Best stone for thermal mass floor

Yeah so I'm still planning the future home... Thinking of the flooring right now, and want to incorporate passive solar as much as possible due to living on around the 55th degree N latitude. I have a faint memory of reading that lava rock has good thermal mass properties, but does anyone know which type of rock or sand that actually has the best thermal properties?

My thought right now is to lay a heat absorbant sand and then bamboo flooring on top of that. It really is crazy how much carbon is sequestered in bamboo... An alternative is hemp boards as these are grown locally. Hoping that heat from the sun will pass through a thin layer of wooden flooring and get absorbed in the sand to then be released slowly during the night.

Also, the standard here when using wooden flooring is to put a layer of foam plastic underneath to give the floor a soft "flex". Is there a good (biodegradable) alternative to that? I'm worrying that this layer, being full of air bubbles, would hinder the warmth from reaching the sand underneath...

I cannot answer your question . . . but

Besides a materials ability to 'conduct' (heat or cold) there is the important issue of whether or not it will be heated to a level where by it will then return a significant portion of that heat into the house.  Conductible materials can also be heat sinks, without proper planning they usually are.  So having conductible materials on outside walls or under the house could be problematic.

What's to stop your sand from receiving the house heat and then loosing it all to the ground beneath?  Those with building experience can comment on this better than I, but maybe you should place that insulation (of whatever type) beneath your sand and above your water barrier, in order to encourage the heat to release toward the floor/house.

Whatcha think?

Hi paddy82, with passive solar you want a material that heats up and stores heat. I don't think lava rock would work as it is not very dense. Mass is the key to thermal mass. A dense and dark material would work better. by adding  foam and flooring you will block the sun from contacting the thermal mass. Jami makes a good point, passive solar and termal mass does not work if you don't get enough sun to actually get a heat gain. Remember stone floors will also conduct a lot of cold from the ground , etc.

Jami:
I think you're very right. I have been pondering something which seems paradoxical to me; I have black stone tile on my englassed balcony but the freakin thing never gets hot, just warm in the height of summer. But on a sunny day in winter it is very very cold. Now I realize, as Castlerock says, the heat is absorbed but lost to the concrete under the tiles and "sucked out" by the cold outside. Ergo important to have a non temperature transmissive layer below the thermal absorbant.

Castlerock:
Lavarock isn't very dense but has bubbles in it, so per theory the argument is the heat is kept in these spaces... I don't know, maybe any sand would work, but if it's below a wooden flooring the color won't matter. I don't know if you should lay wooden flooring directly on sand, but how else would you do it if you want both?
A lot to think about

Paddy82 wrote:


Castlerock:
Lavarock isn't very dense but has bubbles in it, so per theory the argument is the heat is kept in these spaces... I don't know, maybe any sand would work, but if it's below a wooden flooring the color won't matter. I don't know if you should lay wooden flooring directly on sand, but how else would you do it if you want both?
A lot to think about

Air does not hold heat very well. My understanding is that lava rock works well as an insulator, not thermal mass. If you cover a thermal mass floor with any insulator, you will not get much solar gain. In your location, you would need good insulation under and on the edges of the floor and keep it dry. Then, use a fairly dense material like concrete, stone, cob, earthen, etc. Dark colors will help as the floor would absorb a wider band of light energy, light colors will reflect much of the light energy. Again, if you cover the floor with a insulating/light blocking layer like bamboo flooring, you will get little to no solar gain.