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21 September 2017

There is More to Insulation Than Just the R-value

1709 Keith Huntington brian babb 256298

Despite the public's general perception, there is a lot more to achieving a comfortable thermal environment than just the amount of insulation used. In their mind, thermal insulation usually is the primary means, and the effectiveness is measured by the stated 'R-value' of the product.

The R-value of materials is an expression of the ability of heat to move from the hot side to the cold. When this is applied to an assembly of various materials, such as the wall of a house, this is referred to as the 'Construction R-value' of the whole assembly. The higher the figure, the better the material (or construction) is at resisting the transfer of heat energy.

While advertisements for proprietary insulation products and the NZ Building Code (NZBC) suggest that the R-value number is what is important — which it is — the achieving of this number is not a simple matter of shoving the particular numbered insulation product into the particular wall, roof or floor of the building.

R-values can be assigned to any material — the Building Code value of R1.9 for an exterior wall could be achieved with a solid aluminium wall; the problem would be that the wall would be so thick you would need to ask your neighbours if you could construct most of it on their property. When buying/specifying thermal insulation one quickly notices that the same type of material has different numbers which depend upon the thickness. The stated R-values for a particular insulation product are dependent upon a variety of primary factors:

  • First, the thermal conductivity of the material (how easy and fast heat energy can move through it)
  • How dense the material is (squashed fibre-glass pads are less effective than those with the standard 'fluffiness')
  • How thick the material is (how much material there is for the heat to transverse)
  • What the temperature difference is between each face (similar to how a hot coffee loses heat faster than a tepid coffee does when on the same table)
  • And finally, what is the dryness of the material (this is why a dry cotton shirt is warmer than a wet one — water is a very effective heat conductor)

There are other factors such as radiative and convection transfer, and the 'wind-chill' influence, in particular circumstances. So as to have a consistency of description, testing laboratories use a set of standard parameters against which to assign the R-value. This is why it is important to install thermal insulation in accordance with the instructions.

In a building the thermal insulation is not used in isolation, which is why Construction R-values are what is important when looking for thermal comfort. In the same way as you put on a coat on a winter's day when a jersey is not sufficient, the insulation performance of the various materials of, say, a wall are added together to create the final Construction R-value. For example, the NZBC Construction R-values required for Auckland are less than for Central Otago. Unfortunately matters aren't as simple as this, there is the confounding problems of thermal bridges, passive solar gain and thermal mass.

Thermal bridges are the pathways through a construction element which have a different Construction R-value to that through the thermal insulation path. The common situation is the timber wall framing which reduces the overall insulation performance. A simple example is of a fully waterproof bucket which has a small hole in the bottom. Eventually the water will drain out. Heat is lazy; it will not try to struggle through the insulation when there is an easier path through the thermal bridge. Windows are the most important thermal bridges in a building (see my August 2015 blog post; Thermal Drapes: Do They Work As Insulation?, and June 2016 post; More Sophisticated Thermal Performance Ratings for Windows). There is also the quality of the insulation installation. NZ Standard 4246:2006 "Energy Efficiency — Installing Insulation in Residential Buildings" advises at section 3 that gaps etc "… as small as 2mm …" can reduce the insulation value "… by as much as 50% ..."

The above discusses the absolute movement of thermal energy without reference to time, nor to the amount of thermal energy available to be transferred. Also heat does not always flow from the interior to the exterior, it also flows into the building, as in summer. The rule is that heat flows from the hot side to the cold, and the greater the temperature difference, the faster the transfer.

Buildings are not located in laboratories, they are out in the real world and exposed to weather and climate. Although the Construction R-values of the wall/roof elements are constant, the heat energy available to be transferred varies on an hourly, daily, seasonal, annual, etc., basis. This leads to the need to consider these dynamic aspects when discussing insulation.

Looking at the TV weather reports it can be seen that for the majority of the population, the low winter night temperatures are in single figures (unlike the zero degrees or below often used for research purposes). As most houses have carpets on their floors, and standard insulated walls and roofs, the temperature differences driving heat flows are small. During the daytime the low winter highs are usually sufficient for most houses to have a significant passive solar gain to minimise or balance the night losses. Because heat is lost through insulation at a particular 'speed', if there is more internal thermal mass then more time is required for its stored heat to be lost. Where a house has significant thermal mass then the building can be passively re-charging before the stored heat is drained.

Thermal insulation cannot be selected on its R-value alone; it is just one element of a dynamic inter-related system (see my blog posts of 2014 and 2015).

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