How to keep cool in and heat out?
I am not in construction or an architect, just someone who would like to live in/build a small, comfortable, energy efficient home. I don’t need and could never afford a Passive House. Don’t even know if one would work where I live. I live in the low desert southwest near Phoenix. Living is very different here than in cold weather areas. And very different than Florida or humid areas of Texas. Outside temps over 100F in shade 5-6 months; highs 115F; nighttime lows 95F for at least 2-3 months. Low humidity 10 months. Little or no drought resistant shade trees. Most people do not cool their homes below 79F. A lot live/work at home. Not really worried about cold. Portable room heaters work fine when needed so radiant heat designs are not too valid. But…air conditioning is VERY expensive. Yet your description of warm weather homes makes no sense for here. To begin with we would never orient length of home east/west.
I’ve learned a lot reading this blog. Thank you. For example we need to keep heat out rather than keep heat in or keep cool in and heat out and this is apparently handled differently than in cold weather? Is it? But how? That is what I would like to know. I would love to read/learn more about building energy efficient homes for this environment. Any suggestions?
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Anneal,
Here is a link to an article that addresses your questions: Hot-Climate Design.
" To begin with we would never orient length of home east/west."
Really? That really IS the right way to go in a sunny cooling dominated climate:
Orienting the long axis of the house east-west means that in the middle of the day the angle of the sun on south-facing windows is high, which makes them reflect most of the heat, and it's possible to shade them with overhangs to kill most of the direct solar gain.
East and west facing windows get sun on them when the sun is lower in the sky, and the incident angle on the windows reflects very little of that solar energy. By having the longer axis running east-west means less window area on the east and west sides to soak up the heat. If possible, design for NO WEST FACING GLASS, and minimal east facing glass.
Even north facing windows have some amount of net heat gain, but that's the least gain-sensitive side, and the best side for daylighting since it's all shadow-free scattered light, never sharp edged direct sun.
Also, a simple gable roof with an east-west ridge and south facing pitch also allows you to maximize photovoltaic solar. Where once that was a ridiculously expensive option the costs have crashed, and keep on falling. Even if you don't install it on day-1, within 5-10 years the costs are expected to fall by more than half (again!), which will make it one of the cheapest energy sources available. The average installed cost of PV solar right now in much of the US is about $4/watt, or about $40 / square foot. In some places it's getting down closer to $2/watt or $20/square foot. In AZ you'll get between 1.5-2 kilowatt-hours per watt of PV per year, so a 10,000-15,000 watt array would pretty much cover your air conditioning bill if you have an R40-50 attic (with all air handlers & ducts below the insulation), and near-zero west facing windows. It's a lot of money up front, but it zeros out your electric bill (except for the recently instituted 70 cents per 1000 watts of PV array.)
As mentioned I am not a contractor/architect . I guess I was confused as to the definition of long axis. I was thinking the word “facing” is the same as “running”. Now I have figured out that they are actually the opposite! So, we are on the same page regarding the orientation of the house. Thanks for sorting that out for me.
So, is there really any difference between the low desert environment and the Florida environment when it comes to construction? I am a little confused about that too since rammed earth, adobe, and straw bale construction are mentioned in this area for thermal mass (?) but the Florida Hot-Climate Design article mentions only needing 2 inches of rigid foam (R-10) for the walls. I would have thought more than R10 would be needed for the walls? Am I missing something else?
I am not looking for perfection like the passive house. Just what should an average person with a low to medium budget really ask for to build a basic energy efficient home in my low desert environment?
Anneal,
Adding thermal mass to walls is expensive -- it's usually more expensive than adding more insulation -- but it is an effective strategy in hot climates. For more information, see All About Thermal Mass.
It seems like one of your unique challenges is that it doesn't cool off at night. Martin, do you think thermal mass makes sense when it's not getting below 90 at night? Sooner or later the mass will heat up too and then what?
I want to be as accurate as possible regarding temps. The following is from weather underground.com for Phoenix-Mesa Gateway Airport, AZ. We have had a nice June this year :-)
May temp 2011 2012 2013 2014
high 102 °F 109 °F 100 °F 104 °F
mean 90 °F 91 °F 84 °F 88 °F
low 77 77 71 °F 78 °F
June Temp 2011 2012 2013 2014
high 116 °F 113 °F 114 °F 105 °F
mean 98 °F 98 °F 99 °F 91 °F
low 86 °F 86 °F 84 °F 78°F
July temp 2011 2012 2013
high 116 °F 114 °F 109 °F
mean 98 °F 102 °F 96 °F
low 89 °F 89 °F 86 °F
Aug 2011 2012 2013
high 116 °F 116 °F 107 °F
mean 101 °F 101 °F 96 °F
low 89 °F 87 °F 86 °F
Sept 2011 2012 2013
high 108 °F 104 °F 107 °F
mean 98 °F 94 °F 94 °F
low 89 °F 87 °F 84 °F
Oct 2011 2012 2013
102 °F 100 °F 93 °F
90 °F 84 °F 78 °F
80 °F 71 °F 64 °F
Anneal,
Dan is right. Once summer weather sets in, and nighttime low temperatures are higher than your indoor temperature, thermal mass does no good. Under those circumstances, you still want good old-fashioned insulation.
The reason that less insulation is required in Phoenix than in Fairbanks is that your delta-T is smaller. Even when it's 110 degrees outside, your delta-T is only 30 or 40 degrees. In Fairbanks, the delta-T can be 120 degrees.
If it were my house, I would plan on doing most of my cooling with a 12 inch media evaporative cooler:
http://www.homedepot.com/p/Champion-Cooler-5000CFM-120V-Down-Draft-Roof-Wall-12-in-Media-Evaporative-Cooler-for-1650-sq-ft-with-Motor-AD1C5112/100662139
Even at a nighttime temperature of 85F, you can easily cool the house to 60F for about 20% of the cost of conventional AC.
Heat with minisplits. Minisplits may also be required for the "monsoon season" when it is too humid for the MasterCool.
Anneal, The first thing I would do was look at the work of architects well known for building successfully in a similar climate. The Australian Glenn Murcutt is one. Don't concentrate on the building envelope before you have designed a form which is appropriate to the climate and site.
Here is an example of what I mean:
http://www.jetsongreen.com/2009/06/desert-modern-rimrock-ranch-house.html
I think the "Passivhaus-like" strategies work well for almost all climates.
Extremely airtight, ample insulation, not-so-overglazed and careful attention to window orientation/shading.
During the cooling season it is important to block/shade the sunlight BEFORE it passes thru the glazing.
The "Rimrock-Ranchouse" example that Malcolm posted seems to ignore airtightness and window shading.
Overhead doors and sliding glass pocket doors are almost impossible to make airtight.
The Canopy is a cool idea but seems to be doing a very poor job of shading the glass....
Not to mention the concrete slab thermal bridge.
John, Until very recently very culture has modified the basic form of their dwellings to reflect their location and climate. All indigenous architecture does this. A good case could be made that that is what all architecture consists of. It was only recently with the advent of cheap energy that we stopped doing it and made a one sized fits all house form that we spread with minor variations from Alaska to Arizona.
So we end up with an inappropriate basic shape we then try and make work by upping the efficiency of the building envelope. At that point the only way to do this is to create the hermetically sealed enclosure characteristic of Passive Houses. Why not step back to looking at the solutions that were always used before the advent of the ubiquitous modern tract house?
I'm not advocating for the inclusion of all the details in the house I linked to. The idea it illustrates is to first imagine how you want to live on your site. What your relationship with the outdoors will be and modify the house to reflect that. How will you deal with the sun and wind? Should you perhaps first provide shade for the building rather than deal with the sun once it has hit the building enclosure? Do you want to sit in a conditioned enclosure and view your surroundings through three panes of sealed glass? What is the point of living in nature if you aren't allowed to interact with it? And why start with the premiss that the basic form of the house should be the same no matter where it is?
The most disappointing aspect of the Passive Houses I see is that their architectural often seems arbitrary or inappropriate. They sometimes take on the superficial form of the region - say barn-like in Vermont - but none of the essential elements that reflect the local climate.
Thank you Malcolm for The Ultimate Modern Desert House link.
I do understand where you are coming from when it comes to the outside environment, especially the desert.Frank Lloyd Wright has had a big influence out here. Regarding Desert House, to my unprofessional eye but experience in the southwest, this might be a home used mainly during the winter months for people who only come to the desert during that time. The canopy has been used very efficiently to protect the building itself from the sun adding a certain cooling but I wonder if it is used mainly to protect the building from the intense damage caused by the sun itself. It is usually used with mobile homes and used very well. Also for storage hay protection from the sun which might ignite otherwise. It almost looks like they found an abandoned metal building and built this under it.
I love the concept of the wall opening but to live/work in year round I question if it is practical from an air conditioning point. I am quite keen on modular construction and like this concept http://www.lakeflatoporchhouse.com/concept.htm
very much as we have been considering a dog run concept with small living but large working area. I find it interesting that the Lake Flato Porch Front house is a LEED building since it has all those windows.
I could not find out the cost of the Porch Front or other modules. I could do quite a rant regarding the absurdly high cost of modular homes and energy efficient mobile homes. $100,000 for a singlewide? Something is quite wrong there.
Still confused as to insulation vs. thermal mass but I think I just need to think about it more to get it. Thanks, Martin, for your patience.
Anneal, Lake Flato do exactly the type of regionally rooted architecture I was alluding to. You could't ask for a better starting point. Good luck with your build.
Hi Anneal,
I agree that Lake/Flato Architecture is wonderful to look at.
The project you linked to is a great example for a wealthy man's weekend home.
It is not a good model for "Affordable Comfort".
Sure...if you add enough PV this compound could be Net-Zero.
Don't let the LEED rating fool you.
LEED does not denote Energy Efficiency.
https://www.youtube.com/watch?v=mvCP3s7Xq48
Hi John.
I realize I could probably never afford an official “porch house”. I could not find their cost online so I am assuming it is high even though they say it isn’t. That is what the average person thinks now. Energy efficient is expensive so forget it. Attractive is expensive so forget it. I did not know LEED does not mean energy efficient. I am not looking for net zero. I would be over the moon if my total energy cost is (home+ office) max $100 a month at peak heat season. Lower is always better but…
We would love to find an energy efficient single wide to live in and an energy efficient double wide for our shop. Plus a garage or carport. Can they be regionally rooted architecture, too? That would be nice. Will I find something energy efficient I can afford? Probably not. Because energy efficient is expensive.
Hello Anneal,
If you are still interested in design/building a cost effective high performance energy efficient home, the Dept. of Energy (DOE) Build America Program has been doing extensive real world research in all climate zones including our southern Arizona hot/dry climate. You might want to explore the 2013 Build America Top Innovations for net zero ready homes: http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/ba_innovations_2-1-6_zero_energy_homes.pdf
Go to page 2 top left picture and read brief description on right about ARBI research on the home(Left)
The ARBI team, led by Davis Energy Group, worked with La Mirada Homes on this SIP prototype home in Tucson that uses an air-to-water heat pump to heat water for radiant floor heat and chill water for fan coil and radiant floor cooling.
There is a lot more published and (news) story info on this home.
Bottom line is that it attained Passive House performance (31.6kBtu/ft2-year, 38.1kWh/ft2-year is PH min threshold) with off-the-shelf design build features producing R34 SIP composite wall, R41SIP roof and R10 under slab. The structural and thermally isolated concrete floor is temperature controlled which results in controlled ambient temperatures hovering around 75F all the time.
Michael,
Thanks so much for the link. I'll check it out.
Southwest Desert Passive House!
Michael,
The air-to-water pump for cooling the slab is a really interesting way to go for this climate. And I love the idea of utilizing the slab to help with that. Also, congratulations on achieving Passive House performance (31.6kBtu/ft2-year, 38.1kWh/ft2-year is PH min threshold) with off-the-shelf design build features for the desert.
I have a few questions and forgive me if they are sort of stupid. Don’t know if this is the place to ask or not but anyway…
1. How much water does the system use per day? Or is it recycled antifreeze or something?
2. Does the system need to be inserted into the slab or can it be separate on top?
a. I know this may be a dumb question since it is the slab itself that regulates the cooling but I am concerned about leakage, repair, etc.
3. Does the floor have to stay cement exposed to be effective? Currently where we are renting it is stained cement floor and it is hard. Especially since we work and walk on it all day. I was looking into linoleum. Would that work with it?
4. I noticed your SIP’s are from Colorado? Is it worth the shipping expense? (I guess that would be a personal decision?)
5. After reading so much about roof overhang, I am very keen on that. If incorporated into the design would the SIP’s be able to be a lower R-value to save money?
6. I tried to find a cost comparison of the unit/install with traditional HVAC systems but could not find it. Doesn’t mean it isn’t there. My time is limited right now.
7. Does the ceiling height matter for comfort since heat rises?
I’ll probably think of more, but for now…
Anneal, Lake/Flato has this to say about anticipated costs:
"Current projections for hard building cost range around $200-250 per square foot."
Regarding slab cooling etc., bear in mind that complex mechanicals almost always cost more than good insulation and proper passive solar design. Especially in your climate. Keep It Simple.