CPD articles

CPD: Cavity closers and thermal bridging

3 February 2017 | By Adrian Pargeter

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.    

Conclusion

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.

Above: Typical window detail design using premium performance cavity closers which help significantly reduce thermal bridging

Above: Looking down on two windows in plain view - one with a back-to-back cavity closer, one with a single cavity closer. 1. Internal wall 2. Internal window sill 3. External wall 4. Window frame 5. External window sill

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):

HTB comparison

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

Influencing psi-values

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.

Glossary

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.

4. Psi-values

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.

5. Jamb

The vertical components that form the sides of a door frame, window frame, or fireplace, or other opening in a wall.

6. Sill

A horizontal slat which forms the base of a window.

Comments

The British building methods and construction details for low rise buildings is like the "dark ages" compared with North America. With still so much wet trade work and no up and coming tradepeople the Designs and Builders have to get with it. At the same time the secondary education institutes have to come up to speed with the modern world

Roger Ward, 28 June 2017

The article continually talks about the overlap with the frame. The frame of what? Please explain or provide a sketch to explain. I'm sure it is straight forward but I'm struggling to understand.
Thanks

Kym, 28 June 2017

I have difficulty in trying to visualise what Kingspan is putting forward in the article. The door or window will have a cross sectional area and as I understand it is partly this and the make up of the material in the product that is going to give the door/window the uvalue etc. The unit then has to be installed in a prepared opening created by others and the opening as I understand again will generally be constructed larger than the unit to be installed and the gaps filled with other materials. The material filling the gaps will not have good uvalues, plaster materials, etc.
What size should the openings be made to, to ensure the building achieves the best uvalue? There is talk in the article about overlap, does the writer of the article mean plaster overlapping the frame, or what? Is there a tape used to reduce/minimise air infiltration? Please elaborate. some sketches would be beneficial.

Danny Bonner, 29 June 2017

The best way to fill up cavity walls today is closed cell spray foam as it has the best U-value and it is installed fast

James Smith, 29 June 2017

Cavity wall construction in the UK is like being in the dark ages compared with North America Stop using wet trades that are slow and so called skilled Dry construction is faster, provides good R values Get with it
Roger Ward

Roger Ward FCIOB PQS(F), 3 August 2017

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