EWI to just below ground with plinth and ventilation detail

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

Detail for external wall insulation to below ground floor level, with a periscopic vent and air-brick installed through EWI to ventilate a basement or cellar. Required where existing air-bricks to a sub-floor space/void would be blocked by the installation of External or Internal wall insulation. Periscopic vents should ensure continued clear air-path to sub-floor, to ensure adequate ventilation.

Technical description

Section detail. EWI to ground level detail. Insulation just below ground level. Junction of Woodfibre EWI system to 400-200mm external perimeter insulation suitable for below ground use + drainage layer. Periscopic air ventilation to below ground under suspended timber floor. Insulation to suspended ground floor to depth of rafters.

Suggested Insulation Sequence:

  1. Excavate perimeter channel.
  2. Remove existing airbricks.
  3. Fix woodfibre wall insulation insulation.
  4. Install air vents and perimeter insulation. Render EWI system.
  5. Backfill perimeter trench with granular infill.
  6. Insulate internal timber floor (not concurrent with EWI installation).

Reference Specification

Air bricks in external walling: To BS 493, Class 1.

Perimeter insulation suitable for below ground use and drainage layer. To BS EN 13164.

Drainage below ground guidance can be found in Approved documents C, section 3; and Document H, sections 2 and 3 for rainwater drainage.

Drainage layer: Granular infill free from excessive dust, well graded, all pieces less than 75mm in any direction, minimum 10% fines value of 50 kN when tested in a soaked condition to BS 812-111. In one layer no more than one of the following: Crushed rock (other than argillaceous rock) or quarry waste with not more binding material than is required to help hold the stone together/ Crushed concrete, crushed brick or tile, free from plaster, timber and metal. / Crushed non-expansive slag / Gravel or hoggin with not more clay content than is required to bind the material together, and with no large lumps of clay. /Well-burned non-plastic colliery shale. /Natural gravel. /Natural sand.


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 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.

Existing Services/openings

Gas Meters and Inlets: Gas meters and inlets to be left in-situ. Insulation to be fitted round them as closely as possible, whilst ensuring they are still properly ventilated. Care should be taken when excavating close to gas mains.

Parts list

  • Air Bricks And Wall Ventilators [FL011c]

    Plastics air brick. Cranked sleeve to suit and wall ventilator. Re-use existing air-bricks where possible.

    • Manufacturer: Contractor's Choice.
    • Colour: to match existing finishes
    • Work Size: As existing brick/blockwork.
    • Outer Face Position: Flush with wall face.
    • Spacing: Maximum 2m centres to provide 1500mm2 of clear opening per 1m run of external wall; unless otherwise agreed.

    Contractor's choice.

  • 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
    Web: http://www.backtoearth.co.uk/

  • Woodfibre EWI adhesive [EW01]

    Adhesive for use at joint between perimeter/ plinth insulation and external wall insulation.

    Back to Earth. Tel: 01392 861763
    Web: http://www.backtoearth.co.uk/
    Product Reference: UdiSpecial Adhesive.

  • Woodfibre Insulation Between Floor Joists [FL01]

    Woodfibre Insulation.

    • Thickness 200mm.
    • Friction fit between joists to ensure air-tightness- cut batts approx 20mm wider than opening between joists.
    • Supported: Polyester webbing drawn underneath insulation at approx 600mm centres, fixed to joists.

    Unger-Diffutherm. Product Reference:Reference: UdiFLEX
    Supplied by: Back To Earth.
    Tel: 01392 861763
    Web: http://www.backtoearth.co.uk/

  • External Perimeter Floor Insulation [FL03]

    Extruded Polystyrene (XPS) suitable for below ground conditions and for application of render.

    • Thickness: as EWI.
    • Depth: Minimum 600mm below internal floor level.
    • Thermal conductivity: 0.033W/m2k. GWP: <5.
    • Joints: Tight butt joints

    BASF. Product Reference: Styrodur 2800C insulation.

  • Single-sided air seal tape [DP09a]

    Tape for junctions critical to air-tightness. See drawing for locations.

    • Manufacturer: Pro-Clima
    • Size: 60mm
    • Accessories: ORCON F sealant and TESCON PRIMER RP (for use on rough/old surfaces to assist adhesion)

    Green Building Store. Tel: 01484 461705.
    Email: info@greenbuilding store.
    Web: http://www.greenbuildingstore.co.uk
    Product Reference: TESCON VANA/ TESCON PROFIL (single sided tape)


  1. Admin - 2015-04-29 11:44:20 [Displayed version]

    PArts list updated with Back to Earth contact details. DWG file uploaded.

  2. Admin - 2015-04-29 11:43:52

    PArts list updated with Back to Earth contact details. DWG file uploaded.

  3. Admin - 2015-03-27 16:29:06

    Preview image and .pdf file updated.

  4. Admin - 2015-03-27 13:03:39

    FL03 updated in parts list

  5. Admin - 2015-03-24 10:01:09

    Single-sided air seal tape added to parts list.

  6. Admin - 2015-03-23 17:23:45

    General re-formatting. Technical description updated with installation experience. Part list updated.

  7. Admin - 2015-03-13 09:45:43

    Parts list updated.

  8. Admin - 2015-03-12 17:25:03

    Parts list code updated.

  9. Admin - 2015-03-12 17:23:58

    Version 2; Introduction, technical summary updated. Parts list added.

  10. Admin - 2015-02-27 15:43:29

    Version 1

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