Hi,
is it a way to determine U-value calculation in WUFI 2D using the heat flux profiles? if not, how to determine?
would it be the results accurate and feasible to be considered for a project?
Thanks in advance for the response
U-value calculation for walls using WUFI 2D
-
Alberto Morales
- WUFI User
- Posts: 98
- Joined: Tue Mar 29, 2022 10:57 pm -1100
-
Christian Bludau
- WUFI SupportTeam IBP
- Posts: 1313
- Joined: Tue Jul 04, 2006 10:08 pm -1100
- Location: IBP Holzkirchen, the home of WUFI
- Contact:
Re: U-value calculation for walls using WUFI 2D
The U-value is a steady-state parameter. Therefore, to determine it from WUFI 2D, you should define constant indoor and outdoor boundary conditions and run the calculation until the temperature field and heat flows have reached a steady state.
Once steady-state conditions are reached, the U-value can be derived from the calculated heat flow:
\(U = \frac{q}{T_i - T_e}\)
where \(q\) is the heat flux through the construction in W/m² and \(T_i - T_e\) is the temperature difference between inside and outside.
For a 2D detail, you would integrate the heat flux over the relevant boundary to obtain the total heat flow \(\Phi\) per metre of the detail. For a homogeneous wall section of width \(L\), for example:
\(U = \frac{\Phi}{L \cdot (T_i - T_e)}\)
If the model contains a thermal bridge, the resulting heat flow also includes the additional 2D heat flow caused by the thermal bridge. In this case, it is generally more appropriate to evaluate the linear thermal transmittance (ψ-value) rather than assigning a single U-value to the entire 2D detail.
In practice, however, it is probably easier and more straightforward to determine the U-value and, if relevant, the ψ-value using a dedicated thermal-bridge calculation program, as these programs are specifically designed for steady-state heat-flow calculations.
Christian
Once steady-state conditions are reached, the U-value can be derived from the calculated heat flow:
\(U = \frac{q}{T_i - T_e}\)
where \(q\) is the heat flux through the construction in W/m² and \(T_i - T_e\) is the temperature difference between inside and outside.
For a 2D detail, you would integrate the heat flux over the relevant boundary to obtain the total heat flow \(\Phi\) per metre of the detail. For a homogeneous wall section of width \(L\), for example:
\(U = \frac{\Phi}{L \cdot (T_i - T_e)}\)
If the model contains a thermal bridge, the resulting heat flow also includes the additional 2D heat flow caused by the thermal bridge. In this case, it is generally more appropriate to evaluate the linear thermal transmittance (ψ-value) rather than assigning a single U-value to the entire 2D detail.
In practice, however, it is probably easier and more straightforward to determine the U-value and, if relevant, the ψ-value using a dedicated thermal-bridge calculation program, as these programs are specifically designed for steady-state heat-flow calculations.
Christian
-
Alberto Morales
- WUFI User
- Posts: 98
- Joined: Tue Mar 29, 2022 10:57 pm -1100
Re: U-value calculation for walls using WUFI 2D
Thanks for the reply.
The idea is to analyze the effect of the external climate and moisture in materials in the U-value as you know U value is not constant along the year. There are not static conditions inside and outside of the assembly as it is defined in other software like THERM, FLIXO, etc.
so once explained my idea, would it be the procedure the same to analyze the U value?
The idea is to analyze the effect of the external climate and moisture in materials in the U-value as you know U value is not constant along the year. There are not static conditions inside and outside of the assembly as it is defined in other software like THERM, FLIXO, etc.
so once explained my idea, would it be the procedure the same to analyze the U value?
x