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The Essentials of Insulation

There are two types of insulation products that perform very different functions. These products either ...

  • reflect, or
  • trap air

Some products can do both, and these are the reflective cellular type products such as “Air Cell”, “Rhino Cell”, “Protherm”, and “E-therm”.

The reflective products are usually very thin, and are the foil types such as "Concertina Foil Batts”, and “Sisalation”.

Most building wraps that are used in walls are not reflective. Their function is to act as a moisture or vapour barrier, although they still have a slight insulating capacity.

Still air is a good insulator, and that’s why it is important that if air is a part of the design of a wall or ceiling insulating system that it is enclosed. For example, in a wall, air gaps can boost the thermal resistance of the combined materials (i.e. external cladding and internal insulation), but these air gaps must not be vented in any way.

Here’s the vital part that so many building designers and installers get wrong. Insulating materials must not be in contact with the external cladding or the internal wall lining (such as plasterboard), otherwise heat can be easily transferred through the wall. 

Wall batts that are merely stuffed into the wall frame of a building will lose most of their effectiveness.

Much like the characteristics of still air gaps, the trapped air type of insulation is designed to trap tiny pockets of air that then enable insulation to occur. Heat can transmit from one air pocket to the next, and all the way through the material, and if the insulation product is then in contact with an internal lining, then that heat will be transferred to the interior of the building.

An understanding of R value

The R value of an insulation product is a measure of the thermal resistance and is 

  • based on the thickness of the material, or an assembly of materials, to resist the flow of heat
  • inversely proportional to the thermal conductivity ‘k’ of the material 

For a wall or ceiling batt, such as the mineral wool types or sheep wool or wood fibre, the thicker the batt then the higher is the R rating.

The same applies to rigid insulating materials such as expanded polystyrene which are becoming increasingly popular as an external cladding in housing.

Thermal conductivity (k) is expressed in watts per metreKelvin, and is determined iunder laboratory conditions.

The lower the k value the better the material is for insulation.

R value = d (thickness in metres) / k (Watts/metreKelvin)

When you see all those product R values such as R2.5, R3.5 and so on, the R rating has been determined from this formula.

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