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Ψ VALUE ... PROBLEM WITH U VALUE
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Robin Mitchell
Nov 20, 2020, 2:46:19 PM11/20/20
to Kostas Hatzopoulos, THERM
To generate a U-value, you need to add U-factor tags to the boundary conditions.
Robin
On Fri, Nov 20, 2020 at 1:02 AM Kostas Hatzopoulos <xatzop...@gmail.com> wrote:
I WONT TO FINED THE Ψ VALUE .. BUT I HAVE THE PROBLEM WITH U VALUE. PLZ HELP ME
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Robin Mitchell
Building Technology and Urban Systems Division
Energy Technologies Area
Lawrence Berkeley National Laboratory
Energy Technologies Area
Lawrence Berkeley National Laboratory
yal...@gmail.com
Mar 5, 2025, 5:35:59 PMMar 5
to THERM
See section 6.5.2 of the latest NFRC simulation manual.
archi...@gmail.com
Mar 18, 2025, 12:04:50 PMMar 18
to THERM
Hi.
From what you write I suppose you write from Europe (Greece perhaps?) so European standards should apply to your case, in particular EN ISO 10211
Yours is the typical corner Thermal Bridge to which 2 psi values are associated depending on which dimension system is used: gross external or net internal.
Boundary conditions
Example 1. The corner refers to two walls than BCs to be adopted are: external side Rse=0.04 m2K/W, every BC segment's U-factor name could be "External", internal side Rsi=0.13 m2K/W and every BC segment's U-factor name could be "Internal" (see file "Example 1 corner 2 walls Psi.THM").
Example 2. The corner refers to a vertical wall and a horizontal roof than BCs to be adopted are: external side Rse=0.04 m2K/W, every BC segment's U-factor name could be "External", internal side for the vertical wall Rsi=0.13 m2K/W for the horizontal roof Rsi=0.10 m2K/W and every BC segment's U-factor name could be "Internal" (see file "Example 2 corner wall-roof Psi.THM")
Thermal Resistance and thermal Transmittance
According to EN ISO 6946 structures thermal resistance is Rtot = Rsi + sum s/L + Rse and U = 1/Rtot
Example 1. According to the material's thermal conductivity and thickness adopted in the Therm files the brick wall's thermal transmittance is U1=0.141 W/m2K and the concrete wall's thermal transmittance is U2=0.153 W/m2K
Example 2. According to the material's thermal conductivity and thickness adopted in the Therm files the brick wall's thermal transmittance is U1=0.141 W/m2K and the concrete roof's thermal transmittance is U2=0.154 W/m2K
Dimension system
According to EN ISO 13789 buildings' thermal performance may be calculated using externale gross dimensions or external net dimensions. Thermal bridges' Psi-values calculation must be consistent with the dimension system used in thermal calculations.
From the illustration you posted I suppose that you mean to calculate Psi-value consistent to net internal dimension.
In both examples the structures' internal net dimensions are 1,50 m and the gross external dimensions are 1,90 m
L2D Calculation
According to EN ISO 10211 L2D = Phi/deltaT. The thermal bridge's Phi value is found in Therm's calculation results.
Example 1. Phi = 10.3027 W/m, deltaT = 20 K, L2D = 10.3027/20 = 0.515135 W/mK
Example 2. Phi = 10.3415 W/m, deltaT = 20 K, l2D = 10.3415/20 = 0.517075 W/mK
Psi-value calculation
According to EN ISO 10211 Psi-value of a 2D Thermal Bridge must be calulated using the following equation: Psi = L2D - sum(UxL)
Example 1. Psi(i) = 0.515135 - (0.141x1.5 + 0.153x1.5) = 0.074 W/mK, Psi(e) = 0.515135 - (0.141x1.9 + 0.153x1.9) = -0.044 W/mK
Example 2. Psi(i) = 0.517075 - (0.141x1.5 + 0.154x1.5) = 0.075 W/mK, Psi(e) = 0.517075 - (0.141x1.9 + 0.154x1.9) = -0.043 W/mK
From what you write I suppose you write from Europe (Greece perhaps?) so European standards should apply to your case, in particular EN ISO 10211
Yours is the typical corner Thermal Bridge to which 2 psi values are associated depending on which dimension system is used: gross external or net internal.
Boundary conditions
Example 1. The corner refers to two walls than BCs to be adopted are: external side Rse=0.04 m2K/W, every BC segment's U-factor name could be "External", internal side Rsi=0.13 m2K/W and every BC segment's U-factor name could be "Internal" (see file "Example 1 corner 2 walls Psi.THM").
Example 2. The corner refers to a vertical wall and a horizontal roof than BCs to be adopted are: external side Rse=0.04 m2K/W, every BC segment's U-factor name could be "External", internal side for the vertical wall Rsi=0.13 m2K/W for the horizontal roof Rsi=0.10 m2K/W and every BC segment's U-factor name could be "Internal" (see file "Example 2 corner wall-roof Psi.THM")
Thermal Resistance and thermal Transmittance
According to EN ISO 6946 structures thermal resistance is Rtot = Rsi + sum s/L + Rse and U = 1/Rtot
Example 1. According to the material's thermal conductivity and thickness adopted in the Therm files the brick wall's thermal transmittance is U1=0.141 W/m2K and the concrete wall's thermal transmittance is U2=0.153 W/m2K
Example 2. According to the material's thermal conductivity and thickness adopted in the Therm files the brick wall's thermal transmittance is U1=0.141 W/m2K and the concrete roof's thermal transmittance is U2=0.154 W/m2K
Dimension system
According to EN ISO 13789 buildings' thermal performance may be calculated using externale gross dimensions or external net dimensions. Thermal bridges' Psi-values calculation must be consistent with the dimension system used in thermal calculations.
From the illustration you posted I suppose that you mean to calculate Psi-value consistent to net internal dimension.
In both examples the structures' internal net dimensions are 1,50 m and the gross external dimensions are 1,90 m
L2D Calculation
According to EN ISO 10211 L2D = Phi/deltaT. The thermal bridge's Phi value is found in Therm's calculation results.
Example 1. Phi = 10.3027 W/m, deltaT = 20 K, L2D = 10.3027/20 = 0.515135 W/mK
Example 2. Phi = 10.3415 W/m, deltaT = 20 K, l2D = 10.3415/20 = 0.517075 W/mK
Psi-value calculation
According to EN ISO 10211 Psi-value of a 2D Thermal Bridge must be calulated using the following equation: Psi = L2D - sum(UxL)
Example 1. Psi(i) = 0.515135 - (0.141x1.5 + 0.153x1.5) = 0.074 W/mK, Psi(e) = 0.515135 - (0.141x1.9 + 0.153x1.9) = -0.044 W/mK
Example 2. Psi(i) = 0.517075 - (0.141x1.5 + 0.154x1.5) = 0.075 W/mK, Psi(e) = 0.517075 - (0.141x1.9 + 0.154x1.9) = -0.043 W/mK
Ciao
Fabrizio
Hamid BADI
Mar 20, 2025, 4:13:37 AMMar 20
to THERM
Juergen
Jun 20, 2025, 3:07:07 AM (3 days ago) Jun 20
to THERM
This looks really nice. How do you create this graphical output in Therm? And how do you get the Psi values?
Fabrizio Prato
Jun 21, 2025, 4:54:50 PM (2 days ago) Jun 21
to Juergen, THERM
Hi Juergen.
These images are not made with Therm, but with some sw used in France, I think. Perhaps the author may confirm.
They're certainly a good representation of a thermal bridge calculation, since everything needed to reproduce it is clearly shown: internal and external dimensions (and relative psi values), materials conductivities and BCs.
Very nice ... But it is not Therm.
Ciao
Fabrizio
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