With a global gas supply crunch, issues with renewables and strong demand pushing up energy prices, the energy efficient design of roofing and insulation is vitally important for homeowners, as Lisa Grosse from Cedral explains

In 2025, The Future Homes Standard will come into effect, designed to ensure all new homes are future-proofed, with low-carbon heating systems and higher levels of energy efficiency.

One quarter of all heat energy in a home escapes through the roof. With insulation you can more easily regulate the temperature in your home. The better your insulation, the less heat will escape when it’s cold outside. Conversely in summer, good insulation will keep your home cool when temperatures rise.

It can get very hot under your roof. If your HVAC ducts pass through your attic, they will also require insulation against heat. With hot roof insulation, your ducts can remain cooler as your attic has a more conditioned stable temperature.

Cold outside temperatures often lead to condensation on the inside of your roof if it’s not insulated well. Insulation can also help to prevent moisture problems and ice dams by preventing the heat of the home from melting snow on the roof. 

A WARM OR A COLD ROOF?

There are two ways to categorise how a roof has been set up to manage heat – a warm roof or a cold roof. Knowing which one your property has can provide guidance on measures that need to be taken to ensure the building’s long-term energy efficiency.

A warm roof is normally constructed when living accommodation is within or partially within the roof space. Insulation is installed as part of the roof deck, on top of a vapour control layer and underneath the roof covering. This set up is designed to provide improved thermal performance, with the insulation placed within and/or onto the roof rafters. It reduces heat losses and balances your home’s temperature both in winter and summer. However, this may come at the expense of space and head height if installed within an existing roof or may require a higher roof pitch at the construction stage. You also need ventilation below the roof slates or tiles to avoid condensation. 

Cold roof insulation is placed between and over the wooden joists above the ceiling of the building’s top floor. This insulation above the ceiling keeps the heat or cool air in the summer from escaping from the rooms under it. But as the roof rafters and remaining roof structure are not directly insulated, the loft remains ‘cold.’ If the installation is not correct however, the temperature of the building can be affected through thermal bridging, with the cooler temperature of the roof rafters transferring to the structure below or heat escaping into the loft.

Whether you have a warm or cold roof, correctly installed insulation will provide greater energy efficiency by ensuring the room below is warmer during the winter and easier to regulate during the summer. This makes it easier to control the energy levels required to heat the house, without increasing carbon emissions.

MEASURING THE EFFECTIVENESS OF INSULATION MATERIALS

There are three different values that are used to express the quality of the insulation material.

The R-value measures a material’s thermal resistance – the higher the R-value, the lower the heat loss. Insulation material with a high R-value needs to be less thick than a product with a lower R-value to achieve the
same insulation quality. Or, the higher the R-value, the less you need of the material.

The Lambda value measures a product’s thermal conductivity. It is the opposite of the R-value – the lower the lambda value, the lower the heat loss. For your insulation to be as efficient as possible, you want to use materials with a low lambda value, but a high R-value.

A U-value is the sum of the thermal resistances of the combined layers that make up your roof. The lower the U-value, the better insulated your roof is and the more energy you save. 

DIFFERENT ROOFING SOLUTIONS

In a climate like the UK’s, roofs need to withstand lots of rain. A pitched roof which diverts precipitation from the roof thanks to the slope, is a standard solution for residential buildings and there is a variety of materials to choose from. 

Three commonly available roof materials are concrete tiles, terracotta tiles and fibre-cement slates. Concrete tiles are relatively heavy and notorious for absorbing moisture and water. This can cause frost damage in winter or lead to the build-up of mould, unless they have a protective coating. Terracotta is a natural material and its porous nature of this makes it susceptible to frost damage without an additional protective layer or glazing. Fibre cement roofing slates are increasingly popular. Made of a mix of naturally available ingredients: cement, fibres, sand, water and air, they are robust for all weather conditions, with an excellent fire performance classification, lightweight and easy to work with, and compatible with all of these types of insulation. When a roof pitch is made higher to accommodate a warm roof system – and hence has longer rafters – fibre cement slates are a good option, as they are lighter than natural slates and other heavier materials, and last just as long.

Understanding how the roof contributes to the thermal performance of a home can help you to make your home more energy efficient in the coming years. 

Lisa Grosse is brand manager for Cedral