HEAVYSIDE SPECIAL: INSULATION


A FULLER UNDERSTANDING OF CAVITY WALLS


Cavity wall constructions remain the standard building approach for projects across the UK, so it’s important that anyone working on these constructions will need to understand their choice of cavity insulation. That includes merchants selling to those customers, as BMJ finds out.


F


ollowing the updates to Part L (Conservation of Fuel & Power) of the Building Regulations (England and Wales) and Section 6 (Energy) of the Building Standards (Scotland), ambitious U-values set for both new builds and extensions can mean that significant thicknesses of insulation are needed for building projects. This can impact both the cavity width and the overall wall depth, according to Matthew Evans, head of technical GB, at Kingspan Insulation Limited. He says: “The U-value targets set for projects will depend on the building type, location and scope of works. For domestic new builds, an average area weighted limiting U-value is provided for the different elements such as walls, roofs and floors. This is the worst possible U-value the complete construction can achieve


and should not be treated as a target. In practice, other aspects of the construction, such as the roof and floor U-values, would need to be significantly upgraded to allow a property to reach compliance when using this U-value.” This is why, Evans adds, most housebuilders use the U-values given in the Notional Dwelling/ Building specification as a starting point. This specification is applied to a theoretical building of the same dimensions as the actual project by an energy assessor. From this, software generates the performance targets that the actual building is compared against. “As such, whilst this specification isn’t compulsory, it provides a clearer guide of the U-values needed to reach compliance.”


U-VALUE TARGETS FOR DOMESTIC EXTERNAL WALLS Area weighted U-values (W/m2 Part L 2021 –


K) – external walls Part L 2022 - England - domestic


Notional Dwelling 0.18 Limiting U-value – 0.26


new build


Limiting U-value - 0.18 extensions


Wales - domestic 0.13


0.18 (0.21 – flats and apartments)


0.18


0.15 0.17


0.17 COMPARISON ON A TYPICAL CAVITY WALL CONSTRUCTION


Insulation and residual cavity


U-values for differing blockwork types (W/m2


K) Total


construction blockwork blockwork mK)


(1.13


102.5 mm brick outer leaf Insulation and residual cavity 100 mm blockwork 12.5 mm


skimmed plasterboard on dabs


1. 90 mm full fill phenolic insulation (0.019 W/mK) with 10 mm cavity


2. 75 mm partial fill phenolic insulation (0.019 W/mK) with a 50 mm cavity


3. 175 mm mineral wool (0.037 W/mK) fully filling cavity


32 333 mm 0.17


Lightweight Dense depth


(0.15 W/ 0.18


U-value targets for domestic external walls have been upgraded following the recent updates to Part L and Section 6 so it’s important to ensure customers are up to date with current requirements.


Section 6 (Energy) – Scotland - domestic


358 mm 0.18 0.19 408 mm 0.18 0.20


Evans adds: “The most obvious point to consider is the thermal conductivity (lambda value) of the cavity wall insulation. The lower this is, the more effective the insulation will be at preventing heat loss which can limit the thickness of insulation needed. Phenolic insulation boards, for example, can achieve a thermal conductivity as low as 0.019 W/mK.” Fundamentally, Evans explains, buildings need to adequately resist the effects of driving rain and, where moisture does penetrate the wall,


a suitably sized cavity helps stop it getting to the inside of a building.


“Depending on the wall’s resistance and level of exposure, a cavity might be fully filled, or partially filled with insulation. Partially filled cavity walls typically require a residual cavity width of 50 mm (or 75 mm in very severe exposure zones). Full fill solutions can be fitted with little or no residual gap (although this is usually not acceptable in the most severe exposure zones). Using a full fill product over a partial fill insulation with the same thermal conductivity can help achieve some savings in overall wall thickness,” he says, adding, that it is important to check the wind-driven rain exposure risk for the property


“Some certification and warranty providers may prohibit the use of full fill insulation, require different construction approaches to be adopted and/or require the residual cavity width to be increased. Customers should check the requirements with their provider before selecting the construction materials, residual cavity width and insulation.”


Staying slim


Evans says that getting the most useable space from an available plot is a priority for all projects, whether it be a new build or extension. “Recommending insulation materials with lower thermal conductivities can be a simple way to help customers to minimise the depth of their cavity walls whilst still achieving compliant U-values.” BMJ


www.buildersmerchantsjournal.net November 2023


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