ASHRAE Journal - February 2011 - 62

solar Nzeb project

photo 1 (left): Installation of the insulated concrete form (ICF) foundation blocks around Equinox House perimeter. Note the plastic web that both holds the Styrofoam walls together and supports the rebar. photo 2 (right): ICF foundation wall prior to pouring concrete. Note that the concrete footer below the ICF wall is close to the original grade of the surrounding ground.

Ground Heat transfer
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by ty Newell, Member ASHRAE; and ben Newell, Associate Member ASHRAE

ith few exceptions, visitors to Equinox House ask how much

insulation is below the concrete slab floor. There is no insula-

tion below the floor. Ground heat transfer in buildings is an important

factor, especially in regard to superefficient buildings. But it’s difficult to analyze, model, and predict. In designing Equinox House, we looked at the foundation wall and underslab ground heat transfer in relation to the house loads and energy cost to decide whether to use insulation.
The coupling between a building and the ground is difficult to define due to the transient, multidimensional analyses required along with a general lack of property information for soils in a specific location. For our design assessment, we constructed a finite volume heat transfer model. The simulation is a three-dimensional, transient model that solves an implicit form of the conduction equation for the ground and foundation wall regions. We define a region that extends 30 m (100 ft) horizontally from the building perimeter and 5 m (16 ft) below the floor to a level where we define a constant ground temperature of 10°C (50°F). Computational cells are 1 m by 1 m (39 in. by 39 in.) in horizontal dimensions by 99 mm in depth (3.9 in.). Approximately three years of iterations are required to reach a cyclical thermal steady state.

Soil properties are always a problem. What type of soil? How wet is the soil? The structure’s floor is quite complicated as well. Underneath the slab are thickened concrete regions under the internal bearing walls, some internal piers that tie into the footer but leave the insulated concrete form (ICF) wall intact, some sand in the plumbing drain trenches, fill from who knows where, and crushed rock. Under the house, the soil most likely dries out, while the exterior ground varies throughout the year with changes in moisture and freezing. We did not incorporate freezing or varying properties into the model. We explored a range of soil thermal diffusivities (~3 to 8 × 10–7 m2/s [32.3 to 86.1 ×10–7 ft2/s]), representing dry, low thermal conductivity to wet, higher thermal conductivity soils. Floor surface effects such as carpets or other flooring, along with structures (tables) that radiatively block the floor’s view of other room surfaces have been examined, and we have found that their impact is not significant with the ground’s internal resistance dominating the situation.

this is the sixth in a series of columns. Find previous columns at www.ashrae.org/ashraejournal.
62 ASHRAE Journal ashrae.org February 2011

ASHRAE Journal - February 2011

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