U-Values Explained

When it comes to home insulation, one of the most important yet confusing terms is the ‘U-Value’. What exactly is the U-Value? The U-Value is the measure of how effective a given material is as an insulator and it’s referred to a lot in the building and construction industry.

However it is quite alarming that the vast majority of roofers still don’t understand what U-Values actually mean! For those professing to be able to offer effective heat insulation, that’s just not good enough. However, we’re here to provide an overview of U-Values and explain their importance when it comes to home insulation.

Here are the basics of what U-Value means

In simple terms, the lower the U-Value, the better a material is as a heat insulator. The U-Value allows you to measure how much heat will move through any given material per square metre per hour. Therefore, it illustrates how effective a material is as a heat insulator by indicating how quickly it will lose heat.

The U-Value is measured in units of Watts-Per-Metre-Square-Kelvin (W/m2K). However, understanding the basis of how it is calculated is less important than understanding how to compare materials. Comparing materials by their U-Value is a good guideline to ensuring your home is insulated with the most appropriate, energy efficient materials.

Examples of common building materials and their typical U-Values:

  • Cavity walls most often have a U-Value of 1.6 W/m²
  • Solid brick walls normally have a U-Value of 2.0 W/m²
  • Solid floors normally have a U-Value of 2.1 W/m²

As you can see from the above, a cavity wall is the best heat insulator due to the fact it has the lowest U-Value. Whereas the solid floor is the worst as it has the highest U-Value.

The best insulating materials have a U-Value of close to zero; the closer to zero the better. Under LABC guidelines, the retrofitting of insulation to existing buildings requires the following U-Value targets:

  • Wall – 0.3 W/m2k
  • Roof – 0.18 W/m2k
  • Floor – 0.25 W/m2k

Ensuring the U-Value of your home is as low as possible is important as this means your home is not rapidly losing heat.  It is often an indicator of high levels of insulation, making your home more energy-efficient. If the U-Value is high, then this is often an indicator that there is a need to add additional insulation to your home.

So how can you ensure your home has the best U-Values?

The U-Value of Internal Wall Insulation

This is appropriate for homes built with solid walls, which are most common in pre-1930 builds. Unfortunately, energy efficiency was not on the agenda at this time and therefore this type of home lacks insulation and can be very expensive to heat.

An un-insulated solid wall has a typical U-Value of 2.20 W/m2k but by adding internal wall insulation this can be brought down to approximately 0.3 W/m2k. As a result, not only will internal wall insulation give a warmer, more comfortable home, but it could also save substantially on energy bills.

The U-Value of Cavity Wall Insulation

Cavity walls became common post-1930. Up until the late 1990s, most of these walls remained unfilled, meaning they were un-insulated. An unfilled cavity wall can have a U-Value of anywhere between 1.6 W/m2k and 0.6 W/m2k, depending on when it was built.

By installing cavity wall insulation in your home you can help to improve your U-value and in turn, improve the insulation of your home to about 0.3 W/m2k.

The U-Value of Roof Insulation

25% of your home’s heat is lost through the roof. Even more heat escapes if you have a room-in-the-roof or attic bedroom. It is therefore vital to ensure your roof has the correct U-Value and is as energy efficient as possible. An uninsulated roof has a U-Value of approximately 2.5 W/m2k, however, adding insulation can achieve a value of 0.18 W/m2k.

If your home was built pre-1967 and has an old attic room or loft conversion, a great way of achieving this level of insulation is through our specially designed Eco-Roof Insulated Roofing System. The Eco-Roof is a high-tech insulated roofing system designed to offer homeowners a highly efficient way to insulate old loft/attic conversions. It literally stops your money going through the roof by keeping the energy in and, in turn, keeping heating bills down.

Not only can the Eco-Roof dramatically decrease your roof’s U-Value, but it can also save you money year on year.

A Roofers understanding of U-Values

Speak to the average roofer and ask them what U-Value they will offer you for insulating a roof based on its size and we bet you 99% won’t have an answer. If this is the case we suggest you question why you are working with them. Any roofer these days who is worth his salt should understand heat loss and the implications of using the best energy efficient materials. They should also be able to demonstrate how these materials will effect the energy efficiency of your home.


Hopefully, this blog post has helped you to better understand the ins and outs of the term ‘U-Value’ and how it relates to the level of insulation in your home.

If you have any questions or would like to ask us how we can improve the U-Value of your home, feel free to contact us. 

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