Can vacuum insulation panels (VIPs) be used to insulate pitched roofs?

2 September 2020 Kingspan Insulation UK

When installing roof insulation on an existing pitched roof, the thickness of the material is often a key concern. Insulation fitted below the rafters can cut into headroom within the attic space, whilst installing above them will raise the roofline requiring changes to the verges and eaves. As a result, we receive a lot of questions about how best to tackle these areas and whether our Kingspan OPTIM-R VIPs are suitable for these applications.

In most cases, if the roof is being retiled, then we would generally recommend a warm roof construction with layers of Kooltherm K7 Pitched Roof Board between and above the rafters. Alternativley, if you simply want to raise the thermal performance of the structure then it is typically best to fit a layer of Kooltherm K7 Pitched Roof Board between the rafters (with a ventilation gap of at least 50 mm) with Kooltherm K118 Insulated Plasterboard below. We covered both these pitched roof applications in more detail in this blog.

However, there are a handful of applications where even Kooltherm may be too thick, for example, on a conservation area or historic building where changes to the roofline may impact the overall appearance of the building. In these specialist applications, our technical services team can work with specifiers to create bespoke solutions using our OPTIM-R VIPs.

Why use VIPs?

The primary purpose of insulation is to prevent heat transfer through conduction. As thermal conduction cannot take place within a vacuum, VIPs are able to achieve an insulating performance which is far better than more commonly used insulation options. For example, our OPTIM-R VIPs have a core that is made up of millions of microscopic pours. This is then evacuated of air and sealed within a gas tight envelope. These panels achieve a declared thermal conductivity of 0.007 W/mK, which is up to 5 times better than other commonly used insulation. As a result, project teams can achieve target U-values with a much slimmer thickness of insulation.

VIPs are typically fitted in a system with rigid insulation boards of the same thickness. These can be installed around the perimeter of the surface (whether it be a floor or roof) and to allow any penetrations through the insulation layer.

We offer dedicated systems for flat roof and ground floor applications supported with a design service, where our technicians will use your building plans to create the optimal system layout to meet your desired U-value with the slimmest construction. As mentioned, our technical services team can also work with project teams on one-off applications, such as pitched roofs, although the turn around time for these will be a little longer than for our dedicated systems.

VIP applications on pitched roofs will also take some additional care and attention. To give you an idea of what might be involved, let’s take a look at an example application.

Installing VIPs on a pitched roof

Installing OPTIM-R on a warm pictched roof
This application is on the roof of a mid-Victorian era barn. The owners wanted to retain the original exposed oak beams inside the barn which meat that the insulation layer needed to be installed above newly laid sarking boards. In order to meet the target U-value of 0.17 W/m²K, a 125 mm thickness of conventional insulation would have been required. By switching to OPTIM-R VIPs this was reduced to just 50 mm.

As this is a warm roof construction, a vapour control layer was first fitted above sarking board to allow vapour to pass through from the space below. The Kingspan OPTIM-R VIPs and Kooltherm boards were then laid over this. The Kooltherm boards were fitted around the perimeter of the roof surface with thin strips laid in an alternating pattern with two rows of the OPTIM-R VIPs. These strips provide a point for the mechanical batten fixings to be driven into place.
Installing OPTIM-R on a warm pitched roof
The lowest sections of roof membrane were then rolled out, joints taped and a number of timber battens laid in place. The battens at either end of the section were fixed through the Kooltherm sections and into the structural woodwork at marked points. By running string between these fixings, the installers were able to quickly and accurately fix the intermediary battens in place through the Kooltherm board. Cross battens could then be fixed, and the final slate tiles fitted in place as with a typical construction.

Meeting future challenges

The UK’s commitment to achieving net-zero greenhouse gas emissions by 2050 means that the energy performance of much of our existing built-environment will need to be substantively improved. By adopting advanced approaches such as VIPs, and ensuring they are carefully designed and installed, it is now possible for designers and installers to effectively tackle more challenging applications without reducing the quality or character of the final space.

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