30 October 2014

Insulation, Thermal Conductivity, R Value, U Value Explained


Good quality and well installed insulation helps keep the heat in during winter and keeps it out during summer. This makes your house easier and cheaper to heat properly, it also makes it more comfortable and healthy to live in. Insulation works by reducing the transfer of heat by means of a barrier. Unfortunately in most cases the better the insulation the more it costs, so like most things a balancing act is required. To understand insulation you need to know what thermal conductivity, R values and U values are.




Thermal Conductivity

Thermal conductivity is the measure of how easily heat flows through a specific type of material, regardless of the thickness of the material. The lower the thermal conductivity of a material, the better the thermal performance (i.e. the slower heat will move across a material). See below for some example materials and there conductivity values.

Material      
Thermal Conductivity W/(m K)
diamond 1000
aluminium 205
steel 54
glass 1
polyurethane 0.02
air at 0°C 0.024

So heat flows through diamond 1000 times easier than glass. 



R Value

R-value is a rating which measures how well insulation can resist heat flow. ‘R’ stands for thermal resistance. The R-value depends on the type of material, its density and thickness. R-value is widely used to measure building insulation and is generally available for most products. The larger the R value the better it will insulate.

Insulation requirements

New Zealand and many other countries have minimum requirements for insulation. The roof requirement is the highest at R2.9. Due to heat rising this equates to the largest loss of warmth at around 30-35%. It is important to note these R Values are based on the completed sections, for example the walls requirement at R1.9 would include cladding, air gaps, building paper and internal lining.




These are minimum requirements and it is recommended in practise to go well beyond these. Building tiny means we can afford to have better quality insulation with greater results.

U Value


The U-value is a measure of how much heat is lost through a given thickness of a particular material. The lower the U-value is, the better the material is as a heat insulator. U value is the inverse of the R value but takes into account convection and radiation heat losses. This calculation can be complex and best left to software. A range of possible U-values are indicated below for comparison:


Material Thermal     
U value
Single glazing
4.8
Double glazing
2.8
Low e double glazing
2
Cavity wall with no insulation
10.3
Insulated roof 
0.2





Window efficiencies are measured by U value because of the more complex nature of different materials and airflows. 


Real World


The values of insulation materials are measured in a lab. That would work great - if your home were inside a lab! As the exterior of houses are outdoors they are subjected to outside elements, like wind, humidity, and temperature changes. Meaning a measure like R value can be greatly reduced if your house is full of holes. So having a series of materials that resist heat well is step one. Step two is to design the system well to control the factors that can reduce insulation performance.


Summary


Measure   Dependent on Best number Where used Example expanded polystyrene  Units
Thermal Conductivity Independent of thickness  Smaller better Materials 0.03 (given value which can be looked up) W/mK
R Value Dependent on thickness Bigger better Material with thickness (insulation) Will use 100mm thickness for example 0.1 / 0.03 = 3.3 m2K/W
U Value Dependent on thickness plus convection losses Smaller better Building systems like windows 1 / 3.3 = 0.3 + plus convection and radiation losses W/m2/K

  • Higher numbers are good when comparing the Thermal conductivity and R-Values of products.
  • Low numbers are good when comparing U-Values.
  • The U-Value is the most accurate way to judge a materials insulating ability, however it is more difficult to calculate. The manufacturers value and how it compares is important.


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