There’s a lot of technical jargon that gets thrown around when talking about the performance of insulation and windows when it comes to retaining heat. 

When getting insulation installed or burying it online you’ll likely see and hear references to different values, in particular U-values and R-values.

Both of these are figures used to measure the thermal performance of insulation, but they differ in how it is measured and calculated.

A R-value is given for a single material and refers to how good it is at resisting the transfer of heat. The U-value in contrast is given for an area as a whole, such as a wall, and refers to the flow of heat through that structure.

Thermal Conductivity

Thermal Conductivity

Thermal Conductivity, represented by the Greek symbol Lambda 𝜆 is a measurement of how easily a material allows heat to pass through it. 

This is expressed as Watts per Metre Kelvin W/mK and is independent of any other considerations such as the thickness of the insulation.



An R-value refers to the thermal resistance of a material, a measurement of how good it is at blocking the transfer of heat through it. The higher the R-value of a material, the better it is at resisting the flow of heat. 

The thickness of the material in question is essential for grasping the R-value. The thicker the layer of insulation, the higher an R-value it will have.

An R-value is calculated by dividing the thickness of the material by its thermal conductivity. It is expressed as metres squared Kelvin per Watt (m²K/W).

R = l/  – where 𝑙 is the thickness of the wall in metres and 𝜆 is the thermal conductivity of the material in W/mK.

If you are purchasing insulation for a DIY project, looking at the R-value of different products is an important metric for gauging their performance.

You can add together the R-values of insulation with those of the wall structure to produce a figure for its thermal resistance. However, R-values are limited in that they only take into account the conduction of heat, and not other ways in which heat can be lost from a building. 

What is a U-Value

What is a U-Value

The U-value is an expression of the thermal transmittance of an element of a structure such as a wall or ceiling, outlining how much heat is lost through that structure.

A U-value is the reciprocal of an R-value and is expressed as the rate of heat transfer through a structure divided by the difference in temperature across that structure. It is expressed as Watts per metre squared Kelvin (W/m²K).

When a U-value is being calculated it takes conduction, convection and radiation into account, making workmanship extremely important when installing insulation.

Since U-values indicate how much heat is being lost through a structure, the lower the value the better when it comes to insulating your home.

Measuring U-Values

The U-value of a structure such as a wall is calculated using all of the elements involved in the structure, such as blocks, plaster, and insulation, and looks at all the different ways in which heat can be lost through that wall.

Any air gaps or thermal bridges from poorly fitted insulation will result in greater heat loss and raise the U-value of a building.

The U-value of a structure can be measured manually using a thermopile sensor to measure the heat flow over a period of time. However, calculations can also be made using certified measured values for known materials.

Why It’s Important to Understand U-Values

The most obvious reason why it is important to understand U-values is because it is a determining factor in the comfort and thermal performance of your home.

If you are getting insulation installed on a home here in Ireland, the U-value is mostly what will be used to express the final thermal performance of the area in question once the upgrades are complete.

U-Values are also essential in home energy upgrades due to requirements in Building Regulations and SEAI grant funding.

When it comes to grant funding for home insulation upgrades, the SEAI sets certain U-Value thresholds which must be met in order to receive grants. Of course, there is nothing to say that you can’t exceed those targets and aim for an even lower U-value.

U-Value Requirements for SEAI Better Energy Homes Grants
Insulation Type Required U-Value Target
Ceiling Insulation 0.16 W/m²K
Rafter Insulation 0.20 W/m²K
Cavity Wall Insulation 0.35 W/m²K
External Wall Insulation 0.27 W/m²K
Internal Wall Insulation 0.27 W/m²K

Irish Building Regulations also set standards for acceptable U-Values when carrying out upgrade works to existing buildings.

The relevant U-Values for the purpose of Part L Building Regulations is where the wall, roof, etc separates the interior of the building from the outside, directly or indirectly. Walls between two heated areas (e.g. between apartments or semi-detached houses) are not relevant.

These acceptable values can be relaxed for elements of a building, or even parts of an individual element. However, this should only be done when necessary for construction or design reasons.

 The building regulations also expect you to compensate for this elsewhere in order to still hit that area-weighted average.

Maximum elemental U-Value for Material Alterations or Material Change of Use
Building Element Area-weighted Average Elemental U-Value Average Elemental U-value – individual element or section of element
Pitched Roof – Rafters 0.25 W/m²K 0.35 W/m²K
Pitched Roof – Ceiling 0.16 W/m²K
Flat Roof 0.25 W/m²K
Cavity Walls 0.55 W/m²K 0.6 W/m²K
Other Walls 0.35 W/m²K
Ground Floors 0.45 W/m²K
Other Exposed Floors 0.25 W/m²K 0.6 W/m²K
External Doors, Windows, Rooflights & Curtain Walling 1.40 W/m²K 3.0 W/m²K

Briain Kelly


Briain Kelly is a Leinster based journalist and content creator who has been writing about energy efficiency and renewable energy technologies for nearly three years. He researches the latest news in multiple areas related to solar power, electric vehicles, heat pumps, and home energy upgrades. His writing includes both technological developments and government policy.

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