IWI to new triple/double glazed window (inward opening) [Version ID #243]

Author details

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Detail summary

IWI detail for a replacement triple glazed window and associated perimeter detail with IWI system.

Where windows sit outside the internal wall insulation, it is necessary to insulate the reveals to prevent a cold bridge. This detail uses an insulated reveal board to bring the new window into the thermal envelope of the building.

Target wall U-Value: 0.3 W/m²K

Technical description

Section detail. IWI to perimeter. New window to be replaced with inward opening variant.


Before installation: Remove all timber fixtures: Skirting, Picture Rails, Dado Rails etc. Remove curtain rails. Remove electrical socket/ telephone/cable socket outlets. Re-fix/renew all removed once insulation complete.

1.) Remove existing window. 2.) Install new window. 3.) install insulation to reveal. 4.) Install internal wall insulation. 5.) Plaster to internal wall insulation and reveal.

Before starting work undertake study of form and condition of existing walls and likely resistance to penetrating damp; log requirement of any repairs and/ or additional works to both internal and external condition necessary before installation of new system -e.g. re-pointing or repairs to external render. Internally: Before starting work carry out study of form and condition of the existing walls; draw up schedule of fixtures, finishes and fittings to be removed/relocated in order to install the internal wall insulation system. Attention to be paid to mechanical and electrical services (e.g. radiators, light switches). Refer to detail drawings for treatment of potential cold bridges (e.g. window reveals). All timbers: Before starting work carry out inspection of any structural timbers which enter the wall to be insulated (e.g. floor joists). Report any signs of damp or rot to contract administrator immediately. Record current condition of timbers and moisture content for future reference.

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

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). Doc L: Section 5 and 6 of L1B; including all relevant amendments. Ensure existing joists meet requirements in Approved document A (structural) before starting works.

Cills and framing: Remove existing internal cills and any additional internal framing to window. Scabble surface to perforate paint (create lots of small holes with appropriate tool) and remove all wall-paper etc. Re-fix/renew all removed once insulation complete. Plaster: Where existing plaster is gypsum-based, or where space is required, remove existing plaster finish.

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 Cill [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. All timber to be FSC certified (or re-used from existing on site)

  • 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 [IW02]

    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

  • 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.)

  • 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 : inward opening (unless otherwise stated on drawing).\ 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

  • 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 10:02:18

    General re-formatting. DWG file uploaded

  2. Admin - 2015-03-30 12:00:57

    Preview image and detail pdf updated.

  3. Admin - 2015-03-29 11:06:21

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

  4. Admin - 2015-03-26 22:50:47

    Parts list codes updated.

  5. Admin - 2015-03-26 22:36:34

    General re-formatting. introduction and technical description updated with BS codes and approved docs. Parts list updated with suppliers links.

  6. Admin - 2015-03-16 16:24:30 [Displayed version]

    Introduction updated. Technical description added, and expanded with BS and Approved docs. Parts list updated with relevant codes.

  7. Admin - 2015-03-03 17:14:52

    First Version

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