Thermal Bridges

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Thermal Bridges

• peter warm
• AECB Conference Norwich
• June 2008

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Importance of thermal bridges
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Repeating linear thermal bridges
Repeating thermal bridges
Included in U-value calculation
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Non-repeating linear thermal bridges
Add as Correction to heat loss
Extra Heat loss ?? x length
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Lintels (box)Insulation has no effect!
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Lintels

• Provide separate lintels in each leaf.
• Timber box fixes window.

Brenda and Robert Vale
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Thermal bridges at joins with floors
Insulation
Beam and block
Lightweight blocks being used to prevent thermal
bridge.
Foundation
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Hillcrest Park, Exeter (2004)
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Taking Account of Thermal Bridging

• TAKE HEATING ENGINEERS HEAT LOSS
• U x A
• ADD CORRECTION TERM FOR EACH BRIDGE
• ? x L
• ? linear thermal transmittance W/mK
• L length m

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Taking Account of Thermal Bridging
? values for Accredited Details (SAP 2005)
? values can also be calculated see BRE IP 1/06
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Taking Account of Thermal Bridging

• For unknown thermal bridges
• Default rate of additional heat loss y W/m2K
• If accredited construction details are used
• y 0.08 W/m2K
• If accredited details are not used
• y 0.15 W/m2K
• Total overall additional heat loss is given by
• HTB y x AHL W/K
• where AHL is the total heat loss area m2

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Non repeating thermal bridges not at joins
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Pictures courtesy Malcolm Bell
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Thermal Bridges

• Geometrical and Material
• Conventions internal and external
• Calculation methods
• Therm
• Examples

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Exercise 1

• Building Plan

9m

• If Uwall0.1
• Height 5 m
• What is Engineersheat loss
• Using internal (UK)
• Using external (D)
• dimensions?

8m
8m
9m
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Internal (UK)
Perim 8 4 32m Area 32 5 160m2 Heat
loss 160 0.1 16 W/C
8m
8m
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External (D)
9m
Perim 9 4 36m Area 36 5 180m2 Heat
loss 180 0.1 18 W/C 12.5 larger
9m
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Exercise 2 calculate actual corner heat loss
0.5m
1m
1.5m
1m
0.5m
1.5m
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Therm Setup load and get Therm running, then

• LIBARYS
• Pw10lib and Pw10bc on stick or on server
• Libraries / material lib / load lib / pw10lib.lib
• Libraries / boundary library / load lib /
  pw10bc.lib
• SNAP
• Set to 5mm H and V and set on
• Options / Preferences / grid and H,V 5mm
• UFACTOR names
• Libraries / check Ufactor names / check internal
  and external available, if not add names
• DISPLAY
• Can only input once simulation calculated on
  screen
• Calcs / DispOpts / advanced / colourIR 0 20 no
  auto prog default / colour flux 0 35 no auto prog
  default
• ACCURACY
• recommended settings in options/prefs/therm
  settings
• Quad tree mesh 8 max error 2 max iteration
  10
• Thanks to Kersttin Rosemeier ( NZ)

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Exercise 2 Step 1 draw shape
0.5m
1m
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Exercise 2 Step 1 draw shape
0.5m
1m
1.5m
1m
0.5m
1.5m
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Exercise 2 Step 2 define material conductivities
Material k 0.05085 W/m2K (chosen to give U0.1
with standard boundary conditions)
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Exercise 2 summary internal
Perim 8 4 32m Area 32 5
160m2 Simple Heat loss 160 0.1 16
W/C Complex heat loss Simple heat loss Psi
internal length ( length?) (psi
length?) (Complex heat loss ?) ( change over
simple heat loss?)
8m
8m
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Exercise 3 thinner insulation

• Building Plan

8.5m

• If Uwall0.1
• Height 5 m
• What is heat loss
• Using internal
• Using external dims?

8m
8m
8.5m
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Exercise 4 summary internal
Perim 8 4 32m Area 32 5
160m2 Simple Heat loss 160 0.1 16
W/C Complex heat loss Simple heat loss Psi
internal length ( length?) (psi
length?) (Complex heat loss ?) ( change over
simple heat loss?)
8m
8m
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Exercise 4 summary external
Perim 8.5 4 34m Area 34 5
170m2 Simple Heat loss 170 0.1 17
W/C Complex heat loss Simple heat loss Psi
external length ( length?) (psi
length?) (Complex heat loss ?) ( change over
simple heat loss?)
8.5m
8.5m
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Exercise 5 Poor material choice
0.5m
1m
1.5m
1m
K0.13
0.5m
1.5m
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Exercise 5 Poor material choice
0.5m
1m
1.5m
1m
K0.13
0.5m
1.5m
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Exercise 5 Poor material choice
0.5m
1m
Heat flux
1.5m
1m
K0.13
0.5m
1.5m
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Passivhaus convention
For an overestimate of heat loss Use external
measurements To outside of construction, or to
outside of insulation if large rainscreen voids (
eg lofts) Make insulation continuous no
material bridges, only geometrical (strictly psi
external lt0.01) As thermal bridges are
calculated, psi values tend to be negative, ie
more accurate calculation lowers heat loss. Hence
external measurements good for first estimate.
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psi internal and psi external

• Both give correct L2D and hence total heat loss
• Can be calculated from each other
• Psi ext Psi int - Uwall times difference in
  length
• Check this relations ship out for walls.
• Note critical to define plane of measurements
• Note can have psi for mid insulation plane, or
  any other pane is possible, but value different
• Similarly there generally exists a plane for
  which psizero, but this is of theoretical
  interest only.

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Wall to roof junction

• Note difference in external and internal lengths
  on slope
• Note have wall and roof UA to subtract from L2D
  heat flow

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Window and Internal floor details

• No difference in internal and external
  measurements
• Hence psi int psi ext
• Psi is material only, no geometrical component

window
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Internal floor between flats party wall to
external wall junctions

• Significant difference in internal and external
  measurements
• Hence psi int not equal to psi ext

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Ground floor

• Have to take into account ground
• Usually calculated fro default breadth of 8
  metres
• Requires 20 by 20m lump of earth
• Some different conventions see BRE 497

1m
4m
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Ground floor
4m
20m

• Have to take into account ground
• Usually calculated for default breadth of 8
  metres
• Requires 20 by 20m lump of earth
• Some different conventions see BRE 497

20m
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PH window conventions

• Need to calculate psi spacer for each window type
  and orientation
• Then need psi installation at present single
  value for each type

window
window
window
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Summary we have looked at

• Difference between Geometrical and material psi
• Have used Therm
• Link between psi and measurement plane
• PHPP conventions