Masonry Advantage

Thermal Mass Works

CMU vs Wood: Investigating Effectiveness of Thermal Mass with Insulation

Martha G VanGeem

Concrete masonry assemblies have many attributes, including being a versatile material choice for energy efficiency in building envelopes and providing thermal mass.

Recently completed simulations of realistic single-family houses show that thermal mass walls out perform wood-framed walls whether the insulation is on the inside or outside of the thermal mass in climate zones 3 and 4 in the midsection and western portions of the US.

EnergyPlus simulations were performed on 607 walls in 15 climate zones (CZ) and for a typical one- and two-story house for an analysis period of one year. Most of the walls were constructed of concrete masonry units (CMU) of different unit weights, various grout spacings in the CMU cores, and different amounts of insulation on the inside, outside and in the CMU cores.

Windows in the homes were equally distributed on all four cardinal directions and were 15% of the conditioned floor area, which is 2200 sf for the two-story house. Windows and energy criteria other than insulation in opaque walls are equal to the 2012 International Energy Conservation Code (IECC) for each CZ.

The research held all factors, other than the opaque sections of exterior walls, equal for each climate zone for simulations.

Note that results would most likely be even more favorable had passive solar techniques been employed, such as shifting more win dows to the south side of the house and allowing a larger range in thermostat set points (they are quite narrow in this study). But even in these typical houses, results are favorable.

Work was funded by the Florida Masonry Apprentice & Educational Foundation (FMAEF) and the NCMA Education and Research Foundation. Simulations were performed by Pacific Northwest National Laboratory (PNNL).

Three Wall Comparisons This article focuses on two-story homes compared in CZ 3A, 3B, 3C, 4A, 4B and 4C – from Baltimore MD to Memphis TN to El Paso TX to Albuquerque NM to San Francisco CA to Salem OR. An R13 wood-framed wall is compared to a 115 pcf partly grouted CMU wall with R8 interior insulation and a 115 pcf partly grouted CMU wall with R8 continuous exterior insulation.

Walls have similar overall R-values and U-factors, although the overall R-value for the woodframed wall is 9% greater. Wood-framed wall has R13 batt insulation bet ween wood studs at 16″ on center (oc), with gypsum wallboard on the interior and 7/16″ OSB on the exterior. The U-factor is 0.092 and the overall R-value is 10.87.

CMU wall with interior insulation is 115 pcf CMU with grout every 48″ oc. R3 reflective insulation is between the wood furring. R5 continuous insulation is inside of the wood furring, with gypsum wallboard on the interior and cementitious stucco on the exterior. The U-factor is 0.10 and the overall R-value is 10.0.

CMU wall with exterior insulation is 115 pcf CMU with grout every 48″ oc. R8 continuous insulation is on the exterior, with gypsum wall – board on the interior and with a synthetic stucco on the exterior. U-factor is 0.10 and the overall R-value is 10.0.

Thermostat settings were relatively narrow – 75°F for cooling and 72°F in the winter. This means that the temperature is never allowed to go above 75°F without the air conditioner turning on and never allowed to go below 72°F without the furnace turning on. It is likely, typical houses have a wider range of temperature and therefore the beneficial results for thermal mass are conservative.

Cities were previously determined by Department of Energy (DOE) to be representative of CZ 3 and 4. This means that performing an analysis for these cities provides results that are generally accepted to extend to the entire CZ. Therefore, these results can be assumed to generally apply to all locations in CZ 3 and 4 as shown in Figure 1 (p. 20), although the actual results will vary depending on the actual location in the CZ. This CZ map has been used in all versions of the IECC and ASHRAE 90.1 since 2004.

Cities and results are presented in Tables 1 and 2. Results were generated from project data. The PNNL report1 on the methodology can be found at floridamasonry.com/pnnl-energyresearch.html

The Energy Use Index (EUI) is the HVAC energy usage for heating and cooling, including fan, furnace and air-conditioning. Heating is from a natural gas furnace. Energy is kBTU per sf of house area for one year. In this case, it is for a two-story, 2200 sf house. The Energy Cost Index (ECI) is the EUI multiplied by energy costs. National average energy rates were used with electricity at $0.11 per kWh and natural gas at $1.05 per therm. Project data allows the energy prices to be changed to different national or local prices. Default values were used in the study. Costs are in US$/ sf of house for one year.

Energy Usage ResultsTable 1 shows energy usage results when the woodframed wall is compared to CMU walls with interior and exterior insulation. Results show that the house with CMU walls with insulation on the interior uses 3 to 23% less HVAC energy than the house with wood-framed walls.

Mass walls not only perform well in hot climates, as is commonly known, but also perform well in more mild climates with cooler winter temperatures. Results also show that the house with CMU walls with exterior insulation uses 5 to 35% less HVAC energy than the same house with wood-framed walls.

Table 1 shows that mass not only works well when insulation is on the outside of the mass, but also works well (although not quite as well) when insulation is on the inside of the mass. The inside surface of the mass does not need to be exposed to the indoor air in order for the mass wall to save energy.

EnergyCosts Table 2 shows similar results for energy costs when the wood-framed wall is compared to CMU walls with interior or exterior insulation. Results show that the house with CMU walls with exterior insulation costs 7 to 39% less to heat and cool than the same house with wood-framed walls. Results also show that the house with CMU walls with interior insulation costs 4 to 26% less to heat and cool than the house with wood-framed walls.

Energy and cost-saving results for mass walls are more significant for CZ 3 compared to CZ 4. They are also more significant for the drier western portion of the US – the B and C CZ compared to the moist eastern portion – the A CZ. (Divisor is generally on a line drawn up through the center of Texas.) This is due to greater outdoor daily temperature swings in the western part of the US.

Effects of Thermal Mass Thermal mass has two distinct energy saving methods. First, thermal mass performs best when heat flow is reversed in the wall at any point during the day. In these cases, rather than flowing through the wall, heat flow in one direction cancels out heat flow in the other, resulting in very low heat flow through the wall for many hours.

The balance point of a building is that temperature at which the building does not need either heating or cooling. For houses, the balance point tends to be between 60 to 65°F. Thermal mass works best in climates where the temperature fluctuates above and below the balance point during the day.

In these cases, the thermal mass will have the outer side cooler during the nighttime (heat loss) and warmer during the daytime (heat gain) than the balance point of the building – resulting in reversals in heat flow through the wall – and therefore resulting in periods of very low heat flow through the mass wall. For CZ 3 and 4, the outdoor temperature fluctuates above and below the balance point for most days of the year, resulting in beneficial thermal mass effects.

Second, thermal mass also performs well when the mass is exposed on the inside of the wall. This inside mass helps prevent the inside air temperature from readily chang ing, and therefore delays and moderates times when the HVAC system requires heating and air-conditioning. This effect is generally separate and distinct from the reversals in heat flow through the wall described above.

Therefore, as further shown in this study, the inside surface of the walls are not required to be exposed thermal mass – there are still thermal mass effects when insulation and gypsum wallboard are on the inside surface of the mass wall.

In summary, thermal mass in walls performs well in CZ 3 and 4 and it:

• Doesn’t have to be in a passive solar house with south facing glass and large indoor temperature swings. It performs well in a standard house. • Doesn’t have to be in a hot climate. • Doesn’t have to have concrete or CMU exposed on the inside surface (insulation can be on the inside surface). • Performs well from Baltimore to Memphis to El Paso to Albuquerque to San Francisco to Salem.

The intent is that other aspects of thermal mass in masonry walls from this study, including benefits in other climate zones, will be covered in future articles

Martha G VanGeem , PE, LEED AP BD+C, FACI is a self-employed consulting engineer in the Chi cago area with over 35 years experience. She serves as a project principal investigator and specialize d consultant in the areas of green buildings and infrastructure, energy use in buildings, energy codes, thermal mass and moisture mitigation. In the area of green technologies, VanGeem provides expertise on LEE D and other green rating systems , product category rules (PCR), environmental product declarations (EPD)and life-cycle assessment(LCA).She is a member of many energy and green building standards committees including those with in A SHRAE, ACI and ASTM.She is the recipient of A SHRAE’s 2013 Standards Achievement Award and has authored more than 100 publications.VanGeem received her bachelor’s degree of civil engineering from the University of Illinois (Urbana) and her MBA from the University of Chicago. 847.259.6338 | vangeemconsulting.com

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