The U-Value is an indicator commonly referred to in the green building world so felt it was important to explain it’s use and provide some suggested guidelines.
As insulation is such an important factor in building an eco house the u-value is relevant as it is a measurement of heat transmission (how well heat passes) through a single or grouping of materials.
The lover the u-value the better (ie. Less heat can penetrate) as this represents a greater resistance to heat and therefore the better insulating value it has.
From a technical point of view it actually measures the amount of heat that will pass through 1 square metre of the construction with a temperature difference of 1 degree between the inside and outside of the building (W/m2K)
When quoting u-values it is important to note whether the value quoted is for an individual material or the complete assembly. A window for example will have the frame, glazing and the sealants which may be a higher or lower value than the glass alone.
Some typical values are:
Standard double glazed unit with 20mm air gap has a u-value of 2.8 W/m2.
A 225mm solid brick wall with 10mm mortar joints has a u-value of 2.00 W/m2.
Minimum accepted values of W/m2K are:
Windows, Doors 2.20
The more advanced EEBPH Standard and which also relates to Passivhaus is as follows:
Windows, Doors 0.80
Airtightness m3/hr/ m2 1.00
The plan for our eco house in to achieve similar values to those listed above as we strive toward a zero carbon home. A house that has ratings similar to those advanced standards listed above is going to have less heating needs and hence less cost to run both in financial terms and in regards to reduced CO2 emmissions.