IWI to retained window with insulated reveal board

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

Detail for internal wall insulation to an existing window, to be retained, with a thin jamb profile, requiring a slimmer reveal board to provide continuous thermal insulation.

Target Values

  • Cill: 0.045 W/mK.
  • Lintel: 0.059 W/mK.
  • Wall U-Value: 0.3 W/m²K

Technical description

Section detail. IWI to flush eaves with retained window, thin jamb.

Suggested Installation Sequence:

Before starting work: Remove existing internal cills and any additional internal framing to window. Scabble surface to perforate paint (create lots of small holes with appropriate tool) and remove all wall-paper etc. Re-fix/renew all removed once insulation complete. Plaster: Where existing plaster is gypsum-based, or where space is required, remove existing plaster finish.

Before installation: Remove all timber fixtures: Skirting, Picture Rails, Dado Rails etc. Remove curtain rails. Remove electrical socket/ telephone/cable socket outlets. Re-fix/renew all removed once insulation complete.


  1. Remove existing wall finishes and plaster as required.
  2. Fit internal wall insulation.
  3. Fit air-seal tape to window reveal.
  4. Fit reveal insulation.
  5. Finish with plaster and timber cill.

Reference Specification:

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.

Replacement window installation to BS 8213-4. PVC-U windows installation standard: to BS 8213-4

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.

Rigid insulation: where used Phenolic foam (PF): To BS EN 13166.

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


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

  • Laminate plasterboard [IW05b]

    Phenolic Foam Thermal Plasterboard with integral vapour control layer

    • Plasterboard thickness: 12.5mm.
    • Insulation thickness: 17mm.
    • Edge profile: Tapered edge.
    • Thermal conductivity: 0.21W/m2k.
    • GWP<5.
    • Fixing: Adhesive scored bonding coat to ensure no air-gap to rear of board.
    • Joints: Taped and filled joints.
    • Finish: Prepared for plaster skim coat finish.

    Kingspan Insulation Ltd:
    Web: http://www.kingspaninsulation.co.uk/
    Product Reference: Kooltherm® K18 Insulated Plasterboard

  • Rigid insulation below cill [IW05b]

    Rigid thermoset phenolic insulation board.

    • Insulation Thickness: 15-24mm (to suit window frame/ opening).
    • Thermal performance: : 0.023W/m.k

    Kingspan Insulation Ltd:
    Web: http://www.kingspaninsulation.co.uk/
    Product Reference: Kooltherm® K3 Floorboard

  • Softwood Window Board to Sill [IW05b]

    • Manufacturer: Subject to contractor.
    • Type: Half-round edge.
    • Species: European redwood.
    • Class: Class 2 to BS 1186-3.
    • Finish as delivered: Natural finish.
    • Treatment: Not required.
    • Certification: FSC certified.
    • Fixing: Lost nail heads at 30mm centres.

    Contractor's choice.

  • Internal Wall Insulation System (95mm) [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.)

  • Single-sided air seal tape [DP09a]

    Tape for junctions critical to air-tightness.

    • 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

  • Vapour Control Layer [IW02a]

    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

  • Insulated supports IWI system [IW02a]

    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 10:13:26 [Displayed version]

    General formatting. DWG file uploaded

  2. Admin - 2015-03-30 12:02:12

    Preview image and detail pdf updated.

  3. Admin - 2015-03-29 11:06:17

    IW05b "insulated dry-lining board" replaced with "laminate plasterboard

  4. Admin - 2015-03-26 22:49:47

    General re-formatting. Introduction expanded. Technical description revised and expanded with Approved documents, links and additonal BS codes. Parts list updated with suppliers links.

  5. Admin - 2015-03-16 16:33:35

    Version 2. Introduction, Technical description updated and expanded with BS codes and Approved document information. Parts list updated with relevant codes.

  6. Admin - 2015-03-03 17:21:09

    First Version

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