Woodknowledge Wales (WKW) believes that we can only improve the performance of housing and really deliver zero carbon through the measurement of whole life carbon and testing of building performance. Otherwise we live in the dark.

How do we make building performance measurement practical and affordable?

Woodknowledge Wales and Cardiff Metropolitan University recently carried out detailed performance measurements on two newly built timber frame low rise blocks of flats as part of our Home-Grown Homes Project. The purpose of the work was to test out novel methods of building performance evaluation being pioneered by Build Test Solutions (BTS), who specialise in making practical building performance measurement technologies.

The two building projects participating in the testing were:

  • Pentland Close, Cardiff, a development for Wales and West Housing Association. Built by Hale Construction who procured Sevenoaks Modular as a specialist timber structures supplier using their Trisowarm system.
  • Croft Court, Welshpool, a development for Mid-Wales Housing Association. Built by Mid Wales Properties Ltd, who contracted AC Roof Trusses to provide the timber frame.

Both projects were designed with high thermal performance aspirations, and the measurements have shown that this high performance was delivered in practice. WKW hope that these measurements could provide the template for as-built performance measurement testing in the future.

Testing equipment in place at Croft Court.

The performance testing enables the contractors to demonstrate the quality of their work and providing quality assurance to their clients.

The Clients Perspective

Grant Prosser from Wales and West Housing Association commented that

“these measurements for the first time allow us to quantitively assess the energy performance of the built product at completion, as the performance gap is a significant concern for us as it negatively impacts on our residents this is fantastic quality assurance on this project and could be a great way for us to inform our product selection and work with contractors to provide high quality, low carbon new homes.”

The Manufacturers Perspective

Matt Hall from Hale Construction said that

“it’s been great to be involved in the project and get assurance of the quality of our work. We pay close attention to detailing to achieve the design airtightness and limit thermal bridging and it’s great to see the proof that this pays off”.

Thermal Performance and Buildings

Thermal performance refers to building’s ability to retain heat, so that when the performance is higher the dwelling can be heated inexpensively and with lower consequent emissions. It is measured by the Heat Loss Parameter (HLP), which is a measure of the rate of heat loss per degree of temperature difference between inside and out per m2 of floor area.

The performance gap is a much-researched phenomena in buildings, where the actual thermal performance is typically worse, and sometimes much worse, than the design expectation. For example, the Building Performance Network’s recent State of the Nation report which was part funded by Woodknowledge Wales found that in their sample of 29 buildings measured, 20 performed worse than expected by an average of 18% with an extreme case 100% worse than predicted.

Heating buildings accounts for around a third of all emissions in the UK, and addressing these emissions is therefore a key part of any national decarbonisation plan. At present all policy in this area is based on predicted, rather than measured performance, this is largely driven by a lack of practical methods to measure building performance.

The Pulse air permeability testing equipment.

How to Test Thermal Performance in Buildings

Currently the most widely used method to measure building thermal performance is called the co-heating test, it has been a crucial tool in revealing the performance gap but at a cost of thousands of pounds per test and requiring a building to be empty for two weeks it is not practical on a wide scale.

Build Test Solutions make building performance measurement equipment and methods to address this gap, which they applied alongside traditional methods on these two demonstration projects. The measurements included airtightness using BTS’ Pulse equipment and a blower door test, whole building thermal performance using BTS’ SmartHTC and a co-heating test and the thermal performance of the external walls using BTS’ heat flux plate kit. Richard Jack, a product manager at BTS, said that

“this project is an excellent demonstration of a full range of thermal performance measurements, and an excellent opportunity to engage with clients, manufacturers and contractors to understand how the measurements can help inform their processes”.

Croft Court, Welshpool

In Croft Court, the performance measurements were carried out in a top floor flat, for each measurement the measured performance was very similar to the design value.


This suite of measurements allows not just a judgement of the overall thermal performance, but also allows sources of heat loss to be further broken down through different heat loss paths.


Pentland Close, Cardiff

In Pentland Close, measurements were carried out in a top and bottom floor flat and for each showed close agreement with the design values. Data collection for the SmartHTC measurements in these flats was interrupted by the movement restrictions imposed by COVID-19 which means these results can’t yet be calculated.


Pentland Close. Ground Floor Flat Pentland Close. Top Floor Flat

The difference in performance between the two flats is caused primarily by the adjacency of the ground floor flat to an unheated buggy store. By comparison, the top floor flat (which is of the same dimensions) is next door to another heated flat and hence assumed to have no heat loss through the equivalent wall. This results in an extra source of heat loss through the internal wall between the two and also a higher calculated thermal bridging due to larger exposed area. The breakdown in heat loss for the two flats shows this additional heat loss to the buggy store.

Comparing different Build Projects

Beyond the comparison with the design figures for each flat, it’s also possible to consider what level of performance the flats reach in comparison with others.

At present, fabric performance metrics are not common measures of the energy performance of dwellings, with the Energy Efficiency Rating from the Energy Performance Certificate the most commonly used metric. The Energy Efficiency Rating is based upon the expected cost of fuel consumption to heat a dwelling, calculated using the Standard Assessment Procedure (SAP). This calculation is based upon the thermal performance of the dwelling, but also an assumed occupancy pattern, the efficiency of the heating system and the assumed cost of energy (gas and/or electricity). The Croft Court flat has solar PV panels which significantly offset the cost of electricity, and hence result in a higher (better) EPC rating.

Flat EPC Rating
Croft Court 92 (A)
Pentland Close ground floor 81 (B)
Pentland Close top floor 82 (B)

Measuring fabric performance

A building’s thermal performance is key to its energy performance and thermal comfort, it is fundamentally important because the building will likely last longer than its systems or occupants. As such it makes sense to consider a specific fabric performance metric such as the Heat Loss Parameter, alongside the EPC rating, promoting a fabric-first approach. The HLP has previously been used as a key performance indicator in the Code for Sustainable Homes.

All three flats measured display high levels of thermal performance, with insulation levels similar to those recommended in schemes such as Passivhaus (though with higher air permeability). This is clear when comparing the measured HLPs in these flats, which ranged from 0.65-1.00, with the HLP scale produced by BTS.

The performance measurement demonstrations provide quality assurance on these three flats, and a demonstration of what’s feasible using performance measurement. Diana Waldron from Cardiff Metropolitan University, project partners on the Home-Grown Homes project, summed up the project as;

a unique opportunity to gain further understanding in the area of building performance evaluation methods, aiming to find ways to make them more approachable to all relevant actors in the building industry. All the learning captured during this investigative work will be further disseminated, put into practice and re-tested, in tandem with our main aim: to achieve better quality homes in Wales”.