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Passive design in South Carolina  RSS feed

 
D Brown
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Hello everyone,

I am planning a (probably) cordwood home in upstate south carolina. I want to incorporate as much passive solar design as I can. I am wondering how much my location changes things. Most of the info found online (Lots of south facing windows, block the North wind) is from people in the colder climates. I do want to limit my heating needs, but I also don't want a house that will cook me all summer.

I'm leaning towards a single story round cordwood home with a crawl space. May go with a 2 story. May do partial earth bermed. Not 100% sure on anything at this point which is why I'm trying to get as much info from you all as I can!

Advice?

Thank you!
 
John Elliott
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Welcome to Permies, D!

Even though we are in a warmer climate, the heating load in the winter can be two or three times the cooling requirement in the summer. I have a house that was built with absolutely no solar design, active or passive, and I don't use a whole lot of energy for space heating and cooling, so even some modest passive elements are going to help you out.

If you have lots of shade trees on the WNW of the house, that will keep down the A/C load in the summer, and south facing windows will help a lot in the winter. Don't forget the porch in front of the south facing windows, that is the one bit of design that I have that actually is passive solar (although I'm sure it's coincidence, not intentional).

One thing that I would suggest is to work some evaporative cooling into your design. Although we can't have "swamp coolers" like they do out in the much drier west, remember that each gallon of water evaporating off your roof is 8000 BTUs less of demand on your A/C system. I go out two or three times a day during the hottest part of the summer to spray the roof. If I get motivated enough, I might automate this, like having sprinklers on the crest of the roof, and every two hours they are on a timer and spray a couple of gallons.

As for heating in the winter, what you can't get from solar gain through south facing windows, I'm sure you could make up with a small rocket mass heater.
 
D Brown
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Thank you John!
Where are you located?

Is there a formula for the ideal amount of overhang? I plan on having a wrap around porch on all sides. Since I'm going to build with cordwood and need to make sure water stays away, I figured the larger the overhang, the better. But, at what point do I loose the possibility for solar gain in the winter?

Are there any other important passive design aspects to consider in our area?
 
Brian Knight
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Location: Asheville NC
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D brown, welcome to the fray, we are glad to have you. I agree with John that Passive Solar makes sense in your climate. As for heating vs cooling loads I think they are close to equal in your climate which energy code wise, is building zone 3 but you are close to zone 4 and would consider those code minimums if good passive solar performance is desired.

I dont understand you and John's comments on the south covered porch. Overhangs for your latitude should be roughly 18" deep 12" above the glass. http://www.susdesign.com/overhang/

Wrap around covered porches do not belong on sun facing sides of passive solar designs. I would also point out that covered porches have the effect of considerably darkening any windows to interior rooms underneath and that people rarely make use out of a fully wrapped around porch. East and West are excellent for covered porches. South and North not so much. Obviously, design in this area is largely personal.

I would suggest reading some of the other threads here on this subject but I do offer my two usual major things that people often miss out on with good passive solar design: 1.Airtightness 2.International Code insulation minimums or better. A non-passive solar home that pays strict attention to airtightness and insulation can easily outperform a passive solar design that doesnt, especially in a mixed-humid climate like ours.

Partially earth bermed could be a great asset if you do the right drainage and moisture management details. I strongly suggest you avoid a vented crawlspace like the plague.

Which leads to two very important questions: What is your intended heated square footage and what are the goals and design criteria making you lean towards cordwood walls?



 
D Brown
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Thank you Brian,

So is the "roughly 18" deep 12" above the glass" for the south side or the same on all sides?

I am familiar with the importance of air sealing, and something that I think must be thoroughly planned in a cordwood home.
We are leaning towards cordwood because:
it is efficient - I'm planning on a thick wall with dense packed infill or 2 walls with the inside of the first wall spray foamed
suitable for our area - seems to handle the temp and humidity well
durable
not as labor intensive as some of the other alternative methods - I know it is very labor intensive, but still easier then DIY log home or filling tires.
and it can be done fairly cheaply

The house will be about 1500-2000 square feet. Possible 1 round building, maybe 2 connected buildings.

I am not a fan of having a crawlspace! I love having a basement, but they are expensive to build. The only reason I'm considering a crawlspace is so that I can run the HVAC through the floor since I plan on using whole log beams and tongue & grove roof decking. I don't want to ruin the look by having my HVAC running across the ceiling. I had though that If I went partially earth bermed, rather then having than backing the earth against the wall, I could leave a cavity in there. A) French drain behind the wall would eliminate the possibility of moisture seeping through the wall B) I could then run my HVAN through the back wall. What do you think?

Speaking of, if I went partial earth sheltered, it should be the north side, right?

Thanks again
 
Brian Knight
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Location: Asheville NC
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Do you have the site? The best passive sites slope south but smart design can make it happen regardless. The "berm" should be uphill if theres a slope.

Your HVAC plans sound sketchy. If you do an un-vented crawl, its more accepted for HVAC location. You can always build soffits for duct runs. Keep HVAC out of un-conditioned spaces. I would consider PTAC or mini-split heat pumps with an open plan.

Use the overhang tool for your specific latitude, window size, and orientation. If you do two stories, both need overhangs on the south. Other sides can be whatever but more will protect your walls better, a major concern to me especially with window flashing and cordwood. Try to eliminate west windows altogether and as much east and north as possible.

I think the cordwood could be enough insulation with the right details but have to wonder about airtightness. Havent researched it very much, but can imagine cordwood having horrendous air leakage if not heavily plastered on one side. Anyone know of the best cordwood blower door test?
 
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