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Rob Annable
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PostPosted: Tue Apr 05, 2016 10:04 am    Post subject: Thermal bridge - slab detail Reply with quote

Back with another Ecodesigner thermal bridge question. Is it possible to exam the centre of a floor slab? There is no external air at this point but the first stage demands this is designated before allowing me to proceed.

Do I have to attempt one for the whole slab?



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Vahur
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PostPosted: Wed Apr 06, 2016 9:43 pm    Post subject: Re: Thermal bridge - slab detail Reply with quote

Can you explain what are you going to achieve by such a simulation? Energy loss into slab? Why is there energy loss at all? Some environment, which is always colder/warmer comparing to internal air?
This calculation/smulation is all about thermodynamics. Heat flow is there, where is temperatures difference only. Temperature difference is the neccessary condition.

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laszlonagy
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PostPosted: Sun Apr 10, 2016 11:23 pm    Post subject: Re: Thermal bridge - slab detail Reply with quote

I think the attached image shows a situation where the slab is on soil, which is mostly colder than the interior temperatures. So there is some heat loss in that direction which needs to be taken into consideration.

The above simulation seems to be able to handle with above ground condition. This is why you have to specify the external air. Unfortunately I am not familiar with it enough to say if it can do below ground as well.

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Vahur
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PostPosted: Mon Apr 11, 2016 5:18 pm    Post subject: Re: Thermal bridge - slab detail Reply with quote

I have qualification in this field, so I can explain some things.
Take a look at the following file.
https://www.dropbox.com/s/rzl35vdoya2v3ri/PSI%20Kulmasilla%20arvutus%20ENG.pdf?dl=0
I explain there how to make the proper calculation of PSI value.
There is a bit simplier way, but I wanted to go deeper =)

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Rob Annable
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PostPosted: Thu Apr 14, 2016 6:13 pm    Post subject: Re: Thermal bridge - slab detail Reply with quote

Vahur - thanks for the link, I think I understand it. Very helpful.

Laszlo - that's correct, there is heat loss into the ground at this junction but unfortunately there appears to be no way to calculate it as I can't proceed without describing external air area.
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Vahur
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PostPosted: Fri Apr 15, 2016 6:11 am    Post subject: Re: Thermal bridge - slab detail Reply with quote

Rob Annable wrote:
Vahur - thanks for the link, I think I understand it. Very helpful.

Laszlo - that's correct, there is heat loss into the ground at this junction but unfortunately there appears to be no way to calculate it as I can't proceed without describing external air area.



Ground in this tool is working quite funny way. It is just a material and not considering the constant temperature below freezing level. I would gladly discuss the calculation method with Graphisoft energy efficiency engineers. (if they have one)

Rob, just take a bigger part of building to make this simulation. Our Estonian engineers have a rule to take piece of ground 20 m apart from building and in depth. Temperature constant is +7oC, building floor at least 4m from wall into the building, 3x width of wall height. This is our local rule to define PSI value of wall-floor-foundation thermal bridge.

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laszlonagy
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PostPosted: Tue Apr 19, 2016 6:16 am    Post subject: Re: Thermal bridge - slab detail Reply with quote

I asked the guys at GRAPHISOFT about this.
I was told that you could simply specify the soil fill as "External Air" and specify the temperature of the soil for it. The results should still be correct this way.
Then, in the next Dialog, you would not define a "Soil".
This is a workaround but it will achieve what you are trying to do.

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Vahur
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PostPosted: Tue Apr 19, 2016 8:54 am    Post subject: Re: Thermal bridge - slab detail Reply with quote

laszlonagy wrote:
I asked the guys at GRAPHISOFT about this.
I was told that you could simply specify the soil fill as "External Air" and specify the temperature of the soil for it. The results should still be correct this way.
Then, in the next Dialog, you would not define a "Soil".
This is a workaround but it will achieve what you are trying to do.

This is not workaround. This is the only proper way. The "Soil" environment is not a working tool at all. It is not working as it should do. Currently it works as a simple material, but is not analyzed during the calculation.
Also, there is no point (in the current edition) to define the temperature at all. unless you are interested in the temperature inside the construction. Anyway L2D value (for some reason called PSI in EcoDesigner) has units W/mK. So there must be just the temperature difference, no matter how big. Inportant are the surface resistances.
Maybe Graphisoft wants me as the energy efficiency engineer in their team?
Laughing

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