EWI to existing window, retained, with secondary glazing install

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

EWI detail to head and jamb of an existing window (single glazed or other) where a a secondary double glazed window unit is to be added to the inside face of the window for improved thermal performance.

Technical description

Section Detail. EWI to existing window retained, set within existing frame. Secondary glazing added to existing cill as a proprietary product

Reference Specification

Where the existing position of windows is to be retained when EWI is installed, window reveals must be insulated to prevent cold bridge. Phenolic foam to be used in foam based EWI systems or in woodfibre systems where existing window jamb and head is too narrow for standard 40mm woodfibre reveal boards. All phenolic foam (PF): To BS EN 13166.


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

  • 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

  • EWI (Woodfibre) System With 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-Diffutherm. Product Reference: UdiFRONT.
    Supplied by: Back To Earth.
    Tel: 01392 861763
    Web: http://www.backtoearth.co.uk/

  • Insulation to window (Thin-Phenolic) [EW05b]

    Phenolic foam where existing window jamb and head too narrow for standard 40mm woodfibre reveal boards.

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

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

  • Insulated aluminium Sill. [EW05a]

    Create extended window sill over depth of EWI to match existing window.

    • Manufacturer: As external wall insulation system
    • Type: Aluminium.
    • Installation: Pack insulation beneath cill to prevent cold bridge at existing cill reveal and use basecoat to fix as directed by manufacturer.

    As external wall insulation system manufacturer

  • EWI (Woodfibre) accessories [EW01]

    Corner Drip for render over external corner of EWI above window.

    As EWI manufacturer.


  1. Admin - 2015-04-29 16:36:07 [Displayed version]

    PDF and DWG file updated to latest revision. EWI accessories added to parts list.

  2. Admin - 2015-03-30 12:17:04

    Preview image and detail pdf updated.

  3. Admin - 2015-03-25 13:20:39

    General re-formatting. Introduction revised. Technical description updated and additional notes added. Parts list updated with suppliers links,

  4. Admin - 2015-03-25 13:19:52

    General re-formatting. Introduction revised. Technical description updated and additional notes added. Parts list updated with suppliers links,

  5. Admin - 2015-03-10 12:11:25

    First Version

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