New window to EWI, retrofit installation

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

Detail for a new window with a timber frame, to be mounted into external wall insulation, in place of an existing window. This detail is designed to reduce thermal bridging and improve air-tightness whilst spreading the structural load of new window on the EWI, and maximising glazed area.

The timber box also acts as self-finished internal reveal. (May be painted/ varnished as required). This detail is designed so that the window and box can be installed after the external wall insulation system is in place.

Target values:

  • Cill Psi value: 0.045 W/mK.
  • Lintel Psi value: 0.059 W/mK.

Technical description

Section detail. EWI to new replacement window, for retrofit of window after EWI installation, on box into former window location. New triple glazed window to replace existing.

Suggested Installation Sequence:

Windows and window boxes should ideally be installed from outside, with insulation fitted tight up to box and appropriate air-tightness work undertaken as shown on detail drawings. Where this is not possible, windows and boxes can be fitted from inside, with temporary boxing acting as place-holder within the insulation until installation can take place.

Reference Specification:

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

Document L1b 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 during the installation to ensure the continuity of insulation and air-tightness at the junction between window, existing wall and external wall insulation.

All insulation to be continuous to avoid thermal bridging.

Preparation of external wall: Remove any existing loose material and prepare in accordance with manufacturer's requirements.

Any alterations to drainage and other fixtures, or work to prepare the substrate, to be agreed with contract administrator prior to work being undertaken. Prior to start on site inspect the form and condition of the structural substrate; institute repairs and/ or additional works necessary to make the substrate suitable to receive EWI system.

Parts list

  • New Timber frame, triple-glazed windows [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:+44 (0)1484 461705.
    Product Reference: Eco-Contract Ultra Outward Opening Casement Windows.

  • External Wall Insulation (Woodfibre), rendered finish [EW01]

    Composite woodfibre board for use on solid and hard to treat cavity walls.

    • Insulation back board: UdiTherm SK.
    • Insulation top board: Diffutherm NF (T&G, renderable).
    • Thermal conductivity: 0.045W/m2K. GWP 0.
    • Base Coat: levelling on substrate: 90% contact required around edge of boards between board and substrate to prevent thermal bypass. If substrate is very uneven, use base-coat material around edge of board to achieve contact.
    • Fixing: Fixed mechanically to substrate in accordance with manufacturers recommendations.
    • Render/carrier: to manufacturers standard.
    • Render: Udi Perl
    • Decorative finish: tbc
    • System guarantee: 25 years - ECO compliant insurances and guarantees.
    • System Certification : ETAG

    Unger. Product Reference: UdiFRONT.

  • Self-finished softwood box to mount into EWI [EW04a]

    • Manufacturer: Contractor’s choice.
    • Size: To fit existing structural opening. Box to be constructed to same outside dimension as window opening. Window box depth as detail drawing. Inner edge of reveal to protrude 20mm inside the room, beyond the existing plaster face.
    • Species : European Redwood.
    • Section Sizes: As detail drawing.
    • Adhesive: to contractor’s specification.
    • Fixing-to window: Mechanically pre-fixed and glued to window frame.
    • Fixing- to existing masonry: Window reveals to be fixed back to outer leaf of masonry with frame fixings countersunk and plugged, at between 450mm - 600mm centres to suit window size.
    • Certification : All timber to be FSC certified.

    Contractor's choice

  • 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.
    Product reference: TESCON VANA/ TESCON PROFIL

  • External Aluminium Extended Cill [GD01]

    New window cill over depth of EWI.

    • Manufacturer: As window.
    • Product Reference: Manufacturers standard- Aluminium

    Window manufacturer

  • Air-tight mastic adhesive/sealant [DP09c]

    • Location: as per drawing - critical junctions for air-tightness, especially around windows.
    • Accessories: TESCON PRIMER RP

    Green Building Store: Tel: 01484 461705.
    Product Reference: ORCON F

  • 9mm Softwood bead to window surround.

    Used to cover junction of window reveal to wall.

    • Location: to s/w window reveal box. Over existing plaster.
    • Species: contractor's choice
    • Finish: unfinished as per window box.

    Contractors choice.

  • 20mm woodfibre reveal board [EW05]

    External wall board- thin profile.

    • Insulation thickness: 20mm.
    • Thermal conductivity: 0.023W/mK
    • Finish: render as adjacent EWI.
    • Render stops/corner beading: Render stop required at junction with EWI. Manufacturer's standard.

    Kingspan Insulation Ltd.
    Product Reference: Kooltherm® K5 External Wall Board.

  • Injected foam insulation.

    To achieve air-tight seal around window frame.

    Contractor's choice/ or to Window manufacturers reccomendations .


  1. Admin - 2015-04-29 15:28:28 [Displayed version]

    DWG file uploaded. PDF file updated to latest revision. Parts list codes amended to match drawing.

  2. Admin - 2015-03-27 17:39:44

    Preview image and .pdf file updated.

  3. Admin - 2015-03-27 15:11:08

    S/w bead, foam insulation and 20mm reveal board added to parts list. codes updated.

  4. Admin - 2015-03-25 11:01:02

    General re-formatting. Introduction text revised. Technical description updated with links to approved docs and additional specification notes. Parts list updated with suppliers links.

  5. Admin - 2015-03-25 11:00:12

    General re-formatting. Introduction text revised. Technical description updated with links to approved docs and additional specification notes. Parts list updated with suppliers links.

  6. Admin - 2015-03-13 16:36:59

    Technical description updated. Part list updated with codes.

  7. Admin - 2015-03-10 10:13:36

    Version 3- technical description updated

  8. Admin - 2015-03-10 09:56:57

    Version 2 - title updated

  9. Admin - 2015-03-10 09:55:06

    First version

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