Until the 1970s there was no maximum U value for walls. It was then set at 1.0W/m²K, which does not actually need any insulation. From the 1980s the U value was gradually reduced, requiring the introduction of insulation into the cavity.

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With the release of the 2014 Building Regulations we have a situation where all the elements in the fabric envelope – the roof, walls and floor – need to have virtually the same U value. The maximum U value for walls is 0.3, although in reality you would want to beat that — perhaps down to 0.15.

adding 69.5mm of Kingspan Kooltherm K14 insulated plasterboard to a solid brick walls

Adding 69.5mm of Kingspan Kooltherm K14 insulated plasterboard to a 215mm solid brick wall will result in a U value of just 0.25W/m²k

Insulation Types Explained

Rigid Polyurethane (PUR), polyisocyanurate (PIR) or phenolic foam.

There is not much to choose between these in terms of insulative quality as they all have a thermal conductivity of around 0.021 to 0.023W/mK.

Polystyrene is either expanded (EPS) or extruded (XPS). Expanded is less dense and has a thermal conductivity of 0.38 and extruded 0.34W/mK. Wood fibre boards are also rigid and have a thermal conductivity similar to EPS.

Celotex PIR board rigid foam

A Celotex PIR board. PIR (rigid foam) has become the mainstream choice in recent years

Semi Rigid

Tends to be mineral or natural wools and wood fibre with resin bonding, polyester or other fibres that give a degree of stiffness. Thermal conductivity is generally 0.038 to 0.042W/mK.


Same as semi rigid but without the stiffening and with the same thermal conductivity.

Rigid and flexible insulations need treating in different ways: rigid will stand up on its own, but flexible needs support to prevent it from sagging.

The thermal conductivity figure shows that almost twice as much flexible insulation will be needed to achieve the same U value. For example, a ‘standard’ cavity wall needs 70mm of PUR to achieve the Building Regulations maximum U value of 0.30 — whereas mineral wool needs 120mm.

Insulation Options for a New Build

A U value better than the Building Regulations minimum will be needed to meet the new demands of a SAP assessment — indeed, this is a complexity of the new Building Regulations. A U value of 0.20W/m²K or better is required and that means either 110mm rigid insulation or 170mm of mineral wool (or other natural wool).

Using rigid insulation will result in only a 10mm reduction in overall wall thickness, as mineral wool can be used to fill the cavity, whereas a 50mm cavity is still needed with the rigid boards. With structural insulated panels (SIPs) and timber frame, the inner leaf of the wall replaces the block skin and is virtually all insulation. This means that the same U value can be achieved with less wall thickness.

Kingspan Kooltherm K8 cavity board 100mm

100mm Kingspan Kooltherm K8 cavity board will achieve a recommended U value of 0.16W/m²K in a traditional brick and medium-density block construction

Insulation Options for Renovators

The same figures for thermal conductivity and U values apply to renovation projects as new build. In some circumstances the Building Regulations also require the same standard as for new build. The difficulty is that the walls are already up and less easy to insulate.

Insulating the External Surface

External insulation will need some form of external cladding – typically render or timber – over the insulation itself. This acts as weatherproofing and prevents any moisture penetration. The options are limited to rigid foams or wood fibre, which are usually mechanically fixed to the wall, although some can be glued.


Illustration showing typical Celotex PIR solid wall (external) insulation build-up, including 100mm-thick SW3000 board. Expect to pay £4.50–5/m² excl VAT

The insulating effect is broadly the same as internal insulation, although external insulation has two further benefits:

  • It is more continuous, covers a larger area and helps overcome cold bridges
  • It wraps the wall and allows the mass of the wall to act as a thermal store. That in turn helps to take the peaks and troughs out of the heating cycle

Both reduce the heat demands, but external insulation tends to be more expensive than internal insulation.

vacuum insulation panel from Kingspan

Vacuum insulation panels can allow problem areas to be insulated. For example, a 40mm board (ie. the Kingspan OPTIM-R external wall system, shown) behind 10mm thick render on a render carrier board, will achieve a U value of 0.25W/m²K on a 215mm solid brick wall

Insulating the Internal Surface

Insulation can be applied directly to the wall or in a new – typically timber stud – wall set off from the existing wall. Which one is best will depend on the condition of the existing wall, if it is a solid or cavity wall, and if there is any moisture penetration.

Kingspan and Celotex both offer products that have a moisture barrier that can be applied directly to the wall. Natural insulation can also be applied this way as they allow any moisture to be wicked away. Otherwise, leaving a gap between the wall and insulation ensures that there will be no damp penetration.

Celotex internal insulation solution for solid walls offering dot and dab application

Celotex’s internal insulation solution for solid walls, offering dot and dab application and thicknesses between 25–60mm (before dry-lining). The GS5000 product is an insulation board and plasterboard in one

A 225mm solid brick wall will have a U value of about 1.7W/m²K and a 400mm stone wall about 1.4. Adding 50mm of rigid insulation will reduce the U value to around 0.4, whereas 50mm of mineral wool or natural insulation will reduce it to 0.5.

Cavity Wall Insulation

Installing cavity wall insulation can reduce U values in walls previously without insulation from 1.5 down to 0.5 and below. That has to be a good thing, but installing it demands careful planning and application.

Horror stories around cavity wall insulation abound, and with some justification. There is no magic or secret about it — some cavities are just not suitable for insulating (for example, if the brickwork is in poor condition). A clean cavity – one free from mortar dollops on cavity ties and from mortar or other debris in the bottom of the cavity – is good to insulate, but a messy cavity isn’t.

The trick is finding out which one you have. The only way yet devised is a visual inspection (typically using a small camera) which should be carried out by a competent insulation installer.

Jablite EPS Dynamic cavity wall

The unique shape of Jablite’s EPS Dynamic cavity wall insulation panels controls the passage of warm air movement from inside the house and minimises heat loss, working best in collaboration with a (mechanical ventilation heat recovery) MVHR system

There are stories about north- or west-facing walls being unsuitable for cavity wall insulation due to the driving weather conditions those elevations are prone to. However severe the weather, it is unlikely to penetrate beyond the outer brick leaf, so what happens in the cavity will not be affected by the weather.

However, Building Regulations Part C (5.15) does still refer to a map of the UK (below), published as part of the Regulations, that shows the country according to risk of driving rain, and effectively prohibits full-fill cavity insulation in these areas (for new builds). It’s worth at least raising this with the insulation supplier.

UK weather map driving rain

This map highlights ‘UK zones for exposure to driving rain’

  • Quick note: Under the ECO (Energy Companies Obligation) scheme any homeowner can now enquire as to whether they are entitled to a free install of cavity wall insulation (it’s no longer income-tested)

Some older buildings may be exceptions but generally insulating a wall is a given and the only questions are how much and what with? And whatever the decision, insulating the walls will have a dramatic and noticeable impact on energy consumption and comfort.

Some older buildings may be exceptions but generally insulating a wall is a given and the only questions are how much and what with? And whatever the decision, insulating the walls will have a dramatic and noticeable impact on energy consumption and comfort.

Insulating Reveals

The key to good insulation is continuous insulation, and that means insulating window and door reveals too. Leave those uninsulated and around half the effect of the wall insulation will be lost.

But installing 50mm of insulation (plus plasterboard or other finishing material) to a window or door reveal is generally not practical. There are very thin materials, like Proctor Spacetherm, that will do the job. They tend to be very expensive but very effective and useful for these small areas.

Price Guide

Prices will vary with the type of insulation, manufacturer, supplier and quantity. The main priority is deciding on the right type of insulation for the job, and then finding it at the best possible price. There really are no quality issues and, on a like-for-like basis, one manufacturer’s products are pretty much the same as another’s.

Prices range from roughly £4-13/m² (wall space). For rough budgeting purposes, expect to pay £5-7,000 for internal wall insulation, and £6,000-10,000 for external wall insulation. Visit just-insulation.co.uk for a good online price list.

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