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Retrofit External Wall Options (Q+A) G#40938

GBE Q+A Retrofit external wall options 4P A01BRM090723

Retrofit External Wall Options Q+A

GBE > Encyclopaedia > Code > Question+Answer > G#40938

Retrofit External Wall Options Q+A
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Retrofit external wall options

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Question Q1

Getting a bit more into the detail for a typical refurbishment of our typical building stock and adding an extension, what would your ideal wall build-ups for the following be?

Q1.1    existing cavity wall

Context C1.1   

  • See HTT in Context 1.2
  • There is probably 50 mm cavity with full fill stone wool, expanded foam plastic boards or injected rock or plastic insulation
  • 100 mm cavity with 50 mm partial fill stone wool or expanded foam plastic boards
  • Or wider and thicker materials if more recently constructed
  • Make sure all causes of damp is cured and walls are allowed time to dry out
  • Vapour open constructions will tend to dry out building fabric over the life of the building
    • The dryer the building the lower the heating costs will be
  • Attend to all perimeter details and other interface installations to avoid thermal bridges in EWI
    • Thresholds of wall need water tolerant components
    • Roof abutments with walls need thermal break and weatherproofing
    • Rain water gutter and down pipes need insulation behind or they need to be repositioned
    • Soil waste and vent pipes need insulation behind or they need to be repositioned
    • Washing lines, Satellite dishes, Gate posts, post boxes, etc. need to be secured competently
  • Attend to all perimeter details and other interface installations to avoid thermal bridges in IWI
    • Internal floors abutment at external walls needs a thermal break inserted for the floor thickness
    • Internal electrical boxes need a thermal break sleeve

Answer A1.1   

I believe existing cavity walls are HTT and possibly the hardest to treat well.

  • Options to consider:
  • Keep any existing full fill cavity wall insulation and add EWI External Wall Insulation or IWI Internal wall insulation or both
    • But do a condensation analysis before choosing which option, which materials, are used, where and their thicknesses
      • Consider doing a WUFI WÄRME UND FEUCHTE INSTATIONÄR condensation check
      • A modeling tool for calculating hydrothermal moisture movement performance in structures over time
      • Suppliers of hygroscopic thermal insulation will offer these calculations with sales
    • In a vapour closed construction is adopted:
      • The BS 5250 1:5 rule should be adopted:
      • Internal construction should be 5 times more resistant to moisture vapour than external construction
      • Moisture vapour should be resisted from entering the walls from inside and can move outwards more readily
      • This would suggest you consider:
        • IWI: An expanded foam plastic insulation
        • Possibly as an internal drylining composite with gypsum plasterboard with vapour barrier
        • EWI: A vapour open insulation and render
        • Possibly using wood fibre without battening and fixing cavity and with lime render finish
      • If a vapour open construction is adopted,
        • It is potentially compromised by the existing cavity wall insulation, rock or plastic
        • Moisture vapour can readily move inwards or outwards depending upon conditions
        • This would suggest you consider:
          • IWI: A vapour open insulation and clay or lime plaster or clay board and clay finish
          • Possibly using wood fibre rigid board insulation without battening and fixing cavity
          • With proprietary mineral, silica, clay or lime plaster finish internally
          • EWI: A vapour open insulation and render
          • Possibly using wood fibre rigid board insulation with soft compressible backing to mould to the inside surface of the outer wall
          • With proprietary mineral, silica or lime plaster finish
        • Keeping any existing partial fill cavity wall insulation will mean
          • Any additional EWI will be ineffective outside of the cavity
          • Any additional IWI will be effective
        • Replace existing cavity wall insulation with another material
          • If the existing insulation is stone wool replacing it with an injected plastic with better k value
          • If the existing insulation is injected foam plastic it may have been an ozone depleting plastic
          • To empty the cavity requires a tool to be inserted into the cavity to shred the plastic to powder releasing the remainder of the ozone depleting gasses and then the powder vacuumed out
          • This is definitely not recommended, putting our ozone layer as more risk
          • There are specialists who have the kit and know-how to do this
          • I doubt they can be effective at vacuuming out, capturing and safely disposing or destroying the gasses

Q1.2    existing solid brick wall

Context 1.2

Somebody in UK Hierarchy describes solid walls as Hard To Treat (HTT).

  • Due in part to ornamental details that will complicate the addition of insulation either ‘crumble’ outside or ‘frosting’ inside
    • These ornamental ‘crumble’ details are at eaves, plinth, stringcourses, around openings, etc.
    • These ornamental ‘frosting’ details are at cornice, dado, around openings, perimeter, at beams and centre of ceilings, around chandelier hooks or lights
  • Due in part to thermal bridging caused by reduction of the insulation thickness fitting over the crumble and frosting details
  • Due in part due to solid masonry (brick or stone) walls being moisture permeable and more challenging to insulate competently without causing failures, including: embedded timbers becoming vulnerable to rot and masonry becoming vulnerable to frost damage
  • In reality if you choose appropriate materials wisely and address the details well, they are not too hard to treat well

Answer A1.2 Options to consider:

  • Keep existing masonry wall and add EWI External Wall Insulation or IWI Internal wall insulation or both
    • But do a condensation analysis before choosing which option, which materials are used, where and their thicknesses
      • Consider doing a WUFI WÄRME UND FEUCHTE INSTATIONÄR condensation check
      • A modeling tool for calculating hydrothermal moisture movement performance in structures accelerated over time
      • Suppliers of hygroscopic thermal insulation will offer these calculations with sales
    • In a vapour closed construction is adopted: expect failures as noted above
    • If a vapour open construction is adopted,
      • Moisture vapour can readily move inwards or outwards depending upon conditions
      • This would suggest you consider:
        • IWI: A vapour open insulation and clay or lime plaster or clay board and clay finish
        • Possibly using wood fibre rigid board insulation without battening and fixing cavity
        • With proprietary mineral, silica, clay or lime plaster finish internally
        • EWI: A vapour open insulation and render
        • Possibly using wood fibre rigid board insulation with soft compressible backing to mould to the inside surface of the outer wall
        • With proprietary mineral, silica or lime plaster finish
        • Or both

Q1.3    New masonry cavity wall (block cavity brick)

Context C1.3

  • Masonry cavity walls have evolved since 1919 when they were introduced, 10” (250 mm) overall with 2” (50 mm) empty cavity with wall ties spaced to suit and lintels bonding the leaves over the cavity
  • 1962 regulations started requiring insulation to be added, 50 mm initially stone wool slabs, needing cavity tray damp proof courses adding
  • 1974 CFCs linked to the breakdown of ozone layer
  • 1985 British Atlantic survey discovered holes in the stratospheric ozone layers 15 – 50 km above earth at polar regions, which when intact protect us from harmful ultraviolet radiation from the sun
  • 1987 Montreal Protocol and later 1997 Kyoto Protocol were introduced to phase out ozone depleting chemicals
  • 2004 EU Ozone Depletion Directive and UK Legislation finally stopped their use in UK, EU and mostly internationally, but there are still permissions in developing third world countries
  • Progressively regulations demanded better U values and manufacturers made every effort to maintain the 50 mm cavity, inventing other materials to fill the cavity, expanded and extruded plastics and using more environmentally damaging blowing agent and blended gasses to avoid clashing with legislation until the last possible date.
  • Despite manufacturers efforts with irresponsible behavior, cavities finally started to increase in width to accommodate ozone benign thicker insulation
  • If you want to take climate change seriously you need to consider:
    • 200-400 mm of insulation in external walls,
      • thickness depending upon material choice: plastics thinner, stone wool thicker
    • No more cement, mortar or concrete (9% of man made CO2)
    • No more steel (7% of man made CO2) accessories making cavity wall possible
    • No more plastic (4.5% of man made CO2) accessories making cavity wall possible
  • Bricklayers were trained in the art of bricklaying and mortar bonds;
    • Were not originally and probably still aren’t trained in constructing cavity walls with all the elongated accessories that are now needed to accommodate large cavities including:
      • Keeping a 300 mm cavity insulation clean of mortar droppings as moisture bridges
      • Long and two piece wall ties, low thermal conductivity options
      • Wide or two piece lintels with or without thermal breaks
      • Windows sills and wide cavity closers or window boxes
      • Door thresholds and wide cavity closers or window boxes
      • Wide cavity tray DPC, cutting insulation to slope accurately and carefully installed
      • Water and frost resistant insulation below ground
      • Acoustic, smoke and fire cavity barriers
      • Telescopic ventilation ducts
      • Balanced Flues
      • Insulated pipes for: ASHP, etc.
      • Let alone making them airtight: especially at openings, skirting level, service penetrations
    • With decreasing levels of site inspection by:
      • Architects, Technicians/Technologists, Surveyors and Building Control,
      • Poorly informed workmanship and materials substitutions will deliver incompetent cavity walls
      • Performance gap identified by Zero Carbon Hub will continue long after ZCH no longer funded
    • In my opinion the cavity wall has had its day,
      • But NHBC certification still like them
      • Low rise housing developers know what they know
      • A large workforce are familiar with them and learn to live with their complexities
      • Most merchants stock these materials locally
      • So they will continue

Answer A1.3

  • There are limited choices of insulation materials that are compatible with new cavity wall construction:
    • All insulation needs to be inspected using IRT Infrared thermography and errors corrected
    • All insulation should be installed in accordance with manufacturers instructions, recommendations
    • All BBA Certified insulation must comply with the certificate limitations
    • Brick and block provide solar radiant heat protection so the insulation does not need these properties
    • Cavity fire barriers often made of high density stone wool are essential across the whole cavity
    • Polystyrene insulation balls blown into cavity:
      • Provides air pathways through the interconnected interstices
      • Modifying a wall for new opening can lead to the insulation pouring out of the wall
    • Glass wool may be permissible
      • is better left to acoustic applications, but there are better acoustic options
    • Stone wool fibre blown into cavities at low densities
      • We now get phone insurance claim campaigns to replace failed stone wool
    • Stone wool batts at low density, built in,
      • With or without partial fill residual cavities, appear to be permissible
      • Manufacturers claim the non directional fibres prevent inevitable rainwater penetration into the cavity for bridging the insulation
      • Moist insulation has reduced thermal performance
      • Wall ties still need drips, but in full fill the drips will encourage any moisture penetration build up within the insulation thickness
    • Dense wood fiber:
      • There was a in the EU market but it is not in the UK market
    • Expanded and extruded combustible plastic insulation
      • Post Grenfell:
      • Above 18 m and now 11 m high rise residential building HRRB are no longer permitted to use plastics
      • There have been fires below 11m that suggest plastics should not be used at all
      • Post-Grenfell buildings with incompetent brick outer leaves exposing the insulation are being re-clad and insulation replaced with stone wool
      • Sending much early recipe ozone depleting insulation to landfill to be systematically crushed releasing the ODP chemicals to attack the ozone layer again
      • Will all buildings with plastic insulation within cavities eventually be routed out?
        • That would be significant
      • Cellular glass e.g. Foamglas, is non-combustible and competent for a long life building, but expensive
    • Bill Duster’s BedZED project used:
      • 3 x 100 mm stone wool full fill cavity insulation with brick and block
      • 3 x 100 mm stone wool in timber framed timber clad construction top floors

Answer A1.3.1

  • Alternative methods of construction to consider: all vapour open
    • Solid wall with EWI with extruded hollow block inner, clay plaster
    • Solid brick outer wall with IWI options described above
    • Diaphragm wall with brick outer, extruded block inner, hemp-lime cavity insulation
    • Light timber frame with hemp-lime, lime render and clay plaster
    • Light timber frame with bio-based insulation, lime render and clay plaster
    • CobBauge with lime render and clay plaster
    • Straw bale with lime render and clay plaster

© NGS GBE GBC GRC ASWS BrianSpecMan
8th July 2023 – 9th July 2023

© GBE GBC GRC GBL NGS ASWS Brian Murphy aka BrianSpecMan **
9th July 2023

Retrofit External Wall Options Q+A
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GBE Q+A Retrofit external wall options 4P A01BRM090723

Retrofit External Wall Options Q+A
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© GBE GBC GRC GBL NGS ASWS Brian Murphy aka BrianSpecMan **
9th July 2023

© GBE GBC GRC GBL NGS ASWS Brian Murphy aka BrianSpecMan **
9th July 2023

Retrofit External Wall Options Q+A G#40938

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