Construction simulations with inside and outside insulation of temperature and vapor

Transcription

1 Construction simulations with inside and outside insulation of temperature and vapor 1 Research goal To separate the controlled indoor climate from the fluctuating outdoor temperature and reduce energy demands by HVAC-systems, the exterior walls should minimize undesirable energy exchange. Isolation materials are applied globally to increase the thermal and/or vapor resistance of exterior constructions. The place of the insulation material in the wall construction should also be chosen carefully. The goal of this study is to visualize the effects of outside versus inside insulation of an exterior wall on the thermal and vapor behavior in the construction.

2 3 Method We use the software program COMSOL Multiphysics 3.5 to make computer simulations of heat and vapor flow problems in exterior wall constructions. Herewith we try to visualize which influences the position of the thermal and vapor insulation material has on the thermal and vapor distribution in an exterior wall construction. First we simulate a thermal insulated single wall construction, which is exposed to a daily periodic temperature curve on the outside. In model 1 the insulation is placed on the inside of the wall and in model 2 it is placed on the outside. Then we simulate a regular Dutch thermal insulated cavity wall under stationary winter conditions. Both heat and vapor problems are concerned in these simulations, so the relative humidity can be determined, which represents the risk of condensation problems. We also simulate this cavity wall with a vapor resistant layer on the inside and once with this layer on the outside. The exact properties of the simulation models and input parameters in COMSOL are presented in the model section below. 2

3 4 The model The physical modules in COMSOL that are applied in this study, to solve the heat and vapor problems are: Heat Transfer by Conduction Diffusion Geometry and mesh properties of the first simulation study Dimensions [m]: Concrete wall (WxH) = 1 x 0,2 Insulation (WxH) = 1 x 0,05 Fig. 1 shows the two-dimensional models of the insulated single wall constructions, which we have used for the heat flow simulations. The colours in these pictures indicate the different densities of the materials. The model on the left hand side is insulated on the inside, the insulation material is indicated as the bleu rectangle. The wall on the right side is insulated on the outside. Fig. 1 Visualization of the inside (left) and outside (right) insulated single wall models, used for the simulations Mesh settings: The mesh geometry of the two models consists of 3648 elements and is shown in Fig. 2. On the right side the other mesh statistics are present: Fig. 2 Mesh geometry of the models Fig. 3 Mesh statistics 3

4 Boundary Conditions of the first simulation study Convective Cooling: Air temperature outdoor (outside boundary) This temperature range is quite extreme within a daily period, but it makes the effects more clearly. Thereby the wall surface temperature on a winter s day can quickly rise when it is directed at the sun. = *sin(2*pi*t/(24*3600)) K Surface heat transfer coefficient outside = 25 W/m 2.K (outside boundary) Air temperature inside = 283 K; (inside boundary) Surface heat transfer coefficient inside = 7.7 W/m 2.K (inside boundary) Thermal insulated boundaries = Left and right vertical boundaries Material Properties and Initial Condition of the first simulation study Thermal conductivity k: Concrete wall = 2 [W/m.K] Insulation material = 0.04 [W/m.K] Density ρ: Concrete wall = 2000 [kg/m 3 ] Insulation material = 50 [kg/m 3 ] Heat capacity at constant pressure C p : Concrete wall = 840 [J/(kg*K)] Insulation material = 840 [J/(kg*K)] Initial temperatures = 283 [K] 4

5 Geometry and mesh properties of the second simulation study Dimensions [m]: Brick (WxH) = 0,08 x 0,8 Air cavity (WxH) = 0,04 x 0,8 Mineral wool (WxH) = 0,06 x 0,8 Concrete wall (WxH) = 0,08 x 0,8 Fig. 4 visualizes the two-dimensional models of the cavity wall constructions, which we have used for the heat and vapor flow simulations. The colours in these pictures indicate the different thermal conductivities of the materials. The model on the left hand includes no vapor barrier. The vapor barrier is applied on the inside in model 2 and on the outside in the model on the right side. The outer leaf is indicated as dark red rectangle, followed by a cavity of 4 cm width, which is also adjacent to the insulation material (dark blue). The inner leaf is shown as green rectangle with a thin plaster layer against it on the inside surface. Fig. 4 Visualizations of the cavity wall models; left: without vapor barrier, middle: inside barrier, right: outside barrier. 5

6 Mesh settings: The mesh geometry of the three construction models consists of elements. Fig. 5 visualizes this geometry and also the mesh statistics are present at the right side: Fig. 5 Mesh geometry (left) and statistics (right) of the three cavity wall models Boundary Conditions of the second simulation study Convective Cooling: Air temperature outdoor = K (outside boundary) Surface heat transfer coefficient outside = 25 W/m 2.K (outside boundary) Air temperature inside = K (inside boundary) Surface heat transfer coefficient inside = 7.7 W/m 2.K (inside boundary) Indoor vapor pressure = 1170 Pa (outside boundary) Outdoor vapor pressure = 520 Pa (inside boundary) Thermal insulated boundaries = Bottom and top boundaries. 6

7 Material Properties and Initial Condition of the second simulation study Thermal conductivity k: Outer leaf brick = 2 [W/m.K] Inner leaf brick = 1 [W/m.K] Insulation material = 0.04 [W/m.K] Air cavity = [W/m.K] Plaster = 1 [W/m.K] Vapor resistant layer = 5 [W/m.K] Density ρ: Outer leaf brick = 2500 [kg/m 3 ] Inner leaf brick = 2000 [kg/m 3 ] Insulation material = 50 [kg/m 3 ] Air cavity = 1.2 [kg/m 3 ] Plaster = 1800 [kg/m 3 ] Vapor resistant layer = 1300 [kg/m 3 ] Heat capacity at constant pressure C p : Outer leaf brick = 840 [J/(kg*K)] Inner leaf brick = 840 [J/(kg*K)] Insulation material = 840 [J/(kg*K)] Air cavity = 1000 [J/(kg*K)] Plaster = 840 [J/(kg*K)] Vapor resistant layer = 840 [J/(kg*K)] Vapor resistance factor µ: Outer leaf brick = 100 [ - ] Inner leaf brick = 15 [ - ] Insulation material = 1.5 [ - ] Air cavity = [ - ] Plaster = 30 [ - ] Vapor resistant layer = 4000 [ - ] 7

8 Vapor permeability air = 1.8 e-10 [ s ] Initial temperatures = 283 K Solver Settings Simulation study 1 Time dependent solver (transient): Solver: Direct (UMFPACK) Time stepping: Range(3600*24*3,900,3600*24*6) 6 days Type of analysis: Heat transfer by conduction (ht) Relative tolerance: 0,001 Absolute tolerance: 0,01 Simulation study 2 Stationary solver: Solver: Direct (UMFPACK) Type of analysis: Heat transfer by conduction (ht) and Diffusion (di) Relative tolerance: 0,

9 5 Results This section presents the results of the computer simulations. We begin with the simulations of the insulated single wall: Inside or outside insulated single wall: A single wall of 20 cm exposed to an outdoor daily periodic temperature curve. In model 1 the insulation material of 5 cm is placed on the inside of the wall, and in model 2 on the outside. The figure below depict the temperature of both constructions during a winter s day: Fig. 6 Construction temperature during a winter day; inside insulation (left), outside insulation (right) The figure below shows the temperature of both constructions during a summer day: Fig. 7 Construction temperature during a summer day; inside insulation (left), outside insulation (right) 9

10 The graphs below belong to the single wall simulation that is insulated at the inside. On the left side, the graph shows the temperature distribution of the construction from inside out. Each line represents another time during the day. The graph on the right side shows the dynamic temperature change at certain positions (legend) in the construction during a period of three days. Graphs 1 Temperature distribution of an inside insulated construction; left side: from inside out at different times, right side: at certain positions during an interval of three days The graphs below show the same information, but in this case the wall is insulated on the outside: Graphs 2 Temperature distribution of an outside insulated construction; left side: from inside out at a certain time, right side: at certain positions during an interval of three days 10

11 Typical cavity wall without vapor barrier: A double wall, insulated against the inner leaf with an air cavity of 4cm and a thin plaster at the inside. The three figures below show the construction temperatures of the cavity wall in three different ways. The figure on the left indicates the whole surface temperature, while the figure in the middle shows only the isotherms. The graph on the right side indicates the temperature distribution (blue) through the construction from outside to inside along the horizontal red line, which is visual in the left figure. The red line in the graph shows the dew point temperature according to the prevailing vapor pressure. Fig. 8 Construction temperature; Whole surface (left), isotherms (middle), from outside to inside (graph) 11

12 The two figures below indicate the vapor pressure distribution in the construction. The figure on the left indicates the vapor pressure over the whole surface by colors. The graph on the right side indicates the vapor distribution through the construction from outside to inside. The red line in this graph shows the saturated vapor pressure according to the prevailing construction temperature. Fig. 9 Vapor pressure indicated by colors (left) and plotted in a graph including the saturated vapor pressure(right) The figures below indicate the relative humidity (R.H.) in the construction. The figure on the left shows the R.H. over the whole surface by colors, while the graph on the right side depicts the R.H. distribution through the construction from outside to inside. Fig. 10 Relative humidity in the construction, indicated by colors (left) and plotted in a graph (right) 12

13 Typical cavity wall with vapor barrier: A double wall, insulated against the inner leaf with an air cavity of 4cm and a thin plaster at the inside. Vapor barrier is placed at the inside (variant 1) or outside (variant 2). The two figures below indicate the vapor pressure in both constructions. The figure on the left shows the vapor pressure over the whole surface by colors. By the construction on the left the vapor barrier is placed at the inside (right side), by the right construction on the outside (left). The graph on the right hand side indicates the vapor distribution through both constructions from outside to inside. The black line in this graph shows the saturated vapor pressure according to the prevailing construction temperature. Fig. 11 Vapor pressure indicated by colors (left) and plotted in a graph including the saturated vapor pressure (right) The figure below on the left side shows the R.H. over the whole surfaces by colors, while the graph on the right side depicts the R.H. distribution through the constructions from outside to inside. Fig. 12 Relative humidity in the construction, indicated by colors (left) and plotted in a graph (right) 13

14 6 Conclusion & Discussion Conclusion In the first simulation results we can clearly see that the insulation material subdue the outdoor temperature fluctuation. For both inside and outside insulated constructions the indoor surface temperature is nearly the same, but in case of inside insulation, the wall undergoes much larger temperature fluctuations, while in case of outside insulation this fluctuation is just a few degrees. The steady state simulations during winter conditions with the typical cavity wall show that there is a condensation risk inside the construction. Fig. 9 shows that the vapor pressure in this case exceeds the saturated vapor pressure, which will result in condensation. In real life this vapor pressure cannot exceed the saturated pressure, because the vapor will condense. Therefore the relative humidity in Fig. 10 is limited at 100%. Fig. 11 and Fig. 12 show that a vapor barrier at the inside prevents this condensation. The condensation problems were not resolved when this barrier was placed at the outside. Because condensation risk is dependent on the temperature as well as the vapor pressure, the thermal insulation has big influence on it, so the right place for a vapor barrier depends on the location of the thermal insulated layer too. Discussion The best location of the thermal insulation material still depends on the situation. It is often a tradeoff between advantages and disadvantages. With existing buildings the options are often limited. Visual aspects can also determine the choice. Further one should consider whether it is desirable to keep the thermal buffer capacity of the walls in the controlled indoor climate or leave it out. As we have seen by internal insulation, the wall construction undergoes larger temperature fluctuations and the overall relative humidity in the wall increases due to lower temperatures and less drying capacity during winter. This can result in construction damage caused by thermal stresses or internal frost. When the insulation material is placed at the outside, it has to survive the weather conditions. 14