Understanding Common Insulation Terminology

3 April 2020 EcoTherm Insulation

The insulation industry comes with lots of technical terms and other jargon which you may not have come across before. This handy blog will help to get you up to speed, ensuring you’ll soon be able to tell your Psi-Value from your U-value.

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What is thermal conduction?
In thermal conduction, heat is transferred as hotter (more active) particles from one body of either a solid, liquid or gas collide with cooler (less active) particles of another body. The effect of these collisions is to make the particles in the second body become more active and therefore hotter.

What is a thermal conductivity?
Thermal conductivity (also known as a λ / lambda or K-value) is a measure of how well an individual material conducts heat.

The process of thermal conduction is the most common type of heat transfer through building elements. Materials that have lower thermal conductivity values are more resistant to this type of heat transfer and this makes them more effective as insulants. 

What is an R-value?
An R-value (also known as thermal resistance) is the measure of how well a particular thickness of material resists heat transfer through thermal conduction. To calculate this, we take the material’s thickness in metres and divide this by its thermal conductivity value. 

For example, Eco-Versal has a thermal conductivity of 0.022 W/mK. The R-value of a 90 mm thick board could be calculated as follows:
0.09 / 0.022 = 4.09 m2K/W 

What is a U-value?
A U-value (also known as thermal transmittance) measures how well a complete construction (wall, roof, floor) resists heat through thermal conduction and thermal radiation. The lower the U-value, the more effective it is at preventing heat transfer.

U-values are used throughout the construction industry to assess the performance of the entire building envelope and to help set performance targets. For example, the Building Regulations include worst-case U-values which cannot be exceeded.

To calculate a very simplistic U-value, the R-values for all of the materials which make up a construction from outer and inner surface are added together and divided by one. In practice, however, most U-value calculations are more complex than this and will factor in adjustments for aspects such as fixings. In constructions with voids or cavities, the calculation also considers the thermal emissivity of facings.

To easily identify what thickness of EcoTherm Insulation product you need to meet your project's U-value requirements, please visit EcoTherm's Online U-value Calculator - simple to use, easy to tweak and provides an instant U-value.

What is thermal emissivity?
Thermal emissivity measures how well a material emits heat in the form of thermal radiation. Materials which are poor emitters are also poor absorbers and will reflect away heat from thermal radiation. For this reason, many EcoTherm PIR insulation boards are faced with a silver foil which has a low emissivity. This foil helps to retain heat within a property during winter, keeping properties warmer and can help to reflect away thermal radiation from the sun in the summer, keeping properties cooler.  

What is thermal radiation?
All bodies emit and absorb infra-red radiation. The hotter the object, the more radiation it emits. Unlike thermal conduction, radiation is transferred through electromagnetic waves rather than particles. These waves are able to travel through a vacuum which is why we get heat from the sun.

What is building fabric?
Building fabric (also known as building envelope) are the elements which make-up the outer shell of a building. These include walls, roofs, floors, windows and doors.  

What are thermal bridges?
Heat trying to escape from a building will always look to take the path of least resistance. This typically means gaps in or penetrations through the insulation layer. We call these faults thermal bridges (or cold bridges) as they literally act as a bridge which heat can escape through. Thermal bridges can badly undermine how well an insulated property retains heat and can also lead to issues such as surface condensation or interstitial condensation so it is important that they are addressed. 

The most common types of thermal bridges are repeating thermal bridges (such as timber studs) and non-repeating thermal bridges (which typically occur around windows, doors and other junctions). 

They can be addressed by creating a continuous insulation envelope and ensuring insulation butts tightly around openings with no gaps. For example, to prevent timber studs acting as a repeating thermal bridge installers can fit an insulated dry-lining board such as Eco-Liner.

What is a Psi-value?
A Psi-value is the measure of heat loss across the length of a non-repeating thermal bridge. A junction with a lower Psi-value is more effective at limiting this type of heat loss. 

What is surface condensation?
As you might expect, this is the condensation that forms on the surface of a construction. This condensation appears when moist air within a home meets a cold surface, cools to its dew point and turns into liquid moisture. This type of condensation is often found on the inside of windows in single glazed properties but it can also form around thermal bridges. Surface condensation can cause internal finishes to bubble or flake and encourages mould growth.

What is interstitial condensation?
This describes when moist air reaches its dew point whilst inside a building element. If not addressed, this can lead to serious issues such as rotting of timber joists, so it is important that steps are taken to either prevent this moist air from entering the construction, or to safely ventilate it away. 

More Questions? If you have any other queries relating to our insulation or how it is used take a look at our FAQs or get in touch with our technical services team.

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