Matthew Evans of Kingspan Insulation GB discusses what to think about when it comes to insulating your self-build.
Ensuring your new home is effectively insulated is an important consideration for all self-builders. It can help to significantly reduce your long-term running costs, keep spaces at a more comfortable temperature across the year and allow low carbon heating technologies (such as heat pumps) to perform efficiently. It is also typically much cheaper to get this right during the initial build than trying to upgrade the insulation at a later date, so it is worth considering properly right at the start of your project.
To achieve the best results, you need to think about the level of performance you want to achieve, which insulation products are most suitable for delivering this, and ensure that the products that are specified by your architect are actually used and fitted correctly on your project.
Insulation fundamentals
The measure of how well a complete construction element (floor, wall or roof) resists heat loss is known as a U-value. The lower the U-value of a construction, the more effective it will be at preventing these losses.
A good starting point when deciding what U-value the different elements in your home should achieve are the values contained in the Notional Dwelling Specification. This is a building performance specification that is used to confirm whether a property complies with the current energy efficiency requirements in the Building Regulations and Standards. The specification for your project is applied to a theoretical building with the same dimensions as the home you intend to build. The software then generates performance targets that your home will need to achieve (such as expected carbon emissions). These U-values are not compulsory but provide a good indication of what you should aim for.
England, Scotland and Wales all use different specifications within their Notional Dwelling which are shown in the table below. Where possible, it is worth looking towards the more ambitious values used in Wales and Scotland as representing best practice for your home.
Insulation options
Once you have a clear idea of what U-values you would like to achieve, you then need to consider which insulation products will be most suitable for delivering this. The key performance measure for insulation products is their thermal conductivity (lambda value). As with U-values, the lower this value is, the more effective the products are at preventing heat loss. This can mean it is possible to achieve a desired U-value with a slimmer thickness of insulation, which can be beneficial for your project.
For example, in cavity wall constructions, the insulation is installed within the cavity between the inner and outer leaf of brick or blockwork. By specifying insulation products which have a low thermal conductivity, such as rigid phenolic insulation boards, it is possible to achieve the external wall U-values shown above with a slimmer thickness of insulation than may be possible with worse performing alternatives. This can help to limit the depth of the wall construction, allowing you to get the maximum floorspace from your building footprint.
Keep in mind that insulation materials which look similar can have quite different thermal conductivities, so it is important to confirm that what is being fitted on your home actually matches what you or your architect have chosen.
Ensuring best practice
Alongside choosing your U-value and insulation products, it is also important to ensure the insulation is designed and fitted to minimise any thermal bridges. These are areas where materials which are better conductors of heat than the insulation are allowed to form a ‘bridge’ between the inner and outer face of a construction. This bridge can act as a fast track for heat to escape from your home and can account for as much as 30% of total heat losses.
Thermal bridges commonly occur because of gaps or inconsistencies in the insulation layer and are often found at junctions between the wall, floor and roof, and around openings such as windows and doors. The heat losses
at these junctions are measured as a Psi-value.
To limit these bridges, it is best practice for your architect to plan the interaction of the insulation at key junctions in what is known as a detail. This should include a calculated
Psi-value. Rather than having to create each of these themselves, there are a range of manufacturer-created and industry-created details with pre-calculated Psi-values which can be used. These are typically supported by process sequences which provide a step-by-step explanation for installers on how to correctly fit the insulation in line with the detail. It is important that these are followed, and that insulation is carefully installed with no gaps between the boards or rolls of insulation for it to perform correctly.
A solid base
While selecting insulation may not be the most exciting part of a self-build, considering this area right from the start can really pay off in the long run. By targeting low U-values, making use of products with low thermal conductivities, and ensuring the insulation is well designed and installed, you can help ensure your home will be comfortable and affordable to heat.
Matthew Evans is director of technical and regulatory affairs at Kingspan Insulation GB