Technical
So as to more fully understand the total efficiency of the dwelling we need to examine the building elements in the dwelling and how their application affects the CDER and consequently the BER of the dwelling. This section deals with each individual building element such as walls, windows and roofs etc and the amount of heat that is lost through the fabric/element.
U value
- The U-value measures how well a building element, e.g. a wall, roof or a window, keeps heat inside a building
- The U-value is a measure of the heat flow through a building element
- A house built with low U-value building components will use less energy and improves the BER
The higher the U value the higher the rate of heat loss through the fabric.
Typical U Values for new dwellings as specified in TGD L
- Walls 0.27 W/ m2K
- Ground floors 0.25 W/ m2K
- Roofs 0.16 W/ m2K
Building Elements – Heat Loss
- Material U value (W/ m2K)
- Area of Material (m2)
Coefficient of Heat Loss for material is U Value x Area (W/K)
Temperature difference (inside to outside) is in degrees Kelvin (K)
Heat Loss = Coefficient of Heat Loss x Temp Diff.
Heat Loss is in Watts (W)
Thermal Bridge
Component through which the heat is transferred at a substantially higher rate than the surrounding building envelope. Break in building elements.
How Thermal Bridging is calculated in the DEAP energy ratings software :
At junctions between elements and around openings using a general bridging factor which is entered into the software in the building elements section
This thermal bridging factor has the same units as the u-value and calculates a transmission heat loss coefficient for non repeating thermal bridges
- Default is 0.15
- Option 1 is where construction does comply with building regulations = 0.11
- Option 2 is where construction is classified as meeting additional requirements = 0.08
The Heat loss coefficient due to thermal bridging = Y × Total exposed surface area (W/K)
Compliance with Building Regulations Part L
TGD Part L gives two alternative methods of showing acceptable levels of insulation
The overall heat loss method
Or the elemental heat loss method.
The Overall Heat Loss Method:
- Calculates the total exposed elements area
- Calculates the heat loss coefficient for each element (U-Value x Area)
- Sum the heat loss coefficients together and divide by total exposed area to get the average U-Value
- Calculate the ratio of Total Exposed Area/Volume = At/V
We use this ratio At/V and Table 1 (TGD L) to determine if the average U-Value is less that the Maximum permitted U-Value.
We also need to check individual U-Value Limits
Roofs = 0.25
Walls = 0.37
Ground Floors = 0.37
(No maximum U-Value limits specified for Doors and Windows)
