So I recently purchased a uniquely designed home in northern Vermont. It is a strange mix of contemporary/geodesic dome. The slab in question is part of a post and beam addition about 25x35 ft. From a brief phone call with the original owner who had a murky memory about the construction I gathered he designed to room as a large heat sink. The slab is at least 9" and possibly more in spots but here is where things get weird. The slab has a number of registers on the floor a few around the edges and then two 8 inch round holes that extend at least 3 feet deep or more very close to the center of the room. When I bought the house I assumed that there was ducting placed down before the pour and that the center round holes with a few feet of pipe on them would from draft pull the cold air down the registers and up the 8 inch stacks. Well today I pulled out the years of junk that had collected in the holes and low and behold its not like anything I have heard of before. Starting with the outer registers after getting the debris out I found no ducting pipe nothing just lots of med size rocks 3 to 6 inches more or less and a cool draft blowing upward out of the hole. I assume this area had been sealed of for 20 plus years and could imagine the plague or who knows what billowing out of there. Now if thats not strange enough I moved to the round holes in the center of the room. After digging out a full arms length deep of debris I came to a metal grate it was basically a 8 inch stove pipe a few feet long with a heavy screen at the end set in the concrete. There are 2 of the holes about 15 inches apart with a masonry chimney in the middle. Best I can tell without a telescopic camera is that there is some sort of poured box type area below a few feet square and there must be (god there must be!) a center support for the chimney. So today talking to the owner he tells me he had dug out three feet and filled it with river rock before pouring the slab about 15 loads he says! So with this new info it looks like the idea was to pull the air under the warm slab and rocks and back into the room. This has got me thinking of things like radon mold mildew etc as he did put down plastic before the pour but the air circulates under that and is in contact with the ground. He told me this was yet another of his unfinished ideas and he never got to complete it. I have heard of tubes or ductwork in the slab it self but never this set up. Anyone have thoughts or seen anything like this. Sorry for the long post hard to put in words.

First, do a radon test to see what your levels are. If it is really low, you have options. If not, you'll want to seal up all these holes and caulk any cracks in the slab, and retest. Rock beds with various methods of moving air into and out of were pretty common in passive solar homes back in "the day". Some worked, some didn't. Most that I'm aware of were designed by the owner-builders who had ideas but limited if any experience. I built a rock bed with fans, etc designed by an engineer; same people still live there but haven't used it in decades. One interesting question is whether the air coming out of the rock bed is room temperature (60's, 70's) or closer to earth temps (50's).

Yes I plan to do a test asap and for the air temp it seems about room temp a bit less maybe its kinda hard to tell by feel in the room there is a large soapstone stove running and it heats the floor very very well. I will take some readings overnight and in the morn (forcast is for 21deg. tonight). Most of the walls are covered in glass to heat the floor so im very interested to see the temps with a day without sun. Thinking about it though there could be a plastic sheet under the rock bed as well im just not sure but it does seems pretty dry below from what I can tell. Tempted to bust out one the grates and dropping a camera down in! I will report the temps tomorrow.

You have a rockbed storage system that was meant to carry over seasonal heat into the winter. The idea was to use the addition through the summer and fall as a solar collector, raising the temperature of the stones by blowing room air through them and tapping that heat via radiation in the fall and winter. They fell out of favor for a number of reasons. Heat storage was limited; without insulation, much of it traveled down rather than up. The other problem was mold. Blowing humid air over cool stones isn't a great idea. I'd buy a spore test kit as well as a radon kit.

What you have now is a floor that will defy pretty much every and any manner of heating. The sun is too weak in northern Vermont to make a dent in all of that mass. You'll get some seasonal time shift at least. It will be at its warmest now, and coldest next March, give or take. Slippers not optional.

7:20pm
edge 57.8 to 61.2
center 62.6

The air coming out is very very slight but noticeable I cant seem to figure out where the air is coming from no obvious cracks in foundation etc but there are a few places where there is a gap between the slab and footing where you can feel some coming out as well. Keep in mind that I would think these temps might be low as its been sealed up for years prior to today. I was able to hook up a small fan to the center holes and it would push or pull from the outer registers.

Well for the sun although I have not been here through the winter yet. So far it seems with the way the rooms windows and many sliding glass doors are situated the lower then sun gets in the winter the more floor area it will hit. In the middle of summer it only comes in a few feet on the floor where as now it covers much much more. Furthermore slippers are needed (to hot!) within ten feet of the stove in the center even for a while after the stove cools a bit makes for nice mornings after neglecting the stove overnight. And if it helps the slab is painted a brownish color and seems to absorb the sun pretty well. I was expecting a musty odor once I opened it all up but was pretty odor free surprisingly.

Just back of the envelope, 25x35x3=2635 cubic feet x 150 pounds/cubic foot =~400,000 pounds. At concrete's storage efficiency of 20 percent, you'd need 2 million btus to raise the mass by one degree fahrenheit. The previous owner probably did not do the math.

the stove cools a bit makes for nice mornings after neglecting the stove overnight.

IMO, blowing stove heat down there is the best use for the thermal mass. But figure 80kbtu/degree for the rocks, not 2 million.

Well, no. If it was 400k pounds of water it would take 400k btu/degree, since a btu is the energy necessary to raise a pound of water by one degree F. But masonry is roughly five times less effective as a heat store so, yes, six zeroes. Even then, if there is no insulation around the stem walls or under the rocks, you won't get back what you put in.

According to the OP, the slab is 25’ x 35’ and is at least 9” thick. 9” divided by 12”/foot is 0.75’ thick. 25’ x 35’ x 0.75’ is 656 cubic feet. The density of concrete is 150 lbs per cubic foot. 150 lbs x 656 cubic feet is 98,400 lbs. The specific heat of concrete is 0.22 BTU/lb-deg F. 98,400 lbs x 0.22 BTU/lb-deg F is 21,648 BTU/deg F. Being able to read and do simple math are really worthwhile skills…even for a GC.

Bob/Todd, you are exactly right.  This old style of passive solar slab is potentially unsafe because of the radon and mold risk.

Todd did the calculation wrong (less effective means less heat to raise the temp, not more) and sailaway numbers are OK for the slab, but not the 3' of rocks.

You won't get mold if you seal it up and dehumidify it in the summer.

My bad. Thanks for fixing my mistake. I wish I was wrong about the weak sun in the Northeast. My solar slab does not heat too quickly -- more like not enough.

Todd, if you don’t mind me asking and providing, what are the specifics for your passive solar design (latitude, solar fenestration orientation angle relative to true south, solar fenestration SHGC, solar fenestration area, height of roof overhang above solar fenestration, depth of roof overhang from solar fenestration, height of solar fenestration above floor, exposed slab area, slab thickness, and R-value of slab insulation)?  Do you have any local terrain obstacles (e.g., trees, mountains, etc) reducing your solar fenestration exposure?  If so, what do you estimate the azimuth angle(s) and solar altitude blockage angle(s) to be?

Wow thanks for the responses. I kinda think I may just tape all the holes back up for now (other heating issues to deal with). After the first temp reading i dropped the thermometer down a bit deeper and since the other night it has stayed at 60.9 or 60.8 thats taken about three feet below the slab. Either way I think the slab does well taking up the suns heat sure it cold be more but way better than wood floors etc.

I used google earth to model insolation in the great room, so I have as much coverage as possible on 1k sf of 4 inch slab covered with a dark green matte finish porcelain tile. The slab sits on 4 inches of xps and inside Logix ICF stem walls.

The rest isn't ideal. The house is positioned 15 degrees west of solar south by necessity. Latitude= 39 degrees. The house is surrounded by trees and mostly shaded until mid Nov. The bare trunks and branches still shade the windows by some unknowable fraction. A mountain to the west of us takes out the last hour of sunlight. So I put in 10 4x6 casement windows, or 240 sf of glass, to compensate. SHGC=60.

The late start doesn't help. With no hvac, the slab was in the 50s before the leaves fell. With a mini split I can hold it in the 60s. But the problem is cloud cover. According to UCLA's HEED software, IIRC, the average daily insolation in Dec, through all that glass is 7k btu/hr. I knew this when I built the house -- for its Frank Lloyd Wright inspired architecture rather than passive solar. I have radiant in the slab, and my wood stove boiler should be running next month. So I should get some benefit from Jan on when the skies are clearer. But the lesson so far is don't fall behind the curve in a high mass house.

I don't want to hijack this thread, but if you PMed me the specifics, just for grins I would be happy to run the numbers to see what we forecast for your heat gain during a clear sky Winter irradiance period. We can also determine the amount of heat stored and released by the slab after the irradiance period along with forecasting the hourly slab temps to allow comparison/validation to your actual slab temps. However, with your hydronic radiant floor heating system operating, we couldn't directly compare your actual slab temps to this forecast.

There really isn't much you can do about cloudy days if you don't have stored surplus solar heat available to use during these periods. How does your passive solar heating perform on clear sky Winter days? Ideally you shouldn't need any supplemental heating then, but you shouldn't experience overheating either. Your SHGC and R20ish insulation are very good. Having the building oriented 15 degrees west with a mountain also to the west is not good, but there wasn't anything you could do about that. If your passive solar doesn't provide adequate heating on clear sky Winter days, I would surmise that your 240 sf of window area is likely inadequate to offset your building orientation and your terrain obstacles. You may be able to reduce the blockage by the trees and do some ground cover landscaping to increase the ground reflectance coefficient.

Thanks but my feet are a pretty reliable indicator. Harrisburg, Pa in Dec. has maximum cloud cover of 86 percent on Dec. 27 according to Weatherspark, and never less than 80 percent that month. One year, I saw the sun twice in Dec. Not to discourage the OP, but Burlington Vt is worse at max 92 percent in Dec, and not less than 90 percent that month. The Northeast is strictly passive solar low mass country. Again, I knew that but the view is worth it, and wood fired radiant with free wood is pretty sweet too.

Ground reflection isn't in the cards for me either, because it's straight down hill 300 feet -- can barely walk it -- to Friends Creek. Naturally, there is a Friends Creek effect, localized fog in fall and spring.

Well, it sounds like you have a very lovely place. About the only way to beat hydronic radiant floor heating via a renewable onsite wood source is via a renewable onsite hydroelectric source, which is our personal favorite. Nevertheless, we still love onsite wood and our personal favorite for using it is via a masonry heater that provides long duration radiant heat with mid 90s fuel efficiency with near zero air pollution, and also has a baking oven that is available about 6 hours/day when the heater is used to supplement the passive solar and hydronic radiant floor heating. My husband prefers to fly with just the passive solar and hydronic radiant floor heating. I prefer to back off on the hydronic radiant floor heating so as to just keep the floor warm without significantly supplementing the passive solar heating, and instead use the masonry heater to supplement the passive solar heating. Usually just 10-15 pounds of wood via a single 60 minute duration burn does the job each day.