Chartered Institute of Building Magazine of the Chartered Institute of Building

CPD articles

CPD: Insulating flat roofs and balconies

3 June 2015

As demand for balconies and roof terraces above habitable space grows, vacuum-insulated panels are one way to meet the technical challenges they present. Jake Watkins explains

Buildings consume almost half of the energy used in the developed world, so it is understandable that environmental issues influence the construction industry at every stage of the building process. Government legislation and the Building Regulations have become more stringent with every update, requiring more sustainable approaches, and driving the industry to design, construct and deliver more energy-efficient buildings.

Architects, clients and contractors cannot solve all the world’s energy problems, but they can contribute by creating buildings that run at lower levels of energy consumption. A thermally efficient building envelope can make a significant contribution to reducing heating and energy usage, and in turn minimise the impact on the environment, economy and society. Optimum performance can be achieved by using the right combination of product, surface preparation and application procedures.

In response to the growth in demand for improved thermal performance, while keeping the building fabric as thin as possible, waterproofing and green roof specialist Radmat has created ProTherm Quantum Vacuum Insulated Panel (VIP) system – an ultra-thin thermal insulation system for inverted roofing applications, where the waterproofing layer lies beneath the insulation instead of above it, as in other forms of roofing.

Vacuum-insulated panels at 40 mm of thickness (left) provide greater thermal resistance than 200 mm of extruded polystyrene insulation (centre) and 220 mm of expanded polystyrene (right)

Balancing thermal efficiency with constructibility and functionality in use is often a challenge – and nowhere more so than on a building that has a balcony or roof terrace above a habitable space.  Here the requirements to achieve thermal performance compliant with Part L of the Building Regulations, to meet the criteria set out by NHBC Standards chapter 7.1 “Flat roofs and balconies” and, third, to provide a level threshold as required by Part M (Access to and Use of Buildings) of the Building Regulations are particularly demanding.

The limitations of traditional products can make it difficult for an architect or designer to insulate above a habitable space. With such little space for insulation, the solution has often been to insulate both on top of and on the underside of the balcony or terrace. This can be time consuming, and impact on ceiling space and building services, as well as posing a condensation risk.

VIP package

Vacuum-insulated panels (VIP) are fast becoming the product of choice to insulate these challenging balcony and terrace areas. Traditional natural insulation can require 350–450 mm of material to achieve the same performance as 120–300 mm of today’s artificial materials. By contrast, only 20–70mm of vacuum-insulated panels are required to achieve the same level of performance, so VIPs are being specified on an increasing number of projects.

Although not a new technology, the design and use of VIPs has developed significantly in the 85 years since the first rubber-enclosed panel was patented in 1930. Originally developed for use in refrigerators, freezers and cold-shipping boxes, VIPs have evolved to become widely used across a range of applications. However, the science behind VIPs has long remained the same.

An insulating material’s effectiveness is measured by its thermal transmittance, or U-value, which is the rate at which heat is lost (W/m²K), and its associated lambda value (W/mK), which describes the quantity of heat (W) that is conducted through a wall 1 metre in thickness. The better the insulation, the lower the thermal conductivity and lambda value.

Heat transfer through the insulation occurs by three modes: convection, conduction and radiation. Common methods of insulation prohibit heat transfer by convection – by using a matrix of fibres in the case of mineral fibre insulation, and using cells (bubbles) in plastic insulations such as polystyrene and polyurethane.

Minimising the depth of insulation allows level access between the flat and balcony

1.10 mm waterproofing layer
2. 5 mm rubbercrumb protection layer
3. Quantum VIP
4. Extruded polystyrene (XPS) layer
5. Black filter sheet

As a result, a product’s insulation capacity is limited by the conductivity of air, which itself has a lambda value of 0.025W/mK. By combining the conductivity of the air with the matrix material itself, a typical lambda value of 0.03W/mK or higher is certified: which in today’s market is simply not enough for some applications.

Replacing air with a less-conductive gas such as pentane improves the performance of the plastic insulation material, leading to a minimum lambda value that is closer to 0.02W/mK. However, removing air or gas completely further removes conductivity from the core, improving thermal performance even further, and creates a VIP with a lambda value of 0.007W/mK.

When used in an inverted roof insulation system, such as Radmat’s ProTherm Quantum, VIPs give a dramatic reduction in thickness with no loss of thermal performance. This enables architects and construction professionals to provide balconies and terraces that conserve resources, save energy and meet the needs of the construction industry.

Building balconies and terraces over heated space is problematic for architects, designers and contractors: in new-build applications, stringent regulatory requirements and long-term economic viability have created greater demand for improved energy efficiency, while roofing designers and installers must strive to keep overall construction as thin as possible.

Space is often at a premium on construction sites, so every additional square metre of floor space offers added value to the client. In multistorey developments and high-rise buildings, extra floor-to-ceiling height can also make a huge difference to saleable space. High-performance insulation already meets some of these requirements, but there remains demand for thinner products that will have a smaller impact on constructibility and function in use.

Radmat’s ProTherm Quantum Inverted Roof System can achieve a U-value of 0.15 W/m2K while being up to 80% thinner than extruded polystyrene insulation (XPS). Over a 20-storey development, the specification and installation of this system could reduce the thickness of insulation required enough so that an additional storey could be built within the same-sized envelope.

Manufactured in the UK, the Radmat Protherm Quantum VIPs consist of a microporous core from which air and moisture are removed before it is encased and sealed in a thin, gas-tight special hybrid aluminium. This combination provides thermal transmittance of 0.007 W/m2K even at 40 mm thickness. This compares with 0.034 W/m2K for 200 mm thick XPS and 0.038 W/m2K for 220 mm expanded polystyrene (EPS).

Accessible areas

Part L of the Building Regulations sets the levels of thermal insulation required for both new builds and refurbishments. Expressed as a U-value, this required standard will depend on the location, type of building and the application. For flat roofs, this varies from 0.11 W/m2K in new builds to 0.18W/m2K in refurbishments – with the typical requirement for a balcony or terrace of 0.15 W/m2K.

As well as meeting the thermal performance requirements of the Building Regulations, designers must ensure that accessible balconies and terraces comply with Approved Document M: Access to and Use of Buildings. This document says there must be a smooth transition from the internal space onto the terrace or balcony without any step or change in floor height.

Vacuum-insulated panelling allows for a range of finishes at varying thicknesses for residential balconies

It states: “People, regardless of disability, age or gender, should be able to gain access to buildings and to gain access within buildings and use their facilities, both as visitors and as people who live or work in them.”

Another key specification in the design of a balcony or terrace in residential apartments is stipulated by the NHBC Standards. These set the benchmark for acceptable levels of design, material specification and workmanship for newly built homes registered with NHBC. Chapter 7.1 “Flat roofs and balconies” sets a minimum void of no less than 75 mm required between the surface of the insulation and the underside of the sill. With this in mind, it is even more important that the insulation specified is as thin as possible.

As demand grows for flats with private outside spaces and non-residential buildings with accessible terraces, Radmat has worked with contractors to help solve problems on projects where achieving U-values, level thresholds and waterproofing integrity initially did not seem possible (case study, below).

Finishing touches

In contemporary architecture and design, balconies and terraces must also offer aesthetic flexibility. To meet clients’ changing needs, the Radmat Quantum system has been designed to accommodate a series of finishes, including gravel ballast, paving slabs, decking and green roofs.

  • Gravel ballast should be 20-40 mm diameter, washed and rounded, and installed to a minimum depth of 50 mm. Installation of the ballast layer should happen as soon as possible after installation of the thermal filter sheet to ensure the membrane is always protected and that excessive heat build-up or high winds do not damage the Quantum VIP panels. The diameter of the gravel is important: this size has been found to be the most resistant to wind scour.
  • Where paving slabs are used, they should be a minimum 50 mm thick and laid over the thermal filter sheet on proprietary paving slab supports of a minimum diameter 175 mm (or equivalent base area). The paving support pads maintain drainage below the slabs and ensure that moisture vapour can escape from within the system. Gaps between the paving slabs and the upstands should be filled with washed, rounded gravel 20-40mm in diameter.
  • A green roof system can also be installed over the ProTherm Quantum Inverted Roof Insulation System. A root barrier – unless provided by the waterproofing layer – should be loose laid on or bonded to the waterproofing membrane with all laps sealed prior to the installation of the first layer of Radmat Regupol acoustic protection panels. The root barrier should be turned up at the edge of the roof insulation and sealed under flashings.

In conclusion, the Radmat Quantum VIP system is specifically designed to dramatically reduce the depth of the traditional inverted roof, providing a solution to counter low upstands against the increasing depth of traditional EPS and XPS required to meet stringent thermal demands. By designing and supplying every element of the inverted flat roofing systems, Quantum can help the contractor and client to achieve the desired U-values, waterproofing integrity and long-term performance.

Jake Watkins is senior technical manager at Radmat Building Products

A slimline tonic for a flat roof

Radmat has worked with contractors to help achieve U-values, level thresholds and waterproofing integrity.

In one such challenging residential application space was limited because issues with slab heights meant it was not possible to meet the target U-value of 0.16 W/m²K with traditional insulation products. The original specification called for a high-performance 200 mm thick Radmat ProTherm G insulation board with a 10 mm thick waterproofing layer to provide a  U-value of 0.15 W/m²K.

But with a total system depth of 285 mm – inclusive of the NHBC-compliant 75 mm void – a thinner insulation solution was required.

To resolve the height issue a compromise solution was considered: using 100 mm Radmat ProTherm G insulation board above the waterproofing with an additional 60 mm thick Kooltherm insulation board beneath the slab. This reduced the system depth above the slab by 100 mm and provided a U-value of 0.16 W/m²K, but also introduced a potential condensation issue and took up valuable ceiling void space.

Finally the two ProTherm Quantum Systems were evaluated: Hybrid and Pure. The Hybrid system combined 40 mm thick ProTherm Quantum VIP and 40 mm expanded polystyrene insulation (XPS) to deliver a U-value of 0.14 W/m²K with 185 mm system depth. This exceptional thermal performance was achieved without compromising the internal upstand height, meeting NHBC requirements, and maintaining a level floor between both internal and external spaces to satisfy Approved Document M.

However, the ProTherm Quantum Pure option, which removes the XPS layer and replaces it with a second layer of ProTherm Quantum VIP 50 mm thick provided an even thinner system: achieving the 0.15W/m²K target U-value with only 145 mm thickness from slab to finished floor level.

This optimum solution for these insulated balconies began with a high-quality, British Board of Agrément-certified 10 mm thick waterproofing system installed by an approved contractor. Next, 5 mm Radmat Regupol acoustic protection panels were installed over the waterproofing, followed by the two layers of Radmat Quantum VIP inverted roof insulation panels with Thermoset infill board at perimeters.

A further layer of Regupol acoustic protection panelling was installed before completion with TS Thermal Filter Sheet and paving slabs on paving spacers. The completed ProTherm Quantum Pure build-up delivers a U-value of 0.15 W/m²K with a total system depth of just 145 mm.


Radmat Building Products is an independent British company whose range of waterproofing systems will provide a lifetime’s protection for any building. The full product range includes PermaQuik Hot Melt, EshaPlan single ply, EshaFlex RBM, EshaGum RBM, EshaUniversal single layer, ParaFlex liquid applied, TredWay paving and MedO green roof systems.

The company is committed to providing technical support to the specifier and construction team, with the elements of service that give comfort and a satisfactory waterproofing solution to meet the parameters given.

Radmat offers extensive technical support at all stages of the construction process, and is committed to supplying first-class materials, installed by an approved list of contractors and
agreeing designs that are both practical and cost effective.