EWI with Recessed Plinth and Periscopic Ventilation

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

Variation on an external wall insulation detail for periscopic ventilation to a below ground cellar; where the building's internal floor can be insulated to reduce cold bridging through the external wall and below ground. For this reason the EWI to the external wall need not extend below ground level.

A recessed plinth is to be formed with 180mm perimeter insulation below where the 200mm thick woodfibre EWI stops with a minimum of 300mm above ground level; in order to protect the woodfibre insulation from damp and water ingress. A 20mm step provides a feature course as the EWI carries around the perimeter of the building.

A ventilation air brick and sleeve is required where existing air-bricks to sub-floor void will be blocked by floor insulation works. Periscopic vent should ensure continued clear air-path to sub-floor, to ensure adequate ventilation. Insulation to suspended timber floor.

Technical description

Section detail. EWI to ground level detail, with insulation to ground level ground. Junction of woodfibre to below EWI: 400 - 200mm external perimeter insulation suitable for below ground use + drainage layer. Persicopic ventilation to ventilate below ground cellar/basemen for use with internal suspended timber floor.

Suggested Installation Sequence:

Preparation of external wall: Remove any existing loose material and prepare in accordance with manufacturer's requirements.

  1. Excavate perimeter channel
  2. Remove existing air-bricks
  3. Fix woodfibre wall insulation insulation.
  4. Install air vents and perimeter insulation.
  5. Backfill perimeter channel to underside of perimeter insulation.
  6. Render.
  7. Fit internal floor insulation (see relevant detail drawings).

Reference Specification:

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

Perimeter insulation suitable for below ground use and drainage layer. To BS EN 13164-2012.

Air bricks in external walling: To BS 493-1995, Class 1. Gratings/ Ventilators in internal walling to BS 493, Class 2.

Pipe sleeves:

  • Extend through full thickness of wall or floor, position accurately.
  • Clearance around pipes: generally should be 20 mm (maximum) or diameter of service, whichever is the lesser.

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 812-111-1990. 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.

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.

Services and existing 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.

Soil Pipes: An assessment of the condition of soil pipes should be undertaken prior to external wall insulation being undertaken. Assume soil pipes will remain in situ unless agreed in by contract administrator. However in some cases pipes may need to be replaced/ moved. Any changes to existing pipework to be agreed in writing by the contract administrator before work starts.

Rainwater Pipes and Waste Water Drainage: Rainwater pipes and waste water pipes will be moved to the outer face of insulation as standard, and swan-necked back into gulleys where possible. Any work additional to this should be agreed in writing with the contract administrator.

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.

  • Granular Backfill [EW01]

    • Material: 20 mm washed and graded aggregate providing 30% voids.
    • Minimum Thickness: As drawing.

    Contractor's choice


  1. Admin - 2015-04-29 11:50:29 [Displayed version]

    New contact details added for EW01 and FL03. General formatting changes. DWG uploaded.

  2. Admin - 2015-03-30 12:32:57

    Title amended.

  3. Admin - 2015-03-27 16:30:09

    Preview image and .pdf file updated.

  4. Admin - 2015-03-24 10:48:48

    General re-formatting. Introduction expanded. Technical description updated with additional notes, approved document links and installation sequence. Parts list updated with suppliers codes. Double sided tape added to parts list.

  5. Admin - 2015-03-13 09:46:53

    Introduction, technical description, revised and expanded. Parts list updated.

  6. Admin - 2015-02-27 15:54:44

    Version 3- Title updated

  7. Admin - 2015-02-27 15:53:31

    Title updated

  8. Admin - 2015-02-27 15:52:57

    Version 1

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