What is a U-value & Why is it Important?

A building’s thermal performance is an important aspect to consider in order to stay compliant with regulations set out in the UK’s Building Regulations: Approved Document L. 

In construction, the U-value is commonly used to understand a building’s thermal performance in terms of energy performance. U-values offer a more accurate representation of heat loss, as these values measure thermal transmittance (heat loss). 

U-values also consider conduction and convection when looking at how the insulation of buildings and their building materials perform. 

Continue reading to learn what is a U-value, and why is it important in construction? We’ll also cover how to calculate U-values. 


What is a U-value?

U-values, or thermal transmittance, measure the heat loss through building materials. Essentially, it represents the effectiveness of a material as an insulator. With U-value calculations, we can broaden our understanding of how the insulation properties of building components are performing. 

Why Are U-values Important?

It’s crucial to understand how efficiently buildings retain and conduct heat. Buildings with lower energy requirements are much better for the environment. This is why the government has implemented strict laws that aim to encourage the construction of more energy-efficient properties. U-values are important for many reasons:

  • U-values help to determine how energy-efficient a property design and specific building materials are. 
  • They’re essential for SAP and Energy Performance Certificate (EPC) assessments. 
  • By calculating U-values before construction, you can get an estimate of the projected energy performance and the insulation levels your project requires. This data will allow you to amend your design to reduce the rate of heat transfer.

It’s also crucial to examine the U-values after construction. The calculations will prove whether or not your strategies successfully reduced heat loss. The aim is to notice minimal or no performance gap, showcasing that the projected insulation performs as calculated. 

Calculating U-values is not just a tick-box exercise. Overall, U-values help us create energy-efficient and sustainable dwellings with lower total energy use or consumption rates. 

What is a U-value Measured in? 

We measure U-values or thermal transmittance in units for Watts per square metre kelvin or W/m²K. 

The units look at the rate of heat flow (in Watts) through 1m² of a structure. Then consider the temperature difference across the structure of 1 degree (K or C).

How Are U-values Calculated? 

The formula for calculating U-values is as follows: 

U = 1/Rt

Where U is the U-value, and Rt is the sum of the R-values or thermal transmittance of the different components making up the element layers 

Now, let’s break it up. 

R-Value

Calculating the thermal resistance is crucial to achieving U-value calculations. Thermal Resistance calculations consider different aspects.

The formula: R=d/k

  • R is the thermal resistance measured in m²K/W.
  • D is the material thickness measured in metres. 
  • K is the thermal conductivity of the material calculated in W/m K. 

Rt 

This is the total of all the elements being tested to gain accurate U-values. 

The formula: Rt = Rse + Rsi + R1 + R2 + RN….

Here’s how to read this R-value formula: 

  • Rse represents the fixed thermal resistance of the exterior surface.
  • Rsi represents the fixed interior surface thermal resistance. 
  • R1, R2, and N indicate the thermal resistance of each layer you’re considering.  

To sum up: For every layer, you’ll have to do an individual R-value calculation. Add them together with fixed external and internal resistances. 

Additionally, you will still need to take into account and measure any air gaps that appear for a more comprehensive understanding of U-values. 

The above calculation methods may leave some feeling dizzy and disoriented, but there are better ways to measure a building’s projected thermal performance. The most time efficient and innovative way is to make use of efficient online software — like AutoPSI, which can give you accurate outputs for every single calculation, allowing for more design revisions to find the most energy-efficient solutions. 

Now that you know how to calculate the U-value, let’s take a look at how to measure the actual performance vs the projected performance. 

How Are U-values Calculates Post-Construction?

Post-construction measurements are important calculations to help support the theoretical U-value calculations completed in the design phase. You can use different calculation methods to measure the way building elements resist heat flow. 

It is possible to measure the U-value of a wall, roof, and floor using relevant equipment and sensors. A heat flux metre used with heat flux plates can help measure U-values for roofs and walls. It works with a sensor that monitors the heat flow from inside the building to outside.

In order to calculate the thermal transmittance of existing dwellings, you need to divide the average heat flux by the average temperature difference between the interior and exterior. 

It’s important to note that several elements can affect the accuracy of your U-value calculation. Here are some observations that may lead to more accurate results.  

  • Cloudy weather conditions provide better results than sunny days. 
  • Larger temperature variations can lead to more accurate heat loss measurements.
  • The test area needs good adhesion for thermopiles. 
  • A large number of test points will help combat anomalies. 
  • The data needs to be collected continuously for at least 3 days and optimally for around two weeks. 

Building Regulations and U-values

Building regulations specify the optimal U-values that building elements need to meet in order to optimise energy efficiency. With the heating and powering of buildings accounting for about 40% of energy use, the government is focusing on reducing heat loss and consequent energy consumption to improve energy efficiency. 

Building Regulations Document L aims to ensure every dwelling uses less energy, requires less power for operation, and reduces its carbon footprint. The latest regulations focus on providing specific U-values that the entire building must achieve in order to improve its energy performance. 

If a construction element does not meet the minimal U-value as stated in the regulations, you should replace it with a building element with better thermal resistance and less thermal transmittance. Here is just a brief look at some examples of the updated Part L regulations document and the maximum U-value per building element for new-build dwellings. 

  • Party walls — 0.20 W/m²K 
  • Whole window — 1.6 W/m²K
  • Roof light —- 2.2 W/m²K 
  • All roof types— 0.16 W/m²K 
  • Floors —  0.18 W/(m²·K)

What Is the Difference Between U-Values and PSI-Values?

Both U-values and PSI-values measure how a building loses heat. However, PSI-values give us a more detailed look into the total heat loss of the building. PSI-values take into account non-repeating thermal bridges and junction meetings, which U-values do not take into account. 

Both values need to be taken into account when assessing the overall energy efficiency of a building. Achieving accurate PSI-values and U-values is crucial to passing SAPs. Passing thenecessary assessments becomes simple when you use dynamic thermal modelling tools, like AutoPSI. 


AutoPSI: Robust & Speedy Online Thermal Modelling Tool 

U-value calculations can be extremely complex, and the best way to achieve accurate results is to use purpose-made online calculators. That’s where AutoPSI’s online thermal modelling tool comes in. The simple drag-and-drop interface can help you accurately measure projected heat energy transfer. 

AutoPSI brings innovative tools to the construction design industry. AutoPSI’s dynamic online tool helps save time and ensure regulatory compliance with fast and accurate results. Choose AutoPSI to streamline and enhance your construction design processes and create compliant buildings that perform according to the latest energy performance requirements. 

Try AutoPSI today and achieve optimal U-value and PSI values with simple and precise heat loss calculations. Sign up for a free demo today to experience how AutoPSI can help you.  


FAQs About U-values

By now, you should have some comprehensive knowledge of how to find the U-value. However, if you still have some lingering questions, here are the answers to some commonly asked questions. 

What is the difference between the R-value and the U-value?

There are a few related terms that can cause some confusion to those not familiar with the construction industry. The R-value metric. The R-value measures the thermal resistances of the building envelope but doesn’t take into account surface heat transfer or other elements that U-values consider. 

However, the U-value is the essential metric we’ve been discussing throughout this guide to calculate heat transfer. 

Should You Aim for Higher or Lower U-values? 

Lower U-value metrics indicate that the material has less thermal conductivity properties and thus provides good insulation. Therefore, the lower the U-value, the lower the heat flow from the inside to the outside. Less heat loss means better insulation of the building elements. Simply put, low U-value metrics equal better performance of the building fabric. 

Although there have been updates to the previous regulations in the UK, your goal should always be to achieve the recommended value or have a U-value measurement below what regulations suggest. 

These lower values indicate that you’re consciously designing a dwelling with beneficial thermal properties that contribute to improved energy efficiency.  

How to achieve the best U-values? 

If you have an existing dwelling, there are a few ways in which you can improve your property’s U-values. 

  • Improve the insulation thickness of the internal walls for a warmer and more comfortable interior. 
  • Check for an empty cavity wall and add insulation material to any unfilled cavity walls for better internal insulation.
  • Enhance the insulation of the roof to reduce heat loss, avoid high energy bills and improve loft room insulation. 

If we’ve not covered a question “What is a U-value?”, feel free to reach out to our team for more detailed information.