Insulation Terms You Need To Know: Part 3 – U-Values

by Brian Flax | Oct 24, 2016

In part one of our insulation series, we focused on insulation materials and applications. In part two we discussed the necessity of adequate roof, wall and mechanical ventilation. In part 3, our latest installment, we’re going to dive a bit deeper into the technical terms associated with insulation, more specifically U-values and what these numbers mean.

What does the term U-value mean?

In simple terms, a U-value is a measurement of how effective building material or building element (floor, wall roof) is at reducing heat loss. The lower the number, the better the material is when used as an insulator. In more technical terms, the U-value is a measurement of thermal transmittance through either a single or a composite material(s). The thermal transmittance is divided by the difference of the building’s temperature and expressed as watts per square metre, per degree kelvin, or the shortened version W/m2K.

What’s included in the calculation of a U-value?

There are many building elements and materials that contribute to the calculation of a U-value specific to your project, but first you need to understand thermal conductivity (lambda), thermal resistance (R-value) and thermal transmittance. All three are equally important if you want to calculate an accurate U-value.

1. Thermal Conductivity (Lambda or λ)

Also known as Lambda, thermal conductivity expresses the rate at which heat will pass through a material. You’ll see this measurement in watts per square metre of surface area, with a temperature gradient of one kelvin for every metre of thickness. Remember, thermal conductivity is not affected by the actual thickness of the material, and the lower the number, the more thermally efficient the material is as an insulator.

The thermal conductivity of KORE EPS insulation varies for each one of our products. There are many factors that can play into this, like the raw material we use during manufacturing, to the overall density of the finished product. Here’s a few examples of the thermal conductivity for our EPS insulation:

You can view the full range of product thermal conductivities by joining the KORE Resource Centre. The thermal conductivity is listed on each product’s Declaration of Performance Certificate.

2. Thermal Resistance (R-Value)

The thermal resistance, or R-value, measures a material’s ability to prevent the flow of heat through a given thickness. This is opposite of thermal conductivity, where the thickness of the insulation does not matter. The R-value is calculated by dividing the material’s thermal conductivity by its thickness (in metres). You’ll normally see this expressed as m2K/W.

In buildings, the R-value normally includes the thickness of the entire building element, and not just the insulating material itself. For example, a building’s wall would normally consist of several layers, including the surface of the wall, the solid block, the wall cavity and the insulation. Each layer when added together would total the overall R-value, with the higher the number, the more efficient the insulation.

Here’s a sample R-value calculation using KORE Fill Bonded Bead with a thermal conductivity of 0.035W/mK.

KORE Fill 0.035W/mK 150mm in thickness

0.15 metres ÷ 0.035W/mK = 4.28 m2K/W

You would then add the calculated R-values for the remaining building elements to find your overall R-value. Keep in mind that R-value calculations only take into account conduction. As heat moves in several ways (conduction, convection and radiation) you’ll really want to find the U-value for a more accurate representation of the insulation’s efficiency.

3. Thermal Transmittance (U-Value)

A U-value measures the rate of heat loss of a building component, also known as thermal transmittance. The U-value takes into account the thermal resistance (R-value) of all the building elements. Using a wall as an example, the U-value would take into account surface resistance, air spaces and the emissivity of the surfaces facing the air space, thermal bridging, air gaps and any fixings. Unlike thermal resistance, U-values show heat loss associated with conduction, convection and radiation.

U-values are express as W/m2K, or watts per square metre, per degree kelvin.

How do I calculate a U-value?

U-values are complex calculations when air gaps, thermal bridges and fixings are taken into account. While it is possible to calculate general U-values, to be most accurate it is best to use U-value calculation software. With that being said, here are the steps you’ll need to take to calculate a U-value without adding in the more complex elements:

  1. Calculate the thermal resistance (R-value) or each building element,
  2. Add in the Resistance of the internal and external surface resistances (Default surface resistances can be found in BR 443) The internal surface resistance is shown as “Rsi” and the external surface resistance is called “Rse”.
  3. Add together all the R-values, including air gaps, surface resistances, and calculate the reciprocal. You can calculate the reciprocal by dividing 1 by the total of all R-values. This will leave you with the U-value.


KORE Fill 0.035W/mK 150mm in thickness and KORE External EPS70 0.031W/mK 100mm in thickness, with concrete 2.0W/mk 150mm thickness

Rsi R-Value 0.13 m2k/W

+ KORE Fill R-value of 4.28 m2K/W

+ 150mm Concrete R-Value 0.05 m2k/W

+KORE External R-Value of 3.22 m2K/W +

+ Rse R-Value  0.10 m2k/W

1 ÷ (Rsi 0.13 + 4.28 m2K/W + 0.05 m2K/W +3.22 m2K/W + Rsi 0.10 m2K/W) = U-value of 0.128W/m2K

Thermal conductivity, thermal resistance and thermal transmittance are all important terms you should know when working with insulation or construction in general. Remember:

  • Higher numbers are good when you are comparing the thermal resistance, or R-value, of building materials and products
  • You’ll want low numbers when comparing the U-value of building components. While finding U-values can more complex than simply looking at the R-value, it doesn’t take into account all the different ways that heat loss can occur. While you may be able to do simple calculations by hand, we recommend using a U-value calculation software for the most accurate results.

Of course, you can always contact our technical team with the details of your project. We can help calculate U-values using KORE EPS insulation and provide an accurate quotation for your insulation needs. Our team of technical consultants are BBA approved Competent Persons for calculation of U-values and Condensation risk analyses.