CPD: Improving thermal performance of flat roofs

Thermographic image of the Scottish Parliament

As firms start to gear up for compliance with the 2018 minimum energy performance standard, the roof can be the best place to start. Steven Cookson explains.

When it comes to thermally upgrading an existing roof, what is the best approach to specification and compliance, particularly in light of the proposed minimum energy performance standard (MEPS)? The MEPS, introduced under the Energy Act 2011, is part of the government’s drive to reduce the UK’s CO2 emissions by at least 80% from the 1990 baseline before 2050, by encouraging owners to improve the quality of the fabric of their existing buildings.

From April 2018 it will be unlawful to let properties that fail to achieve a prescribed minimum energy performance standard until qualifying improvements have been carried out. The MEPS regulations mean that landlords will not legally be able to rent a building that has an F or G Energy Performance Certificate (EPC) rating. According to data from the national EPC register, 18% of all UK buildings fall into these categories.

The detailed regulations are currently being considered by parliament, with many commentators believing that the government is going to go even further than previously expected. And two planned shake-ups to the EPC rating system itself mean an even greater percentage of buildings could fall into the lower EPC ratings, possibly making the impact of the MEPS regulations even greater.

The financial impact of MEPS cannot be underestimated: when the legislation comes through the impact will be felt far and wide. It will leave many buildings unlettable and have a severe impact on a portfolio’s value for the ultimate owner. With just over two years before the proposed regulations come into force, many property owners are anticipating the change and upgrading their portfolios to meet an EPC rating of E or even better. Of course, this presents a huge opportunity for contractors.

From the top down

Although some buildings may be unable to achieve the anticipated E rating required, for most there will be a range of upgrades and refurbishments to consider and the roof is a great place to start. Heating and lighting systems have often already been upgraded, while planning consent is usually required to make changes to the external walls and windows of a building.

For many commercial properties the roof can be vastly improved, with little disruption or negative visual impact, while the building remains occupied. Savings of between 10-35% can be achieved depending on the current roof make-up and chosen specification.

There are many considerations when upgrading a building to ensure it performs to the correct thermal standards. For buildings with a flat roof it is possible to leave the current build-up in situ and bond new insulation and waterproofing to the existing substrate, to remove the costs of replacing the whole roof. To do this, however, it is vitally important to ensure the existing insulation is checked to ensure it is performing to the expected standards set out in Approved Document L – Conservation of Fuel and Power.

A roof system that is operating well should contain little or no water. However, when a leak develops water will enter the assembly and will remain in the structure for a long time – sometimes for the life of the roof. Depending on the type of insulation used, system moisture is either absorbed by the insulation or runs to the cracks between the non-absorbent insulation.

Moisture compromises the thermal integrity of the insulation and its location needs to be identified before developing a strategy to improve the energy-efficiency of the building. In the past the roof inspection would have required a very time-consuming grid-type contact search using multiple core samples and strip-ups. Of course this is financially costly. Non-contact and non-destructive infrared technology can now be used to highlight the extent of what needs to be replaced and how the current status is affecting the thermal integrity of the building.

A scientific approach

A specialist thermographic survey can be used to quickly and cheaply identify not only where leaks exist but also the extent of moisture ingress. As the materials in flat roofs have relatively low thermal capacitance compared to water, dry roof insulation heats up and cools down faster than wet roof insulation. This means that the temperature of the roof after dark can be used to determine moisture with a thermographic photographic survey exposing the temperature differential between wet and dry roof insulation.

Areas of damp or saturated insulation will reveal themselves on the images as hot spots, looking red and yellow. This is due to two factors. First, wet insulation is not as effective as dry insulation and, second, the thermal capacitance and emissivity of water is very high, meaning water is excellent at retaining and radiating energy. The cameras can detect temperature differences down to 0.07 degrees C, so areas that may seem dry, even to the touch, can be identified with a high-end thermal camera.

Sika’s interactive carbon dashboard allows users to move sliders and see potential savings on three “speedometers”

Quality and innovation are just two of the characteristics replicated throughout Sika Liquid Plastics – visible in its products, manufacturing processes, and research & development.

Since 1963 we have produced an extensive range of high performance, technologically advanced coatings and membranes for the protection and waterproofing of roofs and balconies.

These products are leaders in their field because of their unique properties and ability to outlast and outperform traditional counterparts. Also available is a complete range of cold applied built-up roofing systems including warm roofs, inverted roofs, and green roof finishes.

As a member of the Sika worldwide group, the demand for our products stretches across the globe and many successful projects have been completed in the Middle East, USA, and Europe.

Sika Liquid Plastics is leading the way with innovative products for the future, as the construction industry evolves towards safer applications and protection systems with design lives as long as the building itself.

A survey is carried out at night, to minimise the effects of reflected radiation, either by an operative on foot or, if access or health and safety is an issue, using a remote-controlled drone. The surveyor then uploads the images to the office for analysis and quantification.

Infrared roof surveys are appropriate for the majority of flat roof types, including:

  • built-up roofs with polyisocyanurate insulation
  • roofs that include membrane systems
  • roofs with rigid insulation boards
  • solar chipped felt roofs.

Any roof can be thermographically surveyed if the waterproofing can be seen. But one type of roof that does not lend itself to an infrared survey is an inverted roof membrane assembly, where the waterproofing layer is beneath the insulation, instead of above it as in other forms of roofing.

For inverted roof insulation, boards are loose-laid on top of the membrane and then weighted down with paving slabs or gravel ballast. For these roofs to be surveyed successfully the client needs to be willing and able to clear areas back to the waterproofing.

Where this cannot be achieved, infrared surveys can be undertaken internally, provided the underside of the deck can be seen. However, care must be taken to ensure the survey is not simply imaging deck deflections and standing water. Best practice would be to always survey externally when possible.

Cold roofs, where the insulation is placed below the structural deck, also present a problem as the cameras can only see the temperature on the surface of the roof.

The three steps to calculating the return on investment of a roof upgrade are:

  1. Carry out an initial infrared survey to establish the current performance of the roof insulation and any areas of concern
  2. Analyse and quantify the data to arrive at the correct roof specification
  3. Re-survey once the project is complete to ensure the energy saving is as expected.

The surveying process delivers high-quality images of defects to help all parties see the problem and the need for the refurbishment.

By adding values to normal infrared images, the impact of the roof defects can be calculated in terms of kilowatts, carbon and cash. “Monetising” the images in this way can help to convince the client of the need for the refurbishment in the first place.

To help with step 2, Sika has developed an interactive carbon dashboard (see above). This is essentially a simple website where users can move sliders and see savings on three clear “speedometers”. For building owners upgrading to meet MEPS requirements, this analysis will show the likely impact of the retrofit on the EPC rating of the building.

The flat roof of this shopping centre shows no outward signs of moisture to the naked eye (top). However, an image of the same area (above) taken during a thermographic survey of the roof reveals the extended areas of saturation, showing as yellow and red, spreading from an air-conditioning unit that shows on the photograph as white.

Then, by commissioning a second infrared survey in step 3, the client can rest assured that they are getting what they paid for, that they are futureproofing the building and that they have chosen a specification that is fit for purpose. The after-survey is the final touch which means that no one will have to sit through a “blame game” meeting about the roof.

The combination of the best specification and a thermographic report will offer best value to the client, reducing the performance gap, contributing to environmental targets and helping the buildings’ tenants to reduce their operating costs.

The ever-changing world of specification has created a performance gap for building owners. There is an assumption in the industry that if you read the drawings in conjunction with the specification, then you know what the building is made of and can measure its theoretical lifespan, value and maintenance cost. But clients are often expected to make bold investment decisions on information that is unverified, a dangerous and risky position.

Business doesn’t like risk – it’s a well worn cliché. But a business that owns a multi-million-pound portfolio of buildings underwritten by long guarantees on the fabric could be about to realise that, due to legislative changes, the portfolio could halve in value and be impossible to let without investing tens of millions.

This may well be the reality in 2018. It is an issue that hasn’t yet been properly quantified – and one that is likely to catch out many property managers if they remain unaware.

Steven Cookson is technical services manager at Sika Liquid Plastics

A valuable lesson in energy loss

Working in collaboration with an infrared company, Sika Roofing – which comprises Sika Liquid Plastics and Sika Sarnafil – recently surveyed a primary school in the north-west of England and discovered that its 1,100 sq m roof revealed large regions of poorly maintained waterproofing as well as ponding water.

To help determine the extent of the trapped moisture and damaged insulation, thermal imaging and refurbishment calculations were carried out, which were based on bringing the properties up to current building standards.

The survey revealed the extent of damage and the roof’s real condition. While the roof was in better condition than expected with 660 sq m in good condition, a further 253 sq m had 50% saturation of insulation and the remaining 187 sq m had 100% saturation of insulation.

The estimated total energy use for this building based on the original construction would consume 317 kWh/sq m per year.

The survey identified 440 sq m of damaged insulation, meaning that the building would consume approximately 330 kWh/sq m per year. By overlaying the existing waterproofing and adding an additional 90mm of insulation – the new roof system would reduce the yearly consumption to 265kWh/sq m or 20% per year.

For the school this meant a potential saving of £28,600 over 10 years, and an improvement of the EPC grade from E to D. This was an operational saving that was most welcome to the client and shortened the return on investment for the roof itself.

Following a survey the felt roof of Eden Park Primary School Academy was replaced with a Sika liquid waterproofing membrane

The flat roof of this shopping centre shows no outward signs of moisture to the naked eye (top). However, an image of the same area (above) taken during a thermographic survey of the roof reveals the extended areas of saturation, showing as yellow and red, spreading from an air-conditioning unit that shows on the photograph as white.

This article has been created by Construction Manager in partnership with Sika Liquid Plastics

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