Hello, I am designing a passive solar house. I need thermal mass in the floor in my south facing room but am trying to stay away from a 4" concrete slab. My house has a full basement so a concrete slab for this 14X29' piece would cost about 6K! Do you think that a thin concrete slab, with pex tubing that goes to a basement water tank that recirculate could work? The water act as the thermal mass since it is always changed with the cold water from the tank... Would it work as well as 4" or concrete? I don't  mind a thin slab since i can just beef up the joist, but the 4" slab is just too much for a DIY with the steel decking and...

 

I don't know if I'm clear, english isn't my first language. The goal in that is to allow the basement water tank to act as thermal mass in my south facing room floor.

 

thanks

 

Alexis

Alex, This is a very good question that has many factors to be considered. Water is a good medium for transferring heat (and cold ) from/to sources. The problem with using water as your primary storage of heat is the volume of water needed per square footage of desired heating space. using a hot water tank as the only means of thermal storage will drive an increased demand for traditional heating (oil,gas or electric ) to heat the water when you will want it most, in the evening , night and morning during the cool /cold months when your solar gain will be low..
I would recommend a strategy of supplementing the water storage with a thermal mass.

We're doing a passive solar with radiant heat in a 4" slab. We have no basement, so the slab can be on grade. I did a quick calculation at the beginning of the project, and agree with Ratman: the heat storage provided by a typical 4" slab is VERY difficult to match with water storage. I understand your worry about the $6K for the suspended slab. Have you considered gypcrete? I don't know much about it, but I believe it's much lighter than concrete, and the tubing can be embedded in it. Of course the lightness also works against you, since the mass of something (combined with aspects of the material itself) governs its ability to store heat. -- Jerry

I looked into gypcrete but i don't think it would be sufficient thermal mass...

I am getting estimates for steel floor joist and metal decking... I will keep you posted on this but I think it will be less expensive. I also looked at hambro/speedfloor and just the price for the joist was 3,5K!

Alexis,

Keep us posted as you move forward. I think the metal substructure is a good trade off and I would still suggest you use the theramal mass in the basement as a storage mass..

I might also mention for thought... Have you desided on the type of flooring you will be using? As you may know each material has a different COE and will move as your radiant floor changes temperature. If you are using a wood based flooring you must also think about the moisture levels during the heating and cooling months as wood is not only effected by temperature but also swelling and shrinking due to moisture.. 

 

FWIW, I think radiant heat works best with direct contact between the heated slab, and the air of the building/feet of the dwellers/etc. I lived for short periods in a building with radiant heat in a slab that was pretty much covered with carpeting. The carpet seriously impeded the flow of heat from the slab to the occupants, made the place cold in the mornings and hot in the afternoons (when the heat had finally migrated from the slab up through the carpet) and, I'm sure, increased the energy costs for the structure. On the other hand, a building I lived in for several years in the '50's had vinyl tile applied directly on the concrete, and worked well.