Costs and benefits of retro-fitting houses to reduce heat loss

Dr Robin Russell-Jones May 2020


The U value is the ability of materials to transmit heat (Thermal Transmittance) and is measured in energy lost per area per degree and expressed as Watts per square metre per Kelvin (Wm2K). The lower the U value, the less heat is lost.

New buildings and extensions are covered by Building Regulations which stipulate the maximum U value for different materials. Thus:

Walls : 0.18 Wm2K

Floor 0.13

Roof 0.15

Windows ( and glazed doors) 1.4

However older buildings were built to less exacting standards with higher U values

In addition there may be heat loss through chimneys, vents and poorly fitting doors or windows

This synopsis analysed the potential remedies available to render older buildings more energy efficient and identifies the ones that are most cost-effective.

Eliminate drafts

  • Use weather-proofing strips and or caulking to seal any air leaks around doors or windows

  • Use door snake for gap under external doors

  • Plug chimney when not in use

  • Seal access door to attic

Two other cheap solutions should be considered for all houses:

Install loft insulation: Average cost £300 at a saving of £150 per annum (Pay-back 2 years)

Use thick curtains for single glazed windows: Lowers U value from 4.8 to 3.6

Insulating walls

Prior to the 1930’s, the great majority of houses in the UK were made of solid brick. Unfilled cavity wall construction was used from 1930-1995

Cavity walls filled with insulating materials has been used since 1996

Approx 28% of all houses in the UK have a solid wall construction which equates to 7 million homes (Ref: National Energy Foundation : NEF). The U value of a brick wall depends on the thickness Average is around 2, but may be as high as 2.7 for walls that are only 225 mm thick

Solid walls can be insulated externally (Apply insulating material and then render) or internally (insulation boards or a stud wall filled with insulating material). Internal insulation is cheaper but reduces the floor space of the room.

40 mm of EPS insulation will reduce the U value to 0.65

100 mm of EPS insulation will reduce the U value to 0.3.

For an average 3-bedroom house with solid walls:

Internal insulation costs 5.5-8.5K: savings £400 per annum Pay back 17.5 years

External insulation costs 9.5-13K with savings of £490 per annum: 23 years

Cavity Wall Insulation

U values have gradually improved over time but the values below demonstrate why the UK housing stock is the least energy-efficient in Europe

Unfilled Cavity Walls: Filled cavity Walls

Prior to 1900: 2.0 Wm2K 1996-2012 0.45

1900-1975 1.6 2003-2006 0.35

1976-1982 1.0 2006-2010 0.3

1983-1995 0.6 2010- present 0.2

Cavity wall insulation can reduce U values to around 0.5 at best for houses built before 1976. Costs depend upon the type of house and the number of external walls. Even so the pay-back is less than 5 years for any property using a gas-fired boiler with April 2018 fuel prices (Source: Energy Savings Trust)

Cost Annual Savings Pay-back

Detached property £725 £245 2.9 years

Semi-detached £475 £145 3.3

Mid-terrace £370 £90 4.1

Bungalow £430 £100 4.3

Flat £330 £70 4.7

There are potential problems with Cavity Wall Insulation

It will reduce condensation inside the house as the inner wall is no longer cold. But condensation can occur inside the external wall. If the insulating material is blown fibre then this allows the condensate to trickle down to below the damp proof course If not then damage to the building fabric can occur before the problem becomes visible.

Problems can also arise if a thermal bridge connects the inner and outer walls: either a load-bearing lintel or lumps of mortar (snots) that have accumulated on the metal ties. This can allow damp to penetrate to the internal walls. It is therefore important that the insulating material used is breathable so that it allows ventilation within the cavity. Cavity wall insulation should not be used in areas prone to flooding or walls with poorly pointed or porous brickwork. Ideally a survey needs to be undertaken before cavity wall insulation is used

Windows and doors

Single-glazed windows are responsible for much of the energy inefficiency of UK houses. A mandatory U value of 1.8 for new build was not introduced in the UK until 1973 (following the oil crisis) and was not made mandatory for extensions until 2002. The effectiveness of double glazing varies widely depending on the technology; thickness of glass: width of gap: air or argon gas. Argon gas is denser than air so provides better sound and thermal insulation. High quality double glazing can achieve a U value of 1.2 : one quarter that of single glazing

The U value of a solid timber door is approx. 3.0

Single-glazed window >4.8

Single glazed plus curtains 3.6

Single glazed plus shutters 3.0

Secondary Glazing 2.9-3.4

Double Glazing 1.2-3.7

Triple glazing <1.0

To replace a single glazed window with a high quality aluminium double glazed window (900 x 1200 mm) costs approx. £650 of which £250 is labour

uPVC windows are 30% cheaper : Triple glazing is 30% more expensive. Fitting second floor windows is 15% more expensive because scaffolding is needed.

To upgrade the average 3-bedroom house using uPVC windows costs £5000 approximately and produces savings of £150 per annum (Pay-back 33 years)

The long pay-back reflects the fact that windows only represent 15-20% of the exposed area of the average house


The above data allows us to make comparisons between different options in terms of pay-back.

Sealing drafts 1 year

Loft insulation 2 years

Cavity Wall insulation <5 years

For Comparison

ASHP 10 years (with RHI)

Solar PV 11.5 years (With export payment)

Solid Wall insulation

Internal 17.5 years

External 23 years

Double Glazing 33 years

The above calculations, and the savings generated by installing renewable technologies does depend upon the current price of fossil fuels. Clearly this may change. However the costs of renewable energy have been falling whilst the move away from gas and possible carbon taxes indicate that fossil fuels will become more expensive in the future.

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