• Magnus Pilgaard Sloth
4. semester, Indeklima og Energi, Kandidat (Kandidatuddannelse)
The thermal transmittance, U-value affects the performance of building envelopes. This thesis reviews the existing literature for both in-situ and laboratory methods for measuring the thermal transmittance of wall sections. Both steady-state and transient methods are reviewed and compared to be used in a guarded hot box. As it was not possible to finish the measurement setup, a numerical study on the effect of moisture transport, convective heat transfer due to airflow inside a wall section and solar radiation has been conducted, as ISO 9869 identify these as possible factors altering the thermal transmittance.

Several steady-state and time-dependent simulations have been made on three layered two-dimensional models with different boundary conditions to assess the influence of the parameters. They feature heat transfer, moisture transport, laminar flow, porous media flow and solar radiation.

It is found that the U-value is significantly influenced by the investigated parameters. Using only heat transfer and radiation physics, the U-value is decreased as much as 23%, compared to the theoretical value of the wall section. When including moisture transport, an increase in the U-value up to 17% is obtained. This is most significant in steady-state simulations. The effect is decreased in time-dependent simulations using representative weather data for outdoor conditions. Simulations, that also include a closed loop with forced air flow inside the insulation layer or in an air gap produced the most significant influence on the U-value. The largest decrease in its value is 45% with a slow air flow, and the largest increase was 215% when having a higher air flow velocity. It occurs in the wall with an air gap around the insulation. Time-dependent studies generally decreased the U-value compared to steady-state ones, making the heat loss smaller. The results show a high sensitivity regarding air flow, meaning that this phenomenon should be examined more in the future, especially in built wall constructions as well as the possible velocity range created by thermal buoyancy.
Udgivelsesdato10 jun. 2020
Antal sider126
ID: 333921863