Blog Post

Resilient Design: Dramatically Better Building Envelopes

A resilient home is a highly energy-efficient home that will maintain livable conditions even during power outages or interruptions in heating fuel.

A superinsulated "Passive House" being built by Dan Whitmore in Seattle. These wall trusses provide about a foot of insulation. Photo: Dan Whitmore. Click on image to enlarge.

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.

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. To keep up with his latest articles and musings, you can sign up for his Twitter feed.


Published January 3, 2012

(2012, January 3). Resilient Design: Dramatically Better Building Envelopes. Retrieved from

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November 2, 2012 - 12:39 pm

I live in a 1929 era Craftsman bungalow that had no insulation.  We first used 90 tubes of caulk and 20 cans of foam to work on the air sealing issues. We insulated the existing exterior walls with blown in cellulose. Then we built new walls 5-1/2 inches inside of the existing exterior walls, and insulated that cavity, giving us 9 inches of cellulose insulation in the walls and we put 14 inches in the attic.  We installed double pane argon filled low-e coated windows. Then we built R-20 insulated interior shutters for each of the windows out of Rmax insulating panels. We've never done a blower test to check the airtightness, but we are at 106% of the passive house limit for energy use per square meter (the new construction standards, we are below the standard for retrofits).  We haven't insulated the floor because there is only 6 inches of crawl space under most of the house. We haven't insulated the foundation because it appears to be brick and there is considerable vegetation growing close to the house, and an engineer friend said it was not prudent to think about digging all that out to install insulation.  We are thinking about building a new insulated floor inside.

We also converted the south wall of our utility room from brick to glass.  During the winter at night we cover it with large 4 x 8 R-20 insulated panels. During the summer we put those panels up during the day.

We specifically wanted a house that would work well if the electrical grid went down.  We don't have natural gas service. We have a wood burning stove that we use if cloudy days persist, but we get most of our heat most of the winter from the sun.  We are in Oklahoma City.  We also use a couple of small oil-filled electric heaters for zone heating.  During the summer, we keep the entire 1548 sq ft house cool with 2, 5K btu window AC.

We consider what we've done to be kind of a blue collar redneck passive house.

January 6, 2012 - 6:27 am

Robert is probably right that I overemphasize terrorism. I tend to lump into this category political upheaval. With the first oil embargo in the 1970s, some European countries were caught totally off-guard. Sweden saw heating oil and gasoline prices go up several fold very quickly, and that led the country to institute building codes that require triple glazing--thirty years ago!

I read in the New York Times the other day that if Iran succeeds in closing the Strait of Hormuz to oil tankers, that action could result in a 50% price increase for heating oil and gasoline almost overnight. Similarly, if terrorists or political opponents of the Saudi government were to disable the massive Saudi Aramco oil refinery, petroleum products could become unaffordable to many in the U.S. Perhaps actual shortages wouldn't occur, but higher prices could result in effective shortages for many consumers.

If something like that were to happen, pundits would be shouting about our lack of preparedness. "Why haven't we required that houses be well enough insulated that interruptions in heating fuel wouldn't be a big deal," they would demand to know?

Let's adopt a policy of resilience now, so that we'll be prepared for not only natural disasters and some of the predicted impacts of climate change, but also to prepare for political upheavals or terrorist actions that could leave us without affordable petroleum fuels or electricity. And then we can reap all the other benefits of resilient homes and communities, including comfort, low energy costs, and local food production. (More on the agriculture side of resilience in a future blog.)

January 4, 2012 - 4:01 am

Talk of terrorism seems to be a way to get people's attention and it's become so pervasive in our news that it's difficult not to mention it when discussion resilient homes.

Terrorist are divas. They like to be seen. They like to make an entrance. They are targeting iconic targets that are photogenic. I'd be more worried about foreign governments. China hacking into our electrical network scares me. Any future big wars will include cyber attacks. It doesn't matter where the tanks or bombs are I expect our electrical supply to be interrupted often because it will disrupt our communications and manufacturing abilities.

January 3, 2012 - 11:44 am

Alex, I continue to object to your focus on terrorism, which - contrary to your assumption - I suspect most people in the US don't give much thought to at all, nor should, since the probability of death by terrorism worldwide (775 per year) is about 3% of the likelihood of getting killed by lightning (24,000 per year). The probability of being discomforted by a terrorist attack is probably similarly negligible. Let's focus on natural disasters, which we know are going to occur with increasing frequency and ferocity.

I also don't know why you would consider "typical energy-efficient practice" to be R-10/19/30, when the current code minimum for New England is R-15/20/49. And I question either the need or the efficacy of tightening homes to 1 ACH/50 (John Straube recommends 2-3 ACH/50) and then requiring mechanical ventilation systems which fail during the very power outages you're encouraging us to design for. If we are no longer willing to rely on uncontrolled ventilation, then "cracking a window" is hardly the solution - using semi-passive ventilation systems (exhaust-only with passive inlets), however, is.