Vacuum Insulated Panels

VIPs or Vacuum Insulated Panels are the best thermally performing product in the flat roofing market. The high level of thermal performance is due to the panels being vacuum sealed meaning they cannot be cut. If the panels lose the vacuum seal, they lose the high level of thermal performance. VIPs are often specified where there is a height restriction and/or to reduce the overall thickness of insulation required to achieve the U value requirement.

As the panels cannot be cut, a detailed panel layout must be designed to understand how much of the roof area can be covered by VIPs. The remainder of the roof is then infilled with PIR insulation which can be cut to suit. The VIP system is then overlaid by either a Flat board or Tapered insulation scheme.  This provides protection to the VIP and a membrane compatible top surface.

A common problem with VIP specifications is that a thickness of VIP/PIR is specified, often without this being calculated accurately. Some VIP manufacturers allow clients to specify a thickness using an online U Value calculator based upon the assumption that a given percentage of the roof can be covered with VIPs (typically 75%). This can be misleading:

To determine the required U value we need to ascertain what percentage of the roof can practically receive VIP panels, and what ratio of VIP/PIR is required to achieve the U. Value . This can vary dramatically from project to project due to roof size, roof shape and the number and size of roof penetrations i.e. SVP’s. This can only be achieved by carrying a detailed design.

Clarity at an early stage of the project design can minimise issues with other details, most notable door thresholds, ensure that the U.Value is achieved with the overall thickness and ensure the correct price is tendered upon.

The U Value required is a much better parameter to work to than a specified thickness, however knowing both parameters ensures that we can design within the maximum allowed insulation thickness.

Edging % → 5% 10% 15% 20% 25% 30% 35% 40% 45% 50%
Achieved U.Value 0.15 0.17 0.18 0.2 0.21 0.22 0.23 0.25 0.26 0.27

The table above illustrates the indicative (subject to deck type/build-up) Area Weighted U.Value when using a 40mm VIP, with a 25mm Tissue Faced PIR Overlay.  The ‘Edging’ percentage is the area of the roof ‘infilled’ with PIR, where VIP panels won’t fit.

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