t’s common knowledge that a significant proportion of heat, up to 25%, produced by a boiler will escape through the roof of a house. 35% of heat also escapes through walls and gaps around windows and doors, meaning that 60% of a property’s total heat will escape through the building’s envelope. As this leaves the homeowner with only 40% of the heating they’re paying for, it’s no surprise that the average cost of heating is becoming increasingly expensive for UK households.
So, as architects, what can we do to help reduce this figure?
For roofs, in particular, insulation plays a key part in ensuring maximum thermal performance by minimising the amount of heat loss that occurs via radiation. However, rooflights also play an important part in improving the energy efficiency of the space. As the maximum U-values for walls, floors and roofs is 0.15W/m²K, architects should be choosing rooflights that offer maximum energy efficiency through comprehensive insulated frames and triple glazing.
With regard to the design and build of an energy-efficient roof, the structure itself must be entirely airtight, requiring high levels of insulation and the elimination of all thermal bridges to ensure the maximum U-value of 0.15W/m²K isn’t exceeded. The roof’s specifications will, of course, be dependent on the build itself, but they must also ensure a maximum of 0.6 air changes per hour at 50 Pascals pressure (ACH50) to ensure adequate airtightness is achieved.
The highest U-value for complete window installations remains at 0.85W/m²K. For the design of roofs, in particular, this requires the specification of rooflights that are either triple or quadruple glazed to ensure they do not exceed the maximum U-value. For example, Roof Maker’s latest Passivhaus-accredited fixed flat rooflight offers a U-value of 0.2W/m²K, providing installers with the confidence that the product can help to achieve complete energy and solar gain efficiencies.
Typically, the installation of windows, doors and rooflights is where the greatest level of energy is expected to be lost, due to the glazing present and the breaks between the frame and the glass offering the opportunity for heat to escape. Therefore, the frames must be well-insulated and fitted with low-E glazing filled with argon or krypton to prevent heat transfer. For most cool-temperate climates, this requires a U-value of 0.80W/m²K or less, with G-values around 50%.
In many homes, the roof space of a property is now often used as an additional living space, usually an extra bedroom. To ensure it’s as thermally-efficient as possible, the thermal comfort of the space must be met during both winter and summer, with no more than 10% of the hours in a given year over 25°C. The premise of thermal comfort is that it can be achieved solely by post-heating or -cooling the fresh air mass through efficient indoor air quality conditions. This removes the requirement for additional recirculation of air through manual ventilation systems such as opening rooflights and windows. To ensure this, it requires adequate insulation of the building’s envelope to ensure the desired level of warmth can be achieved within the property.
To offer the cleanest air possible, the roof space must also feature an adequate ventilation strategy, allowing for good indoor air quality and energy savings. At least 75% of the heat from the exhaust air should be transferred to the fresh air again by means of a heat exchanger. This should be situated within the roof of the building to ensure the successful exchange between the exhaust air within the building and the fresh air outside of the building.
By incorporating the above ecological design principles into new-build and renovation projects, architects can ensure they are helping customers significantly reduce heating costs and improve overall indoor air quality, without compromising on the overall aesthetics or quality of the build.