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Thermal Resistance of Linear thermal bridges.

asked 2018-08-21 04:22:30 -0600

lukanuts's avatar

updated 2018-08-21 07:32:56 -0600

Hi following this discussion

i have decided to create a customized Material with No Mass, in order to input only the R-value of each the opaque areas affected by the thermal bridges. Since the definition of linear thermal bridge is W/mK, but eplus takes into account W/m2K only, in your opinion is correct to convert the psi value in W/mK into a W/m2K by taking into account the dimension of the portion of the affected surfaces ?

to be clear. E.g. psi 0.32 W/mK, width affected 0.7m, equivalent U= 0.32/0.7=0.4571 W/m2k and equivalent R=1/0.4571=2.18 m2K/W

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answered 2018-08-21 15:00:19 -0600

Julien Dutel's avatar

E+ (and most of the other energy simulation programs) cannot calculate a U value, it's need to be manually calculate before passing it to the program.

U value taking account of linear transmittance is a relatively new topic, but the report AHSRAE RP-1365 seems to reach consensus. This study generate a new method which is well resume in "Building Envelope Thermal Bridging Guide". With your calculation you need to introduce the Area of the wall concerned by the linear transmittance. check on the guide p18/37:

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answered 2018-08-22 03:07:42 -0600

lukanuts's avatar

updated 2018-08-22 06:09:17 -0600

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.

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Thanks for the research with E+. I agree that simulate a unique layer compose with a "No mass Material" would not be an accurate approach.

Does the wall have multiple layer or a unique layer (like a castle with stones) ? For a unique layer, here is how I would manage it - on one part you specify the density, thickness and specific heat of the stone. These three values will account for the thermal mass effect. - with the thickness now fixed, adjust the conductivity to match the U value you've calculated (Clear field depreciated by the linear transmittance)

Julien Dutel's avatar Julien Dutel  ( 2018-08-23 08:33:20 -0600 )edit

What about surface resistance? Should I have to consider the same used for the clear field (inner and outer layer) in order to match the assumption of the used standard ?

Yes the wall will have multiple layers and i will correct the Rvalue of the insulating material. A question arises, though: If I had no insulating material but only material with relevant density and specific heat should i have to consider to adapt the conductivity for the material in between or whatever I want? I would say the first, wouldn't I?

lukanuts's avatar lukanuts  ( 2018-08-23 09:42:16 -0600 )edit

With multiple layer I would adjust only the thickness of the insulation layer but with consideration of the existing U value of the other layer. In other words: the thickness adjustment of the insulation layer should make the total U Value of the wall equal to the U value you've calculated (Clear field depreciated by the linear transmittance).

Without Insulating material, I would adjust the material with the lowest conductivity

Julien Dutel's avatar Julien Dutel  ( 2018-08-23 11:40:02 -0600 )edit

Hi julien, I am experiencing a case where the effect of the thermal bridges per surface is very relevant that the corrected u value is doubled.In this case to match the expected U value is to adjust the insulating conductivity (same thickness) and changing the thickness of massive elements but this affects surface heat storage calculations.What do u think is better to have. A disregarding low mass insulation, and the new u value is just 10% higher than expected.or B managing insulation conductivity and only massive elem thickness. Which is the best solution for you?

lukanuts's avatar lukanuts  ( 2018-10-31 07:45:22 -0600 )edit

To decide which is the best between A and B which Eplus variables would you check ? the overall loads or the ones related to surfaces like surface heat storage to see what changes in the in the two cases?

Thanks in advance

lukanuts's avatar lukanuts  ( 2018-10-31 07:54:44 -0600 )edit

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Asked: 2018-08-21 04:22:30 -0600

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Last updated: Aug 22 '18