Insulation to eaves installed internally [Version ID #277]

Author details

Original Author:Admin Organisation:

Detail summary

Internally installed insulation designed to deal with potential cold bridge at eaves - where loft or roof insulation meets cavity, or other wall, insulation. Minimum ventilation gap of 35mm provided over insulation to allow sufficient ventilation to eaves.

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.

Target U-value: 0.18 W/m2K

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.

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 \ 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.

    Tyvek \ 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


  1. |
  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 [Displayed version]

    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

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.