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What are U-Values?

What are U-Values?


U-Values measure how effective a material is an insulator. Also known more formally as ‘thermal transmittance,’ to calculate the U-Value of any type of insulation, you need to divide the rate of heat transfer through the structure by the difference in temperature across the area in question. The lower the U-Value, the better the material functions as an insulator.

How insulation works

To understand how insulation works, it is essential that you understand the way that heat flow works. Heat flow involves three basic mechanisms: conduction, convection and radiation.

Put simply, conduction is the way heat moves through materials, convection is the way that heat circulates through liquids and gases and radiation refers to the build-up of radiant heat which travels in a straight line, and heats anything solid in its path that absorbs its energy.

What are U-Values Home Logic UK Spray Foam Installer

The U-Values of Key Areas in your Home

-U-Values of solid walls

Solid walls were the norm in most properties built prior to the 1930s. Unfortunately, at this time energy-efficiency wasn’t really on the radar, since fuel was very cheap. To insulate a solid wall, you can either insulate internally or externally.

-U-Values of cavity walls

Cavity walls became common in the 1930s. However, until 1995, we assume that they were built but left unfilled. Post 1996, all cavity walls were assumed to have filled cavities as part of more regimented building regulations.

-U-Values and roof space

It’s well-known that significant heat loss occurs through the roof, so insulating your loft is one of the best ways of improving energy-efficiency.

-U-Values of windows

No matter how close legislators set the required U-Values of windows, they are never going to get close to that of walls, since you can’t simply add insulation to them.

This has resulted in builders reducing the size of windows in new-build properties to the detriment of the homeowner, since it means less light is able to enter the home.

New-build windows must achieve a maximum U-Value of 1.6W/m2k under Part L of the Government Building Regulations set out for all new build properties.


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U-Values and Energy-efficiency Ratings

The Windows Energy Rating not only takes into account the U-Value of windows, it also focuses on the overall solar gain benefits to the household in question. The Windows Energy Rating applies to the window as a whole, taking into consideration both the pane of glass and the frame surrounding it.

Factoring R-Values into the equation

An insulation material’s resistance to conductive heat flow is measured in terms of its resistance to heat, which is also known as an insulation’s R-Value. The higher the R-Value, the greater the insulation’s effectiveness.

Still, that doesn’t automatically mean that more insulation is better as is commonly assumed. Although R-Value is proportional to insulation thickness, it’s equally important to take into consideration the following factors: the type of insulation material used, its thickness, and the density of its overall composition.

factoring u values

Traditional insulation materials

Needless to say, traditional insulation materials (such as fibreglass and cellulose) are not only bulky, but also resist conductive heat flow in any building cavities.

To select the best insulation for your home, you’ll need to know in advance the specific places in your property where you want the insulation installed, as well as the specific R-Value that you want the installation to achieve.

Other key considerations may include: the impact of insulation on indoor air quality, life cycle costs, recycled content and the ease of installation.

How much insulation you need for your property

This depends on: the climate, type of heating and cooling system already installed in your home, as well as the specific areas in your property that you’ve prioritised for insulating.

How to measure the effectiveness of insulation

The effectiveness of your chosen insulation’s resistance to heat loss also depends on how and where the insulation is installed. For example, insulation that is compressed will not provide its full-rated R-Value.