IWI to new triple/double glazed window. [Version ID #242]

Author details

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Detail summary

IWI perimeter detail to head and base of new window in an existing window position where the window sits outside the internal wall insulation. As such it is necessary to insulate the reveals to prevent a cold bridge.

Target wall U-Value: 0.3 W/m²K

Technical description

Section detail. IWI, perimeter, to window to be replaced.


1.) Remove existing cill, window and any existing insulation at head. 2.) Fit new window. 3.) Air seal tape. 4.) Fix internal insulation directly to existing internal plaster. 5.) Fix cill. 6.) Plaster to finish. 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.

All Expanded polystyrene insulation (EPS): To BS EN 13163. All fixings manufactured in accordance with BS1210. Mineral wool (MW): To BS EN 13162; reaction to fire rating to: BS EN 13501-1.

Rigid insulation: where used Phenolic foam (PF): To BS EN 13166.

All vapour control layers/ air tightness layer to durability requirements of EN 13984.

Please refer to Approved documents: B (fire safety): B1 section 1 (material alterations)/ B2 section 3 (internal fire spread), and Appendix A (material properties). Approved document F (ventilation). Document K, sections K4 and K5; and Doc L: Section 5 and 6 of L1B; including all relevant amendments.

Parts list

  • Insulated Dry-Lining Board- Jamb & head [IW05b]

    Phenolic Foam Thermal Plasterboard with integral vapour control layer (plasterboard thickness: 12mm, insulation: 17mm thickness). Tapered edge. \ U-value: 0.21W/m2k. GWP<5. \ Adhesive scored bonding coat to ensure no air-gap to rear of board. Taped and filled joints. Prepared for plaster skim coat finish.

    Kingspan Insulation Ltd \ Kooltherm® K17 Insulated Plasterboard

  • Rigid insulation below cill. [IW05b]

    Rigid insulation board. Thickness: 15-24mm (to suit window frame/ opening). \ U-value: 0.023W/m2k

    Kingspan Insulation Ltd \ Kooltherm® K3 Floorboard

  • Softwood Window Board to Sill [IW05b]

    Subject to contractor. Half-round edge. European redwood. Class 2 to BS 1186-3. Natural finish. No preservative treatment. FSC certified. Fixed with lost nail heads at 30mm centres.

    Contractor choice.

  • Internal Wall Insulation System (95mm) [IW02]

    Earthwool Eco batt insulation. Thickness 95mm\ Target u-value: 0.3W/m2k. Thermal conductivity: 0.033W/m2k/ 0.130W/mk. GWP<5. \ Fixing: In accordance with manufacturer's recommendations. Supports: EcoStuds - XPS and OSB uprights.

    Knauf \ ThermoShell Internal Wall Insulation System

  • Single-sided air seal tape [DP09a]

    Tape for junctions critical to air-tightness. 60mm (typical). \ Accesssories: ORCON F sealant and TESCON PRIMER RP (for use on rough/old surfaces to assist adhesion)

    Pro Clima \ http://www.greenbuildingstore.co.uk/page--pro-clima-airtightness-tapes-options.html \ TESCON VANA/ TESCON PROFIL (2 release tapes)

  • Vapour Control Layer

    Intelligent breather membrane. \ Fixing: Laps And Edges: Sealed with vapour-tight adhesive tape. At junctions with party/patition walls, lap under insulated plasterboard. At floor and ceiling junctions ensure is appropriately sealed.

    Pro-clima \ Intello membrane

  • New Timber frame, triple-glazed windows [GD01]

    Eco-Contract Ultra Outward Opening Casement Windows.\ Thermal Performance (overall u-value) : 0.79W/m2K.\ Air-tight Size: As schematic.\ Opening Pattern/ Configuration : As existing (unless otherwise stated on schematic).\ 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.

    Manufacturer: Green Building Store. \ www.greenbuildingstore.co.uk \ Tel: +44 (0)1484 461705. \ Reference: Eco-Contract Ultra Outward Opening Casement Windows

  • EcoStud insulated supports IWI system [IW02]

    Vertical supports for IWI batts. Thermally engineered composite of high performance extruded polystyrene bonded to Oriented Strand Board 3 (OSB 3)supports to IWI sheets. Expanded polystyrene insulation (EPS): To BS EN 13163.

    Knauf Insulation. \ http://www.encon.co.uk/sites/default/files/products/downloads/datasheet-ecostud.pdf \ Reference: EcoStud for Solid walls. (Used in conjunction with Earthwool EcoBatt.)

  • M20A Single layer plasterboard & backings [IW02]

    Wallboard, 15mm thickness. Nail fixed as IWI system manufacturers recommendations. Knauf multi-purpose sealant/ taped joints. Beads and stops for plaster in Stainless steel on all external angles and stop ends.

    Contractors choice OR Knauf Moistureshield/ Wallboard/ Denseshield.

  • M20A two coat board finish plaster [IW02]

    Applied to whole system including insulated reveal boards. \ Two coat (undercoat plus finish) plaster system. \ Gypsum bonding plaster undercoat to manufacturers recommendations. Final coat: finish plaster, 2-3mm applied one-coat, smooth.

    Contractor’s choice.


  1. Admin - 2015-04-30 09:57:30

    DWG and pdf file updated.

  2. Admin - 2015-04-30 09:54:26

    DWG file uploaded. general formatting amendments.

  3. Admin - 2015-03-30 12:06:52

    Preview image and detail pdf updated.

  4. Admin - 2015-03-29 11:06:25

    IW05b "insulated dry-lining board" replaced with "laminate plasterboard

  5. Admin - 2015-03-29 10:58:22

    Air-tight sealant added to parts list

  6. Admin - 2015-03-26 22:50:34

    General error removal. Parts list codes updated.

  7. Admin - 2015-03-26 22:28:06

    General re-formatting. Introduction and technical description revised and updated with approved document links, notes and BS codes. Parts list updated with suppliers information.

  8. Admin - 2015-03-16 16:15:50 [Displayed version]

    Version 3. Technical description updated to remove errors in installation sequence.

  9. Admin - 2015-03-16 16:09:34

    Version 2- Introduction updated. technical description expanded. Parts list updated codes.

  10. Admin - 2015-03-03 17:05:12

    First version

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