EWI Plinth Insulation to below ground

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

Detail for an external wall insulation plinth detail to just below ground level. Intended to ensure water-tightness and prevent thermal bridging to zones under an existing floor by providing continuous insulation down to ground level.

Woodfibre EWI to stop a minimum of 300mm from the external ground level, with XPS insulation fixed using adhesive bond to carry down to below ground level, a minimum 300mm below the internal floor level of the building.

Target Values: 0.2 W/m2K for whole construction.

Technical description

Section detail: EWI to junction with perimeter insulation. Below ground: insulation past ground level. Woodfibre EWI junction to XPS insulation at minimum 300mm above ground level.

Suggested Installation Sequence:

  1. Excavate perimeter channel
  2. Remove existing airbricks
  3. Fix woodfibre wall insulation insulation
  4. Install air vents and perimeter insulation
  5. Render
  6. Back fill perimeter channel.

Reference Specification:

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.

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.

Windows and doors: Where windows and doors are to be replaced in insulation please refer to contract/window schedules for details. Where windows and doors are to be retained in insulation please refer to relevant detail drawings to show reveal insulation and cills.

Parts list

  • EWI (Woodfibre), 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/

  • External Perimeter Floor Insulation [EW01]

    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.

  • 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.

  • 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 11:37:48 [Displayed version]

    General formatting changes. DWG file uploaded.

  2. Admin - 2015-03-27 16:26:53

    Preview image and .pdf file updated.

  3. Admin - 2015-03-23 16:52:44

    General re-formatting. Technical description expanded with installation sequence. Links to suppliers provided in parts list.

  4. Admin - 2015-03-12 17:05:35

    Intro & technical description updated and expanded. BS and Approved doc information added. Parts list added.

  5. Admin - 2015-02-27 15:20:49

    Version 1

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