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Should you fully insulate under concrete slabs on or below grade?
Below grade comparison
Downward heat loss tools


Discussion

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 analysis).

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 slab etc.

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 insulation strategies.

BASECALC is a free tool offered by CanmetENERGY  which is part of Natural Resources Canada.

 

Comparison - below grade heated basement slab.
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see also: Effects of tube placement and layout

Running a series of FEA models using the same conditions with the exception of insulation thickness, one can see the effects from a steady state side-by-side comparison between insulated and uninsulated slabs. In this case we've made comparisons of insulation thicknesses ranging from 100mm (4 in.), 75mm (3 in.), 50mm (2 in.), 30mm (1.5 in.), 25mm (1 in.) to zero insulation.


Figure 1 = 100mm (4 in.) R-20

Figure 2 = 75mm (3 in.) R-15

Figure 3 = 50 mm (2 in.) R-10

Figure 4 = 30mm (1.5 in.)R-7.5

Figure 5 = 25mm (1 in.) R-5

Figure 6 = 00mm (0.0 in.)

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