Heat loss through concrete slab foundations

[Amanda_931]

75^oF slab will feel cold.

Pity, because floors are so much easier to take care of if you are barefoot (or slippered) any time you are inside.

The cheapest of the off-grid electricity is usually one of the micro-hydro systems--some are very simple indeed.  And they do weird things like 24 hours a day, seven days a week, 365 (or 366) days a year.  If you're lucky you almost never need the generator.  If you're like a friend in the next county you spend a lot of time trying to get your system that somebody cobbled together ten years ago to work at all.

[JRR]

A little "heads up" for folks considering pouring slabs atop foam insulation:

The compressive strength of most of the foam panels that can be gathered at the local big box store will be in the neighborhood of 15 psi or so ... only suitable for walls, etc., ... not slabs.

However, higher strength foam panels are available in various compressive strengths up to 100 psi.  You can get info at the Owens Corning ("Formular") website.

[Kelly Keilwitz(Guest)]

Hi Neil,
Kelly Keilwitz with Whidbey Sun & Wind, here (thanks for pointing me here, John).
As a geotechnical engineer and renewable energy contractor the house you describe is my dream home that I hope to build some day. I also love the low-profile aesthetics of an earth-sheltered home like you describe.

First, from a geotech viewpoint, understand that a great deal of your budget - maybe 30% to 40% could be in excavation, drainage and concrete (or other appropriate foundation wall) work, and the associated engineering and permitting costs. If you want to go the full-monte you could add on (considerably) more for a reinforced concrete and/or living roof.

Now, solar, passive: You probably already are aware, but your ridge should be facing south with no shading from tall trees

In deciding to go off-grid you need to first consider why you're doing it. If it's to be the most cost-effective, you must consider your energy use and habits, willingness to be actively involved with your energy system, and level of convienience and comfort you want (vs how much you want to spend on the energy system). As a rough guideline it's generally cost effective to go off-grid if you live more than 1/3 mile from power.

Electricity source is the main need for an offgrid home, as thermal needs can be met with fossil fools (oops!)    Solar PV should be your main source, as there are no moving parts, 25 yr warranties and good incentives available. In the wintertime you'll need other production source(s), like fuel genny, wind genny, etc. If you're going to be away a lot in the winter, then a backup fuel genny is the best (most cost-effective) solution. If you have a wind resource, and can put up a tower tall enough to get the wind generator a minimum of 30 feet above the nearest tree within 500 feet (including tree growth for the next 20 years), then wind power is an excellent source to create a "hybrid" system to complement your PV's. As Amanda mentioned the best off-grid resource is micro-hydro, but it is highly site and stream specific.

If your energy requirements are small and you don't mind starting the fuel generator every now and then when it's cloudy for a few days, and do some of the work yourself you should easily be able to install a power system for less than the cost of running power to your site - and have no power bill! You definitely need to think bigger than "a couple of solar panels", though! In considering this cost remember that you're paying up-front for 15 to 20 years of power.

For space-heating, consider solar-thermal and woodstove heat exchanger input into a large water storage tank (usually your domestic water source) with heat exchangers feeding your radiant floor slab. I'm doing exactly this in my (early 70's energy dumb, but sustainably retrofitted) home. Wood IS a renewable resource, as long as you have a woodlot (or live on a tree farm)   !

Good luck!

-Kelly

[glenn-k]

Thanks for popping in here with that, Kelly.  We hope to see more of you here.

[jraabe]

Good to see you here Kelly. Thanks for that thoughtful overview.

Kelly and I have worked together on some past projects including a volunteer effort to solarize and make energy efficient an old community building that was put up by farmers in the 1920's. What was the most cost effective thing we did to an old leaky building that was not occupied and heated all the time? Change out all the lights to CFL and LED bulbs! Really made a difference in the power bill.

[glenn-k]

Have you found LED assemblies that make decent room lights John, or just smaller area lighting?

[Kelly_Keilwitz]

Thanks for the feedback.
This looks like a fun place to hang out. Unfortunately we only have VERY slow dialup available to us here, and are on one line, so I'm not able to spend the time on such forums. We're looking into getting a wireless feed from a neighbor (~1/2 mi) who has satellite broadband - that would help.

I AM interested in the availability and affordability of (white) LED for spot and area lighting, too. We currently use cheap, colored Christmas light LED strings around the ceiling-wall edges of some rooms for low-level area lighting, and I've wired in ceiling outlets in our new addition for just this purpose.
As I understand it, white LED's are so expensive because of the technology needed to make true white light and because that technology is under patent. There's also an issue with using AC to power the LED's, as the diodes can only use half of the wave form. They operate more efficiently on DC. A wall-wart (converter) can be used, but that lowers the efficiency, too.

[glenn-k]

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