EWI- window to eaves with soffit

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

Detail for external wall insulation to an overhanging soffit above a window to be replaced. This detail shows new external wall insulation to meet new loft and roof insulation directly above the new window to prevent a potential cold bridge at the building eaves.

Technical description

Section detail. EWI to existing wall above existing window to be replaced. Existing eaves soffit board removed, new phenolic foam insulation above to join to EWI. Window replacement with triple glazed window with new timber frame.

Suggested Installation Sequence:

Preparation of roof and loft space: Remove any existing loose material and prepare in accordance with manufacturer's requirements.

Any alterations to cabling, existing ventilation, drainage, and other fixtures affected by work to be agreed with contract administrator prior to work being undertaken. Prior to start on site inspect the form and condition of the roof internally and externally, especially any structural timbers; institute repairs and/ or additional works necessary to make the structure suitable to receive works.


  1. Roll back existing loft insulation to reveal junction between ceiling and roof. Remove any debris. Ensure gap between rafters is clear to either eaves board or 200mm beyond outer edge of wall (where no eaves board), whichever is the smaller.
  2. Glue battens to upper surface of the cut insulation boards 100mm in from each edge. On leading edge of the slab taper batten thickness to minimise risk of obstruction on installation.
  3. Once glue has set, push slabs into the gap over the top of the sole plate up to the eaves board or 200mm past whichever is the smaller.
  4. Fit new window and window boards in place of existing. Seal all gaps for air-tightness around window reveal internally and externally. test for air-tightness.
  5. Fix external wall insulation (FAB EW01)- cut to fit to eaves insulation. (Where EWI installed before window: Air seal tape to be installed behind plaster at inside edge of reveal board-see detail. Existing plaster to be removed and Double sided tape applied to window reveal, sealed to existing wall. Plaster over.)
  6. Reinstate loft insulation to underside of rigid insulation.

Reference Specification:

All new replacement timber windows to BS 644, operation and strength characteristics to BS 6375-2. All timber members in windows to BS EN 942. Glazing beads clear finish to Class J2.

Tiling work: Clay tiles to BS EN 1304/ concrete (non interlocking) to BS EN 490. All battens/counterbattens and ridge/hip fixing battens to be timber species to BS 5534, Clause 4.12.1. All fasteners to BS 1202 as appropriate to fastener type

Existing guttering to be replaced/reinstated to BS EN 607, and BS EN 1462, Kitemark certified, clauses 3–7 and National Annexes. Collection and distribution of rainwater: Complete, and without leakage or noise nuisance.

For all works to existing roofs refer to Approved document C (resistance to contaminants and moisture), with reference to Section 2, and section 6.

Reference should be made to Document H (drainage and waste disposal). Approved Document F (ventilation) should be referred to with regards to ventilation to existing roofs and over eaves ventilation. 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 to ensure air-tightness. Seal all service penetrations through EWI and existing wall. Lap and tape all membranes around edges of EWI system.

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 existing roof, to be agreed with contract administrator prior to work being undertaken. Prior to start on site inspect the form and condition of the roof and eaves structure; institute repairs and/ or additional works necessary to make the existing construction suitable to receive EWI system.

Parts list

  • Loose laid loft insulation [L01a]

    Flexible formaldehyde free glass mineral wool. Should be laid continuous to eaves: level, covering all parts of the ceiling to meet any eaves/ internal wall insulation to prevent cold bridge. All service holes sealed, debris removed before laying insulation. Eaves ventilation should be unobstructed. Any electric cables overlaid by insulation should be sized accordingly. Where water cistern platforms located on ceiling joists: Insulation below omitted.

    • Recycled content: 85% (minimum) to BS EN ISO 14021.
    • Thermal Conductivity: 0.040 W/mK. GWP 0.
    • Thickness to 400mm

    Knauf insulation Ltd
    Product Reference: Knauf Earthwool loft roll 40

  • Insulation to Eaves (rigid) [RF06b]

    • Manufacturer: Contractor's choice.
    • Phenolic foam.
    • Size: 50mm thickness
    • Width: cut to suit rafters.
    • Length: 750mm PLUS distance to eaves board or 200mm (whichever smallest).
    • Thermal conductivity: 0.023W/m2k.
    • Installation: Continuous with roof insulation, no gaps, to avoid cold bridging. Friction fit between rafters from inside loft, being careful not to damage existing roof membrane or structure or block existing ventilation paths.

    Contractor's choice

  • 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.

  • Insect Mesh behind verge board [EW07]

    • Type: Stainless steel insect mesh
    • Position: secured behind verge board to close cavity behind.
    • Fixing: Stainless steel staples.

    Contractor's choice.

  • Softwood window boards [GD01]

    • Manufacturer: Contractor's choice.
    • Type: Half round edge.
    • Finished size: To suit opening.
    • Species: Manufacturers standard.
    • Class: Class 2 to BS 1186-3.
    • Preservative Treatment: Not required.
    • Finish As Delivered: To match window.
    • Fixing : Countersunk screws at 400 mm centres and plugged.
    • Certification : All timber to be FSC certified (or re-used from existing on site).

    Contractor's choice.

  • Double/Single-sided air seal tape [DP09b/a]

    Tape for junctions critical to air-tightness.

    • Location: see detail drawings - generally critical junctions for air-tightness. Dependant on construction sequence. Size: 60mm (Single sided)/ 85mm (double 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 - 2 release tapes (single sided) \ Contega SL (double sided)

  • EWI Accessories [EW01]

    • Type: Adhesive render edging strip for seal to external edge of window. Render drip for overhang.
    • Location: as per drawing.

    As EWI manufacturer.

  • 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/


  1. Admin - 2015-04-29 09:51:37 [Displayed version]

    DWG file uploaded. Back To Earth contact details added to parts list for EW01. General formatting changes. EW01 part added.

  2. Admin - 2015-04-29 09:48:25

    DWG file uploaded. Back To Earth contact details added to parts list for EW01. General formatting changes.

  3. Admin - 2015-04-22 14:27:14

    Pdf file updated as per latest revisions

  4. Admin - 2015-03-27 16:19:27

    Preview image and .pdf file updated.

  5. Admin - 2015-03-27 12:01:09

    Technical description updated for air seal tape location. Insect screen added to parts list. EWI accessories added to parts list.

  6. Admin - 2015-03-23 14:41:57

    General Error removal

  7. Admin - 2015-03-23 14:26:29

    General formatting. Technical description added and expanded with detailed installation guidence and approved document links. Parts list updated with links,

  8. Admin - 2015-03-13 18:13:13

    Technical description and parts list updated.

  9. Admin - 2015-03-12 15:14:07

    Version 7. Technical description updated

  10. Admin - 2015-03-12 14:58:38

    Version 6. Title updated,description and technical description updated and expanded. BS and Approved doc information added. Parts list added. Preview image and pdf file updated with revised linetype colours /line-weights.

  11. Admin - 2015-03-03 14:52:53

    Preview image updated.

  12. Admin - 2015-03-03 14:50:18

    Preview image updated. Parts list added.

  13. Admin - 2015-02-27 12:19:01

    Version 2- title updated and .dwg file updated

  14. Admin - 2015-02-26 15:27:09

    Second version- title changed to indicate window to soffit, rather than eaves.

  15. Admin - 2015-02-26 15:15:55

    First Version

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