Retained window to EWI, with sloping cill [Version ID #605]

Author details

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Detail summary

Detail for external wall insulation to an existing window, to be retained, with an existing sloping cill.
This detail allows for the profile of the sloping cill to be retained whilst improving the thermal efficiency of the building envelope by allowing insulation to be carried up to the window frame underneath the tiled cill. Used in instances where the original window jamb is reduced in depth.

Technical description

Section detail. EWI to existing retained window, sloping cill profile retained. Insulation to reveals

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

  • 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. Product Reference: UdiFRONT.

  • 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:
    Product reference: TESCON VANA/ TESCON PROFIL - 2 release tapes (single sided)

  • Insulation to window reveal (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.

  • Flashing let into window [EW01]

    Aluminium flashing let into window over junction between tile and window frame, bonded.

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

    Window manufacturer.

  • New cill tile [EW07]

    Tile Cladding.

    • Type: Terracotta tile cill (Rosemary) or re-used existing tiles if possible.
    • Replacement tile size: As existing.
    • Minimum Headlap: 35 mm.
    • Fixing: Fully nailed.
    • Features: As existing.
    • Fixing: Double layer, mastic sealed and lapped

    Contractors choice.

  • Insulation below tile [EW01]

    • Woodfibre offcuts from external wall insulation.
    • Installation: Insert into gaps between existing wall and tile cill to ensure continuity of insulation.

    Back to Earth: Tel: 01392 861763.
    Product: Diffutherm


  1. Admin - 2015-05-01 09:02:03

    DWG and pdf file updated to match title as appears on site. Amended from "thin-jamb" to "sloping cill" to avoid errors.

  2. Admin - 2015-04-29 16:20:44

    Insulation below tile added. Amendments made to flashing detail GD01. Codes amended to match drawing. DWG file uploaded. PDF file updated to latest revision.

  3. Admin - 2015-04-29 16:20:12

    Insulation below tile added. Amendments made to flashing detail GD01. Codes amended to match drawing. DWG file uploaded. PDF file updated to latest revision.

  4. Admin - 2015-04-29 16:19:23 [Displayed version]

    Insulation below tile added. Amendments made to flashing detail GD01. Codes amended to match drawing. DWG file uploaded. PDF file updated to latest revision.

  5. Admin - 2015-03-30 12:21:29

    Preview image and detail pdf updated.

  6. Admin - 2015-03-25 13:21:11

    General re-formatting. Technical description updated and expanded. Parts list updated with suppliers links.

  7. Admin - 2015-03-13 17:42:15

    Technical description updated. Parts list updated with codes.

  8. Admin - 2015-03-10 11:37:47

    First version

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