Fixing those Energy Leaks
If I proposed that you buy your next automobile from an auto parts store, you would be justified in calling me a nutcase. Yet many people think it’s perfectly natural to build or improve a house by treating it as an assembly of parts. It is -14ºF outside as I write and it is tempting to wish for a better heating system, or better windows, or better patched air leaks – whatever feels immediately in need of improvement – as the grand solution to this less than comfortable, expensive to heat old house. The fact that people try to sell me whatever they happen to have to sell – a new heating system, replacement windows, blown-in insulation, a different fuel – only reinforces our tendency to treat a house as a collection of parts.
But houses are systems. No single-issue fix of any of their parts makes sense when trying to improve their performance. Replacing a 75% efficient boiler with a 90% efficient one, for example, will improve boiler efficiency 15%, but a leaky house will still leak exactly as much as it has before, last year’s ice dams will still threaten to cause leaky ceilings, and we will never know whether the new boiler was the best investment we could have made in reducing our heating costs.
Sometimes overlooking how some marginal improvement affects the house as a whole can even create more problems than it solves. This is because heat, air and moisture interact in a heated building and dealing with only one might exaggerate problems with another. For example, air-tightening a house will exacerbate moisture problems, since moist, warm air can no longer escape through leaks. Therefore tightening beyond a point specific to each house must be accompanied by increased ventilation to remove excess moisture, even as heat contained in the exhausted stale air is retained within the building envelope.
Thanks to lots of research, we know how to build houses that consume no more energy than 1/10th of what conventional houses consume. We know how to reduce the energy consumption of existing houses to a similar degree. This should be our common sense base line: why shoot for anything less?
Buildings that achieve this level of performance – newly built or renovated – all share the following key features: (1) Meet a predetermined performance goal; (2) Optimize all building materials in the context of the specific building as a system; (3) Super-insulate – in Maine this means R-60 all around the heated envelope; (4) No thermal bridges – through wooden framing members, metals, masonry; (5) Airtightness – 0.6 air changes per hour maximum, at 50 Pascals indoor-outdoor pressure difference, as measured by a blower door; (6) Heat recovery ventilation; and (7) High performance “warm” windows.
In the right combination, these characteristics ensure that a building (new or old) will use 90% less energy than its conventional equivalent. The remaining 10% can then come from a local renewable energy source. Buildings that achieve this degree of efficiency are usually system-optimized with the help of computer software. Existing buildings require a professional energy audit to establish their current detailed energy profile as a baseline from which all improvements can be planned in the same fashion. Before anything is done to the house, it is wise to invest in this comprehensive planning process to answer the question “what combination of measures will produce the greatest energy/economic result?” Not every house is a candidate for a 90% efficiency improvement. For example, while a conventional heating system may be replaced by a less expensive alternative, the cost of modifying an existing structure (to create room for more insulation, for example) may be too high to make economic sense.
It is useful to compare the monthly occupancy cost of a house as is (mortgage + heating + taxes, etc.) with the same costs after projected energy-efficiency improvements. With judicious planning and the right financing it is usually possible to minimize occupancy costs by choosing the most energy efficient combination of systemic improvements to the building; in effect investing funds currently spent on heating fuel in permanent energy efficiency improvements instead. Let’s say I burn 1,000 gallons of oil @$3.60 a gallon, spending $300 per month on fuel. I borrow $14,000 @5% for 10 years and spend it on improving my home’s efficiency 50%. Now I pay $150 per month on fuel plus my monthly loan payment of $149, a total of $299. When the oil price rises to $3.70 (as it just did), I will pay $154 for fuel, but the loan payment remains the same. My new monthly total is $303, $5 less per month than if I had done nothing to improve the house. Every time the fuel price rises (the historic average price inflation is just above 6% per year), my comparative monthly cost will decrease, because my loan payment remains the same. Interest rates are currently low. It may be a good time to act. Do your own numbers.