Conclusions

Dynamic Thermal Performance and Energy Benefits of Using Massive Walls in Residential Buildings

CONCLUSIONS

Finite-difference computer modeling validated by dynamic hot-box tests was used to examine the steady-state and dynamic thermal performances of massive wall assemblies. Six U.S. locations were considered during computer dynamic modeling. Four series of multilayer massive walls were analyzed. Data for 19 walls was cataloged in four groups representing walls of the same steady-state R-value: R - 3.03 m² K/W [17.2 hft²F/Btu], R - 2.29 m²K/W [13.0 hft²F/Btu], R - 1.58 m²K/W [9.0 hft²F/Btu], and R - 0.88 m²K/W [5.0 hft²F/Btu]. All walls contained four main wall material configurations:

•    Concrete on both sides of the wall, wall core made of the insulation material.
•    Insulation on both sides of the wall, massive concrete core of the wall.
•    Concrete on the interior wall side, insulation on the exterior wall side.
•    Concrete on the exterior wall side, insulation on the interior wall side.

Two additional wall configurations were modeled as low R-value modifications of the material configurations represented by these main groups.

The results of the dynamic computer analysis show that the most effective configurations are massive walls with thermal mass (concrete layer) being in good contact with the interior of the building. Walls with the insulation material concentrated on the interior side of the wall showed the least favorable dynamic thermal performance. Dynamic thermal performance of walls with either the concrete wall core or the insulation placed on both sides of the wall falls between the above two constructions.

Dynamic thermal performance of massive walls is also a function of climate. The most favorable climate for application of the massive wall systems is in Phoenix. Relatively worst location for these systems is in Minneapolis (especially for less insulating walls).

It was found that in buildings containing low R-value walls (an average R-value below 0.7 m² K/W [4.0 hft²F/Btu]) the use of massive walls is ineffective in all considered locations except Phoenix. It is more efficient to use a lightweight wall of the same steady-state R-value.

Complicated three-dimensional heat transfer can be observed in most masonry units and some of the ICF forms. These assemblies have to be simplified to one-dimensional forms to be used in such whole building simulation programs like DOE-2 or BLAST.

Detailed three-dimensional thermal computer analysis proved that the application of the equivalent wall technique helps to generate accurate one-dimensional replicas of complex building envelope assemblies. An example ICF wall had a complex three-dimensional internal structure. There was a three-dimensional network of vertical and horizontal concrete channels inside the ICF wall form. Several horizontal steel components additionally complicated heat transfer in this wall. Calibrated finite difference computer code was used to generate an equivalent wall for the complex ICF wall. It was shown that response factors, steady-state R-values, and thermal structure factors were essentially the same for the complex ICF wall and equivalent wall. A simple one-dimensional approximate model was also made for the ICF wall. The thickness of each material layer was estimated as the average thickness for the ICF wall. It was found that this one-dimensional approximate model of the complex structures based only on geometry simplifications was inaccurate, both in terms of R-values and the response factors.