VIEWPOINT
INSULATING WALLS FOR FUTURE HOMES
Matt Neary, National Sales Manager at Knauf Insulation
LAST MONTH, WE examined how wind-driven rain impacts the design and construction of external walls in housebuilding, including the importance of cavity size, insulation selection, and masonry facing, which vary across different UK climates. However, this is just one of many factors your customers will need to consider when choosing wall insulation.
Selecting the appropriate insulation product is essential for both external masonry cavity walls and internal walls. This presents an opportunity for merchants to offer advice and expertise on the regulations, practical applications, and key considerations for insulating different wall types.
Considering compliance Approved Document L: Conservation of Fuel and Power was updated in 2022, requiring external walls in new dwellings to have a maximum U-value of 0.26 W/m²K. The requirements for fabric elements in existing homes differ, and airtightness must also be considered. These updates were made in anticipation of the Future Homes Standard (FHS). Although some of the detail and timings of the FHS are still subject to change, it’s clear carbon emissions in new homes will need to be reduced by 75-80% compared to those built under 2013 regulations. Fire safety regulations are also a crucial consideration. Approved Document B: Fire Safety defines fire resistance times for different elements, including walls, and specifies fire reaction classifications for insulation products. Additionally, acoustics, ventilation, and moisture management should comply with relevant regulations.
correctly than other comparable products such as rigid PIR boards. Mineral wool is also flexible enough to move with the construction and is less prone to air gaps, helping to maximise performance. Switching to 150mm cavities also allows the use of insulation that is non-combustible and low carbon while still meeting thermal performance requirements. DriTherm®
Cavity Slab 32, for
example, has the best possible Euroclass A1 reaction to fire classification and is made from glass mineral wool, which has the lowest embodied carbon of any mainstream insulation material.
Internal insulation Internal walls do not require additional thermal insulation. Instead, internal wall insulation should focus on acoustic performance to meet Approved Document E: Resistance to Sound and reduce noise transfer between rooms.
To reduce both external and internal noise transfer, three key principles apply: increasing mass, creating separation, and adding absorption. Internal walls achieve mass through the plasterboard, separation via the cavity, and absorption with noise-reducing materials such as mineral wool insulation.
Separating walls Walls that separate dwellings within a building, known as party walls, require both acoustic and thermal insulation to prevent thermal bypass and minimise noise transfer.
Approved Document L stipulates a maximum U-value of March 2025
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0.20 W/m²K for party walls. But solid walls or fully filled cavity walls can achieve a U-value of 0.0 W/m²K, meaning zero heat loss, if they are built correctly. For a fully filled cavity party wall, your customers must ensure effective sealing around exposed edges and align the insulation with layers in abutting elements.
External masonry cavity walls
Thermal performance is of course even more crucial for the external walls of the building. There are several ways to approach insulating external walls, but housebuilders are increasingly moving towards fully filling a 150mm cavity. There are several reasons for this shift to wider cavities. Firstly, a lower U-value in the external walls gives housebuilders flexibility in other parts of the whole house recipe. Secondly, a 150mm cavity is an ideal application for mineral wool, such as DriTherm® Cavity Slab 32, which is easier to install
Timber frame walls Timber frames are gaining popularity due to their cost- effectiveness, reduced embodied carbon, and superior thermal performance compared to masonry walls of similar thickness. However, timber frame walls have lower mass, which reduces their sound absorption capacity. To comply with Approved Document E and offset this weakness, it’s particularly important to use sound-absorbing insulation materials such as mineral wool for timber frame walls.
Future homes
There is no single best method for insulating walls. The ideal approach depends on regulations, thermal performance goals, fire safety, acoustics, and construction techniques. With the updated Part L and the impending Future Homes Standard, housebuilders must prioritise achieving low U-values, airtightness, and sustainability to reduce carbon emissions. By keeping up with regulatory changes and best practices, merchants can help their customers create homes that are not only compliant but also comfortable, sustainable, and future-proof. BMJ
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