IWI to new triple/double glazed window (inward opening)

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

Detail for internal wall insulation to the head and base of new window, to be fitted in an existing window position.

Where windows sit outside the internal wall insulation, it is necessary to insulate the reveals to prevent a cold bridge. This detail uses an insulated reveal board in order to prevent cold bridging through the external wall at the head and base of the new window.

Target Values Wall U-Value: 0.3 W/m²K

Technical description

Section detail. IWI, to new triple glazed window, inward opening variant. Replace existing window. Insulated board to window reveal.

Suggested Installation Sequence:

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. existing lintels). Report any signs of damp or rot to contract administrator immediately. Record current condition of timbers and moisture content for future reference.

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 existing cill, window and any existing insulation at head.
  2. Fit new window.
  3. Air seal tape.
  4. Fix internal insulation directly to existing internal plaster.
  5. Fix cill.
  6. Plaster to finish.

Reference Specification:

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

Refer to approved Document K, sections K4 and K5; on for guidance on window to replace existing.

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.

PVC frame windows (white & surface covered):

  • Standard: Manufactured to BS 7412 from extruded profiles.
  • Manufacturer: Registered under a quality assurance scheme operated by a certification and inspection body accredited by the United Kingdom Accreditation Service (UKAS).
  • Operation and strength characteristics: To BS 6375-2.

Wood frame windows:

  • Standard: to BS 644,
  • Manufacturer: registered under a third party quality assurance scheme.
  • Operation and strength characteristics to BS 6375-2. All timber members in windows to BS EN 942. Glazing beads clear finish to Class J2.

Wood members for purpose made windows and subframes: Timber: Generally to BS EN 942.
Appearance class to BS EN 942:

  • For opaque finish: Glazing beads, drip mouldings and the like, class J10; all other members, J40 or better.
  • For clear finish: Glazing beads, drip mouldings and the like, Class J2; other members, as specified.

Knots on arrises and finger jointing in timber: Not permitted where exposed to view.

  • Polyvinyl acetate (PVAC) to BS EN 204, class D4.
  • Thermosetting resin to BS EN 12765, class C4. Installation

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.

Cills and framing: 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.

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

  • New Timber frame, triple-glazed window to BS 644 [GD01]

    Triple glazed Softwood window.

    • Thermal Performance (overall u-value): 0.79W/m2K.
    • Air-tight installation: Install windows to achieve maximum air-tightness, using appropriate air-tight tape and sealants at all junctions and potential air-leakage points. (expanding PU foam will not be sufficient to achieve adequate air-tightness).
    • Size: As opening.
    • Opening Pattern/ Configuration: As existing (unless otherwise stated on detail).
    • Sash/Frame Material: FSC 100% Redwood.
    • Drip Rail : Aluminium.
    • Finish- Interior: Factory finished paint/varnish system.
    • Finish - Exterior: Factory finished paint/varnish system.
    • Glazing Unit: Triple, Argon Fill, 44mm.
    • Glazing Type: as standard triple glazed unless stated.
    • Light transmission : 72%.
    • Solar factor (g): 52%.
    • Handle Type: Manufacturer's standard, factory fitted.
    • Security: Secured by Design.
    • Accessories: Egress Hinges and Child Safety Restrictors.
    • Guarantees: Manufacturer's standard.

    Green Building Store: Tel: 01484 461705.
    Web: http://www.greenbuildingstore.co.uk
    Product Reference: Eco-Contract Ultra Outward Opening Casement Windows.

  • 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 10:02:18 [Displayed version]

    General re-formatting. DWG file uploaded

  2. Admin - 2015-03-30 12:00:57

    Preview image and detail pdf updated.

  3. Admin - 2015-03-29 11:06:21

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

  4. Admin - 2015-03-26 22:50:47

    Parts list codes updated.

  5. Admin - 2015-03-26 22:36:34

    General re-formatting. introduction and technical description updated with BS codes and approved docs. Parts list updated with suppliers links.

  6. Admin - 2015-03-16 16:24:30

    Introduction updated. Technical description added, and expanded with BS and Approved docs. Parts list updated with relevant codes.

  7. Admin - 2015-03-03 17:14:52

    First Version

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