Gravel vs. Marble for Storing and Radiating Heat

I am planning on building a passive solar home and am considering a gravel bed for heat storage during the winter. My understanding is that gravel is "efficient" in that it heats up quickly during sunny days, which I believe also means that it will radiate out its heat quickly as well. To extend gravel bed heat storage over a greater number of sunless days, deeper gravel beds are needed. So I thought of marble as an alternative to deeper gravel beds. I believe marble takes longer to heat up ("Why does marble always feel cool?", etc.) but also longer to cool down, meaning it would store heat for more sunless days. Is this correct? Would it make sense to have TWO beds - one gravel and one marble - which would complement each other in terms of thermal properties? I have not been able to find any literature about this anywhere and I don't understand the thermal conductivity equations. Thank you all for any help.

Howdy Bill , welcome to permies !

Might take a while to read through it but here is some info.

http://www.eolss.net/ebooks/Sample%20Chapters/C08/E3-14-02-00.pdf

http://www.ehow.com/info_8279095_rocks-absorb-heat.html

And there were several sites that talked about using electrically heated marble, so there must be something good about using marble !

http://www.radiatorplus.ie/default2.asp?active_page_id=81

Thank you for your prompt and very useful input, Miles. I had read the UNESCO document and am STILL trying to work through the formulae to figure out how much stone I will need. I had not seen the "Which Rocks Absorb Heat" article, which is clearer than others I have read. And I will look further into marble heaters! What I like about gravel (and marble?) storage is that the heating part can be totally passive, and the stone can be used for cooling too.

The "Which Rocks Absorb Heat" article mentions that two types of rocks may be best for a given application. That is precisely what I am trying to explore, but I can't find anything about good combinations, or even where such a configuration has been tried, if anywhere. That makes this search fun. The concept of two types of stone with different thermal characteristics makes good intuitive sense to me, but I struggle with interpreting the tables showing material density, heat capacity, thermal conductivity, etc. Are there any Science Teachers out there who can help?

Thank you again, Miles. I was very pleasantly surprised to see a notification from Permies that a reply had been posted!

Your welcome Bill, I wish I could have given you more help with the equations. Maybe someone else will chime in when they have the time. Lots of folks here are pretty busy in their gardans and such, this time of year. In the mean time be sure to take a look around at some other threads. Maybe you will run acrss an old conversation about some of the same things.

Howdy Bill Iam never too busy to tell people how much they are wasting their time! Iam only half joking of course. I think its great you are going passive solar and wish more people would as well.

To put it a nicer way, take all the time, money and effort that you would be spending on extra thermal mass and put it into air sealing, continuous insulation and possibly windows. Most of the science and experiments into thermal storage have played out since the 70s and 80s when people first started figuring this stuff out. Do a 4" concrete slab (or equivalent earthen floor) insulate all around it and call it a day. Most passive solar designs dont provide enough BTUs to heat more mass than that in a given day so there is no point in spending extra to try to squeeze out more performance in that area. Not with so much lower haning fruit around. 12% high shgc glass to floor area and a slab on grade. Simple. Maybe a slab on the upper level too or a real stone accent wall but the cost effectiveness is debatable.

Air sealing on the other hand is incredibly effective for what you put into it. Some homes are so airtight and super insulated that they can maintain comfortable temps from body heat, appliances and lighting with no passive solar or any other energy input.

Hope Iam not missing something here. You are planning on a mass floor right? Your not trying to actively circulate solar heat to mass storage bins are you?

I live in passively solar heated earthen buildings and I agree with everything Brian says above.

Rebecca, thank you for your post. And Brian Knight! I appreciate your humor, have read your articles and even discussion threads, including those with Ted Clifton (ref. the upper-floor slab), which prompted me to visit him personally two months ago. (I live abroad, but perhaps I could interest you in getting together for coffee or lunch this October ...) I totally appreciate the thinking behind and success of the "insulate (very!) tight ventilate right" school of thought and have read about heating homes via body heat. Very impressive. My challenge, though, is that I ALSO want to open the house up to the lovely outdoors (e.g. Nana-type doors). I'm not sure the two design concepts are compatible but am definitely open to wise voices of experience. Yes re the thermal slab and the windows, and my exploration of gravel/marble includes strategies for passive cooling. I agree with you that humidity is my greatest challenge to address passively. Have you seen the Leaf House desiccant waterfall out of UMD? Not totally passive but very interesting. And I keep thinking that Vanderbilt and his summer guests were somehow ok without air conditioners and dehumidifiers. There MUST be strategies which work.

Rebecca, covenants I will have to follow disallow earthen buildings but I do appreciate their thermal properties combined with low-embodied energy. Good for you and again, thank you for your comment.

Finally, sincere kudos to PERMIES for helping me make these connections!!

Would love to meet you Bill. Interested in what you were able to glean from Ted, he's doing some really good stuff from the looks of it.

It is a challenge to open high performance structures to the outdoors in certain climates at certain times of the year but I think its worth it to sacrifice performance for enjoyment of home, nature and life in general. Ive been hearing about some of the work University of MD is doing but I havent found or sought out any real info on it other than some short blurbs about it in articles about their solar decathalon entry which I think is the leaf house? Would appreciate any links more specific to the technical side of what they are doing if you or anybody else has them.

I hear you on the Vanderbilts and people from that time period in general. How in the hell were people always wearing wool suits back then!? In the summertime with no AC. I just dont get it.

Bill Tucker wrote: My challenge, though, is that I ALSO want to open the house up to the lovely outdoors (e.g. Nana-type doors).
.... I agree with you that humidity is my greatest challenge to address passively. Have you seen the Leaf House desiccant waterfall out of UMD? Not totally passive but very interesting. And I keep thinking that Vanderbilt and his summer guests were somehow ok without air conditioners and dehumidifiers. There MUST be strategies which work.

Rebecca, covenants I will have to follow disallow earthen buildings but I do appreciate their thermal properties combined with low-embodied energy.

Oh well. Earth buildings are also sort of magical in their ability to modulate humidity as well as temperature.

In my experience, growing up in centrally heated, well insulated wood frame houses in the US, and then 20 years living in passive solar earthen houses in the high desert, and also visiting friends' houses in Ladakh that are heated and have stone or cement slab floors and/or cement plaster, here's my opinion. The wonderful thing about earth or wood or plasterboard surfaces in the winter is that they are not highly conductive, so if you have to lean against them or stand on them in socks, they don't suck much heat out of your body. Stone and cement surfaces may be walls and floor of a reasonably heated room, ie room temperature, not icy cold, but because they are highly conductive, they suck the heat out of your flesh very quickly and make you feel cold at any point of contact (such as standing on the floor).

The more thermal mass your house has, the better it will sail smoothly through a couple of weeks of what would be heating or cooling weather in a wood-frame insulated house. That's how those ancient Indian palaces kept their occupants reasonably comfortable through the intense hot seasons, and warm through the brief but chilly winter.

About opening your house up to the outdoors -- be careful. If you put TOO much glass on the south-facing wall, especially without sufficient thermal mass blocking and soaking up the rays, you will suffer from daytime overheating, especially in the autumn and winter. It's really not comfortable to have overheating.

Thank you for your reply, Rebecca. I hope you enjoy Ladakh, which I have never visited, but I could never seem to get enough of the delicious Kerala food when visiting there!

Re opening up the house, I do not intend to exceed Brian's glass-to-floor ratio. And I envision a vine-covered trellis to block summer and fall sun. What I meant about opening the house up was sliding glass walls such as these: http://www.nanawall.com/ I feel that there must be a reasonable, cost-effective alternative to sealing oneself off from the outside. Air-conditioning has only been around for about 50 or so years...

I particularly appreciate your comments about cement and stone floors, the apparent materials of choice for thermal mass. And yes, I definitely prefer the thermal mass approach and see that Ladakh weather is harsher than where I intend to build, so their bulding experience is important.

You already have been given most of the answer. Square feet of sun exposure, insulation, some sort of mass and well sealed is the primary key to passive. Kind of mass really doesn't matter as long as there is enough of it. The one thing I will say on that issue though is color of the mass being heated by the sun does matter. A light colored mass can reflect heat back outside through the very same windows and the rest ends up scattered in the room in low mass things which raise air temperature. It is better to have dark colors that directly capture energy in the mass instead. That way you don't see as wild a temperature swing daily in the house. If you heat the air instead it poorly transfers the heat back to the mass because of the small temperature differential. For example on a concrete wall with light green paint vs a concrete with brown paint there is a 5 degree F surface temperature difference in the sunlight. This results in a wall that is a degree or 2 warmer by evening. Thus instead of wasting the heat by house being to warm you are holding that heat.

To C. Letellier: thank you very much for your post. I was intrigued by the different thermal properties of marble chips vs. gravel in order to extend the number of days of heat storage. However, I have come to learn that in high humidity environments such as where I will be building, disease-promoting organisms thriving on the gravel is a threat. I will be incorporating Healthy House elements into my design so this particular health risk trumps the storage advantages. I wanted to stay away from (the more efficient) water as storage because I am concerned by leaks over time. However, I may end up designing an in-ground storage area which slopes away from the house if leaks ever do occur. Thank you again for your interest!