IWI to upper floor with perpendicular joists

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

Detail for internal wall insulation to an existing timber floor, where the existing floor joists are perpendicular to the wall to be insulated.

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 Upper floor, with access from above. Perpendicular joists. IWI continuous between floors. Taped and sealed all edges.

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: Before starting work 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: Before starting work 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 immediately. 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. Fix IWI to reach ceiling below.
  5. Intelligent membrane and plasterboard with skim continued into floor void.
  6. Seal round joists as it penetrates.
  7. 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 thick) [IW02a]

    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.)

  • 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.
    Product Reference: EcoStud for Solid walls.(Used in conjunction with Earthwool EcoBatt.)

  • 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 single sided tape \ Contega SLdouble sided tape

  • 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.
  • 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.

  • Insulation to joist ends (Flexible) [RF06c]

    • Offcuts from woodfibre wall insulation.
    • Installation: Insert into gaps between wall plate/top of wall and rigid insulation to ensure continuity and avoid air movement.

    Back to Earth: Tel: 01392 861763.
    Web: http://www.backtoearth.co.uk/
    Product: Diffutherm

  • Air-tight adhesive/sealant [DP09c]

    Sealant in critical junctions for air-tightness.

    Green Building store: Tel: 01484 461705
    Web: http://www.greenbuildingstore.co.uk/
    Product reference:ORCON F


  1. Admin - 2015-04-30 09:30:01 [Displayed version]

    General formatting changes. DWG file updated.

  2. Admin - 2015-03-30 11:54:15

    Preview image and detail pdf updated.

  3. Admin - 2015-03-29 10:44:36

    Air tightness sealant and loose insulation to joist ends added to parts list.

  4. Admin - 2015-03-26 16:57:16

    Target u-values added.

  5. Admin - 2015-03-26 16:55:28

    General formatting. Introduction revised and updated. Technical description updated with approved documents and additional notes. Parts list updated.

  6. Admin - 2015-03-16 15:17:56

    Version 3- Technical description updated.

  7. Admin - 2015-03-16 14:51:56

    Version 3- Introduction updated. Technical description revised with Approved doc and BS information. Parts list updated with relevant codes.

  8. Admin - 2015-03-03 16:16:58

    First version

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