Roof Insulation- Existing membrane LR

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

Roof insulation to existing rafters, where the existing roofing membrane below tiles/slates is a Low Resistance (LR) membrane. [See Technical Description for definition as of BS 5250: 2011]

Insulation in plane of rafters where existing roof membrane is low resistance vapour permeable. Need to ensure adequate ventilation behind insulation.

Insulation boards and proprietary basecoat finish with vapour control render applied to inside face of insulation.

Technical description

Section Detail. Roof insulation to existing roof where existing vapour permeable membrane: low resistance underlay (LR).

Existing roof membrane classified as Low resistance (LR), if the airflow in m3/h is at a differntial of 2Pa is greater than 7.8Ar (where Ar = the effective area of the testing rig in m2); as per BD 5534: 2002 Annex L, given in BS 5250: 2011.

If the airflow is not greater than than 7.8Ar then ventilated openings must be provided in the roof void below the underlay, or a 25mm minimum ventilation gap must be provided to the ridge below the membrane: as per BS 5250: 2001.


  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 rigid insulation with render carrier board over rafters.
  5. Tape joints and ensure airtightness.
  6. Apply render to face of carrier 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.

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

  • Insulation Boards/ Batts Fitted Between Rafters [RF02b]

    Compressed woodfibre insulation batts.

    • Thermal conductivity: 0.040W/m2k.
    • GWP 0.
    • Thickness: As rafter depth.
    • Width: to suit rafter spacing.

    Back to Earth: Tel: 01392 861763.
    Product Reference: Udi-FLEX

  • Insulation boards fitted over rafters [RF02b]

    Compressed woodfibre insulation batts.

    • Thickness 100mm.
    • Fixing: Fixed back to rafters.
    • Thermal conductivity: 0.040W/m2k.
    • GWP 0.

    Back to Earth: Tel: 01392 861763.
    Product Reference: Diffutherm & (render carrier board)

  • Proprietary finish & vapour control [RF02b]

    Breathable render finish.

    • Type: hydraulic lime skimming plaster (for painting with clay/water based paints).
    • Thickness: 4mm.

    Back to Earth: Tel: 01392 861763.
    Product Reference: Udi MULTIGRUND BAUMIT KGW20

  • Intelligent air and vapour control layer [RF05b]

    Intelligent breather membrane.
    Fixing: Laps And Edges: Sealed with vapour-tight adhesive tape. At floor and ceiling junctions ensure is appropriately sealed.

    Options for intelligent membrane


  1. Admin - 2015-04-30 10:57:17 [Displayed version]

    Intelligent membrane contact details re-formatted. DWG uploaded.

  2. Admin - 2015-03-30 11:44:34

    Amendments to technical description

  3. Admin - 2015-03-30 11:37:45

    Title updated. Technical description updated with LR definition. Preview image and pdf detail file updated.

  4. Admin - 2015-03-28 18:00:40

    Parts list expanded to match drawing

  5. Admin - 2015-03-04 14:47:04

    First version

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