Pex layed in sand between concrete & insulation
MnDel
| Posted in Energy, Heating & Insulation on
In northern Mn, we have reasonably cheap off peak elec. and a common way to gain more heat sink – to even out the temps of a radiant floor system on rippled heat – is to lay down 2″ of XPS, staple pex to it, and add several inches of sand before leveling and pouring concrete for an on-grade slab.
Sand dosent conduct like concrete, and I know of no study of how deep a sand heat sink is advisable. Any experience out there in this?
Secondly, in this layered system where would you put the vapor barrier? or 2?
Del
Replies
Rippled heat??
What are you trying to do w/ the sand? Not sure that it will appreciably 'even out the temps'. What uneven temps are you trying to even out? Sand is not much unlike concrete as far as conduction goes, although I'm not sure that conduction is even much of a factor in your situation. If it doesn't conduct as much as concrete, it would seem you'd have more uneven temps. If you are simply trying to add thermal mass thinking it will affect energy efficiency in some way, generally you are probably wasting your time/money. If you are trying to get better temperature distribution, I think you will gain very little. Radiant slabs distribute heat quite well.
Never thought you could 'staple PEX' to EPS insulation and have it stay in place.
A vapor retarder, I THINK, would go below the insulation to simply prevent ground moisture from migrating upwards.
He's using off-peak electric, which is why he needs the thermal mass.
A little odd there is off peak electric rates in the heating season. But I see the point, now. Depending on the time of day, it would seem easy to have the concrete slab coast through the peak period (normally mid late afternoon). I'm going to cogitate (is that a word?) about this a bit more and maybe add some thoughts.
careful, cogitate too much and it'll fall off..........
Sure, you laugh now.
I've found various conflicting values for the insulating value of sand, but they seem to center around 0.6 R/inch for DRY sand (and much lower for damp/wet). This is compared to 3-5 R/inch for foamboard.
You DON'T want an insulating value in the sand that's comparable to the foam, as that will cause more heat to be lost downward into the soil. Rather, you want to keep the total sand/concrete R value below maybe 1/5th of the total R for the foam.
PEX in sand *below* the slab?
That's a new one, I've never heard of that before.
IMO if you want more thermal mass I would pour a slightly thicker slab and forego the sand. I would place the PEX a few inches below the surface of the concrete, not lower, tied to the top of a rebar mat on dobies or wire mesh on chairs. I have never heard anyone complain of thermal striping from slab floors, more likely from low-mass floors like Warmboard.
Heat from your tubing is trying to escape down into the ground. Insulation slows that escape but does not stop it. The closer you put the tubing to the soil, the more heat you lose into the ground.
If you are in the design stage of a radiant system I would strongly suggest you consult designers who are used to working in your area and your climate. It may be that what you want is a boiler with thermal mass rather than trying to store heat in the floor. I have seen boilers installed locally that operate overnight, during off-peak rates, but continue to circulate heat during the day, without heating the water further. They're basically boilers full of bricks, and they're larger than the average mod/con.
PEX in sand below the slab?
I have yet to find any website which espouses this & rarely even a mention of this sandwich method. In this discussion
http://www.healthyheating.com/bb2/viewtopic.php?f=25&t=801&sid=dc125c812d2c2dbc7d6acb6f2f7a130f
a fellow points out figures showing concrete to have 3 time the conductivity of dry sand. Sounds like a no brainer then to put pex in the concrete. But here we easily get rippled for 8 hours, and if it's -40F outside thermal mass becomes kind of important. It is hard to imagine a storage boiler with such a large capacity. Lots of folk seem quite happy with this method here.
I have to decide by July 13, am just having a hard time getting my head around putting pex in a poorly conductive substance.
A second reason here (heavy clay country, zone 2b, -45F bottom), that it is a popular method for workshops (like mine to be) is that if heat is ever off for a winter and you have the misfortune of gaining a buckled floor - the pex isn't in it to get broken along with the concrete.
thanks fo r any replies, Del
I'm not sure what you mean by "rippled"
I would assume that you'll run your system with glycol in it, and that freezing wouldn't be a problem, but maybe not.
One thing to be aware of if you decide to pour over sand.... bleed water from the concrete will wet the sand, and the bottom of the slab will stay wet for a long time, giving you greater risk of slab curl as the top dries. I would keep the top very wet for a long time if you go that route. I much prefer to pour directly on XPS foam.
Though concrete may have 3 times the conductivity of sand, the conductivity of sand is still pretty high. The real question should be: What is the thermal mass of sand vs concrete (or whatever other storage medium might be used).
Shoulda listened to your mother.
And what's this "attitude" you display on occasion-when you reply to posts? Having a bad time lately?
What attitude?! Eh sometimes I get triggered by some lame/stupid comment maybe so then I come across a bit sarcastic (I'd say) maybe. I often log in early in the morning, so I'm not quite awake and maybe I read something upside down, too.
Excuse #47 in my rolodex ... maybe I have a stressed day and something I read sets me a bit off.
Heck, I've no idea. Didn't see much of an attitude in this thread, if that is what you are referring to. This can be a difficult communication medium sometimes. Directness is often seen as 'attitude'.
I am PO'd at the old Breaktime here for 'improving' their forum site. I come here to lend some food for thought, but frankly, the new format is junk IMO.
Hope I didn't offend you somewhere. Unless you deserved it ;) I do try to be very respectful and suscinct in my writing and approach. You apparently have some respect for me ... hence your note.
difficult communication medium.................?
this place?
I understand most angles in this place-much like my building carreer-you been along for this much time-even someone with no clue should get the hang of it. So yeah, I realize that mistakes can be made in the way we take things written here.
But, I try not to be too sarcastic-or caustic-most times. I guess for the reason that there is no personal contact through this medium. There's no smirk or wink or eye to eye contact. I don't do emoticons.
At any rate, I noticed you continually prodding other replies in some posts (not in this thread-I don't believe). To me it seemed beyond conversation. But hey, what business is it of mine? nonewhatsoever..........
So just trying to make some conversation-you know, idle thoughts.
Now one way to get a better picture of posters here is to meet up face to face. And what better location than a FEST!
You really aught to make one.
Hey, thanks for the heads up. I continually try to monitor and improve my conversational/communication skills. If I 'continually prod' someone, I usually have a reason and am trying to get a response to enable better feedback. I've been traveling the last few weeks, so maybe my mindset hasn't been the best for responses ... always felt like I was hurried?
I recall a year ago maybe a 'discussion' w/ Riversong ... he had little respect for other's point of view and thought his point of view (his science) was the only point of view to have. That got quite heated. Like I didn't disagree w/ him, but he always said I was dead wrong. Go figure. While science is simply science, there are different ways to look at things. Science can be complex (duh), so that lends itself to different ways of looking at things.
Del
What you are thinking of doing is what we are already doing in the solar side of heating. I am building a solar heated house in st paul Mn using FOUR FEET of sand below the slab. This sand has it's own pex tubing for injecting the heat from the hot july and august sun, which will provide heat for the house through the new year!
In my house, the sand bed is fully insulated on all sides, top, and bottom, so it is a little different than what you are doing. Additionally, the slab has it's own pex heating loop since it does not get heat directly from the sand.
What you are doing is very similar to the common design in central wisconsin advocated by Bob Ramlow. In his design, the sand below the slab is not insulated from the slab and it acts like a big 'flywheel' to hold heat energy for a few days and sometinmes months, if the sun doesn't shine. This is a well proven design with many examples up and running for many years.
Sand is not an optimal material for moving or storing heat. We use it because it is cheap, easy to work with and it won't leak all over the place causing your house to collapse. My gut feeling is that you want a bit more than two inches to make it through the day. That depends on the heat loss of your building, how hot you get the sand, and whetherit is a house or a utility building. The farther down the PEX is, the slower the response to a heat call will be. I think the Ramlow method uses pex in the sand and pex in the slab, but the slab PEXmay be optional. His followers tend to use two feet of sand, but thay are banking weeks worth of heat, not just the 8 or 10 hours you need.
You are on the right track. You need to do a heat load calc and analyze the needs of the building occupants before you can calculate whether you need more sand. If you are heating with electricity, wouldn't it be cheaper just to put a storage tank on the boiler, or get a bigger hot water heater if you are using one?
While sand may not be the most conductive material, since you have a circulating loop in it, it's less of an issue. And like you say, it's inexpensive, available, and it's easy to work with. I don't think sand compacts like soils and some gravels. A reasonalbe choice IMO.
? So if the other poster's sand is 4 feet deep as he said he used ... what should his insulation thickness be? I tend to not agree with what you're saying. Thermal mass is often insulated using say 2-4" of EPS (R-10 to R-20).
If you're paying (even at reduced rates) for the heat, you don't want to lose any, or you mess up the economics. (Somewhat different from when you're storing "free" solar heat, etc.)
So it's a simple question of how much you're willing to lose. I'd guess most people would want to loose no more than about 1/5th of the heat, so you want 5 times as much insulation below as above. Four feet of dry sand is roughly R20 (maybe even R30), so you really want something approaching R100 below. (Though you can take a little credit for the insulating value of the soil below the foam.)
And actually it's a little worse than that, since you need to take into account the temperature of the "sink". For the soild side you can figure 50F, while the room side would be 60-65F. So the losses to the soil will be made slightly worse.
Like you say, there's no free lunch.
My reaction to that is that has to be just about the most ridiculous rule of thumb I've heard in the energy field! I can't imagine anyone ever insulating to that level!
Your insulation is simply to reduce heat loss from a given condition on the 'storage' side of the insulation. I'd guess you should be storing heat in the sink at say 90 degF for the heating side and like you say around 60 for cooling. The heat loss is to the ground. The value of the insulation needs to be substantially more than that of the concrete covering the storage mass, not the R-value of the mass itself. The concrete is your heat exchanger w/ the space in the house.
The goal of the insulation is to minimize loss to the ground and promote loss to the house. This has little to do w/ the storage mass itself, really.
If the pipe is below the sand, then the sand is acting as insulation. No way to get around that fact. And when you do the arithmetic the heat will flow as directed by the insulation.
Place the pipe in the middle of the sand and you'll change the equation significantly.
(And if you can disregard the insulating value of the sand, you can disregard the even lower insulating value of the concrete. By your argument, you really don't need any insulation below because all the mass above is "storage" and its insulating value can be disregarded.)
Even w/ say 2 inches of EPS below, the insulating value of a couple of inches of sand and the concrete slab will be far less than the R-10 below. The math says the heat will tend to flow up ... i.e. the heat loss up will be greater than the heat loss down. And yes, the pipe in the center of the sand is a much better way to distribute heat into the mass.
I've no idea what you mean in your third paragraph. The insulating value of concrete and sand are [relatively speaking] much lower than insulation, so the heat will tend to go upwards. I never said anything about eliminating insulation below because of the thermal mass of the sand/concrete or that we would disregard the insulation value ... doesn't even make sense to me. You're taking my words and changing them to suit your argument in some fashion.
Even w/ say 2 inches of EPS below, the insulating value of a couple of inches of sand and the concrete slab will be far less than the R-10 below.
Yes, with a couple of inches of sand and a normal concrete slab 2 inches of foam is probably sufficient. But you said:
So if the other poster's sand is 4 feet deep as he said he used ... what should his insulation thickness be?
I said it should be considerably thicker, and I stand by that.
But that is like saying ... the volume of air and other materials inside my house has an insulating value and I need to account for that and make sure that my wall insulation is 5 times that. I'll think about this a bit more and respond. Biannual storage systems routinely use the earth as the storage w/ no insulation below and like 2 inches above. Assuming your storage is much like a water heater where you charge/discharge somewhat evenly, then your insulating argument doesn't really apply. For low delta T storage applications, more than a couple of inches of insulation wouldn't make economic sense.
This is not like a "biannual
This is not like a "biannual storage system" where the heat is essentially free. In the OP's situation the heat is being paid for at reduced rates -- probably about half the standard rate. Thus it doesn't take very much inefficiency to make the whole thing a money-loser.
A spin-off conversation at Green Building Advisor
FYI, there is a related conversation on this topic at GBA
Dan
Pex layed in sand between concrete & insulation
I would look into a hot water storage tank "reservoir" with heavy insulation, connected to the heating system with a timer and shunting valves. Heat the water tank in off hours. During peak hours timer opens valves to allow the zoning pumps to draw the water from the storage tank through a mixing valve until tempeature drops to pre-set "low temp cutoff". Ther are additional engineering details to build into the system but this is the basic idea. I think it would be simpler and more effective.
Virginbuild
I certainly dont see simpler with a water storage system.
I think the OP just wanted to just use electricity off peak only. Therefore any costly system would offer him no benefit. Personally, I believe a concrete slab with a heat sink under it (like the sand) would be simplest and cheapest. I also believe the reason for pex in sand is to heat the sand up to maintain the heat in the slab to a relatively static temperature during the entire day. I think it's an excellent idea.
Extending your mass and storing heat into a sand bed
Here in Northern New Mexico at the base of the Rocky Mountains about 8000 ft elevation near Taos NM, I know a Solar Architect/Builder who has a house with the Footings completely insulated with 24 inches of compresed earth also insulated - (laid on top of 4" rigid foam). within this compresed earth is pex tubing for the summer zoning in which he dumps heat into this mass from May until November. He also has pex tubing in the slab above this mass which he uses this zone during the winter. He has this all instrumented with temperature sensors below the mass in the ground, within the compressed earth and in his slab.
He has found the the extra mass will eventually heat up to about 70 degrees by November, when he switches o the winter zone on his passive heating system. He is able to maintain sufficient heating during the winter there.
He also has thermal mass in all his walls with R40 insulation in walls and R80 in the Roof. That might help..
Doug Patrick
Sand dosent conduct like concrete, and I know of no study of how deep a sand heat sink is advisable. Any experience out there in this?
This statement is exceedingly unsupportable. Sand, rock and concrete are all similar in their heat transfer properties, as well as many other physical properties
sand bed solar articles
Hi,
Here are some articles on sand bed heat storage as used on solar homes, but I guess the idea is the same even if the heat source is different:
http://www.builditsolar.com/Projects/SolarHomes/SandBed/SandBed.htm
I think it may prove to be difficult to control the heat release so that it comes up through the floor when you want it to.
Some of the articles seem to point to a significant delay between putting heat in the sand bed and getting it out.
To me (personally) this is not a very appealing scheme in that while it reduces heating costs (maybe) it actually increases greenhouse gas emissions, because not all of the stored heat goes up toward the floor. I wonder if better insulation, a more efficient heating system, a heat pump, ... might not be better way ways to get to lower heat costs?
Gary