Not sure if this belongs here or in the solar section but i know some of you have experience with this so here goes.  I'm building a small 20 by 28 foot three story house with 6 inch sips for the top two floors and a partially back filled Superior walls first floor/daylight basement. The superior walls sit on a 16 inch or =/- bed of 78m gravel and i am planning on poring a 4 inch slab with pex as the floor on the first floor and either staple up or gypcreted pex on the 2nd and third floor. My Superior walls are now up and to proceed i must complete my pour witch brings me to my question. We are planning on domestic solar hw and radiant.  Would it make sense for such a small house to size up my solar to supliment my radiant.  If so, should i now excavate some gravel, say one foot and place foam and sand and a pex "shunt" to have a hi mass storage under the slab.  I dont have much room for a large water tank and thought that a 14' by 24' by 12" bed of sand might help. I know it is much smaller than typical but i cannot undermine my superior walls by making it any larger.  I have learned so much from this forum over the past year or so and i am open to all sugestions. I have read "radiant Floor Heating" By  Dodge Woodson which was somewhat helpful and "Solar Water Heating" by Bob Ramlow from MOTHER EARTH NEWS   that help a ton.  My peoject is near Charlotte NC.    Joe lstiburek"s SIP book lists Charlotte as 3341 heating degree days and 1582 cooling. house will also have heat pump.   Thanks ahead of time for any advice or help.   Michael

Hi Brawler,

I have taken many classes that Bob Ramlow has taught on the subject of solar space heating and he would recommend that no tubing be in the slab for your lowest level. Instead insulate sides and bottom of a 24" deep hole encompassing as much as area as you can and fill it with sand that is compacted every 6". The only tubing is the shunt load tubing that is buried 18" below the slab. The idea is that during the latter summer months and early fall months you can heat up that mass with the solar. The mass will radiate heat at a somewhat uncontrolled rate until it is gone or replenished.

Mike Dunn

I would have to disagree with the use of sand as a good mass medium. I worked with a post and beam builder in the early
80's that featured a 4ft under slab thermal mass of sand. After poor results they switched to large stone over cmu's. The reasons were, the sand doesn't accept or give off heat very easily, and the sand was a b.... to dry. Simple analogy, go to the beach on a 95 degree day, sand will burn your feet, until you wiggle your feet around to get to the cooler sand below the surface.

Thanks for the responses guys. Within the next week or so i'm going to have to take some concrete action. Im just not sure that such a small bed of sand would hold suficient heat to warrant the effort. Wet sand would be ok, maybe even better since there will be a vapor barrier between the concrete and sand. If anyone has a recomendation for a company they have delt with that supplies radiant/solar for DIY i would appreciate that info. Thanks again.

Does anyone know if there are any structural issues with cycling the temperature of the subsoil beneath the foundation up and down?

I am also considering building an interseasonal heat store by running the summer shunt heat beneath the house, as where I am located, I am shy at least 1/3 to 2/3 of the required heat input for at least November through February due to lots of cloud cover.

I found the following UK link (http://www.icax.co.uk/) which covers about the same idea, though theirs relies on dry soil as the insulator, rather than something put into the ground as part of building the foundation.  I am still trying to figure out if this is even vaguely economic to do, though.