EWI perimeter+sub-floor insulation with controlled ventilation

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

Detail for insulation to a ground floor and sub-floor, where the existing internal ground floor void is inaccessible, or it is not possible to lift the whole floor.

To allow the existing ground floor finish to be retained: a granular insulation infill is proposed to prevent heat loss through the floor, with perimeter insulation to footings, used to to prevent cold bridging through the external wall, and the ingress of damp and mould.

The sub-floor ventilation is used to ensure a reduced risk of condensation and humidity build up within the void, using a small humidity controlled fan. (This system can also be modified with temperature and humidity monitoring systems below the floor; as well as additional personal controls for the ventilation fan.)

Technical description

Section detail. EWI to ground level detail. ventilated. Insulation past ground down to footings. Woodfibre external wall insulation junction with 400 - 200mm external perimeter insulation suitable for below ground use + drainage layer. Sub-floor insulation using lightweight aggregate insulation. Controlled ventilation inlet to below sealed timber ground floor.

Suggested Installation Sequence:

  1. Inspect sub-floor and timbers for signs of damp and rot. sub-floor of all detritous - especially timber and organic materials. Remove any plastic ground coverings. Note positions of gas pipes, wiring and other services. Where necessary fix services to underside of joists for future access/ safety.
  2. Carefully create an access hatch for approximately each 4sqm of floor (approximately the distance between the joists x 2-3 floorboards) in such a way that these can be put back afterwards; sufficient to gain access to underfloor void as needed and/or use existing floor access hatches. (Denail floorboards and set aside for reuse on site & remove projecting nails from joists after board removal.)
  3. Hatch: Fix pieces of floorboard together to form hatch. Edge trim as necessary to underside to create air-tight seal. Where floorboards exposed, ensure finish is high quality and of sound appearance.
  4. Excavate perimeter channel and remove existing air-bricks.
  5. Fix woodfibre wall insulation insulation. Install air vents, sleeve and perimeter insulation. Tape all joints between wall and vent
  6. Backfill perimeter channel to underside of perimeter insulation.
  7. Render external wall insulation to granular infill below external ground level.
  8. Pour lightweight aggregate insulation in through the hatches and rake across the floor from above to achieve an even spread -taking care not to damage underfloor gas pipes and services.

Reference Specification:

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.

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.

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

  • Ventilation Fan Unit Wall Mounted [FL02]

    Small humidity controlled fan to assist in sub-floor ventilation and avoid excess humidity in floor void.

    • Manufacturer: Contractor's Choice.
    • Type: Fan only - both extract/ intake.
    • Antibackdraught Shutter: Not required.
    • Adjustable Integral Overrun Timer: Not required.
    • Performance: To achieve 0.5 ach (air changes p/hour) in floor void in winter conditions.
    • Accessories: External controller and wall fixing kit.
    • Position: TBC on site.

    Contractor's choice

  • Ventilation Fan Controller Automatic Operation [FL02]

    • Manufacturer: As ventilation fan units.
    • Type: Surface mounted single fan controller.
    • Controls: Humidity sensor located in floor void.

    Contractor's choice

  • Controlled Air-Inlet Vents [FL02]

    • Manufacturer: As ventilation fan units.
    • Type: Automatically opening inlet vents, linked to humidity control and extract fan control.
    • Number: To achieve 0.5ach in floor void when open.

    Contractor's choice

  • Lightweight Aggregate Insulation to Sub-floor (Granular) [FL02]

    Lightweight recycled glass aggregate.

    • Thermal conductivity: 0.08W/mk
    • Depth: As perimeter insulation. (min 200mm)
    • Width: 200mm.
    • Installation: As technical specification.

    Supply options available for recycled glass aggregate:

  • 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.

  • 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.

  • Air tightness works - To Floor [DP04]&[DP02]

    Draught-proof and seal all timber floors to ensure no air-leaks.

    • Location: All timber ground floors.
    • Floor: Draughtex neoprene strips or timber fillet/ mastic seal between skirting and floor.
    • Skirtings and Wall junctions: Seal all junctions and repair any cracks in plaster to ensure no air-leaks using standard decorators caulk.
  • Granular Backfill [EW01]

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

    Contractor's choice

  • Concrete Flag Paving over drainage layer [FL03]

    • Manufacturer: Contractors Choice.
    • Size: to cover drainage layer.
    • Finish: smooth
    • Colour: Natural
    • NBond: Half lip staggered.
    • Jointing: N/A

    Contractor's choice. Use reclaimed paving where possible.


  1. Admin - 2015-04-29 11:55:34 [Displayed version]

    EW01 and DP02 and DP04 parts list contacts updated.DWG uploaded.

  2. Admin - 2015-03-27 16:31:19

    Preview image and .pdf file updated.

  3. Admin - 2015-03-27 12:48:14

    Parts list formatting- codes and nomenclature changed so concurrent with drawing.

  4. Admin - 2015-03-24 10:43:56

    General re-formatting. Introduction expanded and revised. technical description updated with approved documents and additional notes. Parts list updated with supplier and additional parts: [FL03] granular backfill and paving slab.

  5. Admin - 2015-03-13 10:14:34

    Introduction, technical description expanded. BS and Approved doc details added. Parts list added.

  6. Admin - 2015-02-27 16:05:46

    Version 1

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