25

    First version

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