Should you fully insulate
under concrete slabs on or below grade?
Below grade comparison
Downward heat loss
Again the steady state models represent a
specific set of criteria in a point in time and are not
indicative of all projects over a period of time (transient
They do however provide a good visualization
about the flow of energy in the form of heat from warm to cold
via conduction from heated and unheated mass.
In these examples comparing a heated slab with or
without insulation one can see the various thermal zones indicated by
the color changes.
One could, using the uninsulated slab as the base
case, take each area of a certain color and volumetric heat
capacity (in this case assumed homogenous) and compare the heat
energy differences in that zone at that time. This of course
will change based on changes to the heating system (off or on,
and for how long and at what temperature etc.)
In reality, all the influential items affecting
soil heat transfer (listed
in Part I), come into play in the analysis and could be
based on simple assumptions such as no changes in soil
conductivity, constant outdoor and ground temperatures and all
soils settled and compacted under and adjacent to the heated
Likewise they could become complex based on
assumptions to changes in the water table, capillary and
frost actions etc.
In any event, on grade and below grade
foundations currently account for 10% to 40% of the energy used
to heat homes in cold climates and the FEA tools allow us to
analyze these loads with a little more understanding than just
standard conductive calculations.
Having said that, FEA is not an everyday tool for
the designer whereas a tool such as
BASECALC™ can easily and quickly provide accurate heat loss
models for on and below grade foundations based on appropriate
BASECALC is a free tool offered by
CanmetENERGY which is part of Natural