Boundary Conditions for materials in contact with the ground (Therm 6.3)

[marcustait79]

Can anyone please shed light on appropriate boundary conditions, specifically film coefficients, as they pertain to materials which are not immersed in air e.g. EPS to soil, sand blinding or Hardcore etc?  I gather that surface resistance does not manifest the same way in this application?  At the moment I've created a new boundary condition and taken a tabulated soil resistance of 1.5m2K/W (from Dansk Standard 418 page 37) to give a coefficient of 0.66W/m2K but I'm really just stabbing in the near darkness!!!

Also, I'm in process of modelling some tongue and grooved parts as part of floor and roof details and wondered if it is appropriate to treat the assembled tongues and grooves as homogeneous?  Is there a clever way to account for joints like this or is the heat loss negligible?

Many thanks in advance!

[archi...@gmail.com]

Hi.

According to the standard EN ISO 6946 "The surface resistances apply only to surfaces in contact with air. No surface resistance applies to surfaces in contact with an other material".

So external surfaces of floors or walls in contact with the ground have Rse=0.00 m2K/W (film coefficient He=0.00 W/m2K). And this because surface resistances, and film coefficients, depend on convective and radiative components of heat transmission, which cannot exist in the case of a surface in contact with another material.

If you have to calculate the thermal transmittance of a structure in contact with the ground EN ISO 13370 should be used, to correctly take into account the influence of the ground.

About modelling tongue and grooved parts: I think you could consider them as homogeneous only if no closed air cavities are involved, in which case they should be modelled and  should be assigned "air cavities" materials.

But if the cracks let a lot of air through it is a problem of air tightness the Therm cannot calculate.

Ciao

Fabrizio

[Diego Turazza]

I agree completely with Fabrizio, i only point out maybe a refuse: Rse= 0,00 m2K/W means film coefficient He -> + infinity  W/m2K. I set simply 1000 in my program for "ground" boundary conditions

Ciao

Diego

[Fabrizio Prato]

Hi Diego.
Rse=0.00 m2K/W is not a refuse. It is the Rse value considered by the En Iso 6946 in case of structures in contact with ground. Obviously it must be  "translated" with the highest possible film coefficient in Therm. Personally all my BC's film coefficients with Rse=0.00 are set to 99999 W/m2K.
Ciao
F.

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