Retained window to EWI, thin jamb (section)

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

Detail for external wall insulation to an existing window to be retained, where the existing window sits up against the inside edge of existing window jamb. This detail proposes phenolic foam to be used where the existing window jamb and head is too narrow for standard 40mm woodfibre reveal boards.

Technical description

Section detail. EWI to existing retained window, perpendicular cill profile.

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

  • 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-Diffutherm. Product Reference: UdiFRONT.
    Supplied by: Back To Earth.
    Tel: 01392 861763

  • Insulation to retained 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 with upstand [EW05b] .

    Extended window sill over depth of EWI to match existing window. Including built-in upstand.

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

    As EWI manufacturer.

  • 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 single sided tape

  • EWI (Woodfibre) accessories [EW01]

    Corner Drip for exterior corners of EWI render

    As EWI manufacturers.


  1. Admin - 2015-04-29 16:27:00 [Displayed version]

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

  2. Admin - 2015-03-30 12:19:39

    Preview image and detail pdf updated.

  3. Admin - 2015-03-27 15:36:42

    Air seal tape and cill details added to parts list

  4. Admin - 2015-03-25 13:21:09

    General re-formatting. Technical description updated with additional clauses and notes. Parts list updated with suppliers codes.

  5. Admin - 2015-03-10 11:50:16

    First version

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