Sub floor+Perimeter insulation, vented-shallow suspended floor.

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

Detail for insulation of a suspended timber floor, where the existing floor void is not accessible from below, and it is not possible to lift the whole floor to insulate from above.

This solution proposes a aggregate insulation infill laid over the sub-floor, with a controlled air inlet for ventilation. This ventilation below the floor allows the air in the internal floor void to be controlled, ensuring reduced risk of condensation and humidity build up within the sub-floor.

In order to mitigate thermal bridging through the external wall, external perimeter insulation is provided to the depth of the building footings/ or 600mm below ground level (as appropriate).

Target floor U-value: 0.45w/m2K

Technical description

Section detail. Sub-floor insulation with stand-alone perimeter insulation (exterior wall) to suspended timber floor. Including ventilation of sub-floor with air inlet (controlled).

Preparation before works

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.

Inspect any structural timbers, particularly those which enter external walls (e.g. floor joists); Inspect sub-floor, and timbers within for signs of damp and rot. Report immediately if any found. If possible record current condition of timbers and moisture content for future reference. Remove any organic matter from sub-floor and floor void. Ensure all existing air-bricks are unobstructed with clear ventilation path

Suggested installation sequence:

  1. 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. (De-nail floorboards and set aside for reuse on site & remove projecting nails from joists after board removal.)
  2. 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.
  3. Excavate externally to footings/ or 600mm.
  4. Adjust services as required.
  5. Fix perimeter insulation to external wall face with cill and air inlet vents.
  6. Render outer face of insulation above ground level.
  7. Fix cill.
  8. Backfill to create drainage layer.

Reference Specification:

Please refer to Document A (structural safety) as reference for the condition of existing floor joists or members. Ensure existing joists meet requirements in Approved document A (structural) before starting works.

Refer to approved document C (resistance to contaminants and moisture) with reference to below ground works. Refer to approved document H (drainage and waste disposal), section H3, rainwater drainage in addition.

Document F(ventilation) should be referred to with reference to sub-floor ventilation guidance.

Documents L1b of Part L (conservation of fuel and power) should be referred when working on existing dwellings, with particular attention to sections 5 and 6 regarding material alterations. Document L2b should be referred to with existing buildings other than dwellings.

Extruded polystyrene perimeter insulation, suitable for below ground use and drainage, layer; to BS EN 13164.

  • 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.
  • Generally: clearance around pipes: 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. 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.

Parts list

  • 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:

  • 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

  • Sealing to: Floorboards, Skirtings, Wall/Floor Junctions [DP04]

    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.
  • External Perimeter Floor Insulation [FL04]

    Extruded Polystyrene (XPS) to BS EN 13164. 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.

  • Sill To Perimeter Insulation [FL04]

    Self-finished aluminium to suit 200mm insulation.

    • Finish as delivered: Smooth finish.
    • Colour: to suit render finish.

    Contractor's choice

  • Proprietary Render For EWI [FL04]

    To match render to external insulation if elsewhere on property.

    As EWI insulation system manufacturer.

  • Single-sided air seal tape [DP09a]

    Tape for junctions critical to air-tightness.

    • Location: see detail drawings - generally critical junctions for air-tightness. Size: 60mm (Single sided)
    • Accessories: ORCON F sealant and TESCON PRIMER RP (for use on rough/old surfaces to assist adhesion)

    Back to Earth: Tel: 01392 861763.
    Product reference: TESCON VANA/ TESCON PROFIL:single sided tapes

  • Drainage Layer [FL04]

    Lightweight aggregate insulation drainage layer; as internal underfloor insulation.

    • Thermal conductivity: 0.08W/mK
    • Depth: As perimeter insulation.
    • Width: 200mm

    Supply options available for recycled glass aggregate:

  • Optional concrete flag paving [FL04]

    • Manufacturer: Contractors choice.
    • Type: Use reclaimed paving where possible.
    • Colour: Natural
    • Finish: Smooth
    • Size: To cover drainage layer
    • Jointing: N/A

    Contractor's choice. Use reclaimed paving where possible.


  1. Admin - 2015-04-30 13:34:20 [Displayed version]

    General error correction in intro/summary

  2. Admin - 2015-04-30 11:22:24

    General formatting changes to parts list contact details, DWG file uploaded.

  3. Admin - 2015-04-22 14:04:33

    Pdf detail file revised to most up to date edition.

  4. Admin - 2015-03-30 11:08:36

    Preview image and pdf detail file updated.

  5. Admin - 2015-03-30 10:47:30

    Drainage layer and optional flag paving added to parts list.

  6. Admin - 2015-03-26 13:58:35

    Change to XPS on parts list

  7. Admin - 2015-03-26 12:50:19

    General re-formatting. introduction revised and technical description updated. Parts list amended with suppliers links.

  8. Admin - 2015-03-24 10:22:40

    Title amended.

  9. Admin - 2015-03-17 11:41:50

    Title updated to match drawing. Introduction updated with additional data and target U-values. Technical description revised and updated with relevant BS data and Approved doc guidance. Parts list updated with codes, expanded with additional parts.

  10. Admin - 2015-03-04 16:57:39

    First version

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