BUILDING CONSTRUCTION
Fabric performance When targeting lower U-values, it is especially important to consider the thermal conductivity of the insulation. This is the measure of its ability to prevent heat loss through thermal conduction. The lower the thermal conductivity, the more effective the insulation is at preventing this type of heat loss. This means it may be possible to achieve a desired U-value with a much slimmer thickness of insulation, reducing the overall construction depth. Rigid insulation materials such as phenolic and PIR insulation have notably lower thermal conductivities than alternative materials such as rock mineral fibre. For example, take a typical flat roof
application above a 150 mm concrete slab with 50 mm screeded falls, a vapour control layer, and single-ply membrane. In this construction, it is possible to achieve a U-value of 0.15 W/m2
K with
a 140 mm thickness of PIR insulation (thermal conductivity 0.22 W/mK). To achieve this same performance with a typical rock mineral fibre product (thermal conductivity 0.38 W/mK), a total insulation depth of 245 mm is needed – greatly increasing the construction depth and weight.
Other aspects of the design This increased construction depth can have implications for other aspects of the design, such as the structural support requirements and extent of foundations. In external wall applications, it can also notably reduce the available floorspace which can be achieved within a given footprint due to the additional wall depth. The focus on building fabric also means it is worth specifiers considering the advantages of offsite approaches. For example, structural insulated panels (SIPs) feature a rigid insulation core sandwiched between two layers of oriented strand board (OSB), and can be used to form the roof and walls of buildings. These panels offer excellent ‘out-of-the-box’ thermal performance and insulation continuity, helping to reduce thermal bridging, while their jointing supports the construction of highly airtight buildings. In addition to these fabric benefits, SIPs can also provide clear programme benefits. The panels are pre-cut to the specific dimensions at dedicated offsite
Element type
All roof types (W/m2 Walls (W/m2 Floors (W/m2
K) K) Air permeability 3.00 m3 K)
Tapered roofing systems offer a lightweight solution for new and existing flat roofs, combining insulation and drainage in a single system.
facilities. This means that they can be produced and erected to a predictable schedule, and in many cases the outer building shell can often be erected in 4-6 weeks. Once a breather membrane is applied externally, and windows and doors are fitted, this shell is weathertight, meaning internal fit-out can begin while the external cladding and roofing are applied, further streamlining construction programmes.
Refurbishment and extension work The requirements for existing buildings depend on the specific work that is being carried out. Where an existing construction, such as a roof, needs to be removed and replaced, or an extension is constructed, these should typically meet the same worst-case U-values as for new build projects (shown in Table 1). Some flexibility is provided in recognition of the challenges that refurbishment work may pose. For example, in ADL2 2021 in England it’s noted that where insulating a floor may create significant access problems with adjoining floor areas, higher U-values may be acceptable, although these should still meet the threshold values in Table 1. Extension work on large buildings (over 1000 m2
) in England and Wales may also
ADL2 2021 England Notional Building
Specification (side-lit and un-lit buildings)
0.15 0.18 0.15
/m2 /Hr@50Pa. 4.00 m3 Scotland
Notional Building Specification
0.11 0.15 0.13
/m2 /Hr@50Pa.
Table 2: Notional Building in ADL2 2021 in England and TH6 2022 in Scotland (Welsh Part L2 Non-domestic has not yet been published).
trigger consequential improvements to other aspects of the existing building, such as upgrading the thermal performance of building elements. In Scotland, where an existing
unheated building is converted, or major refurbishment work undertaken, specifiers should look to meet the worst-case values in Table 1 where possible, and the threshold as a minimum in virtually all cases. Finally, in England, and under the expected changes in Wales, work on ‘existing elements’ can be completed to slightly relaxed standards. This applies where there is a change of use in the building, where an unheated building is converted, or if 50% of the surface area of an individual element is renovated, or more than 25% of the surface area of the external envelope is renovated. Ideally, this work should look to achieve the improved values shown in Table 3. Where this isn’t technically feasible, or will not provide payback within 15 years, you should look to achieve the best possible U-values achievable. In all cases, you should reach, or improve upon, the threshold values given in Table 1.
Historic or protected buildings In all regions, it is recognised that when carrying out work on historic or protected buildings, it may not always be possible to meet even the threshold values. In these cases, the guidance documents recommend that consideration should still be given to how the building can be sensitively improved, rather than adopting a ‘do nothing’ approach. This includes the use of innovative solutions, such as optimal performance insulation, which can help to address ‘problem areas’.
Insulating ‘problem areas’ On all refurbishment applications, it is not
October 2022 Health Estate Journal 83
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