GBE CWI v IWI (Q+A)

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Following Grand Designs Live 2024 Ask an Expert as a HiiGuru
Email Correspondence with HiiGuru Customer

Hi we met yesterday at Grand Designs Live. You were kindly going to recommend some products after you finished your next ‘ask an expert’, but I’m afraid I had to leave due to the train strikes.

As a reminder, I was the person with these queries:

Will we have a cold spot / condensation issue with alu windows?

Previously aluminium windows were made with aluminium extrusions with single or double glazed sealed units with aluminium extruded spacers between glasses and they suffered badly from condensation on their internal surfaces of glass and aluminium.

Today commercial aluminium windows are made with two layers of aluminium separated by a thermal break of plastic/rubber.

They also have options of double or triple glazed sealed units with plastic ‘warm edge’ spacers between glass sheets in place of aluminium spacers.

I would imagine that domestic windows have the same options available but may cost more for anything above minimum specification.

If so, should we upgrade the windows, or upgrade the ventilation? Answer – yes!

Upgrading from aluminium only to thermally broken aluminium may not be sufficient to avoid condensation altogether.

Upgrading from aluminium only to thermally broken aluminium with ‘warm edge’ spacer glazing may not be sufficient to avoid condensation altogether.

Changing from aluminium windows to steel windows unlikely to avoid condensation.

Changing from aluminium windows to thermally broken steel windows unlikely to avoid condensation.

Upgrading from aluminium windows to plastic windows may not be sufficient to avoid condensation altogether.

For PVC windows to be competent they need a steel core to fix ironmongery to, to prevent premature failure.

The steel core may generate condensation withing the hollow extruded PVC frame, but the window should be self-draining.

PVC windows generate hazardous toxic smoke in a fire, avoid them if possible.

Upgrading from aluminium window to ‘composite’ timber inner and aluminium outer should avoid internal condensation altogether.

Upgrading from aluminium windows to timber window will avoid internal condensation altogether.

There is a UK manufacturer of timber windows with a 20 year life expectancy paint scheme (extraordinary but achieved by making sure the wood is at the optimum moisture content to take the paint) which will tolerate any condensation.

They exhibit at the Self Build shows, https://www.georgebarnsdale.co.uk

Manufacturers of decent composite windows Velfac https://velfac.co.uk

Consider Passivhaus accredited glazing and windows:

https://database.passivehouse.com/en/components/list/group_4?

Install windows with pre-compressed, self-adhesive, self-decompresses over 2 hours, window to wall perimeter sealing tape: Iso-Bloco (there are a few versions, including ISO-Bloco-one and ISO-Bloco-Reno) by ISO-Chemie https://www.iso-chemie.eu/en-GB/

Get good ventilation in kitchens bathrooms bedrooms.

Doesn’t need to be whole house.

We should not be using window materials that cause us to need intermittent or permanent energy input into a mechanical ventilation system to avoid a condensation problem.

Passive or intermittent mechanical ventilation could assist in reducing condensation on inadequate specification windows.

If aluminium windows are used throughout then condensation can occur in any room where moisture is generated: Breathing, Bathing, Cooking, laundry.

And on any side of the building from cold northerly winds and prevailing wet south westerly winds.

Products:

Whole house mechanical ventilation with heat recovery (MVHR) is best, then choose a Passivhaus accredited system.
https://database.passivehouse.com/en/components/list/group_2?

If budget is limited consider:

Passivhaus decentralised systems
https://database.passivehouse.com/en/components/list/group_2? decentralised systems
Redring through the wall MVHR (c£1000) I cannot find it on internet anymore.
Partel IE and Partel UK supply LUNOS Decentalised systems through the wall MVHR https://www.partel.ie/ventilation-systems/

Will we have a damp / mould problem if we go for CWI and if it is incorrectly installed, leaving cold spots?

Is it worth getting CWI given that risk?

Cavity masonry walls:

Introduced before 1919 but became mandatory from 1919.
intended to separate damp outer brick leaf from dry inner brick leaf with a 2” (50 mm) ventilated cavity,
connected and held apart by metal wall ties with water drips between leaves,
with brick returns to close the cavity at windows and doors with a vertical damp proof course separating the inner and outer leaf,
Lintols bridge the cavity and may require a cavity tray damp proof course on its top.
Cavities are filled below ground level with a lean mix concrete with a 45-degree sloping top, down and out.
In the 1960’s and 1970’s (London in the 1980’s) improved insulation standards were introduced.
Cavity Wall insulation (CWI) was an option to fill the 50 mm cavity with insulation that could meet the insulation requirements at that time.
Retrofit CWI into a 50 mm cavity will only provide 1970’s levels of insulation, inadequate by today’s regulations.

Some designers rejected CWI

Since it potentially links the wet outer and dry inner leaves with potential for moisture to pass between insulation joints, along wall ties towards the water drips and beyond.
All materials suffer from inaccurate workmanship, gappy or incomplete installation, potential air, water, moisture vapour energy leakage at joints, thermal bridges through fasteners or wall ties
Retrofit blown in insulation suffers from not being able to reach all corners and needs well placed injection holes and air pressure to blow upwards and sideways as well as gravity filling below.
Built-in partial fill insulation is an option to maintain a residual cavity for moisture to run down the inside face of the outer brick leaf, that requires special ties with retaining disks to prevent insulation slumping and creating bridges for water flow

Retrofit CWI materials options include:

Injected expanding foam plastics: (some early plastics off gassed, were unhealthy and were banned)
Blown in polystyrene balls: (the interstices between the balls create passageways for air, heat, moisture vapour and water, therefore not effective)
Blown in mineral fibres (glass or stone): (the air spaces between the fibres create passageways for air, heat, moisture vapour and water, therefore of limited effect)

New Build CWI material options include:

Built-in expanded foam plastics: (many plastics off gassed, were ozone depleting and were eventually banned)
Built in mineral fibres (glass or stone): (manufacturers claim fibres are random direction and do not permit moisture passage inwards)
Blown in polystyrene balls: (the interstices between the balls create passageways for air, heat, moisture vapour and water, therefore not effective)

Replacement CWI material options include:

Manufacturers claim to be able to remove claimed ineffective or defective insulation by devices to pulverise or fragment it to a dust that can be removed by vacuum suction and replace it with higher performance materials.
Pulverising any foamed plastic that might be ozone depletion potential needs to be questioned, is highly suspicious, its effectiveness checked, its improved performance verified, is difficult to control, and should be highly regulated all the way to safe collection of gasses, microplastics or fibres; then neutralisation, consolidation, disposal or destruction.

Increasing Regulations:

Increasingly CWI had to be tested for moisture penetration affected by wind driven rain and height above ground and British Board of Agrément tested, set limits and certified individual CWI insulation products.
Inner brick leaf was replaced with blockwork, over the decades blockwork recipes improved to provide greater thermal insulation
Eventually insulation regulations increased and cavities and inner leaf blocks needed to be increased in thickness to meet increasing demands.

Incompetent CWI:

Cavity Wall Insulation (CWI) can be installed competently and if rushed incompetently.
We get phone calls asking if you have damp or cold spots so there must be a significant number of failures.

Drying a wet or defective wall

Ensure roofing is competent and draining to gutters or outlets, without ponding or leaks.
Ensure flashings are competent, not cracked, not loose and well attached.
Ensure all gutters and down pipes, soil waste and vent pipes, overflows, gulley are competent or fix them.
Ensure ground level is 150 mm or more below damp proof courses or lower the ground levels until they are.
Ensure any ground and surface water is directed away from the walls.
Ensure any air bricks are clear and not blocked.
Use internal heat to dry out the walls
Carefully remove timber skirtings boards so as to reuse them of not rotten
Remove gypsum plaster from inside face of wall to allow walls to dry out
Apply lime or clay plaster to inside of external walls to help dry out walls (except if doing IWI see Q5 & A5)

Is it ‘worth’ doing Internal Wall Insulation (IWI)?

Yes, but it may create thermal bridges where internal walls or partitions abut inside of external walls.

Insulate the abutting walls or partitions to reduce the thermal bridge.
In France I have seen thermal breaks separating the inner walls from the external walls, but this is plant, temporary works, labour and materials intensive.
Insulate at lease 500 mm or to the chimney breast or next wall abutment.
Insulate to achieve the full U value with the IWI and existing inner leaf only.

Dry the wall out before applying IWI.

See A4 above: Drying a wet or defective wall, bullet 1 to 9 (not 10)
Consider using moisture permeable insulation, lining board and/or finish and decoration they will help to keep the wall dry and continue to dry it out.

Would we have a problem if we only PARTIALLY do CWI

i.e leave some walls as exposed brick? (We have nice bricks).

Is that partial CWI and partial IWI?

That will create new problems: Thermal bridges and moisture and mould potential.
The CWI and IWI will need to overlap a considerable distance.
But retrofit CWI needs to be complete overall the whole building’s external walls, since it is difficult to stop and start it competently.
Once CWI is complete then IWI could be partial, it will make a difference to the internal temperature of the wall locally.
CWI alone will be 1970’s levels of insulation
CWI + IWI should be 2020’s levels of insulation
The differential U values may cause condensation risk, mould risk.
Choose your exposed brick locations wisely, limit extent, consider aesthetics, show it off with lighting, expose in dryer parts of the house, not in kitchen or bathroom unless you have an aga on; not in cold or damp parts of the building.

For the walls you recommended we keep things breathable, but I’m not sure if insulated plasterboard would be a problem.

For solid wall it is essential to use moisture permeable or breathable materials

For cavity wall with variable U values and two layers of insulation (CWI+IWI) in part the interstitial condensation risk is complicated and breathable would be a safer option.
Consider using moisture permeable insulation, lining board and/or finish and decoration; they will help to keep the wall dry and continue to dry it out.

Gypsum plasterboard?

British Gypsum/now Isover on a project I specified claimed their systems were breathing wall compatible, using gypsum boards and vapour control layers and plastic insulation.
All three layers are moisture barriers, and some of their accessories, primers and skim finishes are also moisture vapour resistant so the joints might be the only route out for moisture, but nowhere near vapour open enough.
Unless they have some evidence to support that it’s a load of greenwash.

You also said we should avoid cavities.

For solid wall it is essential to avoid cavities where moisture cannot bridge the cavity and mould can start to grow.
For cavity wall construction a ventilated empty cavity is safe
For any construction a void or cavity is a place for heat to escape to, to avoid heating rooms and people
Once in a cavity heat can create eddy currents and jump the cavity to reach the other side
Heat can thermal bypass through airy insulation

Any recommendations on products or methodologies very welcome

Look at Green Building Encyclopaedia website
Q&A, checklists, Jargon buster, issue papers, suppliers, go to suppliers closest to you, visit NSB&RC in Swindon, see EcoMerchant at NSB&RC, see materials mockups on display at NHB&RC.
Use dense wood fibre IWI, primers, clay or lime plaster, clay or lime decoration