PH_Rev02.txt

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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
- 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
- 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 + (0.425-0.175)/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 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

Author

monqs

249206

Card Set

PH_Rev02.txt

Description

Passive House Day 2

Updated

2013-11-26T19:22:34Z

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