The system angle of energy-saving winter comfort

Posted Wednesday, January 30, 2013 in Sustainable Maine

The system angle of energy-saving winter comfort

Passivhaus Thermogram

by Paul Kando

Let’s compare two houses. House #1 is a zero-energy house constructed to meet the high-energy efficiency standard of 1.4 kWh maximum heating energy per square foot of heated floor area per year. The house is comfortable year round and built to last for many years. Indoor heat, air and moisture are carefully controlled to prevent moist air entering into wall cavities or other structural parts where condensing moisture could cause damage. The house is tight, super-insulated, and heated principally by the body heat of occupants and waste heat from appliances. There are no thermal bridges through the insulated envelope. Heat-recovery ventilation provides fresh air, recaptures over 80 percent of the heat from the exhaust air, and removes excess moisture.

House #2 has been constructed conventionally. Its 2,000 square feet of floor space consume 1,000 gallons of fuel oil per year, which translates to 20.3 kWh of heating energy per square foot of heated floor — almost 15 times that consumed by House #1. Reasons for this huge difference include insufficient insulation, poor quality windows and lots of thermal bridges and air leaks. (Ironically, very old, leaky houses have fewer moisture issues than newer ones because “they breathe,” i.e. allow excess moisture to escape with the heated air through numerous leaks. This is why many drafty, centuries-old wooden structures are still with us. In contrast, many newer, “better insulated, tighter” dwellings have sustained serious moisture damage in just a few decades because of moisture trapped in the structure.) 

Both houses are made of similar materials and components: wood framing, plywood, sheetrock, insulation, windows, doors, wiring, plumbing and so on. The often overlooked difference between them is systemic. House #1 has been designed to meet a measurable performance standard for energy, air, and moisture control. During its design process every material, component, construction method and detail has been optimized to help meet that standard, i.e., they have been carefully chosen based on their anticipated performance. Furthermore, the team that built House #1 focused on meeting the performance goals laid out in the design. Designers, workers and contractors had to collaborate every step of the way, focusing on this common goal.

The design process of House #2 was brief and perfunctory, limited to copies of drawings, specifications and lists of materials “to meet code.” For modern tract housing, what passes for design is often an assembly of “cookie cutter” boiler plates. Mechanical systems are routinely over-sized, “just to be on the safe side.” At the building site, a number of subcontractors — some paid on a piece basis, others on a fixed price contract or by the hour — seek to invest the least amount of passable effort for maximum possible pay. The result is an assembly of more or less well-made parts and marginal overall performance. High-quality framing may be combined with poor-quality insulation and spotty drywall work in a reasonably tight structure riddled with holes bored by electricians and plumbers that nobody is responsible for fixing. In short, House #2 is at best an assembly of parts, not an optimized system.

Most of us live in Houses #2, trying to “plug” their defects as best we can. The result often falls short of what it could be for the price and effort. At times we create new problems. For example, adding a layer of 1-inch foam board under a new siding virtually guarantees moisture damage due to condensation on the inner surface of the outer sheathing. (The foam forms a vapor barrier but is not thick enough to keep that surface above the dew point). Tightening a house without providing energy-prudent ventilation may result in energy savings, but may also create moisture problems and unhealthy indoor air.  

What to do? The next best thing to living in a high-performing house is to address the existing house as a system. Random measures to keep out the cold sometimes create more problems than they solve. Knowing how your house fares systemically permits safe and effective improvements, even if done a step at a time. A well-performing house emulates nature. Nature is frugal, takes a long-term view, relies exclusively on solar energy, and on systemwide cooperation. Poisons are avoided, form follows function, there is no waste, resources used are locally appropriate. A house could hardly do any better. A thorough energy audit will help you understand your house as a system and identify steps to be taken to safely and cost-effectively improve its performance.  

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