Roof insulation to flanking wall of a room-in-roof

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

A roof insulation detail for the flanking wall of a room-in-roof or a storage space in roof, with additional loft insulation buildup as required to a minimum depth of 400mm to achieve the target U-value.

In this detail, insulation is installed to the upright stud walls between a room-in-roof and the loft space; along with insulation and membranes in plane of the existing rafters (where the existing roof membrane is non-breathable). It is important to ensure adequate ventilation over any insulation installed in the plane of the existing rafters.

In most cases, some loft insulation will already be installed (typically to a depth of between 50-200mm) in which case it is only required to add enough insulation to increase this to 400mm.

Technical description

Section detail. Loft insulation. Room in roof, insulation to existing with impermeable sarking.

Insulation to upright stud walls, in plane of rafters where existing membrane non-breathable. Where possible access stud walls from behind through existing loft access. Otherwise create new insulated loft access hatch.

Loft insulation increased to 400mm depth - recycled formaldehyde free glass fibre loft roll with breather membrane topping to reduce thermal bypass. In cases of existing insulation: inspect for damage, leave in-situ if acceptable condition and overlay with new insulation.

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, drainage 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. Check existing walls and make airtight.
  2. Access loft space and fix flexible fibre insulation between timber studs.
  3. Reinstate & Install additional new loft insulation, to flanking wall.

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 breathable membranes to loft and eaves insulation to To BS EN 1928. Durability to EN 1297/1296.

All insulation to be continuous to avoid thermal bridging. Loose laid mineral wool insulation to BS EN 13162, EN 16001 and to conform to BS EN 13501-1 for fire. Phenolic foam eaves insulation: To BS EN 13166 or Agrément certified. Installation of Mineral wool insulation to: BS EN 5803-5.

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 to flanking wall [RF08]

    Glass mineral wool semi rigid batts.

    • Thickness: 200 mm.
    • Thermal Conductivity : 0.034W/m2K.
    • Fixing: Friction fitted between studs and held in place by polyester webbing stapled between studs. (Where existing studs are <150mm, fix additional insulation bats to back face of studs to prevent cold bridging.)
    • Ensure insulation is continuous with loft insulation to prevent thermal bypass and cold bridging.

    Knauf Insulation Ltd. Product Reference: Earthwool Universal Slab

  • 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

  • Insulation Boards/Batts Between Rafters [RF02a]

    Formaldehyde Free Glass mineral wool semi rigid batts friction fitted between rafters.

    • Thickness: 35mm less than rafter depth.
    • Width to suit rafter spacing.
    • U-value: 0.032 W/m2k.
    • GWP 0.

    Knauf Insulation Ltd. Product Reference: DriTherm Cavity Slabs.

  • Loose laid loft insulation [L01a]

    Flexible formaldehyde free glass mineral wool. Should be laid continuous to eaves: level, covering all parts of the ceiling to meet any eaves/ internal wall insulation to prevent cold bridge. All service holes sealed, debris removed before laying insulation. Eaves ventilation should be unobstructed. Any electric cables overlaid by insulation should be sized accordingly. Where water cistern platforms located on ceiling joists: Insulation below omitted.

    • Recycled content: 85% (minimum) to BS EN ISO 14021.
    • Thermal Conductivity: 0.040 W/mK. GWP 0.
    • Thickness to 400mm

    Knauf Insulation Ltd. Product Reference: Knauf Earthwool loft roll 40

  • Breather Membrane/ wind-tight layer [L01a]

    Membrane, laid over top of loft insulation to prevent thermal bypass. Joints to be lapped and taped.

    Dupont Tyvek Product Reference: Tyvek Housewrap

  • 9mm ply board

    9mm ply board, fixed to underside of existing rafters as per drawing to retain insulation to flanking walls.

    Contractors choice

  • Polyester webbing to hold insulation

    Webbing drawn underneath insulation at approx 600mm centres, fixed to joists.

    Bowmer Bond: Tel: +44 (0) 1335 342244
    Fax: +44 (0) 1335 300651 (Sales)
    Fax: +44 (0) 1335 346353 (Admin)
    Product Reference: Polyester Webbing


  1. Admin - 2015-06-25 14:04:06 [Displayed version]

  2. Admin - 2015-04-30 13:16:15

    Minor title amendment - change of " wall in room..." to "wall of room..". Intro summary updated.

  3. Admin - 2015-04-30 10:49:24

    DWG and pdf file updated. General formatting changes. Polyester webbing added to parts list.

  4. Admin - 2015-03-30 11:17:58

    Preview image and pdf detail file updated.

  5. Admin - 2015-03-28 18:00:27

    RF05b amended in parts list

  6. Admin - 2015-03-17 17:16:25

    Introduction expanded. Technical description updated with BS codes and approved docs. Parts list updated and amended

  7. Admin - 2015-03-17 17:12:43

    Introduction expanded. Technical description updated with BS codes and approved docs. Parts list updated and amended

  8. Admin - 2015-03-04 13:35:17

    First version

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