|THE THERMAL MASS ADVANTAGE|
|By Alex Janusz and Kim Pressnail
In the search for more responsible building designs minimizing life-cycle energy use and associated carbon emissions, designers sometimes overlook the advantages of thermal mass. It is easy to do so because thermal mass is a hidden resource. For some, the effects can be difficult to estimate, and therefore, challenging to incorporate into building rating systems such as Leadership in Energy and Environmental Design (LEED).
Yet, studies in Canada and Europe—and several documented examples—have shown thermal mass can have a significant impact on operational energy. Depending on the location and orientation and design of a building, utilizing thermal mass can reduce heating energy use by between four and eight percent compared to a lightly constructed building with low thermal mass.*
Thermal mass leads to energy savings over the life of the building that are significant especially compared to embodied energy since the ratio of operational energy to embodied energy in many buildings designed in Canada today is often three or four to one. This means saving operational energy has a multiplier of three or four compared to saving embodied energy. Couple these savings with improvements in thermal comfort and you have two strong reasons why one should consider designing with thermal mass in mind.
Thermal mass comes in many forms, but it is a feature of structures constructed with reinforced concrete floor slabs and envelope and demising walls built using some combination of masonry or concrete materials.
In one study, it was found the amount of reinforced concrete required for the ‘structure’ provided sufficient thermal mass to manage solar heat gains and to minimize energy-use.** Increasing the quantity of thermal mass in floor slabs beyond the structural needs consistently reduced heating and cooling loads, but the energy savings were a diminishing return. The study also showed floor slab thermal performance was most influenced during the heating season by the intensity and distribution of solar gains transmitted through exterior windows. This means thermal mass performance is most sensitive to window properties, size, and orientation.
A second study, carried out by the authors, examined ways in which thermal mass could be used to reduce energy costs and greenhouse gas (GHG) emissions in electrically heated buildings in Ontario. This study examined how thermal mass could be used to do something called “load shifting.”
Load shifting occurs when heating demands are postponed to periods when electricity is cheaper and the electricity used is less GHG intensive. At night, time-of-use charges are at a minimum and most of the grid-supplied electricity comes from hydro and nuclear power sources that are almost carbon-free.
To the extent that heating loads can be shifted to nighttime, electrical cost and carbon savings accrue, even though the actual heating energy use goes up slightly as slabs and walls are ‘pre-heated.’
The authors also examined ways in which the effect of peak shifting using thermal mass could be maximized in a 1970’s multi-unit residential building. It was found that by coupling the heat distribution system with the floor slab through a radiant floor system, thermal mass benefits could be maximized for various retrofit options. Exposure of the thermal mass elements led to greater savings as well (Figure 1).
The right-most plot in the images in Figure 1 depicts the base or reference case where no retrofit measures were carried out to the building ventilation system or to the envelope. It is clear from Figure 1 load shifting can be a very effective operational strategy for utilizing thermal mass to reduce electricity costs and GHG emissions during the heating season. It is also clear exposed floor slabs with a radiant floor heating system are an effective way to achieve these savings.
Not only can shifting heating demand reduce electricity heating costs and GHG emissions, it can also reduce the stress on the electrical grid.
In many places, peak electricity demand is driven by weather. For example, in Ontario, peak electricity demand typically occurs during hot summer days, or during exceptionally cold winter nights. At these times, electricity is added to the grid by operating natural gas-fired generators or purchasing electricity from other providers who have higher carbon emission rates. Using thermal mass to shift heating or cooling demand in buildings would reduce stress on the grid when it matters most and limit carbon emissions.
Canada lags behind Europe in taking advantage of thermal mass but that is starting to change. Free tools for performing detailed building simulation, such as Google Sketch Up, Open Studio, and Energy Plus, are making it easier for designers to consider and evaluate the effects of thermal mass.
As building codes evolve, so do designs. As Canadians move toward low-carbon energy sources for heating and cooling buildings, demand for electricity will necessarily increase. With this increase, there will be a concerted effort to save energy, as it usually cheaper to save a kWh of electricity than to produce and distribute it. This will inevitably lead to energy conservation and carbon reduction methods, including the use of thermal mass and radiant floor heating systems.
* Consult the 2009 paper, “The influence of the external walls thermal inertia on the energy performance of well insulated buildings,” by N. Aste, A. Angelotti, and M. Buzzetti.
** Read Adam DiPlacido, “A Parametric Analysis of the Thermal Performance of Concrete Floor Slabs in Cold Climates”, M.A.Sc. thesis, University of Toronto, 2014.
Alex Janusz is an energy and sustainability analyst at RDH Building Science Inc., where he conducts energy modelling and performs building science research as a member of the energy and sustainability team. He has a passion for sustainable design and a keen interest in building science. Janusz completed M.A.Sc. at the University of Toronto where he studied ways of using thermal mass to reduce operational energy costs and greenhouse gas emissions in Canadian buildings. Janusz can be reached at firstname.lastname@example.org.
Kim D. Pressnail is an associate professor of civil and mineral engineering at the University of Toronto. He has been teaching building science within the Faculty of Applied Science and Engineering since 1990. His research interests include the design and construction of low-energy buildings and, in particular, utilizing thermal mass to reduce carbon footprint. Pressnail can be reached at email@example.com.
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