[Greenbuilding] sub-grade heat loss was passive haus article

[Alan Abrams]

so John, are you saying that when measuring heat flow from one mass to
another, it does not matter what the density of the cooler mass is, and
that the only thing of significance is the difference in temperature?

-Grasshopper


On Wed, Aug 21, 2013 at 9:42 AM, John Straube <jfstraube at gmail.com> wrote:

> The heat flow under steady state from indoors to 55F soil is the same
> (within a few percent) of indoors to 55F air.  More heat flows to outdoor
> air at 45F because the temperature difference is larger, but for the same
> R-value, the heat loss to 45F soil is the same as it is to 45F air.
>
> The only difference for steady state flow is that the surface film
> coefficients change.  Yup, there is no surface film used for concrete to
> soil heat exchange (we assume the film has an R-value=0) but the value for
> the film on a windy night for the outside of a wall is only R0.2 or so, (a
> rounding error in R20 assemblies).
>
> The only difference of density and heat capacity is caused by dynamic
> swings.  Because the soil is so massive, moderately conductive, and
> connected to a stable deep earth temperature, it definitely does not vary
> much over time, whereas the outdoor air temperature does.  This has no
> impact on assessing the heat loss of over the heating season for a floor
> slab (e.g., the slowly changing soil temperature makes steady state heat
> loss calculations very accurate, whereas mass / heat capacity in walls
> matters more because of the fast changing air temperature).
>
> Not sure what rules of thumb are being used.
>
>
> On 2013-08-21, at 9:26 AM, Alan Abrams <alan at abramsdesignbuild.com> wrote:
>
> > John, your points are well taken, but just to clarifiy: what was meant
> by that is different heat flow rates to materials of different densities
> and specific heat capacities.  the important question is then, does heat
> flow at a greater rate to air at an average winter temperature of maybe 45d
> F, or to the earth at maybe 55d F, per square foot.
> >
> > and is it still air or windy?
> >
> > it goes back to the original point, that there are some limits to rules
> of thumb.
> >
>
> John Straube
> www.JohnStraube.com
>
>
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-- 
Alan Abrams*
**certified professional building designer, AIBD
certified passive house consultant, PHIUS*
*certified passive house builder, PHIUS**
*Abrams Design Build LLC
*sustainable design for intentional living*
cell     202-437-8583
alan at abramsdesignbuild.com
www.abramsdesignbuild.com
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