Sunday, September 15, 2013

Cabin at the Cave


Cabin from Below

The cabin is sited against large rocks that are part of a limestone outcrop.  Fifty feet below the cabin is a cave opening that leads to a continuously flowing underground spring with a waterfall and twenty-foot diameter pool. The site was selected for the water and geothermal cooling potential our cave offered, but it was a compromise location.  A location below the cave would have allowed natural flow of water and cold air down to the cabin, but the 70-degree slope down there was more of a challenge than we wanted.

Cabin Site

The building was constructed to be a workshop and portable sawmill shelter with a wood-heated loft for warmth on cold days and sleeping during intervals of sawmill use and home construction.  Lumber from the mill would be used to build a small home adjacent to the workshop.  The home would use geothermal heating and cooling from the cave, pending proof of concept work at the workshop. Because it looks pretty cool, friends and neighbors refer to the workshop as a cabin, and we’ve pretty much adopted that term for its description.

Cave air and water in our area averages 60 degrees on an annual basis, but with the main cavern roof only ten feet below the surface, the cave should have a peak temperature around 70 degrees six months after the summer temperature peak (July-August), and a minimum temperature around 50 degrees six months after the lowest temperature (January-February). We've confirmed that temperatures vary in this general range, but we make only infrequent entries to not disturb the bats any more than necessary, so we have very limited data. Our measurements also show the floor to be much cooler than the top of the cavern thirty feet above during cold weather, due to cold air coming down from the entry about 15 feet above floor level.

Water condenses in the cave during the summer (it drips continuously in there) to provide dehumidified air when needed for cooling, and it adds moisture in the winter when air in the cabin would be dry. Deeper caves in our area stay much closer to the annual average 60 degrees the year around and don't vary much from the average, compared to our twenty degree range.
In the picture below, the cave mouth is just out of sight below the bottom point of the triangular rock that’s directly in line with the cabin in this photo. The rock (a triangle with twelve foot sides and two to three feet thick) appears to be in position to slide down over the cave entry at the slightest provocation, but probably would require an earthquake.

Cave Location - At Point of Rock Below Cabin
Despite the fact that the cabin site is on a 30-degree slope, we were able to build a winding ATV trail to it that maintains a reasonable 12% grade.   When we began leveling the site, we found that the outcrop behind the cabin began to slope forward once we got about three feet down.  We had to build up the lower foundation a couple feet higher than planned, but it worked out OK.  Keeping the back of the cabin up close to the rocks provides a convenient way to access both the cabin roof and the loft – it’s only a step across.

ATV Trail to Site
The soil we dug out below the outcrop was screened to provide gravel for drainage beneath the floor and soil to build up a terrace below the cabin for a walkway.  We were a bit short of gravel, but found a good gravel bank while hand-digging the ATV trail, so we were able to utilize that.
Our design gives us an 8 by 12-foot sleeping loft with a wood stove for heat in the coldest weather and a fully open 12 by 16-foot lower level.  The loft is accessed by a wooden ladder and has a south-facing 3.5 by 5.5-foot double pane window for light and winter solar gain. We’ve hinged the window at the top side to open for ventilation.  Two windows the same size are mounted in the south wall of the lower level, but they are fixed in place.

The perpendicular roofs allow space at the high edge of the lower roof for ladder access to the loft area.  Both roofs have 3/12 slope and are fiber cement over plywood, sealed with acrylic elastomer.  Siding also is fiber cement for its fire-resistant properties. The lower side of the loft is five feet high and the high side is seven.  The lower level has a rear wall of eight feet and the front wall is eleven feet high. Ceiling below the loft is seven feet high, but the other half has a full cathedral ceiling.

Cabin Telephoto from Opposite Hillside
If we were to do it over again, we’d make both levels a foot higher: upstairs for added head room and downstairs for easier loft access (Our entry via ladder is only three and a half feet high). There is to be a full-sized door into the loft from outside: its floor is at the same level as the top of the outcrop it is built against. During construction, the wall there was not closed until the roof was finished and building materials came straight in off the rock. 
The main door is five feet wide by eight feet tall, designed that large to allow room for our portable sawmill to be housed inside with the rails extending outside. However, after three break-ins, we nixed the idea of keeping any expensive equipment there.  We like the large door for the light it provides when it’s open, though. Screens are not necessary here.  The bat population in the cave keeps flying insects at a very low level.

Cabin from Above

The outer door to the loft, additional siding and inside finish work were put on hold after the third break-in. We were kept busy making repairs, beefing up the door by sheathing it with cement backer-board and hardening up other vulnerable areas. Soon after the third break-in, ice storm damage to trees closed off road access to the cabin. We’ve had no more break-ins, but the only access was by foot until we finally cleared the access road this spring.
Efficient use of the cave’s waterfall for the water wheel is complicated by the water being spread over a wide area as it falls from the rounded edge of the rock from twenty feet above.  

Water falling from Above
We really need to access the water source to be able to utilize the water to power our water wheel.  Once there, it might be possible to install a pipe to collect water.   Hopefully, the twenty feet of head we’d have for the water wheel would let us move both air and water with it. Unfortunately, the slimy wet concave surface that leads to the water shelf is far beyond our climbing skills, and we’ve been unable to get a ladder in there that’s long enough to access the area in a safe manner.

We've discarded the idea of a funnel system because although minimum flow is only about one gallon per minute, after a hard rain, we've observed twenty or thirty gallons a minute.  It really roars! A normal flow of a couple gallons a minute can be seen against the black shadow on the photo’s left side near the top, just as it begins to fall from the curved edge of the rock at top center.  The next photo shows splashes in the pool below over about a two-foot circle.
Water Hitting Pool Surface

Below is a wide angle (thus distorted) shot from across the cavern toward the waterfall source, seen here as stone coated dark black on its wet upper surface. We suspect the black coating may be lead sulfide – water in this area is high in lead, so we use a purification filter designed to remove lead for our drinking water. Above the black rock it looks like six or so feet to the ceiling, and a passage may extend to the left.  The ceiling, as best we can determine without sophisticated survey equipment, is directly underneath the cabin.  As high as the ceiling is, our calculations suggest there’s roughly ten feet of solid rock between the cabin floor and the cavern.

Falls Source Area - 20 Feet Up

Our break-ins have pretty well eliminated the idea of a home there, but we still hope to complete the cabin and fully test our ability to deliver 50-degree air to cool in summer and warm it with 70 degree air in winter, as well as pump water by means other than an electric pump. We may even convert the cabin into mini-home someday.  We’ll get to the water source somehow.
Best wishes,