Hi all. I've read a lot of the other threads but my situation is a little different. Any opinions would be great.

I planning a 40x60x14 workshop in the Virginia mountains.  The building will be sunk in the ground 6 or 7 feet and the extra fill will be used to backfill the north and south wall to about 9 or 10 feet. The East and West sides will be more like a walk-out basement with some back fill on the corners. I'm planning on ICF walls and I haven't figured out the roof yet.

For the radiant floor heat,  I'm thinking 1/2" every 12" or 5/8" every 18". The floor will ultimately be heated by solar panels. A wood stove will be the 'emergency' heat for the building.

The slab is going to be on 8" crushed stone, vapor barrier and then insulation. For slab insulation I was going to use 2" under the slab for the first 4 feet and then 1" for another 4 feet. I was planning on not using insulation in the center. I'm thinking this will help heatsink the slab to the ground temp in the hot summer months but I'm still up in the air. The building will be surrounded by red clay soil by the way.  As near as I can figure, the ground temp here is 57 degrees with a 7 degree swing depending on season. I'd rather the building be cool in the summer naturally but I still want to be able to stand on the slab without by feet going numb .

Around the outside of the walls, I was considering adding some horizontal insulation but I'm not sure if it's necessary since it will be several feet underground.

I'm going to have to excavate at least 5' around the walls. Would it be worth it to bury some tubing around the outside perimeter to help bring some cooling from outside to either the floor or some type of fanned radiator?


What do you guys think?

Kirk

Stick with 12" o.c... spacing is much more important for water temperature consideration than pipe size, and if you're thinking solar, the name of the game is "how low can you go" with water temps.

Walls should be insulated. Under slab insulation should be continuous.

Playing with cooling loops is fine, but then you need antifreeze for the winter.

I pretty much agree with Rob.
Below the frost line there should be no variance in temperature. And that's a key. The soil piled around the walls will freeze(or at least become cooler than the ground temperature) so the main value would be to stop air infiltration. So, there is value, but the expectation of 'warmth' is indeed questionable.

I would definitely advise adding interior insulation to reduce your heat loss.

Thank for the reply guys.

Just so I can clarify. When you guys mention insulation, do you mean extra indoor insulation?? I'm hoping to use ICFs for the walls, are you suggesting even more insulation?

As far as the temperature variance goes, I based my info from this site:
http://www.geo4va.vt.edu/A1/A1.htm#A1Sec2

I live in a zone where the temperature is 57 degrees year round at 30 feet below ground. Based on figure 3, the temperature at 8 feet of depth should fluctuate about 7 degrees during the year.  I realize that digging down to that depth and adding a building is going to change that temp swing, but I have no clue how much. I know it depends a lot on the insulation below the slab though.

My main purpose for backfilling up the walls is to use the thermal mass to minimize intra-day temperature changes and help heat sink the building during the summer, but more importantly for water runoff. (I also need to get rid of the dirt somewhere ;-)

Another concern is the slab freezing. This is a shop without any stable heating system. I need to make sure that during an abnormal cold spell without any heat the slab and the hydronic system in it won't freeze.  There's a trade-off somewhere between insulating the slab to allow better heating and heat sinking the center of the slab so it resists freezing. I just don't know where that trade-off is.

Panelcrafter: There's no expectation of warmth. I am trying to design something that will stay close to under-ground temperature throught the year with minimal intra-day fluctuations. In the winter if I can keep the slab at 50 degrees naturally and then possibly raise it to 60 degrees with some solar panels, that would be great.  I would also be able to run the solar panels cooler like Rob said.

Summer is a different story though. The best way I can think of to naturally cool the building is to use the underground earth temperature and possibly several runs of cooling tubes around the far perimeter.  Any cooling lines would be 8 to 10 feet underground so there's zero chance of freezing at this location.  I would trench the tubes into the ground even further than the foundation and possibly even use that trench as a secondary foundation drain.

Oil hit $97 a barrel yesterday with no end in site. I cannot imagine what it will be in 20 years. I want to limit the HVAC energy use of his building to running small electric pumps.  I don't expect it to be as comfortable as a home but I do want it to be comfortable in work clothing without using fossil fuels.

All this is still in the planning/wishing stage though. Any words of wisdom would be more than appreciated.

I realize this post has strayed from a strictly radiant slab topic. Should I post in the general forum instead or ask to have the thread moved?

Kirk
PS Is there any software that can help model this?

Well if you are going to have ICF walls, you could use precast hollow core concrete panels for the roof. Then add 3'-5' of soil and the garage would essentially be underground. It would be like a bunker, and no doubt fairly comfotable year round w/o using any of those nasty oil products.