Image Credit: Alex Wilson
More Energy Solutions
Over the past month and a half, my blogs been focusing on resilient design — which will become all the more important in this age of climate change. Achieving resilience in homes not only involves keeping them comfortable in the winter months through lots of insulation and some passive solar gain (which I’ve covered in the previous two blogs), it also involves keeping them from getting too hot in the summer months if we lose power and our air conditioning systems stop working. This week, despite the freezing weather, we’ll look at cooling-load-avoidance strategies and natural ventilation.
Orientation and building geometry
With new houses, we can relatively easily control orientation and geometrical form to minimize unwanted solar gain.
The optimal orientation for a house is with the long axis running east-west, so that the longer walls face south and north. This allows the house to benefit from the sun when we want that heat, but keep it out when we don’t want it.
The sun always rises in the east and sets in the west, but in the summer it rises much higher in the sky. By having more windows facing south, most of the sunlight will glance off that glass during the summer when the sun high overhead, while in the winter, with the lower-angle sunlight, most of that sunlight shines through those windows — providing passive solar heating (see last week’s blog).
At the same time, having fewer windows on the east and west makes sense relative to summertime overheating. Significantly more sunlight shines through a square foot of east- or west-facing window during the course of a day in the summer than through a square foot of south- or north-facing window, so limiting east and west windows helps to prevent overheating.
Window selection
The type of glazing in our windows has a major impact on how much sunlight is transmitted through them. This is why it almost always makes sense in well-insulated buildings to “tune” the windows by orientation. By this, I mean using glass (glazing) on the south that transmits a high percentage of the sunlight striking it and glass on the east and west that transmits less sunlight. We refer to this property as the solar heat gain coefficient (SHGC); it is the fraction of total solar energy transmitted through the glass (assuming the sunlight strikes the glass at a normal (perpendicular) angle.
A good rule of thumb is to select south-facing windows that have SHGC values of 0.6 or higher (0.5 or higher with triple-glazed windows), and east- and west-facing windows with SHGC values of 0.3 or lower.
Windows with SHGC values of 0.6 will transmit twice as much solar energy as windows with SHGC values of 0.3. The beauty of recent advances in glazings it that we can now have fairly large window areas (to provide views and natural lighting) without nearly the energy penalty (both from heat loss and unwanted solar gain) we had two or three decades ago.
Shading windows from direct sun
On the south, we can also use simple overhangs or awnings to block virtually all of the direct sun. On the east and west, different shading strategies are better, because the sun is lower in the sky. For these windows, exterior shade screens or roller blinds can be very effective. So can plantings of tall annuals like hollyhocks or vines like clematis, morning glory, and grape.
Designers and builders in the south learned the principles of shading windows long ago. Traditional architecture in hot climates often included wrap-around porches that kept direct sun out of the house, while providing pleasant outdoor living space. (Part of resilient design is looking at how our grandparents or great grandparents built — and returning to some of this vernacular architecture that is so well-adapted to the local climate.)
Reflective roofs and walls
Light-colored roofs and walls reflect, rather than absorb, most of the sunlight striking them. By not heating up as much, less heat is transmitted through to the interior. With high insulation levels in roofs and walls (see below), the need for reflective exterior surfaces is less important, but this strategy can still make a difference.
High insulation levels and tight construction
Just as an energy-efficient building envelope reduces heat loss in the winter, it also reduces unwanted heat gain during the summer — thus helping to control cooling loads and maintain comfort.
If we follow the sort of recommendations for insulation levels for resilient homes that were outlined a couple weeks ago, unwanted heat gain will be very effectively controlled in the summer — as long as windows are closed during the hottest days.
Natural ventilation
Finally, we can achieve resilient homes that won’t get too hot if power is lost and air conditioning doesn’t work through natural ventilation. This strategy is particularly effective at night, when it’s cooler outside than in.
Simple operable windows with screens offer the primary strategy here, but we can go further. In hot, sunny climates, such as the Southwest, one can build solar chimneys that use the natural buoyancy of warm, rising air to pull in cooler outside air — sometimes through inlet tubes buried in the ground (earth tubes). Operable windows high on a wall or skylights can also serve as solar chimneys.
All of these natural cooling strategies can keep a house safe and reasonably comfortable in the summer during power outages. During normal times, such measures will significantly reduce the amount of time an air conditioner has to operate, while keeping the house more comfortable.
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.
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10 Comments
Humidity
There's an old saw: "It's not the heat it's the humidity." This particular old saw is largely accurate.
Here (the SE) temps get down 65-75 at night ... good for night-time cooling, except that the humidity is stifling. Unfortunately, while the old-timers did what they could with shade as you point out, Alex, they didn't have much to say about humidity ... which we deal with now via A/C.
Running a dehumidifier is nearly as wasteful as using the A/C.
Has there been any work done that's produced useful strategies that REDUCE humidity without using A/C?
Come May, I'm going to be looking for some "dry shade."
Reflective roofs question
I live in zone 6a and we have more heating degree days than cooling.Wouldn't a roof that attracts heat be better in my climate? This question has come up when I talk with other architects I know. And none of us have a compete grasp on this issue. Also with super insulated roofs like R60 or so does roof reflectance really have that much impact?
vent panels
In natural ventilation, I notice you haven't mentioned operable vent panels. Any reason? By my calculations, mine is insulated to approx R-26, which is somewhat better than a typical casement window.
Props to Mike Eliason:
http://bruteforcecollaborative.com/wordpress/2011/02/28/vent-panels-the-new-window/
reducing humidity w/o a compressor
JoeW,
I heard mention of an emerging technology that uses a salt to dehumidify incoming ventilation air. Haven't looked into further but I heard about it in the context of the University of Minnesota's entry to the solar decathlon back in 2009.
Vent Panels
Minn. Disaster,
Thanks for the link. I've seen awning windows used for ventilation panels with an insulated panel replacing the glazing. Hardware and weatherstrip already in place and trim and size matches the windows. When closed it looked just like the other windows.
Too many of these articles and comments don't address regional differences.
Here on the Colorado Front Range, all you have to do is properly place windows and open and close them at the right time of day/night, very low tech. No A/C, no salt to dehumidfy incoming air, nothing but a good cross breeze!
Regional differences and humidity
As has been so aptly noted by commenters, there are significant regional differences in appropriate natural cooling strategies. What makes sense in a low-humidity mountain climate with large diurnal temperature swings likely won't make sense as a primary cooling strategy in a more humid, Southeastern climate where it doesn't cool off as much at night. This is all very true, and too often we fail to adequately focus on regional variation in appropriate passive solar and natural cooling strategies.
However, note that the focus of this series of blogs is on RESILIENCE--how to create homes that will provide reasonably comfortable and safe conditions during emergency operation when there are power outages. My feeling is that at these times, our standards for what we consider comfortable will be broader. While we might not normally accept the high humidity that will occur if we rely on natural ventilation in Georgia, if power is lost for three weeks in Atlanta, we are going to be okay with higher-than normal humidity during that period of time. Even if we would normally not use nighttime ventilation in the Southeast, it's a viable strategy as a back-up strategy during times of emergency.
the Dehumidifying Slab
I once asked a very prominent (Cold Climate) Building Scientist what he would suggest for building in a Hot/Mixed Climate....
his answer was a little surprising: "Buildings for Hot/Mixed climates? That’s a good one. The drastic answer is that we must design for the end of air conditioning. Air conditioning cannot continue in perpetuity. So my question is how to achieve summertime comfort without grid electricity. Much of this comes in how to do the foundation. Foundations can, in theory, dehumidify, which would be nice."
So... I was very curious and asked him to elaborate on how to dehumidify with a foundation:
"It’s only for buildings where the soil around the foundation is protected, long-term, against rainwater. Figure that one out and the rest is easy."
A riddle....I give up ...please tell me more...
"The dehumidifying slab is easy. Soil gas in very well-drained soil, a few feet away from the soil surface, has a dewpoint at the well water temperature, usually. If the slab is vapor permeable (no poly, foam only at the edges) then it's easy to get vapor transport across the slab. If the slab is cold, it's much more permeable, and a better conductor of moisture."
well...I sensed that he was too busy for endless questions...so I left it alone
.....but his half baked idea still intrigues me
Humid climate issues
Alex: "Even if we would normally not use nighttime ventilation in the Southeast, it's a viable strategy as a back-up strategy during times of emergency."
Excellent point. And while relying on a/c entirely within a sealed box is obviously not a resilient strategy even in the SE, a mixed/humid home that has good natural ventilation capabilities can also make good use of that in the shoulder season when daytime temperatures can be high but humidity is typically low.
Having said that I'd like to warn against a popular but misguided practice of making use vented cooling at night (often with a whole house fan) and then closing up the house and going to a/c in the afternoon. In a tight well-insulated house, because dehumidication of incoming air uses more energy than cooling it, in the height of the summer it's better to use the a/c either continuously (on a diurnal basis) or not at all.
And I'd like to give a big shout-out to this sentence of Alex's in the OP:
This can't be emphasized enough. They were great low-energy builders - they had to be.
dehumidifying slab
To call this idea half baked is to be excessively generous.
And while no aspect of the human situation can be expected to endure 'in perpetuity', the emergence of cost-effective photovoltaic technologies actually gives us reasonable hope of being able to sustainably power our increasingly efficient a/c systems over the long term.
Natural house cooling in general
This article reminds me of my parents house in northeastern China back in the 1990s, in that most apartment buildings relied on natural cooling (no air conditioning). As the wall materials were mostly concrete, indoors was naturally cool during summer but the opposite during hot, humid day time. The ideas in this article about direction of windows and reflective roofs are primary factors that must be considered for natural cooling systems. I recommend that not only for the case when power is lost, but also when people live in a dry and warm climate regions, that they can save energy by eliminating unnecessary cooling or heating systems of houses. But for some regions that are hot and humid or freezing most of the year, it is not really useful even though some might consider it.
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