I am trying to build a home this upcoming spring that will be at least partially heated with passive solar. I am trying to gain more than just solar tempering and get some mass in the home to boast the solar heating capacity and also prevent overheating , without totally blowing my budget. I initially thought that I could just to do a stained concrete floor in the lower section of home that will be heated via solar gain, however I am going to be on a stem wall foundation due to fact that there is a 4 foot slope from the front to the back of the house and slab foundation just doesn't make sense, plus the soils are somewhat expansive and I have been advised against a slab foundation based on that as well. Well, I just finished talking to two different GCs that told me they guessed the cost of the stained slab as a finished floor, would be around $32,000 to $36,000 versus doing a typical combination of wood, tile and carpet over the floor joist for around $12,000 to $14,000 depending on what I picked out. Wow talk about sticker shock. That is way out of budget and not doable. At 2.5 times the cost of the a "typical" floor I just described, God only knows how long it would take to re-coup the my money with the added efficiency of the solar slab given that this is borrowed money from the bank that I have to pay back with interest. Does anyone out there have any better ideas that are not super expensive on how I can get some thermal mass in the home? And also without converting some of the inside walls to concrete (AKA MUCHO MORE MONEY AGAIN). As a backup plan of mine, in case I can't get or afford the thermal mass, was to duct the hot air from the warm, south part of the house to the cold, north section of the home that that is far away to at least help with the overheating issue and maybe adding some additional hot air to the cold part of the house. Basically I would be creating a thermal air loop that simply pulls hot air from the south side of the house and pushes it into the far opposite side of the house that has no solar heating. This room I have in my mind is quite far away, probably 50 to 60 feet. But there is no doors or anything that would stop the falling air from slowly making its way back down the stairs into the lower south side to be heated again. I realize that this is not optimal by far, but potentially better than nothing at all and I think it would help some. Your thoughts on that would be appreciated as well. The ducts would be in conditioned air space as well:) thanks

So they told you $36k for a concrete slab finished floor vs. $14k for a raw concrete floor covered with carpet?

there is something missing here; something that the two contractors are including in their estimate that is perhaps a misunderstanding. I've had a little experience with staining concrete and it's nowhere near that much. Don't remember exact figures, but somewhere between the cost of carpet and the cost of hardwood. If you are thinking of building a wood floor, pouring concrete on top & staining that, you'd have to add the cost of concrete; probably 1.50-$2.00 per yard installed.

More to the point, why not build a superinsulated house and just eliminate the "cold parts of the house"?

No that was not carpet over concrete. That was carpet, tile and hardwood flooring over the floor joists versus installing a slab of concrete on top of the floor joists and then staining everything.

I think both of those guys were including the cost of having to install a metal floor pan in which to pour the concrete, increasing the size or decreasing the spacing of the TGIs and then the cost of the concrete slab and staining. The sq. footage is about 1800 sq. ft. When I was a hardscape contractor, I would budget $ to $ a square foot just for plain gray broom finished concrete. I think if we stained that, it was about double. I guess if you then add the cost of everything else I would have to do to "perch" the slab onto of the floor joist, it makes up that additional cost? I think that is what they were getting at. As for eliminating the "cold" areas, I was just referring to a section of the house that was somewhat separated from the solar area. Redrawing the house is not an option at this point due to size constraints on where I am trying to situate the house and the fact that I have already spent a lot of money on the design work so far and don't want to go backwards as well. The layout of the house is what is now. What I am trying to do now, is do material selection and system choice selection. Remember, shit here in the Roaring Fork Valley is SUPER EXPENSIVE. It is more than double the cost of what I built my house for Grand Junction 12 years ago for and even GJ is not deemed cheap by national standards. Thanks for the input though Bob.

Many of the guys on this forum like the idea of a concrete floor, and concrete would be an advantage for heat storage. I happen to like the feel of a wood floor in that it has some give. Dancing or running on a hard surface is uncomfortable on my legs, and likewise, just walking around, I like the feel of a wood floor on a wood floor joists. Almost all good dance halls have a wood floor and subfloor for good reason. I can live with either floor type, but my standard production house came with a conditioned crawl space and wood floor joists, and I like the feel of it better than the last house that I had with concrete floors and carpet.

During sunny days with cool outdoor temperatures, I make use of passive solar tempering and use the HRV to recirculate the air into the conditioned crawl space and then to distribute the air throughout the first floor living space. The crawl space has 55 tons of gravel laid on top of R-5 insulation, so I get some indirect heat storage there. (That heat storage is not "direct," but probably more effective than the concrete in an ICF wall that is hidden behind R-11 insulation.) I also use the recirculation option for the hot-air furnace fan to help redistribute the air from the solar tempered south end to the north end of the house. The house is well-insulated, but the temperatures do not equilibrate on their own without some help from the blowers, and even then the south end is warmer than the north end of the house.

Lee, that is an interesting idea. Probably not as efficient of a slab of concrete that is direct contact with the warm air and some light, but nevertheless I see the idea. Also, that's another good point as I have bad arthritis throughout my spine, hips and feet. Standing on hard floors for very long hurts me very badly. That could be a very good reason not to go with concrete I guess in addition to the extra cost.

Lee, also have you done any type of data collection in regards to the gravel temps and the return air coming back out of the crawl space? I would be very interested in this as installing some insulation on top of the soil and then spreading three or four trucks of gravel would not be super expensive endeavor and then my wife would get the wood floors that she wanted anyway) However it is still some cost involved and enough cost that I think I would have to see some hard data to prove its worth.

Say you spend $1400 on the gravel and I am don't know what 1800 sq. ft. of insulation would cost, but I am simply guessing that maybe between $600 to $1000 for that and then some duct work. That would amount to about a $2k to 3k investment. Do you think you are actually getting that much heat back out of that gravel as a return on your investment?

thanks

If you are doing a wood framed floor over a crawl space, you might consider using 1-1/8" Advantech sub-flooring (very dense and very strong material), then 1/2" Durock cement board, then 3/8" thick dark colored porcelain tile (typical thickness for 18"x18" tiles). This would give you some mass in the floor and shouldn't cost much more than standard flooring over a crawl space. Another way to get some mass inside the home is to pick some interior walls and have brick, stone, or stucco applied or have the entire wall made of this material. You would just need to insure that your floor supports in this area are beefed up to support the weight of the wall.

Regarding insulation under your crawl space floor or at the ground level, you are going to have to spend money on one or the other. There are quite a few advantages to insulating at the ground level and up the sides of the foundation and sealing it air tight. Then you could consider pouring a 2" thick rat slab over the ground foam to seal it up and provide additional mass under the floor but inside the insulation boundary. I would use at least R10 (2" XPS or better and less costly would be 2.5" EPS foam) on the ground and around the foundation for this application. Also, if you could work it in so you insulate the foundation walls on the outside with rigid foam rather than inside, then the mass of the foundation walls comes inside the thermal boundary - for instance if you have 2x4 walls with 1/2" sheathing then 1-1/2" or 2" rigid foam attached, this sheathing foam would essentially be flush with the foundation exterior foam insulation.

Arkie6, thanks for the input there on the Durock. I had forgot about that product all together. Yea that might be better than nothing I guess. thanks for the reminder, though how much good do you think it would actually do under a wood floor? As I don't want to tile the entire 1800 sq. ft. or even most of it and hoped to do wood floors for the majority of it, if I cant do stained concrete.

thanks

Posted By beetle55 on 04 Jan 2014 07:53 PM
Lee, also have you done any type of data collection in regards to the gravel temps and the return air coming back out of the crawl space? I would be very interested in this as installing some insulation on top of the soil and then spreading three or four trucks of gravel would not be super expensive endeavor and then my wife would get the wood floors that she wanted anyway) However it is still some cost involved and enough cost that I think I would have to see some hard data to prove its worth.

Say you spend $1400 on the gravel and I am don't know what 1800 sq. ft. of insulation would cost, but I am simply guessing that maybe between $600 to $1000 for that and then some duct work. That would amount to about a $2k to 3k investment. Do you think you are actually getting that much heat back out of that gravel as a return on your investment?

thanks

Don't let me mislead you about the gravel. I did not put the gravel in as a heat storage for the passive solar energy. The gravel was there as a part of the conditioned crawl space (and covered with plastic). I added R-5 of insulation under the gravel to reduce heat losses and added R-10 around the edge between the gravel and the footings. I also designed the HRV so that it draws in the stale air from the crawl to pull conditioned air into the crawl. So on recirculate mode, I pull some of the air from the solar heated south end into the crawl, as well as the rest of the house.

Why only R-5 under the gravel? The soil here is dry sand with some rocks thrown in. Dry sand has a R-value of 0.58 ft^2 degF hr/Btu per inch (http://www.aquatherm.com/sand-as-insulation). Let us say that the rocks in the sand reduce the R-value by 1/3 to 0.39. The crawl space walls are 2.5 feet high, but the footings go down another 8", plus the insulation on the crawl space walls extends out onto the crawl space floor by 1'. Therefore, heat losses through the crawl space floor must travel at least 4.17' through soil to reach the outdoor air temperatures, and much farther for heat lost in toward the center of the crawl space. So the effective R-value of the dirt under and around the crawl space is 19.5 ft degF hr/Btu, and with the added R-5 from 1" of XPS, the lowest effective R-value is 24.5, with higher R-values in toward the center of the crawl space, so similar to my above ground walls. The cost, including labor, of the 1" XPS under the crawl and 2" around the inside of the footings was $1489 (http://www.residentialenergylaboratory.com/costs.html). If I had it to do over again, I would use EPS rather than XPS for reasons give by Dana1, the global warming potential of gases used to make XPS, and make it 2" thick under the crawl.

I doubt that you could justify the expense of the gravel and the insulation based on heat storage for the passive solar, but you've got to put something down there under your house, and insulate under your first floor or insulate the crawl. It is popular now to do conditioned crawl spaces, so that is one option with gravel or a rat slab. In either case, you need a vapor barrier and possibly insulation.

You must also include in your design some approach for radon mediation in the "crawl space" area. Garfield County has potential indoor radon levels above the standard of 4 pCi/L (http://co-radon.info/CO_radon_map.html). Buried in my 8" thick gravel bed are two 4" perforated pipes running the length of the crawl that connect to 2" vertical PVC pipes that go up through the house and roof. That approach was insufficient, since the radon level on the first floor was 8.8 pCi/L, so I added blowers to convert from a passive to an active system, and that reduced the average to 1.3 pC/L.

You asked if I have data for the temperatures in the gravel. I put 1-wire temperature sensors into the bottom of the gravel and under the XPS during the construction process (so essentially on each side of the XPS so that I could measure heat transfer rates), but the RJ-45 connectors that I used seem to have lost connectivity. Since the crawl space is usually a few degrees cooler than the first floor (heat is only directly added to the first floor), I suspect that any gravel heating is pretty hard to quantify. Keep in mind that I am pulling air into the crawl from all parts of the house, not just the south end. Right now it is 74F in the south end room and 67F in the north with the furnace turned off (outside is 23F), so the HRV is redistributing some of that heat, but I need to turn on the furnace fan to have more impact on the redistribution.

Putting the gravel on top of the insulation defeats the purpose of the gravel. If you don't need the gravel for drainage or radon collection, just pour the concrete on the foam.

FBBP, I think you missed a good chunk of the thread there bud.

I am trying to gain more than just solar tempering and get some mass in the home to boast the solar heating capacity and also prevent overheating

Too much fixation on concrete floors. If your site design is not conducive to a simple concrete slab at ground level then you need to abandon that and move on to something else. The best way to prevent overheating is to control insolation, not by adding mass.

Posted By beetle55 on 06 Jan 2014 02:05 AM
FBBP, I think you missed a good chunk of the thread there bud.

*** The crawl space has 55 tons of gravel laid on top of R-5 insulation, so I get some indirect heat storage there.***

*** I added R-5 of insulation under the gravel to reduce heat losses and added R-10 around the edge between the gravel and the footings***.

*** I would be very interested in this as installing some insulation on top of the soil and then spreading three or four trucks of gravel***

All I was pointing out was that this is not a good idea. The gravel is there to provide a pressure relief from soil waters and a way of collection and removal. If you put the foam under it, you can still have hydrostatic pressure lifting of the slab as the strength of the underslab water is stronger then the weight of the gravel and slab. There is much better ways and places to get thermal mass. Lee has very dry soils so he can get away with it but it still has a risk factor.

Posted By FBBP on 06 Jan 2014 10:15 AM
*** The crawl space has 55 tons of gravel laid on top of R-5 insulation, so I get some indirect heat storage there.***

*** I added R-5 of insulation under the gravel to reduce heat losses and added R-10 around the edge between the gravel and the footings***.

*** I would be very interested in this as installing some insulation on top of the soil and then spreading three or four trucks of gravel***

All I was pointing out was that this is not a good idea. The gravel is there to provide a pressure relief from soil waters and a way of collection and removal. If you put the foam under it, you can still have hydrostatic pressure lifting of the slab as the strength of the underslab water is stronger then the weight of the gravel and slab. There is much better ways and places to get thermal mass. Lee has very dry soils so he can get away with it but it still has a risk factor.

There is something that I must not be understanding about Archimedes Principle. If I take a 1" thickness of XPS combined with 8" thickness of gravel on top of it, then even assuming a zero density for the XPS combined with the average density for gravel (including the void fraction) of 1682 kg/m^3 (http://www.simetric.co.uk/si_materials.htm), then the average density (of XPS plus gravel) is computed to be 1495 kg/m^3. The water that is displaced by this has a density of approximately 1000 kg/m^3. So exactly how is the water going the float the XPS with the gravel on top of it??

Plus the fact that Lee is in sandy soils with considerable aggregate makes it unlikely that water would ever be an issue anyway would be my guess, less not enough that would float that much weight.

Yea but I am doing what I can to get the best solar gain I can from the site, which has excellent soil exposure and reduce the heating bills by doing that. And of course yes highly insulating the house and sealing will be in the plan as well. but nothing in my mind beats passive solar heat. never breaks, nothing to maintain. however I am not going to spend 40k to make it that way. I am not going to be in the home in 10-12 years quite likely.

Yeah - can't say if you understand old Archie or not, but it might be that he has nothing to do with this situation. Could be we are talking about pressure rather then buoyancy.

I doubt I'm the only contractor on the forum who has had the dubious pleasure of breaking out a concrete floor that has lifted due to hydrostatic pressure.

As I mentioned, you can get away with it as your situation will not develop hydrostatic pressure but my concern is that others (like OP) will read your post and say "that's a great idea". No, it's not a great idea and you had the decency to say that. I'm not sure OP understands that you told him not to get carried away with the gravel.

***plus the soils are somewhat expansive*** further complicates the issue. Water can be his friend or his enemy or both. Being able to add or take away moisture might be a big deal to him.

Being the OP, might not be as naïve as you might think, he has worked for 15 years in the earthworks, grading and drainage, irrigation and hydraulics, and I simply said that it seemed like maybe a good idea to look into , and before I would do anything I would spend time looking into it before just acting on an idea without thought and time. Obviously I was not sold on the idea, that why I was asking questions.

Believe me, water is not a problem where the lot is. Lack of water is more than anything. I have installed miles of drain lines and pumps, etc and understand soils and water very well as my livelihood depending on it. No need in trying to talk over my head. I said that was "an interesting idea" , NOT that is a great idea. Big difference. And after my comments you say you think I am going to carried away with the gravel?

glad you understand.