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[EnergyPlus_Support] Re: U-factor floor in contact with ground
3d Slab preprocessor is there for exact analysis...checkout the auxilary programs docs. However, much more important is the film heat transfer coeff on the inside between slab and room air. If you are calculating this by hand, don't forget about it. As far as I know there is no "film" coeff between the underside of the slab and the ground as they are in direct contact. "ground temp" in energy plus defines the temperature of that under side boundry condition. Convention is ave inside conditioned temp - 2 degC = "ground temp". This holds for most cases where slabs are about 150 - 250 mm. It varies a tiny amount over the year, but inaccuracies here are often negligable in energy transfer due to the normally small dTs and large resistive path.
--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "FrancescoP" <direzionecontraria@...> wrote:
>
> Hello Jason,
> you are right: to calculate the U-Factor EnergyPlus considers the one-dimensional model. Do you think that the "Building envelope requirements" of ASHRAE (Table 5.5) consider only the built part of the construction, i.e. they do not consider the thermal resistance relative to the ground? I thought that a model which considers also the thermal resistance relative to the ground, i.e. a three dimensional phenomenon, should be used. I mean, a model similar to the model which is proposed by the European technical standards. See the paragraph 9 of ISO 13370:
> http://wiki.pato.metid.polimi.it/@api/deki/files/637/=ISO_13370-2007.pdf
> Best Regards
> Francesco
>
> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, Jason Kirkpatrick <jason.alan.kirkpatrick@> wrote:
> >
> > Francesco,
> >
> > If you know the thermal conductivities (k) of the layers of the floor, and
> > the thickness (L) of those layers, then you can calculate the thermal
> > transmittance (U-Value) for the individual layers as
> >
> > U = k / L where U is the thermal transmittance (W/m2-K) k is thermal
> > conductivity (W/m-K) and L is material thickness (m)
> >
> > Additionally, the thermal transmittance of the assembly for materials in
> > series is
> >
> > 1 / U (assembly) = 1 / U (layer1) + 1 / U (layer2) ...
> >
> > Jason
> >
> >
> >
> > On Sat, Jun 16, 2012 at 4:32 AM, FrancescoP <direzionecontraria@>wrote:
> >
> > > **
> > >
> > >
> > > Thank you for your advice, Jason. I read that part of the Engineering
> > > reference.
> > > Therefore if I know the F-factor I can calculate the U-factor or if I know
> > > the U-factor I can calculate the F-factor, but how can I do if I know only
> > > the dimensions of the building, the thermal conductivity of the soil,
> > > thickness and thermal conductivities of the layers composing the
> > > slab-on-grade floor?
> > > Best Regards
> > > Francesco Passerini
> > >
> > >
> > > --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, Jason Kirkpatrick
> > > <jason.alan.kirkpatrick@> wrote:
> > > >
> > > > Again you should read page 131 of the engineering reference, which gives
> > > > the following in terms of R value
> > > >
> > > > F-Factor x Exposed Perimeter Wall Length / Zone Floor Area =
> > > > Effective U Value
> > > >
> > > > rearranged gives
> > > >
> > > > F-Factor = U-Value x Zone Floor Area / Exposed Perimeter Wall Length
> > > >
> > >
> > >
> > >
> >
>
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