U Value
U Value
U Value : U-Value
The U value is the measurement of heat transmission through a material or assembly of materials. The U value of a material is a gauge on how well heat passes through the material and the lower the U value, the greater the resistance to heat and therefore has a better insulating value. The U value is generally used in the building and construction industry to specify assemblies of components which provide a suitable insulation and energy efficiency value. U-Value is expressed in units of W/m2 and relates to the amount of heat lost in watts (W) per square metre of material. Basically if a wall material has a U-Value of 1 W/mē K, for every degree of temperature difference between the inside and outside surface there would be 1 Watt of heat energy flowing through each meter squared of its surface.
U-Value and Heating Bills
The lower the u-value means less heat loss, better insulation and effective lower fuel bills. Building regulations regarding the specifying of u-values has become very stringent over the past 10 years to give buildings better insulation and reduce heating due to global consumption.
Example U-Values:
Standard double glazed unit with 20mm air gap has a u value of 2.8 W/m2.
Standard double glazed unit with 20mm air gap and 1 pane of low e glass has a u-value of 1.8 W/m2.
A 225mm solid brick wall with 10mm mortar joints has a u-value of 2.0 W/m2.
Typical cavity wall with no insulation has a u-value of 1.6 W/m2.
When quoting u-values it is essential to note whether the value quoted is for a individual material or a complete assembly. A window for example will have the frame, the glazing, and the sealants which may be a higher or lower u-value than the glass alone. Assembled materials can have their u-value reduced considerably by adding insulation. Insulated plasterboard can be applied to the inside face of a brick wall to considerably reduce the u-value of the structure. Glass can have special thermal coatings applied which can allow the heat into the building but restrict the flow of heat out of the building. Low e glazing has become very popular over the last few years as glazing in a building is where large amounts of heat can be lost making the whole building less thermally efficient. The cavity between the double glazed units can additionally be filled with a gas such as argon which is denser than air alone, again lowering the u-value and increasing the insulation.
Material Thermal Conductivity..
Internal Brick 0.77 W/m-K.
External Brick 0.56 W/m-K.
Mild Steel 50 W/m-K.