Insulation to suspended floor, thin joists, membrane below.

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

Detail to show floor insulation, laid between floor joists, to insulate a suspended ground floor. Applicable where the existing joists are not deep enough to accommodate a minimum 200mm of woodfibre installation; or not deep enough to achieve the target U-value for the floor. Additional timber is fixed to bottom of the existing joists to increase their depth and allow the fixing of a breathable membrane to the underside.

Target values: U-value: 0.28W/m2k

Technical description

Section detail. Floor insulation between joists with breather membrane below. Accessible from below. To achieve target U-value of 0.2W/m2k.

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

Remove any organic matter from sub-floor and floor void. Ensure all existing air-bricks are unobstructed with clear ventilation path

  1. >li>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 additional timber to joists support new ceiling plasterboard to underside of existing joists.
  6. Fit insulation butts between joists.
  7. Install rigid board insulation.
  8. Plaster finish over.

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.

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

  • Vapour Control Layer [FL01c]

    Intelligent breather membrane.
    Fixing: Laps And Edges: Sealed with vapour-tight adhesive tape. At floor and ceiling junctions ensure is appropriately sealed.

    Options for intelligent membrane

  • 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.
  • Woodfibre Insulation Between Suspended 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


  1. Admin - 2015-04-30 13:23:14 [Displayed version]

    Intro/summary updated.

  2. Admin - 2015-04-30 11:10:49

    General reformatting. DWG file uploaded.

  3. Admin - 2015-03-30 11:11:39

    Preview image and pdf detail file updated.

  4. Admin - 2015-03-26 13:26:47

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

  5. Admin - 2015-03-17 10:27:08

    Detail updated. errors removed.

  6. Admin - 2015-03-17 10:23:23

    Version 2. Introduction expanded. Technical description updated and expanded with BS codes and approved docs. Parts list update with codes.

  7. Admin - 2015-03-04 15:47:03

    Version 2- Parts list updated

  8. Admin - 2015-03-04 15:45:55

    First Version

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