Roof insulation- Existing non-permeable membrane (HR)

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

Detail for the insulation of roof rafters, internally, where the existing roofing membrane within a roof is impermeable/ High resistance (HR underlay to BS 5250: 2011); and as such: a minimum ventilation gap must be provided to the eaves in order to ventilate the roof space.

This detail demonstrates the installation of new insulation, to a depth of 200-250mm to achieve the target U-value, to the existing rafters with a 50mm ventilation gap behind to ensure adequate ventilation of the new insulation.

Target U-value: 0.18W/m2K

Technical description

Section detail. Roof insulation. depth to required U-value. Internal installation. Existing sarking non-permeable as per BS 5250: 2011, based on testing to BS 5534: 2003 Annex L; where airflow in m3/h at a differential of 2 Pa is less than, or equal to: 7.8Ar (where Ar is the effective area of the test rig in m2.)

Suggested installation sequence:

Preparation of roof and loft space: Remove any existing loose material and prepare in accordance with manufacturer's requirements.

Any alterations to cabling, existing ventilation, and other fixtures, affected by the work to be agreed with contract administrator prior to work being undertaken. Prior to start on site inspect the form and condition of the roof internally and externally, especially any structural timbers; institute repairs and/ or additional works necessary to make the substrate suitable to receive insulation.


  1. Removal of any existing light fittings, set aside for re-use. Remove any existing timber mouldings.
  2. remove existing ceiling to expose rafters.
  3. Check condition of existing roof membrane.
  4. Fit insulation batts between rafters. Fit insulated dry-lining board over.
  5. Tape joints and ensure airtightness.
  6. Plaster to face of board.
  7. Re-instate/replace and fixtures/ fittings/ mouldings as required.

Care should be taken to ensure air-tightness. Seal all service penetrations through insulation and roof. Lap and tape all membranes around edges to existing membranes or as shown on detail.

All insulation to be continuous to avoid thermal bridging. Phenolic foam eaves insulation: To BS EN 13166 or Agrément certified.

For all works to existing roofs refer to Approved document C (resistance to contaminants and moisture), with reference to Section 2, and section 6.

Approved Document F (ventilation) should be referred to with regards to ventilation to existing roofs and over eaves ventilation.

Any work should be checked that it does not affect existing routes of escape, access and facilities, internal and/or external fire spread to linings and/or structure in event of fire. Refer to: Approved document B (fire safety) for guidance for both dwellings and non-dwellings for existing buildings.

Approved Document A (structural safety) should be referenced when working with existing roof timbers or other roof structural members.

Document L1b 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.

Parts list

  • Kooltherm® K18 Insulated Plasterboard [RF02a]

    Phenolic foam backed plasterboard.

    • Thickness: 100mm with 12.5mm plasterboard thickness.
    • Thermal conductivity: 0.020W/m2k. GWP< 5.
    • Adhesive scored bonding coat to ensure no air-gap to rear of board. Joints taped and filled.
    • Integral Vapour Control Layer: ensure all joints between sheets detailed and taped to minimise vapour transmission using vapour-tight tape.

    Kingspan Insulation Ltd.
    Product Reference: Kooltherm® K18 Insulated Plasterboard

  • Insulation Boards/Batts Between Rafters [RF02a]

    • Type: Formaldehyde Free Glass mineral wool semi rigid batts friction fitted between rafters.
    • Thickness: 35mm less than rafter depth.
    • Width: to suit rafter spacing.
    • Thermal conductivity: 0.032 W/m2k. GWP 0.

    Knauf Insulation Ltd. Product Reference: DriTherm Cavity Slabs.

  • M20a Single Coat Board Finish Plaster

    Gysum finish plaster. Tape joints before plastering. 2-3mm thickness applied in single coat. smooth finish.

    Contractor's choice.

  • Softwood battens [RF06a]

    To form ventilation space below HR membrane. Manufacturer: Contractor's choice. Species: Contractor's choice. Size: to provide minimum 25mm clear ventilation space under drape of existing roof membrane.

    Contractor's choice.


  1. Admin - 2015-04-30 10:59:08 [Displayed version]

    DWG file uploaded. General re-formatting

  2. Admin - 2015-03-30 11:50:09

    Softwood battens added to parts list. Preview image and detail pdf file updated.

  3. Admin - 2015-03-30 11:48:50

    Softwood battens added to parts list.

  4. Admin - 2015-03-19 13:43:07

    Version 1- Detail added to pattern book.

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