Ask the Expert: Cut your losses and save CO2


Improving insulation is at the heart of the country’s mission to reduce CO2 emissions in new homes. Matthew Evans of Kingspan Insulation answers key questions around the materials and details to consider to avoid heat loss in your build, and lower its environmental impact

What’s the right thickness of insulation for my project?

To answer this, we need to look at the science behind insulation. The main role of insulation in your home is to prevent heat conduction, i.e. heat escaping. Different materials conduct heat at different rates and this is measured through their thermal conductivity (also known as a lambda value). For example, a material such as copper, which is very conductive to heat, will have a high thermal conductivity. This is why it makes a great choice for pots and pans.

To calculate how well a complete construction in your home (such as a wall) prevents heat loss from conduction, we first measure the thickness of each of the materials used (bricks, insulation, plaster etc) and divide this by their thermal conductivity. This gives us a thermal resistance value for each material. We can use the values for all the materials, along with some additional parameters for things like fixings, fasteners, air gaps and other materials, to then calculate the total U-value (thermal transmissance) for the construction. The lower the U-value is, the better insulated the construction will be.

As insulation materials with lower thermal conductivities are more effective at preventing heat loss, you can fit a slimmer insulation thickness and still achieve a low U-value. A product’s thermal conductivity should be clearly identified on its label. You can also find free U-value calculator tools online, which let you enter details about your home’s construction and target U-value, and will provide the correct insulation thickness for a particular product.

What’s the right insulation material for my build?

There are a wide range of materials on the market today, and the right choice will depend on your budget and the specifics of your project. Below are some of the most common options.

Glass mineral wool and rock mineral fibre are relatively cheap, with good acoustic properties. This can be helpful if your home is near an especially busy road. The downside is they have relatively high thermal conductivities, meaning a thicker layer of insulation will be needed.

PIR (rigid boards) are designed with much lower thermal conductivities than mineral wool or fibre insulation. They’re also lightweight, making carrying and cutting easier. 

Phenolic boards have all the benefits of PIR insulation but typically have an even lower thermal conductivity. This means you can insulate your home to a really high level while keeping constructions slim.

Vacuum insulation panels (VIPs) are a specialist solution with extremely low thermal conductivity. They can be a great problem solver for applications like insulating above an existing solid floor while minimising any loss of headroom. As the panels themselves cannot be cut or pierced, they’re typically supplied in different dimensions together with rigid insulation of the same thickness which can be cut to fill gaps.

What other things do I need to consider?

It’s important to be aware that how your insulation is fitted will have a major impact on how it performs. In well-insulated homes, as much as 30% of heat losses can occur because of thermal bridging. This is where materials which are more conductive to heat than the insulation create a ‘bridge’ between the inner and outer face of a construction. These bridges often occur around windows and doors, or at junctions between the wall and floor or roof due to gaps in the insulation layer, or inconsistencies in its thickness. In addition to allowing more heat loss, they can also lead to issues with condensation and dampness.

To prevent this, your architect should create drawings of all of the key junctions (or use existing ones from manufacturers), to check that the insulation will limit thermal bridges. Your installers then need to carefully follow these, ensuring the products they fit match those used for the calculations.

What are the implications if I install a heat pump?

Heat pumps operate most efficiently (and affordably) at lower flow temperatures than conventional gas boilers. This means it is crucial to ensure all parts of your home are well insulated so it retains heat more effectively, allowing you to get the full benefit of these systems. 

Good insulation levels are particularly important if you’re fitting an air source heat pump. These use external air as their source of heat, and as temperatures drop, they have to work harder to extract it. If your home allows lots of this heat to escape, your bills could increase significantly. It’s also much more complicated and expensive to improve insulation in areas like floors or walls at a later date, so it pays to get it right from the start.

Do I need to consider overheating?

As our climate warms, we’re likely to experience heat waves which are more frequent and intense, increasing the risk of properties overheating. This shouldn’t stop you insulating your home to a high level, or mean you have to compromise its energy efficiency. Instead, you should look at design features to minimise heat gains. 

These can include adjusting the orientation of your house, changing the size or orientation of windows, and fitting windows that don’t allow as much solar heat in, or adding external shading, insulating hot water pipes, and even fitting more energy efficient appliances which will release less heat when running.

Matthew Evans is GB head of technical at Kingspan Insulation