Roof insulation at rafters

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

Detail for insulation installed to the ceiling of a room in roof, to replace existing thin insulation.To increase thermal efficiency and prevent heat loss through the roof of the building. Insulation is shown in-plane of rafters where existing roof membrane is non-breathable. Ensure adequate ventilation between insulation and roofing membrane- minimum: 25mm or as shown on detail.

Target U-value: 0.18W/m2K

Technical description

Section detail. Roof insulation. Insulation to existing room, replace ceiling thin.

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. ensure existing ceiling in decent condition and existence of clear vent route behind. Check condition of existing roof membrane.
  3. Fit flexible insulation between rafers.
  4. Fix rigid insulating dry lining board over.
  5. Ensure air and vapour barrier airtightness. Plaster to face of board.
  6. 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

  • Insulation Boards/ Batts Fitted 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.\ Reference: DriTherm Cavity Slabs.

  • 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.
    Reference: Kooltherm® K18 Insulated Plasterboard

  • M20a Two Coat Board Finish Plaster [RF02a]

    Two coat (undercoat+finish) plaster system.

    • Undercoat: Gypsum bonding plaster. 5mm thickness
    • Surface preparation: Tape joints
    • Final Coat: Gypsum finish plaster 2-3mm applied in single coat.
    • Finish: Smooth.

    Contractors choice


  1. Admin - 2015-04-30 10:51:46 [Displayed version]

    DWG uploaded. General re-formatting.

  2. Admin - 2015-03-30 11:19:06

    Preview image and pdf detail file updated.

  3. Admin - 2015-03-18 10:29:04

    Version 3- Parts list html. text updated to remove errors.

  4. Admin - 2015-03-18 10:04:15

    Version 2- Introduction expanded with U-value targets. Technical description expanded with BS and approved doc links. Suggested sequence and reference spec. expanded. Parts list updated with codes and links.

  5. Admin - 2015-03-04 13:49:00

    First version

Leave a comment


  1. For those who have seen the Earth from space, and for the hundreds and perhaps thousands more who will, the experience most certainly changes your perspective. The things that we share in our world are far more valuable than those which divide us.

    User name

Leave a comment


A total of 14 users have left feedback about this detail.

Share your thoughts about this detail with other users:

Feedback summary

  • 80% of users found this detail easy to implement.
  • 50% of users found it easy to source skills for this detail.
  • 60% of users found this detail hard to understand.
  • 100% of users found the overall cost for this detail to be low.
  • 90% of users found materials for this detail easy to source.