IWI to retained window with secondary glazing

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

Detail for internal wall insulation to an existing window (single glazed), with a minimal frame depth. Where windows such sit outside the internal wall insulation, it is necessary to insulate the reveals to prevent a cold bridge. A secondary double glazed window unit is added to the inside face of the window for improved thermal performance.

Target Values:

  1. Psi cill value: 0.03W/m2k.
  2. Psi lintel value: 0.03 W/m2k.
  3. Target wall value: 0.3W/m2k.

Technical description

Section Detail. IWI to existing window with secondary glazing installed as part of insulation works.

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. Clean existing window.
  2. Fix secondary glazing.
  3. Fix air-seal tape in place.
  4. Install internal wall insulation.
  5. Install insulation to reveal.

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.

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 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.
    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 - 2 release tapes (single sided)

  • 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

  • Secondary glazing systems-proprietary product [GD06a]

    Secondary glazing layer to existing window; to help achieve target U-value of 1.2 W/m2K. Glazing type: as standard. Size: To fit existing opening. Type: Horizontal slider, heavy duty frame. Frame: Heavy duty. Colour/finish: TBC Glazing: 20 mm low-e coated argon filled double glazing unit with warn edge spacers – toughened inner pane. Flyscreens: Not required. \ \ \ Finish: White semi-gloss polyester powder coated.

    Duration Group: Tel: 01268 681612
    web: http://www.duration.co.uk
    Product Reference: Secondary Glazing

  • 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.

  • 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.)


  1. Admin - 2015-04-30 10:17:14 [Displayed version]

    DWg file uploaded. General formatting changes.

  2. Admin - 2015-03-30 12:03:25

    Preview image and detail pdf updated.

  3. Admin - 2015-03-29 11:13:53

    IWI supports added to parts list

  4. Admin - 2015-03-29 11:06:13

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

  5. Admin - 2015-03-26 23:02:29

    General re-formatting. Introduction adjusted. Technical description updated and expanded with additional BS codes and approved docs. Parts list updated with suppliers links.

  6. Admin - 2015-03-16 13:54:25

    Introduction, and technical description updated with BS and approved docs. Parts list expanded and codes added.

  7. Admin - 2015-03-03 17:28:59

    First version

Leave a comment


  1. For those who have seen the Earth from space, and for the hundreds and perhaps thousands more who will, the experience most certainly changes your perspective. The things that we share in our world are far more valuable than those which divide us.

    User name

Leave a comment


A total of 14 users have left feedback about this detail.

Share your thoughts about this detail with other users:

Feedback summary

  • 80% of users found this detail easy to implement.
  • 50% of users found it easy to source skills for this detail.
  • 60% of users found this detail hard to understand.
  • 100% of users found the overall cost for this detail to be low.
  • 90% of users found materials for this detail easy to source.