Learning From the Ancients
Can your home pass the survival test?
Engineering studies have confirmed what some ancient societies knew all along: homes with exterior walls made of heavy materials experience greater comfort summer and winter, and more importantly, higher survivability during disasters.
The typical pine stick frame home is the norm in Alberta and can be characterized as low mass and has low capacity to sink or release heat energy relative to the volume of the building. In Alberta, it should be the housing industry's goal to construct energy efficient, comfortable, quiet, safe, healthy, and reasonably affordable homes to purchase and operate. Understanding the role of both internal and building envelop mass in achieving the goals noted above should be a priority in all home designs, but it is not.
Today, the assessment of the role of increasing a home's overall mass and its heat capacities towards achieving the objectives of energy efficiency, comfort, quietness , and safety as noted above are seldom given meaningful consideration by builders or home owners. As noted, the role of a house's total mass and related winter safety issues seems to be totally missed in the Alberta Building Code regardless of the repeated news stories of communities in Canada suffering through severe winter storms without power. It is troubling that building codes in Canada and Alberta have not addressed the issue of residential home safety during extended power or natural gas outages. It seems prudent that homes built today in Alberta's often bitterly cold winter conditions should meet some measurable building standard with respect to providing safe shelter for the occupants (18-24 hours or more?) during any extended local, community or provincial wide power outage. Although the annual probability of such an event is not high, the consequences could be extremely serious. Today, most Albertan would be at the mercy of circumstances in the case of a wide spread and prolonged power outage. Public Safety Canada recommends that all home owners should plan to be self reliant for 72 hours during disasters.
Related Theory and Principle Tid Bits to Understanding Survival Time
Heat is a form of energy and a method of describing that which is transferred in materials, such as a concrete floor, drywall, storage tank of water, and furnishings between a system (i.e., a house) and its surroundings (outside) as a result of temperature differences only.
Energy is defined as the ability to do work at some future time. Heat energy flows from warm places to cooler places and flows through different materials at different rates. For example, heat flows through water (high density) more quickly that through air, which is why you cool off faster in cool water than in air at the same temperature.
Temperature is a measure of thermal energy in materials, but merely knowing the temperature of something is not sufficient to determine how much energy is stored in it or the building materials' capacity to store energy. You also have to know how much of the substance there is and its heat capacity characteristics. This quantity of substance is called its mass.
The total amount of heat energy in a substance is proportional to both the temperature and the total mass of the substance (matter). This means that a given amount of heat energy (i.e. 1.0GJ/ 1.0 million Btu) can be stored in a small amount of mass at a high temperature (masonry fireplace-oven, or water tank), or in a large amount of mass at a lower temperature (concrete floor slab floor or insulated concrete wall system).
The amount of heat a material can store is commonly referred to as the material's heat capacity. Operationally speaking, the amount of heat energy needed to produce a given rise in temperature is called the heat capacity of that substance. More explicitly, the heat capacity of a body consisting of some substance is numerically equal to the amount of heat needed to increase its temperature by one degree. The heat capacity per unit of mass of a material like steel, copper, wood or water is called its specific heat.
For example, the specific heat of water is 1.0 and that of white pine is 0.42 (Btu/lb degrees F).
An important measurement parameter of a building's dynamic performance is its unique survival time constant (the heat capacity of a building divided by the specific heat loss rate). Every house is a complex system and its survival thermal time constant can be easily determined by measuring the rate of heat loss over time (winter conditions) in the absence of any additional heat sources. The time constant is a parameter which describes in its easiest way the energy performance and thermal dynamics of a house. This survival time constant describes the decay rate of indoor temperature with respect to outdoor ambient temperature when no additional energy is added into the home (system). This test can be performed more quickly when the indoor Vs the outdoor temperature difference (?T) is ? 40 degrees C during a typical Alberta winter evening/night. In the case of an extended loss of power in the winter, the stored thermal energy if large enough in a well constructed home will dramatically slow the rate of decline of the temperature inside, providing the occupants with much more time to safely respond to any external loss of utility sourced energy. Most homes in Alberta would perform poorly if tested in this manner, becoming very uncomfortable in 2-3 hours or less and unsafe after a few more hours.
In water, a lot of heat energy per cubic meter of mass can be stored; concrete less so; and air relatively little (approx. 800 times less). It takes a lot of heat to warm a cubic meter of water or concrete through some significant difference in temperature. Once heated, high density materials in your home, materials with high mass such as heated water in hydronic heating systems, stone, or heated concrete slabs will slowly release their stored energy, releasing the same large amount of energy required to heat them. This flywheel effect of mass being able to release its stored heat energy (providing work) back into the occupied space provides for more even and stable interior temperatures, decreasing both heating and cooling peak loads. When cooling is not used, the building's thermal mass can clearly decreases peak indoor air temperatures in the summer time.
Disaster Preparedness--will your home provide safe shelter?
Are you and your family now at the mercy of circumstances? When a typical low density 2x4 or 2x6 wood framed home's interior is a comfortable twenty degrees C, there is still precious little stored energy in its low density low mass exterior walls, its wood floors, inside air and contents. Most home owners in Alberta are unaware that on a typical cold winter day (i.e. minus 20C or colder), any extended loss of power (gas furnaces will not function) or loss of natural gas to low mass homes will result in very rapid drops in internal room temperatures (occupants could find themselves literally shivering in the dark in a few hours) as the limited internal energy meets little resistance as it flows through the home's envelop. This was recently experienced by thousands of homeowners in Sylvan Lake in Feb. 2008 when the power went off for 4-5 hours. It is true that with care and attention during construction, the exterior walls of today's newer 2x6 wood framed homes can be enhanced to improve their thermal resistance-conductance properties and lower air infiltration rates. This enhanced construction quality will slow down the rate interior temperature falls, but the overall lack of any significant structural mass or mass in its contents (little energy stored) will still lead towards the same very cold end result during extended power outage, just delaying the inevitable internal freeze by a few hours.
In a short period of only 3-4 hours in the cold winter power loss scenario noted above, the majority of home owners in Alberta will be experiencing their homes' quickly becoming alarmingly cold. Making this situation more dangerous, there may be little or no water pressure if the problem is city or province wide. Many home owners will then face the unfolding reality of imminent property damage and a potentially life threatening crisis should the power remain off longer than 5-6 hours.
In a winter power loss scenario, seniors and young children will be at most risk to hypothermia, and there will be precious few buildings in the community capable of helping any of the ten to hundreds of thousands of citizens all seeking heated shelter. Many people will feel the initial effects of hypothermia when they awake to a very cold home if the power is lost after they have gone to bed. Adding to this crisis situation, gas stations will be unable to pump fuels. Although the annual probability of a city wide or provincial wide power outage is relatively low, the consequences will be very severe in the winter months. By definition, then the risk to Albertans is high from this causal event because the severity of the consequences is severe.
It should be noted here that a worst case scenario is beyond any professional's ability to accurately estimate because of lack of related data and experience. It is understood by professional risk professionals that in the worst case the consequences losses are always much worst than anyone can envision. Regardless, it is not very difficult to envision the range of losses and then estimate the time for a community to recover from a city-wide power or natural gas infrastructure failure that has extended beyond 18 hours on a typical cold winter day or more.
Take The Survival Challenge
If you want to test how well or how poorly (survival time constant) your own home will provide your family protection, select a cold winter evening when competing the following test process. Send the kids and spouse off for a dinner and movie when the outdoor temperature is in the minus 20-30 C range. With your house at a comfortable 20 C, trip off all the breakers except for the main floor family room, put on your coat, find a good book, pour yourself a "single malt", turn off the TV and all of the lights except for one low wattage reading lamp, and begin recording the main floor's temperature every 15 minutes for the next three hours. From this simple data gathering task, you'll be able to determine the rate the temperature decreases and infer how well your home would protect you in the noted scenario. Stop the test if the interior temperature drops below 5 C before the three hour test is completed. As the difference in temperature between the outside and inside decreases, the rate of decline in interior temperature should begin to slow. After the data gathering part of the test is completed (make a note of the outdoor temperature), graph the indoor temperature data (y axis 0 to 20 C) over time (x axis--15 minute increments). From this line graph, you can easily extrapolate in hours when your house will likely reach the freezing point. Remember, long before your house reaches the freezing point, your home will have failed to provide you and your family with safe shelter, you will then likely face a serious crisis with few options.
In the causal event of an extended city wide or province wide power outage exceeding 12-18 hours during the winter, the impact on Albertans would be very costly and likely precipitate casualties. Today only a few homes in Alberta would remain reasonably comfortable and safe to occupy over a 24 hour period without any active energy supply. Knowing this reality, mindfully seeking cost effective methods of adding mass to our new or renovated homes in addition to significant improvements in overall home envelope performance should become the norm and not the exception in Alberta. Adding more mass with high heat capacity to our homes will provide occupants with more comfort, can lower annual energy costs, and should provide added security during extended periods of interruption in energy supply. Home owners might sleep more securely knowing that their high mass homes will afford them more protection and time to respond during any extended energy supply loss. A long survival time constant might some day become an added asset when selling homes. Of course there are other relatively low cost methods home owners should evaluate in their search for a solution.
