shnkn
commented
3 years ago SFH: Attic Problem
The attic was modeled without any fenestration surfaces. Also, this zone was not ventilated/cooled -nor heated. Therefore, the zone must have been heated via the gains from outside (roof/wall surfaces) and the adjacent living zone’s ceiling surface.
I tried to figure out what caused the high and low temperatures within this zone by examining both existing and newly defined output variables’ and their relationships. For the comparison of these variables, I visualized the eso files via DesignBuilder Results Viewer. However, I needed to make a couple of arrangements in the variable reporting frequencies defined on the IDF file because the app gave errors while parsing the eso file. I would like to share this information with you to prevent any future/possible problems because the same eso file was not appropriately opened in another visualization software. In the IDF file, using other options rather than defining a “detailed” reporting frequency for output variables solved this parsing problem.
In this issue, there are two questions that should be answered: 1- Why does the zone have a lower indoor temperature in winter compared to the outside temperature at night? 2- Why does the simulation give very high-temperature results in summer for the zone (e.g., 57 C)? A comparative image is attached below for zone mean air temperature differences between the zones.
Comparison 1: Site Ground Temp/ Site Sky Temp/ Zone Mean Air Temp/ Outdoor Air Drybulb Temp
The first comparison I have conducted was to understand whether the indoor temperature changes in the attic zone followed a similar trend with the outside air conditions. From here, it can be understood that the zone temperature is highly affected by the outdoor air conditions and both of them are following similar paths. The reason for this could be related to a high infiltration rate or very poor insulation. Still, if the infiltration or poor insulation caused this, I would have expected more overlapped or very similar results but the indoor temperature values in summer are so much higher than the outside.
If we take a closer look at the wintertime (January 6) values, we see that the zone air temperature is lower than both the outside dry-bulb temperature and ground temperature. The zone has no direct interaction with the ground, so this variable is irrelevant to explain the results. The inclusion of the site sky temperature variable to the comparison helps us to clarify a misunderstanding. I have checked the entire year and seen that the zone’s temperature never fell below the sky temperature. Also, we can see that the sky temperature is colder than the outside dry bulb temperature. The zone is probably losing more heat from the envelope through radiation compared to the heat gains from the envelope via conduction. In this way, question 1 was answered (the results are accurate, and seeing lower indoor temperature values in winter has a reason).
Comparison 2: Roof Surface Inside and Outside Face Temperatures/ Zone Mean Air Temperature/ Roof Surface Outside Face Incident Solar Radiation Rate
In order to answer the second question, a roof surface’s outside and inside temperatures along with its incident solar radiation rates were compared. According to the below graph, it can be understood that the main reason causing overheating is related to the solar radiation introduced from the outside surfaces of the zone. Accordingly, the solar radiation heats the outside surface of the roof (up to 60 C), then the heat is transferred to the inside surface, and the room gets overheated from the interior surfaces of the zone (there is also conductive heating from the outside but the radiative heating is more dominant in the overheating problem).
Therefore, a possible solution for the overheating could be to prevent overheating first from the outside; so it would not be able to affect the interior surfaces and zone temperature. For this, the roof construction and its material properties are needed to be reviewed (Possible solution 1).
Comparison 3: Infiltration Rate
In order to understand what happens to the heated air in the attic unit, we should check the zone’s infiltration rates. According to the below graph, we see that the heated air is trapped inside and the infiltration rate defined in the zone is not enough to evacuate it. By taking into consideration that there is no active or passive ventilation is available in the zone, this is the main reason why we see these high-temperature values inside. A possible solution could be increasing the air change per hour by intervening infiltration values or by defining a ventilation system similar to the living unit (Possible solution 2-3).
How can we prevent overheating in the attic?
Possible solution 1 (Intervention to the materials (e.g., increasing the building’s roof surface reflectivity)
The first possible solution is to change the properties of the defined roof construction. I have checked the baseline’s construction and replaced the Asphalt Shingle and OSB ½ inch materials according to the values defined in the Ashrae’s Handbook values from the Design-Builder software. After I compared the existing construction and the new one, saw that the new values have the potential to reduce overheating approximately 2 to 4 degree Celcius but I do not think it is enough.
While I was examining the roof constructions, I have realized that all of the BuildME scenarios have the same roof conditions. I believe that changing the roof construction with more energy-efficient materials has a visible impact on the energy consumption of the buildings. Would you consider to include different roof constructions for alternative energy-efficient scenarios (e.g., nzeb)?
Possible solution 2 (Intervention to infiltration)
Other than blocking the solar radiation from outside, the overheating can be reduced by evacuating the heated air from inside. The existing SFH model is already defined with infiltration, however, its rate is not enough to prevent overheating. So, I replaced the effective air leakage area parameter from 370 cm2 to 5000 cm2 roughly to see how this parameter could affect the zone’s air temperature. The graph below compares the existing and new models in terms of zone air temperature. Accordingly, the overheating problem is likely to be solved by changing the infiltration value for the attic. So, I would suggest recalculating this value.
The zone temperature is still higher than the outdoor temperature but I think this is more acceptable than the previous one.
Possible solution 3 (Intervention to ventilation)
I have also tried to define the same ventilation system for the living unit to the attic; however, the results were not changed significantly.
And, I tried to conduct another simulation both by adding a new ventilation system and changing the infiltration values.
nheeren
The attic's defined zone has lower indoor air temperature values than the outside in winter (nights). Moreover, the zone has higher temperature values (e.g., up to 57 C in the 4C Climate zone for the USA) than outside in summer. The problem could be related to the heat transfer rate differences between the envelope surfaces and ground/air/adjacent zones.
I will try to update this thread once I come up with a meaningful explanation.