CPD: Cavity closers and thermal bridging
St Catherine’s Primary School in Worcester used Kingspan Kooltherm Cavity Closers throughout
Adrian Pargeter, head of technical at Kingspan Insulation, outlines the results of a study of jamb and sill details in buildings.
The performance gap between the designed and “as built” energy performance of properties has been a topic of much discussion in recent years. As insulation levels within buildings increase, details around windows and doors take on greater significance in the overall thermal effectiveness of the building envelope.
Cavity walls are one of the most commonly used constructions in the UK, and one of the simplest and most effective ways to prevent heat loss from thermal bridges in these areas is by installing insulated cavity closers.
Thermal bridging is a key contributor to heat loss within a building. Bridging occurs when two exposed thermal elements meet, which allows heat to transfer through conductive materials to the outside. This can be, for example, where a ground floor meets an exposed wall, an external wall meets a balcony, lintels and sills, window or door jambs.
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This CPD discusses a thermal modelling study by BRE Scotland of jamb and sill details and the best way of using cavity closers in construction of buildings. Specifically, the study analysed different cavity wall constructions using Kingspan Insulation’s premium performance cavity closer.
The study took place over a number of years and was headed up by BRE senior scientist Tim Ward. It highlights the issues with relying on the traditional measurement for heat loss – psi-values calculated using the accredited construction details (ACDs), and provides a simple route for builders to achieve enhanced psi-values with Kingspan cavity closers.
Cavity closers are insulated extrusions for closing wall cavities at openings such as window reveals and door reveals. They can come with various types of insulation, but using a cavity closer with a premium performance insulant may provide improved fire resistance as well as improving the overall thermal efficiency of the building envelope. It can also include an integral damp proof barrier, effectively preventing the problems frequently associated with traditional methods of closing cavities and thermal bridging.
It is important to note that since sill details are usually identical in build-up to jamb details (there is an exception when a sill board is used instead of the standard plaster or plasterboard on dabs), the heat flow through a sill is less than that through a jamb. Therefore, in BRE’s opinion, the psi-values calculated for jamb details in this study are applicable to the equivalent sill details.
The results of the BRE study clearly show, by installing premium performance insulated cavity closers, it is possible to easily achieve significant reductions in thermal bridging. By increasing the amount of overlap on the frame, further reductions can be made.
To truly close the performance gap, it is crucial that installers ensure all detailing is carried out correctly. Failure to do so may not only impact the building’s performance but also the installer’s long-term reputation.
For those serious about designing low energy buildings, attention to detail is critical, yet the devil need not always be in the detail.
Psi-value parameter comparison
Worst case ψ value
Several construction parameters influence the psi-values of a jamb and sill which were tested one by one, identifying the option which yielded the worst psi-value. This would be taken as the “worst case” parameter and used in all subsequent calculations. This defined a series of worst case psi-values, plus the range of constructions for which they are valid.
This ensured that the other construction parameter options, which gave better psi-values, would also be covered. After the worst case construction parameters were determined, psi-values were calculated for the different frame depths and overlaps at U-values of 0.18, 0.20 and 0.25W/m2.K. The range of psi-values set out here are applicable to the following wall constructions (with worst case parameters highlighted in italics):
- Internal finish of either lightweight plaster, standard plaster or plasterboard on dabs;
- Wall cavity insulated with either partial fill cavity insulation with a residual air gap of ≥ 50mm, or full fill cavity insulation;
- Check or normal reveal;
- Premium performance cavity closer (λ ≤ 0.22 W/m.K) either as a single section or coupled sections, fully filling the wall cavity width;
- Inner leaf of either ≥ 100mm warm steel frame or ≥ 100mm blockwork (λ ≥ 0.11 W/m.K) (worst case - 100 mm lightweight blockwork (λ - 0.11 W/m.K); and
- Outer leaf ≥ 100mm brickwork (worst case - 100 mm brickwork (λ - 0.77 W/m.K))
Overall heat loss associated with thermal bridging
As part of the overall study another experiment was done to quantify the significance of the improvement that can result from the adoption of cavity closers; the transmission heat transfer coefficient (HTB) using the “safe approximation” psi-values needed to be compared against the approximated psi-values derived from the ACDs (shown in the SAP Table K1 “Approved” column).
Using SAP 2012, the HTB was calculated for four different new-build houses. These buildings were representative of current new-build dwellings and Building Regulations in England.
The HTB for each dwelling type was calculated using the psi-value from the SAP Table K1 “Approved” column and the “safe approximation” psi-values that incorporate the premium performance cavity closer. The two HTB values were then compared and the % improvement was calculated.
The comparison is detailed below. It shows clearly that the HTB calculated using the “safe approximation” psi-values is significantly better than that calculated using the psi-values derived from the ACDs. It can also be seen that the greater the depth of overlap between the frame and the closer, the greater the improvement.
Depth of overlap
The results also show that the depth of the overlap between the frame and the cavity closer has the greatest influence on the psi-value.
For the purpose of simplicity, when calculating the HTB for a building, the psi-values shown right can be used as a “safe approximation”, for the full range of constructions described and wall U-values ≥ 0.18 W/m2.K.
Case study: St Catherine’s Primary School, Kier Construction
St Catherine’s Primary School is a £5m primary school built by Kier Construction for Worcestershire County Council under the West Midlands Construction Framework.
To maximise the fabric performance of the cavity walled structure, Kier fitted rigid phenolic insulated cavity closers around openings in the building envelope.
The Kingspan Kooltherm Cavity Closers helped to minimise linear (non-repeating) thermal bridges. This was a key project consideration as Kier Construction project manager Ali Long explains: “The cavity closers provided a cost-effective solution which helps the building achieve improved energy performance, complementing other energy efficient design features. Kier and Kingspan Insulation worked closely to ensure the solution was delivered on time and to budget.”
The cavity closer also helped meet Building Regulations requirements regarding internal fire spread. The specified product has passed a 30-minute UKAS-accredited fire resistance test with PVC windows in thicknesses up to 150mm. In addition, its PVC-U J-section acts as a positive damp proof barrier.
1. Linear thermal bridging
Linear thermal bridging describes the heat loss or gain at junctions between elements and around openings where there is an interruption in the insulation layer. Linear and point thermal bridges are considered when calculating the energy performance of a building.
2. Cavity closers
Insulated extrusions for closing wall cavities at openings such as windows and door reveals, reducing heat loss and prevent thermal bridging, condensation and mould.
3. SAP 2012
The Standard Assessment Procedure (SAP) is the methodology used by the government to assess and compare the energy performance of dwellings. SAP calculates typical annual energy costs for space, water heating and lighting, plus the CO2 emissions of a dwelling.
Psi (Ψ) values are a measure of the linear thermal transmittance of a thermal bridge
and have units of W/mK where m is the linear metres of the thermal bridge.
The vertical components that form the sides of a door frame, window frame, or fireplace, or other opening in a wall.
A horizontal slat which forms the base of a window.