How can you reduce thermal bridging at little cost?
U Value = ?
_{esistance}
Rse is what?
External Surface Resistance
Rsi is what?
Internal Surface Resistance
Rsi Floor = ?
0.17
Rse External Wall Surface = ?
0.04
Rsi Internal Wall Surface = ?
0.13
Rsi Internal Roof Surface = ?
0.10
What is Rsi based on?
direction of heat flow
If you have a frame construction:
1) then the conduction of heat through the frame element will be very different than through any intermediate insulating element. The key example is timber frame.
2) Here, you need to take sepearate U values through the frame item and the insulating element and average them.
Calculate U-Value for:
R = Rse + Rp + Ri + Rw + Rsi
Rp = 0.015/0.4 = 0.0375
Ri = 0.2/0.035 = 5.71
Rw = 0.215/0.60 = 0.358
R = 0.04 + 0.0375 + 5.71 + 0.358 + 0.13
= 6.2755
U-Value = 1/R = 1/6.2755
= 0.159
Say 0.16 W/m2K
The resistance of a building material is derived by the following formula:
R = (1/k) x d where k is the conductivity of the building material and d is the material thickness.
or
R = (1/)building x d where k is the conductivity of the building material and d is the material thickness.
How is the U-Value Defined?
The U value is defined as being reciprocal of all the resistances of the materials found in the building element.
The formula for the calculation of a U value is:
U(element) = 1 / (Rso + Rsi + R1 + R2 ...)
- where Rso is the fixed external resistance
- where Rsi is the fixed internal resistance
- and R1… is the sum of all the resistances of the building materials in the constructional element.
What 5 pieces of information do we need to calculate a U-Value?
1) The build up of each element.
2) Sequence of each building material
3) Thickness of each material.
4) Conductivity of each material. This is a measure of its inherent ability to facilitate the passage of heat. It is normally referred to as a ‘k value’.
5) The properties of the internal and external faces of the constructional element need to be allowed for. These are called external resistances and are fixed values.