A Thermal Bridge, is where a penetration of the insulation layer by a highly conductive or noninsulating material takes place in the separation between the interior and exterior environments of a building.
What is a Thermal Bridge also called?
A Cold Bridge
What types of Thermal Bridge exist?
LINEAR Thermal Bridges
POINT Thermal Bridges
GEOMETRICAL Thermal Bridges - placed at the junction of two planes, such as at the corner of a wall
What types of Point Thermal Bridge exist?
Repeating POINT Thermal Bridges e.g. columns in a framed building
Non-Repeating POINT Thermal Bridges e.g. single occurence
Linear Thermal Bridges are measured in?
Psi Value in W/mK
What does the Psi symbol look like?
3 prong fork
Like this: ψ
Point Thermal Bridges are measured in?
Chi Value in W/K
What does the Chi symbol look like?
2 swords crossed (CostX symbol)
Like this: χ
What is the formula for Heat Loss through an area?
Qt = A x U x ft x Gt
Gt is based upon climate data for specific location
What is the formula for Heat Loss through a Linear Thermal Bridge?
Qt = L x Psi x ft x Gt
Gt is based upon climate data for specific location
Steps Psi calculated?
U1 = U-Value of general insulation to area
U2 = Reduced U-Value of general insulation at Thermal Bridge
Delta U = U1 - U2
Psi = Delta U x width of Thermal Bridge exposed edge
or
Psi = (U1 - U2) x w (width of Thermal Bridge exposed edge
Calculate the Heat Loss thru' the Thermal Bridge illustrated below
Calculate the annual Heat Loss thru' a 50m Thermal Bridge with a Psi Value of 0.018W/K and Gt of 82kKh/a
Qt = L x Psi x ft x Gt
Qt = 50m x 0.018W/K x 1 x 82kKh/a
= 73.8kWh/a
If Thermal Bridge Heat Loss = 73.8kWh/a and overall Heat Loss = 9.8W/m2 TFA, calculate Heat Demand (Pt) on worst day of year, when external temp is -10C
Therefore it does comply with Passive House requirements as Heat Demand <10W/m2
Calculate the Payback Period for the addition of a reduced 50m Thermal Break given the following data; Psi without Thermal Break = 0.51W/mK; with Thermal Break = 0.02W/mK; Gt = 60kKh/a; Heat cost = 0.10 per kWh
Qt = Delta Psi x L x ft x Gt
= (0.51 - 0.02)W/mK x 50m x 1 x 60kKh/a
= 1470kWh
Savings per year = 1470 kWh x 0.10 = 147
Cost of Thermal break = 50m @ 20/m = 1000
Payback = 1000 / 147 = 6.8 years
Calculate the additional wall insulation required for 100m2 of wall to compensate for a Thermal Bridge Heat Loss of 1470kWh/a when existing U-Value of wall is 0.15W/m2K and Gt is 60kKh/a and insulation conductivity is 0.04W/mK
Qt(wall) = A x U x ft x Gt
= 100 x 0.15 x 1 x 60
= 900kWh/a
Additional Heat Loss from Thermal Bridge = 1470kWh/a
Total wall Heat Loss = 900 + 1470 = 2370kWh/a
Calculate Equivalent U-Value
U = Qt / (A x ft x Gt)
= 2370 / (100 x 1 x 60)
= 0.395W/m2K
Delta R = 1/U(orig) + 1/U(inc T B)
= 1/0.15 - 1/.395
4.135
Extra insulation thickness = Delta R x Sigma Insulation
= 4.135 x 0.04
= 0.165m
Which as the greater Heat Loss, 2 columns with a Chi value of 0.47w/K or a 7m Thermal Bridge with a Psi of 0.15W/mK
No need to use Qt as time is not involved
Heat Loss Columns = 2 x 0.47 = 0.94W/K
Heat Loss Thermal Bridge = 7 x 0.15 = 1.05W/K
Therefore the 2 columns have the lowest Heat Loss
How is Treated Floor Area calculated?
On the basis of external wall dims i.e. outside to outside
How is Height of Treated Floor Area calculated?
Height is measured from ceiling to underside of ground floor insulation
What is the impact of negative Psi values?
They do not generate negative heat losses, but ar used to cancel positive Psi values to reduce total Heat Loss