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THERMAL BRIDGING
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What is a Thermal Bridge?
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
 NonRepeating 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?

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 = UValue of general insulation to area
 U2 = Reduced UValue 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
 Insert image in Q
 U1tot = 1/R1tot
 R1tot = Rse + Rpe + RB1 + Rpi + Rsi
 = 0.04 + 0.04/0.07 + 0.425/0.08 + 0.015/0.35 + 0.13
 = 6.0968
 U1 = 1/R = 1/6.0968 = 0.16W/m2K
U2tot = 1/R2tot
 R2tot = Rse + Rpe + RB1 +RC1 + Rsi
 = 0.04 + 0.04/0.07 + (0.4250.175)/0.08 +0.19/2.3 + 0
 NB Rsi = 0 here as there is no internal surface
 = 3.819
 U2 = 1/R = 1/3.819 = 0.262W/m2
Delta U = U1  U2
 = 0.16  0.262 = 0.098
 Psi = Delta U x d (depth of Thermal Bridge )
 = 0.098 x 0.180= 0.018 W/K

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
 Thermal Bridge Heat Loss /m2 TFA = 73.8kWh/a / 150m2 TFA
 = 0.49 kWh/m2/a
 Heat Demand = Pt = L x Psi x ft x Delta T
 = 50m x 0.018 x 1 x (20  (10))
 = 27W
 TFA = 150m2
 Heat Demand / TFA = 27W / 150m2 = 0.18W/m2
 Heat Demand for other heat losses = 9.8W/m2
 Add Thermal Bridge Heat Losses = 9.8 + 0.18 = 9.98W/m2
 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 UValue 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 UValue
 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

