Posted By FBBP on 06 Jan 2014 01:25 PM
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.

Since I am not getting through to you, let me be more blunt. I do not have any slab to break with hydrostatic pressure. So your concern must be that my crawl space floor, which consists of 8" of gravel will be lifted by the 1" of XPS under the gravel, and it will all float up. I have shown you that clearly the gravel plus XPS will not be lifted by water. Can't happen. If you do not understand the physics, go build a model and try it. The downward pressure is greater than the upward pressure. The crawl space might flood, but the floor will not lift, and there will be no damage to the stem wall foundation from water trying to lift the XPS plus gravel.

It is a fine idea to put rigid foam insulation under the crawl space floor. It reduces the heat loss. I have shown you that the R-value for heat losses through the crawl space and the soil to the outside air is now similar or better than through the above-ground walls. This approach is not limited to dry climates.

It is also a fine idea to put gravel at the bottom of a conditioned crawl space. It keeps one from slopping around in the dirt or mud when doing work in the crawl space. It also provides an ideal way to put in a radon remediation system.

The conditioned crawl space with ducts in the crawl space with insulation on the walls and under the floor is a reasonable approach from both a technical and financial standpoint.

beetle55-

There was a previous thread started by robert.thompson who is building in Ontario, Canada (?) and who was building on a sloping lot like you are. However, he had a lot of solid rock in the planned building area, which you may not have. He was considering various approaches to the foundation. There might be information of interest to you in that thread. The search function on this site is not good. The recommended approach is to use a Google search like: greenbuildingtalk.com: robert.thompson foundation

Lee, yes thanks for the tip on Roberts foundation. I did happen to see that one. However I don't have solid bedrock to go on, its just a bunch of fractured volcanic rock that is a monster to dig thru. I think I am just going to abandon the idea of the slab on top of floor joist as it just appears that I am going to incur a lot more cost than can be justified. In addition, after looking at the amount of glazed window area I have versus total square feet of the house, I am not convinced now that I even enough windows to warrant having additional mass of that size in the house. I think I am just going to go with the air circulating thru the house and insulate well and seal it up as best I can from air infiltration and I should have a fairly low heat load. Thanks for all the input though as I also thought initially that I might use the crawl space as a conditioned space for various reasons.

Convective air circulation is easy to build. I have an unheated north entry way, and a south facing sun room also unheated in any fashion. I installed a 14" insulated flexible conduit from the top of the vaulted sunroom through the attic to the north entryway, and put a small squirrel cage blower in the center of the conduit to encourage the flow of warm air from sunroom to north entryway. It works very well. I can sit on the steps ( lower than rest of house by 3 feet) of the sunroom and feel the flow of cooler air moving from the entryway into the sunroom. I think the whole deal cost me <$200. You do need to be able to open and close doors to facilitate air flow. I can raise the room (23' x 32' with 14' ceilings) temperature 4 degrees, using the convective heat from the sunroom. YOU DO NEED A SUNNY DAY for this to happen.

DAND, yea I figured that you need a fairly sunny day for that to happen well. We are quite lucky here were we get 300 days of sunshine a year! I am stoked on that. Just curious, how long is your run of the 14 inch duct? what CFM are you moving?

have you considered a fish tank? watter can hold over twice the heat compard to concrete, doubt the window in question is large enough to boil a small tank, and you'd be surprised how much a larger one can hold, in front of the glass would work like a trombe wall.

there is another way to lay floor tile on joists floor in cost wont be any different than whatever other flooring you may like, it involves tar paper, expanded sheet metal, and a mix of 60 shovels sand 1x50pound portland, barely moist, 1 -2 inches deep depending how bad the carpenter did, perfectly level, the craftsman I was working for called it MUD, he used modified rototillers to mix it, you need someone that knows the trade most these days go for the durock liquid nails and screws method ( no coment ), HD classes are great but they are not trade shools

AS far as the fish tank, yea I thought about that for about 10 seconds but I would think that for the fish tank idea to actually be worth while, it would need to be a GIGANTIC fish tank. thanks for the suggestion though.

it's hard to tell without any details on the window size/type/orientation HDD etc, I just got into it on a different forum and turns out a plain double pane for me would actually be a net loss even oriented true south, that beeing said the real question is how much net heat gain will that window give you, for me the fishis would freeze, even in your area if you got a bit better than the cheapest window the net gain may not be enough to warm up 10 galons of watter a couple degrees, the initial though of a concrete floor on metal pan could be complete overkill, bottom line how much heat do you expect to store, ceramic tiles liquid nailed to the subfloor may be to much mass for all I know ( bad quality job but just an example )

Georgec, I have no idea what your first part of your paragraph means at all. For the rest of it, are you talking in a Cryptic code? I am not following you at all. The one thing I did pick up on was that I could not heat up 10 gallons of water. That is completely wrong. I have actually tested a 5 gallon container of water and I was able to heat it up from about 70 to over 90 degrees on a December day with plenty of sunlight without even having a optimal shaped container. Have a good one.......

its complicated as I found out, but in short a window will give you heat gain as well as it is a major heat looser probably the worst in the whole house, the gain happens over a period of max 6 hours the loss is 24/7 for my instance turns out a plain south facing double pane would actually loose more heat than I can ever expect to gain from it, your experiment is great but you are confusing heat with temperature, they are not the same thing, run the math 20F over 5 gallons is not that much heat. all I am saying is you may not need that much mass as a concrete floor for example, after all it's your house do what you will

South widows that are in correct ratios to the size of the house and that are high quality windows are a NET POSITIVE. All other aspects a net loss. There is some solar homes that heat almost exclusively with a large amount of correct south facing windows and very little window openings on the other aspects. 5 Gallons of water holds a lot more heat than 5 gallons of air. It was just a simple experiment. I am sure if I had the room, that I could have heated up hundreds of gallons. One degree rise in water temperature is an exact amount of energy. I don't care how you want to measure it. Temperature, heat, BTU, etc. Have a good one and enough of this.

And the old standby for thermal mass....just double up the gypsum drywall. Lots of surface area for passive transfer with the air.

Lots of surface area for passive transfer with the air.

If you really want to get some mass into a room, try a thin stone veneer wall. I used it on a south-facing wall in a sunroom and not only does it add dimension and beauty to the room, it probably has triple the mass of double drywall and double the surface area, all thermally bonded together without sheets of paper insulation in between.

Examples would be "Ledgestone" or "Shadowstone" panels by RealStone Systems.

Yea I forgot to mention, I was already thinking of the double drywall idea, just was wanting something more than that, but it is for sure a start and easy to do. As for the veneer, its not a huge amount but I like that I idea and that would for sure add a bit of mass to the room and really liven up the room, providing it is done nicely. I will look into that ICF, at this point my budget is melting like a stick of butter in a hot pan and I am going to have to make some hard decisions on where to spend my money and I think air sealing and insulation is probably going to out weigh spending more money on mas at this point. Bummed, just don't quite have the funds to do the house I really want, so I have to compromise quite a bit on it. thanks guys

Adding the stone veneer makes a nice weekend project somewhere down the line. If you put cement board up instead of gypsum and plastered it lightly, you might be able to tolerate it until the funds allow the veneer.

that's a point, always a possibility