These days it’s all about the U-Values, so you better recognise! Everyone loves a nice low U-Value, or is it a high one….? Here are the quick tips you need to make sure you are bang on with your U-Values!
U-Value – is the measure of the overall rate of heat transfer, by all mechanisms under standard conditions, through a particular section of construction.
Unit is measured in W/m2 K
This is broken down as: The rate of heat flow (in Watts) through 1m2 of a structure when there is a temperature difference across the structure of 1 degree (K or ˚C)
Wall 1 with U-Value of 0.3 W/m2 K will lose heat at half the rate of Wall 2 which has a U-Value of 0.6 W/m2 K
So, the LOWER the U-Value, the BETTER.
The lower the u-value the more efficient the construction is at keeping heat flow through the structure to a minimum.
Currently the Building Regs Part L1A for U-Values are as follows:
Roof: 0.20 W/m2 K
Wall: 0.30 W/m2 K
Floor: 0.25 W/m2 K
Party Wall: 0.20 W/m2 K
Windows, roof windows, curtain walls, pedestrian doors: 2.00 W/m2 K
These are worst acceptable values, and it is always advisable to strive to achieve u-values that comply with BREEAM “Excellent Rating” by seeking out high standard products that are at the forefront of sustainable design.
If you choose to take your standards a step higher, aim for the BREEAM standards of:
Roof: 0.16 W/m2 K
Wall: 0.2 W/m2 K
Floor: 0.22 W/m2 K
Or you could look at the Passiv Haus where in Sweden, to achieve passive house standards, the insulation thickness would be 335 mm (0.10 W/(m².K) and the roof 500 mm (U-value 0.066 W/(m².K)!
Generally, my view on this is make it the best it can be.
Although there are a couple of free u-value calculators online, and some more intelligent modelling software will calculate the u-value for you, it is worth learning how to do it yourself, just in case it pops up in an exam. After all, it is pretty simple when you know how.
To calculate the u-value of a particular part of the building construction you need to know a little about each element of the construction.
Thermal Resistance (R)
U-values are calculated from the thermal resistances of the parts making up a particular part of the structure. Transmission of heat is opposed in varying amounts dependent on material and surface. Thermal Resistance is defined as a measure of the opposition to heat transfer offered by a particular component in a building element.
In order to calculate thermal resistance, you must know the thickness of the material and the Thermal Conductivity (K) value. These values can be found in the Metric Handbook, or the Architects Pocket Book (the Architects Pocket Book is extremely useful, I would recommend every architect student has a copy of this book).
R= Thermal Resistance (m2K/W)
d= Thickness of material (in Metres – very important)
k= thermal conductivity of the material (W/m K)
You must know the thermal resistance (R) in order to calculate the u-value. If you are specifying standard products it is often easy to find the resistance values for these elements. Sometimes it is worth having a look at specific manufacturers websites for these details.
Total Resistance (Rt)
Ra is airspace cavity and values for this can also be found in Architects Pocket Book.
How do you know the Rso and Rsi Value?
Rso is the outside surface resistance, and Rsi is the inside surface resistance. These values are specified in the Architects Pocket Book as:
|RsiInside Surface||RsoOutside Surface|
Now you have your Rt value, the calculation is simple, one divided by the Rt. There you have your U-Value.
Other helpful bits:Architecture.com U-Value guide BRE Guide to U-Values
http://www.vesma.com/tutorial/uvalue01/uvalue01.htm (This one seems the best)
McMullan, R. 2007. Environmental Science in Building