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GBE Overlaying Loft Insulation (Q+A) G#38593

By 10 July 2020March 8th, 2021Code, Encyclopaedia, GBE Defects, Q&A

GBE Overlaying Loft Insulation Q+A

GBE > Encyclopaedia > Code > Question+Answer > G#38593

GBE Overlaying Loft Insulation Q+A

Overlaying attic insulation

We are installing loft insulation funded by ECO3.

The requirement is 270mm thick rolled insulation.

I have 2 questions really;

Firstly, sometimes we find existing 100mm insulation between joists that has slumped to 70mm.

If this is beneath floor boarding there will be a 30mm gap above the existing insulation.

If we lay 200mm loft roll above the boards, will the 30mm gap be a problem?


Secondly, if there is floor boarding with no insulation beneath. Is it ok to lay 270mm on top of the boarding?


GBE Question & Answers


What do we know?

  • Buildings: Existing Housing
  • Retrofit programme: ECO3 (Third phase of Energy Company Obligation?)
  • Location: Lofts (Pitched roof attics)
  • Insulation location: at flat ceiling level below pitched roof attics
  • Exiting construction: 100 mm ceiling joist depth
  • Existing insulation: between ceiling joists
  • Exiting insulation condition: assumed: was 100 mm. now shrunk to 70 mm
  • Sometimes: there is floor boarding on the ceiling joists
  • Sometimes there is no insulation below the floor boarding


  • Floor boarding is there for access walk/crawl way to services, base for service or for domestic possessions storage
  • If the ceilings were not replaced at the time of the insulation being added or added to then there is no vapour check in the ceiling other than any non-vapour permeable paint.


  • Is the ceiling lime plaster on lath?
    • In which case it is vapour permeable
  • Is the ceiling gypsum plasterboard?
    • In which case its less moisture permeable
  • Is the ceiling gypsum plasterboard with vapour resistant polyethylene/aluminium foil?
    • In which case the ceiling is vapour resistant

Historic context:

  • There have been grants in the past 40-50 years to increase attic insulation
  • My own experience was this was to:
    • add 50 mm or 100 mm insulation for the first time
    • or to increase 50 mm of insulation to 100 mm
  • There may have been other later grant schemes increasing to other depths
    • CESP, CERT, Decent Homes, etc.

Analysis of existing situation:

  • If the 100 mm was made up of 50+50 mm of insulation that has shrunk to 70 mm
    • Has the lowest older 50 mm collapsed to 20 mm
    • Or have both layers slumped from 50 to 35 mm
  • Or has 100 mm of insulation shrunk to 70 mm
  • The collapse may be caused by:
    • The weight of the second layer of insulation (unlikely)
    • The older insulation breaking down over time and collapsing
    • Brittle fibres breaking and collapsing
  • Any collapse or slump in the insulation suggest the density of the compressed/collapsed material has increased and its k value worsened as a consequence.
  • The consequence is the trapped airspaces are diminished, thus reducing the available air space to hold warm air still, thus reducing the performance of the insulation.

Cross ventilation

  • Most pitched roofs have cross ventilation from eaves to eaves to reduce risk of condensation
  • Cross ventilation if misdirected could nullify the insulation
  • Wind at the eaves can blow the heat out of insulation, so a wind baffle is sometimes used
  • Ensure any voids in the insulation build up, do not connect to the cross ventilation or insulation above those voids become redundant

Conductivity Thermal Insulation

  • Insulation works by holding air still and preventing air circulation or preventing thermal eddy currents.
  • Cavities in construction without insulation are a void towards which heat can conduct and then emit from surfaces into the voids.
  • Once in the voids the heat can rise by chimney effect, by the air moving in the cavity, the heat in the air dissipates and then the cooled air can fall in the same void, setting up eddy currents.
  • Larger cavities allow air to move freely and heat to dissipate, whilst tiny spaces within materials can hold the air still and reduce the heat loss.
  • Always lay multiple layers of insulation at 90 degrees to the previous layer, in case of gappy insulation on one layer, so that gaps in the first layer will be covered by the next layer
  • Ideally relay he first layer to close hose gaps first

Ventilation cavities

  • Ventilated cavities allow air in and out, either by chimney effect or cross ventilation, ventilating voids will allow heat out and coolth in.
  • If the void air temperature is lower than the surrounding materials then conductivity will allow heat to travel towards the colder voids, emit from the surfaces and be ventilated away.
  • Ideally an open celled or fibrous insulation should have a membrane to close the surface to stop the flow of heat out of its surface.
  • Ideally the membrane should be low emissivity and possibly heat reflecting.
  • Aluminium foil or aluminized polyethylene exhibit such properties.

Condensation Risk?

  • With a board, which may be moisture permeability resistant, over a cavity, in vapour permeable construction, there is a risk of moisture rising from the below, passing through the insulation, cooling and condensing on the underside of that board
  • In any other situation the underside of board needs to be ventilated to remove the condensation.
  • However with more insulation above the cavity and board means the insulation above the board would become redundant.


Should we keep or remove the existing insulation?

  • In my experience I have found evidence of animal (possibly mice) occupation of old mineral fibre insulation, tunnelling in the lowest layer of insulation and nesting there well in from the eaves, this provides a great route for unhindered heat to leave the building and coolth to enter the building.
  • With any deterioration of the insulation resulting in reduced thickness this reduces the performance of the insulation.
  • In both cases it would be recommended to remove the existing insulation and replace it with new.
  • In all cases the quality of the new replacement materials that go back in, in its place must be better than what came out, including workmanship.


  • Is the requirements is to install 270 mm of insulation or to top us exiting to achieve 270 mm?
  • To add good insulation on top of incompetent insulation will only result in a diminishing of the performance of the complete insulation.
  • Is there an Energy Performance requirement in the Contract?

Redundant Insulation

  • To add 270 mm of insulation on top of the boarding above an empty joist zone
  • The insulation will stop the heat going further but
  • This leaves the empty joist zone as a place for heat to escape to from the room below rather than heat the occupants
  • Being in that space rather than the occupied space below is a waste of energy, money and an increase in CO2 generated
  • Once the heat is in the empty joist zone it can circulate until it dissipates without any benefit to occupants


  • I just wanted to thank you for providing such a comprehensive response.
  • You have raised many issues for further thought but in the meantime it has cemented the following loft insulation strategy for us at Carbon Rewind.


  • Add new 170mm loft roll above the loft boarding


  • Remove loft boarding,
  • remove existing insulation.
  • Install new 100mm loft insulation between joists.
  • Add new 170mm loft roll above joists – crossways


  • Remove loft boarding.
  • Install new 100mm loft insulation between joists.
  • Add new 170mm loft roll above joists – crossways


  • Be careful of the wording
  • ‘Plain English is preferred’
  • ‘Integrity’ may not mean the same thing to all that read it
  • Integrity (slumped) is better but use (slumped) after the first time you use Integrity, or after all times you use it
  • Loss of integrity may also include: gapping, animal tunnelling, dust on its surface, condensation within, damage
  • Be explicit and invite questions when other situations arise

Q         Reusing redundant stone wool waste

  • Another challenge is to address waste created by our installations.
  • Any ideas for reusing loft roll that has lost its integrity?

A          Reusing redundant stone wool waste

  • If it has lost its integrity does not have any value in any other situation?
    • Not as a thermal insulation
  • Manufacturer that engage in remanufacture and refurbishment find they reProduct competes with their new Product and often stop offering the reProduct
  • Stone wool other applications:
    • With potential:
      • Green walls: growing/water transport media
      • Green roofs: growing/bedding media
    • Without potential:
      • Thermal insulation: no the integrity is already gone
      • Ventilation filtres: but broken loose fibres are not desirable in ventilation air
      • Fire proofing: but not at thermal insulation densities
      • Acoustic insulation: but often wrapped in acoustically transparent pillows
        • I would rather use sheep’s wool
        • See Woolley Shepherd for Sheep’s Wool Acoustic clouds

Waste Insulation:

  • Removal of incompetent insulation to make way for new competent insulation will increase waste generated.
  • In an ECO3 project the quantity of waste generated should be sufficient to warrant investigating segregation to return to insulation manufacturers for recycling
  • Manufacturers tend to put barriers in place a common one is:
    • ‘we only take our own off-cuts back, we know what’s in it, to control the recipe of new material’
    • Don’t be deterred, if you do not ask, you do not get
  • If you have many 1000 m3 of insulation this may be of interest to some
    • Check if they have a take back scheme of their own
  • Rockwool Ltd. do have a recycling process where brickettes of stone wool waste is part of the recipe’
  • Make sure the segregation is comprehensive and thorough without compromise
    • Provide a separate 2 or 4 wheel bin or m3 bag and/or skip, or compactor
    • Establish how the recycling manufacturer wants the materials packaging or how they want to receive it
  • Some courier services offer low cost or free use of spare pallet space in vehicles returning with spare capacity
    • If there is no deadline, the materials can sit around in consolidation centres until spare capacity is available going in the right general direction

Other issues

Loft Insulation Defects



© GBE GBC GBL NGS ASWS Brian Murphy aka BrianSpecMan **
13th June 2020 – 10th July 2020 Q&A emails

© GBE GBC GBL NGS ASWS Brian Murphy aka BrianSpecMan **
10th July 2020 – 8th January 2021 This page

GBE Overlaying Loft Insulation Q+A
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10th July 2020 – 8th March 2021

GBE Overlaying Loft Insulation Q+A, G#38593

GBE Question & Answer (Template) Q#14864

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