Technical Corner What do U-Value?

14 TECHNICAL CORNER

www.elmhurstenergy.co.uk

What do U-value?

Luke Smith, Managing Director at Build Test Solutions

U-Value measurement can be used to provide quality assurance on new or newly retrofitted buildings, or better understand the real performance of walls, floors and roofs. But until now U-Value measurements have been few and far between, Build Test Solutions are trying to address this by making tools that are more accessible and easy to use including the exciting new Heat3D iOS app.

U-Values are used to define how easily heat transfers through building elements like walls, floors, roofs, windows and doors. A low U-Value means low heat loss, and hence good thermal performance.

In an existing building, it can be very difficult to determine the build-up of a construction in a non-destructive manner. For that reason, U-Values are often estimated based on the age and approximate construction type of an element or building, as in RdSAP. These estimates are determined from lookup tables, with the tables themselves informed by calculations for similar walls and previous measurements studies for samples of similar walls.

U-Value calculations and lookup tables are valuable tools to help understand and design buildings, but are naturally limited in how accurately they will work for an individual building. Error and uncertainty is introduced through a number of sources.

This means that the actual U-Value of an element is not as predicted or designed. Unexpected performance can mean unintended consequences, and incorrect U-Values are no different. If the U-Value of an element is higher (worse) than expected, this means additional heat loss and could in turn lead to issues with condensation or mould growth. If the U-Value of an element is lower (better) than expected prior to a retrofit, it could mean that the expected benefit in energy and cost savings are not delivered in practice.

All of these factors make in-situ measurement a valuable and insightful tool. By knowing the actual performance the building owner and user can get quality assurance on works carried out, the thermal comfort and energy demand of the space can be better understood and controlled, and designers can create more informed retrofits and feedback can be delivered on what materials and processes work in practise.

Measurement has generally been carried out using ‘heat flux plates’. These are coaster-sized sensors which measure heat flow through them and the surface they’re attached to by very accurately measuring the temperature difference between one side and the other. A complication is that heat is stored in the material of

the wall as well as transferred through it, for that reason the heat flux plates are generally temporarily attached to the inside surface of building elements for around 1 week. By also measuring the temperature on the inside and outside of the element the rate of heat transfer through it can be measured, this is the U-Value.

Heat flux plate measurement is well established and a testing methodology is laid down by the International Organization for Standardization. Build Test Solutions provide a U-Value measurement kit which enables users to easily carry out measurements to this standard through a hardware set and online dashboard. Measurements using heat flux plates have played an important role in informing assumptions within the SAP methodology and helping to understand how buildings really work.

Even heat flux plate measurements have limitations though, in particular the measurement is somewhat invasive and time-consuming, while the result is only for a very small section of the element whereas we know elements are rarely uniform in nature and that performance typically varies across the surface.

The newly developed and patented Heat3D system addresses these issues. Heat3D includes an iOS application and hardware to measure the air and radiative temperatures in a room. Using the Augmented Reality kit in the iOS device (iPhone or iPad), a user quickly builds a 3D model of a room. Infrared images are then ‘tiled’ onto that model using a FLIR One infrared camera plugged into the device, creating a 3D infrared model of the room.

Using its patented technology, Heat3D then quantifies the heat loss through each part of the external walls in the room. This survey takes 10-15 minutes to complete and crucially quantifies how much heat loss there is through the walls, which infrared images alone cannot do. Moving one step beyond that, a time lapse survey can be carried out over an hour to quantify the U-Value of each part of the external walls. Within a couple of hours therefore, the user knows the U-Value over the whole wall and has a brilliant visual model to demonstrate this heat loss to a client.

Heat3D is a revolutionary new product which has been developed and validated by Build Test Solutions in collaboration with Electric Pocket Ltd and the University of Salford. Over the past three years hundreds of Heat3D measurements have been successfully compared with heat flux plate measurements in houses around the UK and in the Energy House research facility at the University of Salford. The underlying technology and science has been independently reviewed by the National Physics Laboratory. This painstaking research has delivered the evidence to be confident in the accuracy of measurements carried out with Heat3D.

Over the coming winter, Build Test Solutions will be releasing Heat3D to engaged early adopters, if you are interested in being an early adopter and offering a new service to your clients please contact enquiries@buildtestsolutions.com.