How to measure thermal efficiency?

thermal effiency measurement

Traditional standard tests

In the 1960s a model for calculating the insulation of buildings was established by the scientific community.

This model relied on using the results from tests using a guarded hot box or a guarded hot plate, laboratory equipment original created for traditional, thick, single layer insulation. This laboratory equipment measures the thermal conductivity of these products at a steady state (in other words, a constant state), this means their capacity to prevent heat loss through conduction.


The method consists in placing an insulation product between two environments at different temperatures to generate a temperature difference (ΔT). Then, the amount of energy needed to maintain a constant temperature either side of the insulator is measured. Once the thermal flow has stabilised, the measurement can be made. The quantity of energy equals the thermal flow passing through the product. This is the thermal conductivity. The thermal resistance or R value defined is a ratio between the thickness (e) & the thermal conductivity (λ) of an insulation product (R = e /(λ)).



Unlike traditional thick insulation - which mainly work to prevent heat exchange through conduction - the specific nature of thin multifoil insulation, combined with its application between two air gaps, forces energy to be transmitted via radiation rather than by conduction or convection. Heat exchange through conduction plays only a minor role in the way thin multifoil reflective insulation works.

This single measurement for heat conductivity is not sufficient to categorise the overall thermal performance of thin multifoil insulation.

In situ measurement

In the absence of any suitable standards, ACTIS measured the thermal performance of its products in situ, in other words in real-life conditions.

It is important to note that in real life conditions, energy is transferred in a variety of complex ways (radiation, convection, conduction). The in situ test model allows for an overall appreciation of these.


The results from in-situ testing carried out in European countries as in France, in the UK and in Germany reveal that the performance of ACTIS thin multi-foil reflective insulation is equivalent to traditional thick insulation product.

This measurement methodology has been defined by BM TRADA, an EOTA member.

BM TRADA Certification's Building Insulation Product Scheme including the BIP-001 in-situ testing test method has been UKAS accredited in 2011.

This in-situ method is the one discussed in the European Technical Approval and European Comity of Standardisation.


This method consisted of insulating identical and standardised buildings in different ways, and measuring, and comparing, the energy consumption required to maintain these buildings at an identical, constant internal temperature, whatever the external weather conditions.

One building was fitted with a "mineral wool" (20 cm, R = 5) type of insulation, whose thermal performance was known, and accredited by the conventional methods.

The other building was fitted with thin multi-foil reflective insulation which ACTIS wanted to test for thermal performance.

The actual duration of the test consisted of between 12 to 14 weeks.


A very specific protocol was established for kitting out the buildings, as well as for monitoring consumption and analysing the results.

Exterior dimensions: The test buildings had external dimensions of 4 x 7 metres and an under-ceiling height of 3 metres.

  • Description of the structure of the test cells:

The buildings had wooden frameworks and were covered in tiles to represent a converted attic. The walls and pitches were given an internal finish of plasterboard.

There were no windows in the test buildings and the ventilation was open to the outside. Each test space was accessible through an insulated airlock in a sidewall. Heat exchange occurred through the walls insulated with the materials being tested.

  • Authentication and quality control:

The dimensions and exposure of the test buildings was monitored by a land surveyor.

Both the test buildings were standardised to ensure that they had the same thermal behaviour. Their energy consumption was first measured without insulation.

The buildings were then insulated and sealed before the tests began.

The mineral wool was fitted by an approved installation company, QUALIBAT, and subject to an APAVE (third party inspection agency) inspection prior to the interior facing being fitted.

  • Data acquisition:

Meters registered energy consumption in each of the test buildings. All the data was collected by a computer programme and analysed, taking into account any factors likely to influence consumption, such as weather data.

This methodology was certified by BM Trada, a British body accredited by the UKAS (United Kingdom Accreditation Service) and member of the EOTA (European Organisation for Technical Approval).

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