U-value measurement of buildings

U-values are a metric of how effective a building element (wall, floor or roof) is at preventing heat from transmitting from the inside to the outside of a building. U-values are an essential part of determining building performance and calculating heat loss.

What are U-values?

A U-value describes the ability of a material to prevent heat loss and is the inverse of an R-value. An R-value is the total thermal resistance of the different layers of materials that make up a particular building element. By building elements we are referring to walls, floors and roofs. The U-value of these elements form a series of heat transfer coefficients which all contribute to the total heat loss of a building.

Also known as thermal transmittance, a U-value is therefore the rate of transfer of heat from the inside of a house to the outside through a particular building element. Heat transfer is measured in watts per square metre, per degree Kelvin and shortened to W/m²K. In simple terms, the lower the U-value the better insulated the fabric of a building is and the lower the heat loss and energy bills will be.

What do I need to know about U-values?

U-values are used internally in any SAP Assessment or Energy Performance Certificate (EPC) using either calculated U-values based on the construction type or assumed U-values using lookup tables. These assumed or calculated U-values don't necessarily reflect the true U-value of the actual building after construction.

The main use case for determining more accurate U-values is to inform the retrofit decision-making process. By measuring U-values pre-retrofit, you can make an informed decision as to the level of insulation required to reduce heat loss and improve thermal comfort whilst minimising cost.

Then by measuring U-values again post-retrofit, it is possible to quantify the level of performance upgrade that the insulation has achieved and provide quality assurance that the insulation is performing as expected.

U-value measurement and surveying equipment

Determine accurate U-values of walls, floors and roofs with our easy-to-use U-value measurement tools. Measured U-values can be input into energy models or help determine what level of insulation to add to houses.

Heat3D mobile app and FLIR ONE Pro on an iPhone

Heat3D Infrared Thermography

An innovative mobile app that allows you to precisely measure heat flow and U-values of walls using a low-cost, quick and non-invasive method.

Learn more about Heat3D Infrared Thermography
ISO 9869 U-value measurement system

ISO 9869 U-value Measurement System

System for measuring in-situ U-values of walls, floor and ceilings according to the ISO 9869 methodology.

Learn more about ISO 9869 U-value Measurement System

Problems with calculating U-values?

Simple U-value calculations can be made by taking the thermal resistance of each material forming a layer in the construction of a particular building element and calculating the reciprocal of the sum. The internal and external faces also have resistances, which are also added as fixed values. There are numerous online U-value calculators available in order to do this.

Calculating a U-value like this may be very simple in terms of new-build when the exact makeup of an element is known and can be monitored but it's not quite as simple when we're dealing with the existing properties when we can only make an assumption or educated guess as to the exact construction of any element without invasive examination.

Compounded to the challenge of using calculated U-values is the fact that standards of installation and workmanship can negatively affect the U-value. Poorly fitted insulation, air gaps, cold bridging from fixings, wall ties etc, the different thermal properties of mortar joints and suchlike all have an effect on the actual U-value but which go unseen within the walls, floors and roof of existing properties.

Why measure U-values instead?

The in-situ U-value of building elements can vary significantly from calculated values. Instead of calculating U-values, it is possible to measure the exact U-value of existing walls, floors and roofs directly using portable equipment and sensors. Measuring provides an accurate reading of the true U-value of building elements without the issues described above. Build Test Solutions have pioneered various U-value measurements techniques which allow you to measure in-situ U-values in as little as 1 hour.

Heat Flux Measurement

The established and traditional method of measuring an in-situ U-value is using a heat flow meter along with one or more heat flux plates.

Our In-site U-value Measurement System is a portable tool that acts as a heat flow meter and consists of a data logger, 2 x heat flux plates, surface temperature sensors, air temperature sensors, and a 4G modem all incorporated into a protective case.

The system can record heat flux measurements over a number of days or weeks in order to deliver greater accuracy and quantify the in-situ U-value of building elements according to the ISO 9869 methodology.

Whilst this is a recognised and accepted method of U-value measurement it does take a minimum of 72 hours (3 days) to collect enough data for accurate results which may not always be possible.

Heat3D Infrared Thermography

An alternative method is to use our Heat3D infrared thermography based solution for measuring U-values. Heat3D is an innovative mobile app that allows you to precisely measure heat flow and U-values of building elements using a low-cost, quick and non-invasive method.

Heat3D is designed to work on iPhones and iPads and uses a portable thermal camera to obtain results in a matter of hours as opposed to days or weeks using heat flux plates. The rapid nature of the measurement enables quantified U-values that are more accurate than manual calculation methods but achievable in a similar amount of time.

The Heat3D iOS app uses Apple ARKit to detect room features such as walls, floors and ceilings and construct a 3D model. When combined with a FLIR portable camera, infrared thermal images are projected onto the 3D model showing the heat signature through each surface. Quantitative infrared thermography is used to calculate the heat loss through these surfaces and, using temperature sensors, a U-value is calculated and presented on the device.

Measuring a U value across an entire element with Heat3D can take as little as 1-hour meaning that the measurement of actual U-values is now possible in all projects.