Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
Hi Julien, thanks for the suggestions. The only drawback i can see is that i will have an entire E+ construction with no thermal mass. For example: considering an entire facade full of Thermal bridges, in order to have an unique equivalent U value (Clear field transmittance+linear transmittance weighted by the area and point transmittances excluded) the only way to model it is to create a No mass material. Since in malta we used to build heavyweight constructions i cannot totally exclude the effect of thermal mass, and the Iso 13790 for detailed simulations requires it.
Is it correct my interpretation of your answer?
PS: From EngineeringReference E+ (version 8.7 pag 66)
[..]In some cases, either due to a lack of information or a desire to simplify input, a user may choose to enter a material layer as a “no mass” or “R-Value only” material. This assumption essentially says that these layers add nothing to the thermal mass of the overall construction and only add to the overall resistance or R-Value of the construction as a whole. While this is not recommended, it is allowed and in some cases is not a poor assumption for extremely lightweight materials such as some types of insulation.[..]
[..]In EnergyPlus, there are two possible cases for the existence of “no mass” layers: either between two other solid, thermally massive layers (multiple “no mass” layers next to each other are simply combined in this approach) or at the inner or outer most layers of a construction. There are potential issues with having a resistance-only layer at either the inner or outer most layers of a construction. A little or no mass layer there could receive intense thermal radiation from internal sources or the sun causing the temperature at the inner or outer surface to achieve very high levels. This is undesirable from a simulation standpoint as there are limits to temperature levels in EnergyPlus that could be exceeded causing the simulation to terminate and is likely unrealistic from a real-world perspective. Thus, for such potentially problematic calculational scenarios, EnergyPlus will continue to convert a “no mass” layer at either the inner or outer most layer of a construction into a thermal mass layer using the properties of air as has been done in the past. The case where a resistance-only layer is defined anywhere except the inner or outer layer of a construction is handled by treating the “no mass” layer as a single node layer.
| | 2 | No.2 Revision |
Hi Julien, thanks for the suggestions. The only drawback i can see is that i will have an entire E+ construction with no thermal mass. For example: considering an entire facade full of Thermal bridges, in order to have an unique equivalent U value (Clear field transmittance+linear transmittance weighted by the area and point transmittances excluded) the only way to model it is to create a No mass material. Since in malta we used to build heavyweight constructions i cannot totally exclude the effect of thermal mass, and the Iso 13790 for detailed simulations requires it.
Is it correct my interpretation of your answer?
EDIT. I have probably found the answer, at pag 30/39 Many modeling programs use construction layers to build up the building envelope assemblies based on material properties. To account for thermal bridging, all the material properties should be left as is, while only the insulating layer R-value should be de-rated such that the correct overall U-value determined from calculation is matched and output by the software. This method allows for the functions that account for thermal mass to be approximated by the software
PS: From EngineeringReference E+ (version 8.7 pag 66)
[..]In some cases, either due to a lack of information or a desire to simplify input, a user may choose to enter a material layer as a “no mass” or “R-Value only” material. This assumption essentially says that these layers add nothing to the thermal mass of the overall construction and only add to the overall resistance or R-Value of the construction as a whole. While this is not recommended, it is allowed and in some cases is not a poor assumption for extremely lightweight materials such as some types of insulation.[..]
[..]In EnergyPlus, there are two possible cases for the existence of “no mass” layers: either between two other solid, thermally massive layers (multiple “no mass” layers next to each other are simply combined in this approach) or at the inner or outer most layers of a construction. There are potential issues with having a resistance-only layer at either the inner or outer most layers of a construction. A little or no mass layer there could receive intense thermal radiation from internal sources or the sun causing the temperature at the inner or outer surface to achieve very high levels. This is undesirable from a simulation standpoint as there are limits to temperature levels in EnergyPlus that could be exceeded causing the simulation to terminate and is likely unrealistic from a real-world perspective. Thus, for such potentially problematic calculational scenarios, EnergyPlus will continue to convert a “no mass” layer at either the inner or outer most layer of a construction into a thermal mass layer using the properties of air as has been done in the past. The case where a resistance-only layer is defined anywhere except the inner or outer layer of a construction is handled by treating the “no mass” layer as a single node layer.
