Resilient Design: Dramatically Better Building Envelopes
January 3, 2012
A resilient home is a highly energy-efficient home that will maintain livable conditions even during power outages or interruptions in heating fuel.
When most people think about resilience--resilience to storms or terrorism, for example--they think only about resilience during the event. Equally important, if not more important, I believe, is resilience in the aftermath of that event. Hurricanes, ice storms, blizzards, wildfires, tornadoes, and other natural disasters not only have an immediate impact, for which we may or may not be able to prepare, but they often have a much longer-term impact, usually through extended power outages.
The same goes for terrorist actions; some suggest that smarter terrorists of the future may target our energy infrastructure or hack into power system controls to wreak havoc (cyberterrorism).
In achieving resilience, I believe that our single most important priority is to ensure that our dwellings will maintain livable conditions in the event of extended power outages or interruptions in heating fuel. (I used to refer to this as "passive survivability," but I came to realize that that term was too negative or dire-sounding to get much buy-in.) Here in Vermont, a resilient home is one that will maintain temperatures of, say, 50 degrees Fahrenheit without supplemental heat. The most important strategy for ensuring that those livable conditions will be maintained is by creating highly insulated building envelopes. I will cover other strategies, such as passive solar heat and solar electricity, in future blogs in this series. Below are the key strategies for achieving exceptionally good energy performance:
Insulate extremely well
We used to think that 2x6 walls insulated with fiberglass or cellulose were perfectly adequate relative to R-value--even defining that house as energy-efficient compared with standard construction (insulated 2x4 walls). It takes far more insulation to achieve the level of resilience needed to ensure that the house will maintain livable conditions without supplemental heat or electricity.
Building Science Corporation, of Westford, MA recommends the 10-20-40-60 rule-of-thumb for insulation levels in homes in cold climates (roughly defined as homes north of the Mason-Dixon Line). This rule of thumb refers to R-10 for basement sub-slab insulation, R-20 for foundation walls, R-40 for above-grade walls, and R-60 for ceilings or roofs. That's a lot of insulation, compared to typical "energy-efficient" practice, which might include no insulation under a floor slab, R-5 to R-10 on foundation walls, R-19 in walls, and R-30 in attics.
Getting to these insulation levels is not easy. R-10 slab insulation requires two inches of extruded polystyrene or 2.5 inches of expanded polystyrene. R-20 foundation walls require four or five inches on the foundation exterior or an insulated 2x6 wall on the interior. Here are two options for achieving R-40 walls: double 2x4 walls held apart enough to achieve a ten-inch cavity and insulating with dense-pack cellulose; or insulating 2x6 studs with cellulose and then adding three inches of polyisocyanurate on the exterior. R-60 in an attic floor requires about 18 inches of cellulose.
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For more on insulation materials (a lot more!), you might be interested in my recently published report: Insulation Materials: The BuildingGreen Guide to Products and Practices. It's available as a downloadable PDF file for $129.
Install top-performing windows
This level of energy performance calls for windows that achieve a unit insulating value of R-5--that's not the center-of-glass R-value, but the average R-value for the entire window, including edges and frame. National Fenestration Rating Council (NFRC) window energy performance labels list U-factor rather than R-value. (U-factor is the inverse of R-value.) Look for an NFRC-rated U-factor of 0.20 or lower.
To achieve such superb energy performance typically requires triple glazing (three layers of glass or two layers of glass and a suspended plastic film) and at least one, but sometimes two, low-emissivity (low-e) coatings and low-conductivity gas in the space between the layers of glass. You can find windows today with unit U-factors as low as 0.15 (R-6.7). Such windows aren't cheap, but they are increasingly available, and they do a great job at keeping energy consumption down and ensuring comfort.
Very tight construction
Really well-insulated buildings should also be airtight. We don't want uncontrolled air leakage bringing outside air in through the walls or basement; we want to be able to control where fresh air is brought in through a properly designed ventilation system. The Passive House certification program, which originated in Germany but is gaining traction worldwide, including in the U.S., requires airtightness of 0.6 air changes per hour at 50 pascals of pressure difference. (We measure air tightness using a "blower door" and often report that air tightness as an elevated pressure of 50 pascals.) I think a reasonable airtightness level for new construction is 1.0 air changes per hour at 50 pascals--not quite as tight as the Passive House standard.
In the event of loss of power so that the ventilation system stops operating, windows can be cracked to provide fresh air, but most of the time ventilation systems should be operated to ensure good air quality in the home.
New vs. existing homes
Achieving highly insulated building envelopes is much easier with new construction than with existing homes. To achieve such performance with an existing home requires what is often referred to as a "deep energy retrofit." More on that in a future blog.
In this resilient design series, I'm covering how to achieve resilient homes and communities, including strategies that help our homes survive natural disasters and function well in the aftermath of any event that results in an extended power outage, interruption in heating fuel, or shortage of water. We'll see that resilient design is a life-safety issue that is critical for the security and wellbeing of families in a future of climate uncertainty and the ever-present risk of terrorism.