Equinox House is very “massive”, thermally. It is similar to an adobe structure, but achieves its thermal massiveness in a different manner. Thermal massiveness is achieved through a combination of insulation and mass. The more insulation, the more massive a building becomes. Thermal massiveness is characterized by a parameter we call the “thermal time constant”. It is a measure of how fast your house will change to the outside conditions if someone unplugs your furnace or air conditioner.
In the 1980’s, Ty conducted a house monitoring study for the State of Illinois on the Washburne Trade School’s passive solar house located in Chicago. The house had a lot of mass, with a concrete slab floor, water storage tanks, a large two story stone fireplace, and a special “phase change” energy storage system (think melting wax or some type of dissolving salt). But, test results over a 4 month period during the 1986 to 1987 winter showed that the Washburne passive solar house was not thermally massive. The average thermal time constant for the Washburne passive solar house was only 18 hours. Unfortunately, many building designers do not understand how the combination of insulation and mass work together to make a building thermally massive.
Equinox House is similar in size to the Washburne house (190 square meters or 2,100 square feet of floor area), but its time constant is 110 hours. In other words, when someone pulls the plug on the Washburne house, you’ll know it in a few hours, compared to days in Equinox House. But, the Washburne house was intentionally built with lots of mass for energy storage, while Equinox House has not added any mass beyond that of normal drywall and a normal 4 inch thick concrete floor. The difference is the insulation, air sealing, and appropriate window design.
Physical mass will not provide “thermal massiveness” unless it is also well-insulated. The Washburne house was poorly sealed and over-windowed (70 square meters, or 750 square feet of windows for an estimated cost of $30,000 in today’s window cost….$10,000 worth of windows would have provided a bright, daylighted home with significant solar heating, with the other $20,000 buying the solar photovoltaic system for powering all of the house’s annual energy requirements!). Windows (even “super” windows with R10 and greater) provide minimal massiveness to a house. It is essential that one matches the physical mass of a structure with the house envelope’s insulation characteristics in order to achieve true thermal massiveness.
So, what does thermal massiveness get you? Two things that everyone wants in a house: comfort and energy cost savings. In terms of comfort, the attached plot shows the interior temperatures and outside temperatures for two similar sunny December days spaced nearly 25 years apart. Notice how Equinox House temperature stays quite steady through the day and night, while the Washburne house increases above comfort levels during the day and then comes crashing down in temperature at night as the furnace struggled to keep the house comfortable. We found that the Washburne house was uncomfortable nearly 30% of the time in December in the upstairs region. Of course, if it overheats in December, other months are even worse. This is a common problem in many “passive solar” homes that used a shoot-from-the-hip design methodology that leads to extra cost for excessive windows and extra mass stuff while providing no energy or comfort benefits.
Comfort is not only determined by temperature level, but also the uniformity of temperature and the constancy of temperature. If the temperature is changing with time, your thermoregulatory system must sense and then adjust your metabolism to achieve a balance in its energy flows. Just when you body thought it was set up for one temperature, things change, and it has to adjust again, which ends up making you a bit cranky when you’re trying to get comfy for reading a nice book. Non-uniform temperatures are also uncomfortable as your feet may be cold while your head is hot, and your brain is trying to decide what to do about it (maybe lay horizontal at the temperature you like best?).
Thermal massiveness lets you save energy and money at the same time (and remember, we spent no extra money to build thermally massive characteristics into Equinox House). In the summer, for example, outside air is cooler at night by 10F on average, and electricity costs less at night than during the day. With a thermally massive house, all of your daily cooling can be done at night, which saves energy because your air conditioning system operates more efficiently, and saves you money by taking advantage of lower nightime electricity rates. This assumes you are on a time-of-day electricity rate, which you will be in the not-too-distant future if you are not currently.
Watch for our upcoming (March, 2011) column in the ASHRAE Journal for more information on building mass.
Photo showing the concrete floors in Equinox House. Frank Lloyd Wright considered concrete to be a “feature” and an important aspect of his “organic architecture” concept, while the appraiser for our bank viewed them as a “flaw”.
Figure showing diurnal temperature swings in the Washburne passive solar house in Chicago and Equinox House on similar sunny days in December.
