IWI to suspended floor, perpendicular joists (access below)

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

Detail for internal floor insulation to a suspended internal floor, accessible from underneath in order to retain an existing floor finish.

This Internal wall insulation system uses an intelligent membrane to regulate vapour diffusion. This membrane should continue across intermediate floors, ensuring continuous insulation, vapour-control and air-tightness.

The detail provides information on the packing of joist ends in wall, and air-tightness measures, in order to prevent cold-bridging at junctions between IWI and internal upper floors.

Target Values

  • Target wall U-Value: 0.3 W/m²K
  • Target floor U-Value: 0.2 W/m²K

Technical description

Section detail. IWI to internal floor Perimeter to insulated suspended timber floor in perpendicular joists.

Before starting work: undertake study of form and condition of existing walls and likely resistance to penetrating damp; log requirement of any repairs and/ or additional works to both internal and external condition necessary before installation of new system -e.g. re-pointing or repairs to external render.

Internally: carry out study of form and condition of the existing walls; draw up schedule of fixtures, finishes and fittings to be removed/relocated in order to install the internal wall insulation system. Attention to be paid to mechanical and electrical services (e.g. radiators, light switches). Refer to detail drawings for treatment of potential cold bridges (e.g. window reveals).

All timbers: carry out inspection of any structural timbers which enter the wall to be insulated (e.g. floor joists). Report any signs of damp or rot to contract administrator. Record current condition of timbers and moisture content for future reference.

Suggested Installation Sequence:

Before installation: Remove all timber fixtures: Skirting, Picture Rails, Dado Rails etc. Remove curtain rails. Remove electrical socket/ telephone/cable socket outlets. Scabble surface to perforate paint (create lost of small holes with appropriate tool) and remove all wall-paper etc. Re-fix/renew all removed once insulation complete. Entire area installed with internal wall insulation to be finished with Gypsum plaster.


  1. Remove wallpaper and plaster if necessary, any fixings and skirting board.
  2. Lift floorboards adjacent to wall to allow access to floor void and joists.
  3. Stuff joist ends with off cuts of flexible woodfibre insulation.
  4. Seal round joists as it penetrates wall.
  5. Install Insulated suspended timber floor from below.
  6. Fix IWI to floor level with EcoBatt to foot.
  7. Intelligent membrane and plasterboard with skim.
  8. Fix floorboards and skirting boards and make sure of air-tightness.

    Reference Specification:

    All Expanded polystyrene insulation (EPS): To BS EN 13163. All fixings manufactured in accordance with BS1210. Mineral wool (MW): To BS EN 13162; reaction to fire rating to: BS EN 13501-1

    All vapour control layers/ air tightness layer to durability requirements of EN 13984.

    Please refer to Approved documents: Part B (fire safety): B1 section 1 (material alterations)/ B2 section 3 (internal fire spread).

    Refer to approved document C (resistance to contaminants and moisture) with reference to the condition of internal walls before application on IWI.

    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 iWI and existing wall. Lap and tape all membranes around edges of IWI system.

    All insulation to be continuous to avoid thermal bridging.

Parts list

  • Internal Wall Insulation System (95mm) [IW02]

    Water repellent glass mineral wool slab to BS EN 13162.

    • Thickness 95mm.
    • Target u-value: 0.3W/m2k.
    • Thermal conductivity: 0.033W/m2k/ 0.130W/mk.
    • GWP<5.
    • Fixing and supports: In accordance with manufacturer's recommendations.

    Knauf insulation Ltd.
    Product Reference: ThermoShell Internal Wall Insulation System (Requires: EcoStud for solid wall uprights.)

  • Vapour Control Layer [IW02]

    Intelligent breather membrane.

    • Fixing: Laps And Edges: Sealed with vapour-tight adhesive tape.
    • At junctions with party/partition walls, lap under insulated plasterboard.
    • At floor and ceiling junctions ensure membrane is appropriately sealed.

    Options for intelligent membrane:

  • Skirtings and Wall/Floor Junctions [DP04] & [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.
  • Double/Single-sided air seal tape [DP09b/a]

    Tape for junctions critical to air-tightness.

    • Location: see detail drawings - generally critical junctions for air-tightness. Size: 60mm (Single sided)/ 85mm (double 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 - 2 release tapes (single sided) \ Contega SL (double sided)

  • Woodfibre Insulation Between Floor Joists [FL01]

    Woodfibre Insulation.

    • 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
    Web: http://www.backtoearth.co.uk/

  • Insulated supports IWI system [IW02]

    Vertical supports for IWI batts. Thermally engineered composite of high performance extruded polystyrene bonded to Oriented Strand Board 3 (OSB 3)supports to IWI sheets. Expanded polystyrene insulation (EPS): To BS EN 13163.

    Knauf insulation Ltd.
    Reference: EcoStud for Solid walls.(Used in conjunction with Earthwool EcoBatt.)

  • Single layer plasterboard backings [IW02a]

    Plasterboard backings. Nail fixed.

    • Manufacturer: Contractors choice.
    • Plasterboard: wallboard
    • Thickness: 15mm.
    • Edge profile:Tapered.
    • Fixing: Nail fixed to Manufacturers recommendations.

    Contractor's choice

  • Two coat Board finish Plaster [IW02a]

    • Manufacturer: Contractors choice.
    • Extent: whole system.
    • Plaster: Gypsum bonding plaster
    • Surface preparation: Tape joints.
    • Undercoat thickness: As manufacturers recommendations.
    • Final coat: Gypsum finish plaster.
    • Final coat thickness: 2-3mm applied one coat.
    • Finish: Smooth.

    Contractor's choice.


  1. Admin - 2015-04-30 09:42:00 [Displayed version]

    General formatting changes. Contact detail for parts list updated. DWG file uploaded.

  2. Admin - 2015-03-30 11:55:29

    Preview image and detail pdf updated.

  3. Admin - 2015-03-29 10:51:08

    Duplicate insulated support studs removed in parts list.

  4. Admin - 2015-03-26 17:13:21

    General formatting. Introduction and technical description updated. Parts list updated with suppliers codes.

  5. Admin - 2015-03-16 15:16:46

    Introduction updated and technical description revised and BS and approved docs added. Parts list updated with relevant codes.

  6. Admin - 2015-03-03 16:37:13

    First version

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