Roof Insulation to eaves

Author details

Original Author:Admin Organisation:Retrofit Pattern Book

Detail summary

Detail to show the buildup of new loft insulation to the underside of an existing eaves, installed internally, to allow ventilation of the existing roof over the new insulation. A minimum ventilation gap of 25mm is to be provided over the loft insulation to allow sufficient ventilation at the eaves.

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

Target U-value:0.1W/(m².K)

Technical description

Section detail. Roof & Loft insulation. To eaves. Installed internally at junction with wall insulation.

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. Roll back existing loft insulation to reveal junction between ceiling and roof.
  2. Remove any debris.
  3. Check condition of existing roofing membrane.
  4. Ensure gap between rafters is clear to either eaves board or 200mm beyond outer edge of wall (where no eaves board), whichever is the smaller.
  5. Glue battens to upper surface of the cut insulation boards 100mm in from each edge; on leading edge of the slab taper batten thickness to minimise risk of obstruction on installation.
  6. Once glue has set, push slabs into the gap over the top of the sole plate up to the eaves board or 200mm past whichever is the smaller.
  7. Reinstate/ Install loft insulation, to back of eaves insulation

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.

All insulation to be continuous to avoid thermal bridging.

Loose laid loft insulation should be laid: level, covering all parts of the ceiling; with all service holes sealed, and 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: any Insulation below is to be omitted at risk of moisture damage.

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

  • Loose laid loft insulation [L01a]

    Flexible formaldehyde free glass mineral wool. Insulation cover should be even and continuous, right to the eaves. Ensure meets any eaves insulation and/or internal wall insulation to prevent cold bridge at eaves without blocking any ventilation paths required to loft space/ roof space.

    • 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

  • Insulation to Eaves (rigid) [RF06b]

    • Phenolic foam.
    • Size: 50mm thickness, width cut to suit rafters.
    • Length: 750mm PLUS distance to eaves board or 200mm (whichever smallest).
    • Thermal conductivity: 0.023W/m2k.

    Contractor's choice

  • New battens - to maintain air path [RF06b]

    • Size: depth to provide min 25mm ventilation path (TBC on site by contractor)
    • Grading: Fully factory pre-graded.
    • Treatment: contractors choice.
    • Fasteners: glued
    • Certification: All timber to be FSC certified (or re-used from existing on site).

    Contractor's choice

  • Insect Mesh [EW07]

    If existing soffit not present, or not ventilated. Screen installed behind existing ventilated soffit board.

    • Type: Stainless steel insect mesh
    • Position: secured behind verge board to close cavity behind.
    • Fixing: Stainless steel staples.

    Contractor's choice.


  1. Admin - 2015-06-25 13:50:14 [Displayed version]

  2. |
  3. Admin - 2015-04-30 13:12:23

    Title amended. Intro summary description amended.

  4. Admin - 2015-04-30 11:30:09

    Amendments to intro summary for clarity.

  5. Admin - 2015-04-30 10:37:27

    DWG and pdf file updated. General formatting amendments.

  6. Admin - 2015-04-30 10:36:33

    DWG and pdf file updated. General formatting amendments.

  7. Admin - 2015-03-30 11:16:21

    Preview image and pdf detail file updated.

  8. Admin - 2015-03-28 17:27:45

    Insect mesh added to parts list

  9. Admin - 2015-03-17 17:12:15

    Approved documents and links added to Technical description.

  10. Admin - 2015-03-17 16:11:24

    Version 4- Parts list codes amended to remove errors.

  11. Admin - 2015-03-17 16:08:28

    Title amended to match drawing. Introduction amended with u-values. Technical description content updated and expanded; format altered. Parts list expanded with codes, and format amended.

  12. Admin - 2015-03-04 13:41:07

    Second version: title updated

  13. Admin - 2015-03-04 12:57:02

    First version

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