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Understanding R value insulation

Almost all home insulation materials and particularly bulk insulation products have certain features in common. Each is designed to trap air in tiny pockets or cells, whether fluffy, foamed or loose fill. Some rely on reflecting heat and some rely on a combination of trapped air and reflected heat. RMAX ThermaAdvantage® is an example of a product using both methods of insulation. In this section we focus on those that insulate by using trapped air.

Some of these products using trapped air are more efficient than others at resisting heat flow. All are rated by 'R value'.

The R value, or thermal resistance of a material, expresses the ability of a particular thickness of that material to resist heat flow.

The definition of R value is the reciprocal of the material’s thermal conductance (C value). The R value refers to the thermal resistance of a building material, or assembly of building materials, and is the means of calculating the overall thermal resistance of a building section, by simply adding individual component R values.

Approximate thickness of common insulation required for an R value of 2.0.

Isolite® EPS 80mm
Mineral Wool Batts 86mm
Loose Fill Cellular Fibre 94mm

R value (m2K/W)

Practical calculation of 'R' for a homogeneous insulating material 100mm thick;
Design data:

Thickness- d = 100mm or 0.1 metres
Thermal Conductivity -  k = 0.038 (W/mK) - at 6 degrees C mean temperature for EPS Class SL

Conversely, to determine the thickness - d, required of an insulation material for a specified R value, say 2.5, the calculations as follows, using EPS Class SL.

d = R x k
   = 2.500 x 0.038m
   = 0.095 metres or 95mm of EPS

Long Term R values

The moisture content of an insulating material at the time of testing can have a considerable effect on the value of the thermal conductivity obtained.

Water has a thermal conductivity more than 10 times that of common insulating materials and increased moisture content can lead to a marked reduction in the efficiency of insulation or the long term R value after installation.

The long term minimum R value of many insulating materials is often below that claimed by respective manufacturers, because the insulating value of the materials is reduced by dust deposits, ‘settling’ of the insulation or the take up of moisture.

Moisture Resistance

EPS has a low water transmission rate and it has no capillary action. However it must not be considered as a vapour barrier in the same sense as a polyethylene film.

It has excellent breathability characteristics and allows moisture to escape from a wall or floor element and does not form vapour dams.

In spite of the low moisture transmission rate, it is still necessary to use a vapour barrier under conditions of high humidity and high temperature differentials. The vapour barrier should be installed on the warm side of the structural component, with insulation as near as possible to the cold side.

Of all materials used for insulation applications, EPS is one of the most resistant to the adverse effects of moisture. Condensation, which may build up with any insulation material under critical vapour flow conditions, only marginally affects the thermal performance of EPS. Even if condensation develops through improper use, EPS will retain its dimensional stability and superior insulation values. The following chart demonstrates the effect of moisture on k values on several commonly used insulation materials.


Technical data from ASHRAE and International Institute of Refrigeration, ‘The effects of Moisture on Insulating Materials...’