Up here in the north country, modern construction has insulation (typically
2 inches of foam board) put *under* the slab and between slab and outside walls/footers (only 1" there) to help prevent heat losses. Could be done in warm clients for the opposite reasons I think.:-) Ain't that the truth! Like Prandtl, Reynolds, throw in some Nusselt for the HT and things can be really 'fun'.
daestrom
It is not a standrard practise in warm climates at all. Up north , for slab on grade homes without basements, you will see some perimeter insulation go down past the edge of the slab maybe 2 feet below grade, only noticed insulation below slab on radiant floor projects typically.
Grashoff (sp?) too :)
It's a conservative underestimate.
Oddly enough, I feel no such need :-) BTW, the vapor barrier under the slab was to keep the soil dry, so it's an excellent insulator for upward heatflow.
Nick
Ah, the Mayflower. I have sailed in her - literally.
It estimated nothing
You have need otherwise your schemes remain unproven fantasy.
Mositure in the soil would be attracted to a cool slab, the insulation value of a vapour barrier is negligible.
A arrogant rule of thumb: "If Abby doesn't understand it. It won't work" :-)
I disagree. Moisture is not "attracted" :-)
Nick
It is hard to understand your substitution for considering forced convection , when you are incorrectly applying small temperature differential radiant transfer shortcuts.
To be anal, I believe the constant is 0.1713E-8, however the temperature raised to the power of 3 is the absolute MEAN temperature of the ceiling and the slab. You state the slab is 75 so that would make the ceiling 85. The air would be quite warm then wouldn't you think. Its already established that it is humid so this side track of yours seems to point to a natatorium again.
I will admit you have confused me as you appear to be using a net radiant heat transfer from the ceiling to the slab, to estimate FORCED convection from the air to the slab.
I have seen the radiative heat transfer coefficient used in examples of hot water radiators. Heat transfered by NATURAL convection to room is roughly equal to the heat transfered by radiation. But the slab is cool, natural convection works against you, in fact you are relying heavily on forced convection.
Anyway you look at it, the approach you are trying could possibly estimate the convective heat transfer from a HEATED slab to the air. Or it just shows the net radiant heat transfer from a ceiling to a floor slab, stealing some of your 'coolth' available.
So trying to rationalize things out by assuming temperatures does not seem to work.
This would be a good one to program as you would have to do quite a few iterations to get it all balanced out. Increased heat flux from warm earth into slab, now radiation from ceiling and walls putting heat into the slab balanced out against the heat removed by water evaporating to humidify the home, and the forced convection of warm room air ( mixture of room air and triple digit air infiltrating in) contacting the slab. You would have to keep iterating until you could equate everything and finally come out with the room air temperature and the slab temperature. At least the rising indoor temperature would reduce the sensible heat gain of the space. :)
Sure looks like the conditions found adjacent to an indoor swimming pool in the making to me.
So enough smoke and mirrors, if you want to see what is really happening, you need to establish what the slab temperature equalizes at. Your whole scheme revolves around it. It is the foundation of everything you are hoping to prove so without establishing this, it is yet one more pipe dream.
The Inverted Pool of Pine, with a dripping wet ceiling slab would work better, be unhealthy but more practical at least you would have natural convection working with you. What would work the best, (without DX air conditioning) is a system that cools outside air to a temperature lower than the desired room temperature and then displaces room air outside of the structure to avoid a build up of 100% relative humidity. What a great idea, reduce the sensible heat in triple digit ambient air to the point where it is cooler than the room air, and then calculate the air flow required based on the sensible heat gain of the space.
You should investigate ground source heat pumps a little bit, in particular the effect of MOISTURE MIGRATION in the soil. Typically soil moisture is attracted to cold pipes in heating mode. In the cooling mode, warm pipes dissipating heat below ground tend to drive moisture away from the pipes.
OK You guys. On this same vein...
I have to do a heat loss calculation for my home design with slab heat.
How do I calculate the total equiv R factor for the earth under the slab? I am planning R10 (2 inches of styroboard) and the law says I have to insulate the perimeter somewhat.
I realize convection doesn't really happen down below the 2 feet of styroboard but the heat sinking is infinite after the R10 underlay.
Is there a rule of thumb to use for this?
Let it shine boys...LOL
Thanx
Arrogance again. It might be, were I so doing :-) Pick one: a) an ordinary ceiling with radiation and no fans or setbacks, or b) a low-e ceiling with fans and possible setbacks. Don't pick both!
That's extraordinarily anal, and wrong, according to the ASHRAE HOF and lots of other sources which say the Stefan-Boltzman constant is
0.1714x10^-8 Btu/h-ft^2-R^4.
Also anal. The radiation conductance is a way to discover the temperature diff required to move heat in this case, vs starting with a known diff...
77.5 or 80 F changes the answer infinitesimally. Try it, if you like.Let's try to avoid this passive-agressive waffling.
This is promising, Abby. Confusion can be better than certainty :-) Pick a) or b) above, not both.
That's good for daytime setbacks.
In case b) with fans and low-e surfaces...
It "could possibly do so" because it does :-) With cool slabs as well. Figure 1 on page 22.1 of the 1993 ASHRAE HOF, "Surface conductance for different 12-inch square surfaces as affected by air movement," shows
1.5+V/5 Btu/h-F-ft^2 smooth surface airfilm conductance, with V in mph. I conservatively assumed V = 0.Radiation melts more ice in a) like an ice rink with a plain roof, than b) like an ice rink with an aluminized roof.
Maybe I wasn't clear enough in explaining that a) and b) were mutually exclusive cases, altho there are in-betweens, with moderate emissivity. Foil walls and ceilings (b) can save more water with daytime setbacks, but they aren't everyone's cup of tea.
Not needed, IMO. But knock your socks off, s'il te plait.
No I don't. More arrogant waffling...
More arrogant waffling words.
Please read more carefully. I've never suggested anything like that.
How would that work? Who said anything about 100% RH?
I don't understand. Ignoring your sarcasm, how would that work?
How arrogant to assume that I haven't. PE Norman Saunders writes that the main mechanism for upward heat flow in soil is evaporation from lower layers and condensation above...
All soil contains some moisture. When heat is moving downward, the conductivity of the water speeds the temperature change slightly, even as water's thermal capacitance slows it. A warm wet strata below is another matter. Vapor is always forming, and because of its low density, it rises through the inevitable pores until the lower temperature causes it to condense, releasing the considerable heat from change of state. See Penrod, "Measurement of the Thermal Diffusivity of a Soil by the Use of a Heat Pump," J. App. Phys. 21 May 1950.
Water vapor is only "attracted" to COLD pipes, below the dew point. It's hard to get to the dew point with a dampened slab and "attract" water vapor up through 800' of sand below in the southwest, but it's useful to avoid dampening soil with a vapor barrier below the slab, if only to avoid wasting water.
Nick
Confusion is like fertilizer. It feels like shit, but nothing grows without it. --Carl Rogers
Nick
You have a pattern of getting more and more obscure when you are losing an arguement.
In a case of something such as a hot water radiator, radiant heat transfer and natural convection are of roughly the same magnitude, therefore some one incapable of calculating the convective heat transfer can do a quick calculation of the radiant heat transfer, to be in the ballpark of the NATURAL convection.
So what you are trying to do as far as I can tell is to quickly estimate the net heat transfer from the ceiling to the floor and say this is equal to the convective heat transfer of the room air to the slab. So the value of 1.08 that you round off to 1.1 seems analogous to
1/.92 , heat flow down through a convective air film on the underside of the ceiling. Quite a coincidence how this works out, 80F could be a MEAN temperature of the underside of a ceiling and an air conditioned room.You go on and on but have not been able to prove a single thing, all you can do is abitrarily pick a temperature that SEEMS logical out of your head to plug holes in your scheme or use 'stored coolth', turn on a grainger fan. Everytime.
You need to pick one is the whole point. You have a forced convection situation. Explain how your attempt to work out the radiative heat transfer ceofficient has anything to do with FORCED AIR convecting heat to a cool slab?
you made me blow the dust off of a couple books,my sources are 50/50 on that one. I said I was being anal in the first place.
No not anal at all, very significant. The insignificant changes in MEAN temperature seem to come up with the same air flim resistance for heat flow down from a warm surface to the air beneath it.
But the point is, the approximation has nothing to do with the heat transfer from air to the floor slab. It does point out however the loss of 'coolth' (I love your buzzword) as the ceiling radiates more heat to the floor slab than the the floor slab radiates back.
I don't have that figure, but why rely on some figure when you will not even use a pyschrometric chart?
Since you want to avoid calcualting convection coeffcients why not take the easy way out. Treat it as an air flim resistance with moving air, R0.17. Hey just need the air blasting down from the ceiling at 15 mph and the required differential is 2.16F to get rid of the space sensible gain of 15,550 Btu/hr in a 1225 sq ft buidling.
Get a Big Ass Fan, like used in a barn and blast the air down. Only need 1320 FPM at a 1225 square foot slab.What do the fan laws say about power requirements now at 1.6 million CFM? Plus you need to deal with the sensible heat from the make up air, so you can do the math on how it increases the required temperature differential maybe 75 is a valid slab temperature after all.
Sure looking more plausible once again to directly cool the outside air before it is introduced to the space using a swamp cooler. Air flow is only a thousandth of what your scheme potentially requires. OSHA regualtions would not make the occupants wear eye protection either. , just non-slip foot wear :)
Lol if it is melting ice, then it is stealing your 'coolth'
The case has always been forced convection heat transfer between air and a cool slab, your estimate of NATURAL convection by equating it to radiant transfer has back fired so please move on.
I think if you invested the time to do so you would become disillusioned with your scheme and come to the conclusion that you are replicating the environment found in a natatorium.
I can't force you to do anything, let alone prove anything.
Lol, sticks and stones. The cooled slab is the cornerstone of your plan and you have no real clue how cool it will be or the rate of convection between the air and the slab. So you are there with your beach towell enjoying the natatorium.
You are like the marketting guys in Dilbert. :)
Read through your HOF on evaporative cooling, try it on a pyschrometric chart. You seem weak in the area of pyschrometrics.
I pointed out that your cool slab will increase heat transfer from the ground into the slab and therefore the space. You countered with a vapour barrier which is negligible with respect to insulation value.
So if anything the vapour barrier stops loss a 'coolth' with water flowing from the slab to the earth, but has virtually no effect on the rate that heat conducts into the slab from the ground. Actually from the surface of the surrounding earth through the ground and into the slab.
Wonder what the dewpoint of water in soil is when the when the soil at
42F is in contact with a pipe at 37F.Mositure is drawn to the cold is the point.
I disagree. Then again, you seem to get more obtuse when losing :-)
As far as you can tell :-) But that's incorrect...
I do? Why? Owner's choice. OK, let's pick a)...
Nope. No fans at all. No convection, just radiation.
It doesn't :-) You still seem confused. Maybe that's good.
OK. I'll try it. Gr = 4x0.1714E-8(460+80)^3 = 1.079573184 Btu/h-F-ft^2 at
80 F, so we need dT = 10K/2K/Gr = 4.631459983 F to radiate 10K Btu/h from ceiling to slab. Using a closer mean Tm = 80-dT/2 makes Gr = 4s(460+Tm)^3 = 1.065743771 Btu/h-F-ft^2, which makes dT = 4.691559206 F... also < 5 F. If you consider that 0.06 F temperature difference "very significant," you might enjoy doing another iteration :-)I have no idea what you are talking about. Do you? Air is transparent to radiation, so there is zero "heat flow down from a warm surface to the air beneath it."
Agreed. It only concerns heat radiating from ceiling to slab, but that also bounds the room air temperature.
Exactly. BTW, one special benefit of this system is the reduction of the mean radiant temperature for a person inside the room, which makes for more comfort than cool air alone.
I'm more comfy with simple equations in plain ascii.
That's a bad implementation of plan b). Plan a) has no fans. BTW, my example concerns 10K Btu/h and 2K ft^2, vs 15,550 Btu/h and 1225 ft^2.
No thanks :-)
Would you have any evidence for this article of faith?
You may be projecting here. This is science! Let's use numbers!
But the dry soil beneath is great insulation. And thermal mass.
Sounds irrelevant, in this case.
Nick
I've been watching your thread with Abby N. and I just have one question for him? Just which Law of Physics does this "Mositure in the soil would be attracted to a cool slab" Attraction conform to? Is this "attraction" thermal, electromagnetic, or some neuclar type, large or small? Does this attraction cause movement? Ok well I guess that is 3 or 4 questions......Inquiring Minds want to know.......
Me I am always interested in NEW Physics Laws.........
...
Normals comment is completely unscientific but perhaps it's simply an observation from an unscientific observer. For example, moisture is not "attracted" to a cool glass on a warm day but beads of water form on the outside anyhow. Perhaps this is the phenomena that Abby was trying to express.
Anthony
When the moisture beads are condensed on a cold surface the dry air surrounding it gets mixed with the normal damp air again. This continuous movement toward a cold surface is known as "attraction"
George and We> > snipped-for-privacy@ece.villanova.edu wrote:
slab, the insulation
just have one question
the soil would be
perhaps it's simply
example, moisture
beads of water
phenomena that Abby
And:
The problems of moisture in and under a concrete slab-on-grade are a problem of vapor transmission through the slab. The attraction or flow of moisture to the surface is the normal flow from a point of higher vapor pressure to a point of lower vapor pressure to create equilibrium. By controlling or lessening the rate of moisture transmission in slabs-on-grade, we can successfully use impermeable systems on these surfaces.
A similar phenomenon occurs in concrete and masonry walls/slab floors. Capillary action is a function of the natural attraction between water and the capillaries found in concrete and masonry.
Nice site. Thanx
Or the fact that a clear sheet of plastic on the ground will become damp on the underside. In many areas, water is evaporating from soil on sunny days (and replenished on rainy ones), so anything like a tarp that traps that evaporation gives the appearance of 'attracting' moisture on the underside.
daestrom
Lol, losing, I am just sitting here watching you tap dance.
I keep challenging you to prove your scheme in a realistic situation. I gave you a small home with a modest heat gain, it included gains which you constantly ignore in your cooling schemes, heat from the sun. Cooling is not based on the difference in outdoor/indoor air temperature alone as you have a habit of assuming.
Over and over you have unrealistic low loads to deal with and base your schemes on average temperatures. If you used an appropriate load, then your use of averages would allow you to properly estimate energy consumption. There will be no comfort under peak conditions because your system cannot handle it.
It is a good thing you do not make your living by designing for comfort.
You keep trying to make it a larger home with even less of a cooling load. 10K in a 2000 sq ft home may be theoretically possible, but extremely rare, not many windows. A sensible gain of 15,500 Btu/hr in a
1225 sq ft home with a 106 ambinet exceeds 'good' when compared to insualtion levels and shading typically in use in the SW.So a 15,550 sensible gain on a 1225 sq ft house in the sunbelt with a triple digit ambient is a low load for you to prove your scheme by.
Your previous schemes had you cooling a slab, and then arbitrarily using a 21000 CFM ceiling fan to cool the air by blowing it at a slab.
My arguements are
1) The heat that evaporates the water does not directly come from the air, it comes from the slab 2) The cool slab, draws in heat from the earth 3) Only a small portion of the air being blown down to the floor makes contact with the slab and is cooled by the slab. 4) You allow the heat of the make up air directly to the room air 5) You plug holes in your scheme with unproven claims like 'stored coolth' or randomly picking fans out of a grainger catalog without validating they are even close to being sufficient.You then suddenly change this to radiant cooling, I realize the thread is called "Cool Towers" but you were the one mentioning your wet slab scheme again, and you claim you have a scheme that provides evaporative cooling with minimal air flow.
The main arguement is evaporative cooling can work but you need a swamp cooler to cool the outside air directly. You are making the house a swamp cooler, and ignoring the unhealthy environment you will create, you are going to make the inside of this home as warm and as humid as a natatorium unless you move some significant air.
So you are showing that the ceiling radiates heat to the floor. What does the floor do with this heat? It evaporates water to humidify the space. I already conceded a well insulated ceiling at R40 so the gain here is pretty small, the ceiling will not be cooler than the indoor air.
The only time I have ever seen a ceiling cooler than the space was a dropped ceiling, below an insulated attic, with Airtex Modular Radiant Ceiling Panels. Just had to keep them above the room dewpoint by a few degrees and they did remove some sensible heat from the space, air naturally convected up to them. I guess you would radiate some heat directly to them as well
As I was trying to explain to you earlier, as can be the case with a hot water radiator, the magnitude of heat radiated into the room is a similar magnitude of the heat that convects into the room. So using the temperatures you can quickly calculate the radiant heat transfer coefficient based on the MEAN temperature and equate it as being roughly equal to the convective coefficient.
So for an e=0.9 ceiling, the air film resistance for heat flow down is R0.92, which is roughly equal to 1/1.079573184=0.926. Is this a pure coincidence or have you verified the air film resitance for heat flow down into a space from a ceiling?
see above
The heat through the ceiling would be approximately 2100 btu/hr based on R40 for 1225 sq ft.
Walls - 1600 (forgot to add walls for the 15500 total sensible gain) Glass - 8600 People/Appliances -4800
106 F ambient ,20% outdoor RH, 78 inside, 33N lattitude, 80 F soil 35x35x9 house, slab on grade, R19 walls, R40 ceiling, scroll back for the square footage of double glazing I gave you before on each wall, eaves overhang by 2 ft, eaves one foot above windows, internal shades on windows drawn.People cook and do laundry in their home. I gave you everything but an insulated slab as I do not think there are many of those in the SW.
You want 80F inside then neglect the walls and call it 15,500 that I was saying before.
A lower than average load. 10,000 sensible gain in 2000 square feet, no way. Look at the fenestration and load from occupants and appliances. Bigger home more glass. Sq Lit also states that there is no real overhang above windows in his area and I am allowing for some external shading.
People sitting by a window when it is -40F outside lose heat by radiation and feel cold and those who leave the blinds open in the summer will feel hot, in this situation it may offer them some relief from what you are trying to tell them is comfortable.
Oh you like equations yet you quote a figure. Like I said I do not have that one, perhaps you could scan it and give a link so I can see if you are not taking something newly discovered out of context again. Is it for air flow over a cool plate or a hot plate?
So sticking with equations, then use some equations that show how heat will convect from the air DOWN to the slab.
I used a simple published value, but it shows an extreme amount of air moving. A very reliable air film value too. R=0.17 for 15 mph wind, maybe you want to try 7.5 mph but the R-value increases. So you would need a bigger differential temperature. Maybe for still air it will work out to R=0.92.
15550/1225/0.92= a temp differential of 13.8F. Here let's use this temperature, I will give you this one.Room is 80 therefore slab is 66.2 degrees and you have to evaporate water to get the temperature down to that, water likes to evaporate at its wet bulb temperature so you need to consider that more than you think, plus you get more heat conducting up from the ground. Surrounding ground probably 80 degrees even a couple feet down, a simple flux plot could work out the increased conduction there. But that is for you to figure out, its your proof that is lacking.
AT 66F water will probably require about 1056 Btu/lb. Ignoring the extra load of heat into the slab from the soil and the radiation from the ceiling, for 15,550 Btu/hr sensible gain (excluding the make up air) you would need to evaporate 14.68 pounds of water in an hour.
Don't get excited , water evaporating off of a 66.2 degree slab evaporating into 80 db/66.2wb air sounds like your extreme worst corner of the comfort zone but then you have to exhaust air to avoid the humidity building up. Here is where your ship sinks.
At 80db/66.2wb indoor and 106F 20% RH oustide, for every CFM you exhaust, the make up air adds 1.08x26= 28.08 Btu of sensible heat each hour. So for each CFM, you would have to evaporate an additional
28.08/1056x7000=186 grains of moisture each hour. However each CFM of exhaust only gets rid of 5.8 grains of moisture. NOT GOING TO HAPPEN- natatorium for sure. Swamp cooler going to have a problem here as well.Take SQ lits data then for June 27.
Take a swamp cooler drawing in 106 air at 7% RH. Add 53.1 grains of moisture per pound of dry air and the air is cooled to 72.7. Conservatively assume the fan/motor adds a degree of heat, now you have
73.7 degree air. To maintain 80F, 15500/1.08/(80-73.7)=2,278 CFM. To maintain 78 F 17100/1.08/(78-73.7)= 3682 CFM with 126 pounds of water each hour.Less air, less water, outside air is filtered and don't have 2527 CFM of triple digit air infiltrating in, moving by like a sirrocco wind. Hope that attic is air tight! The floor is dry, no mold spores 'germinating' on the floor, swamp cooler has a chance of working.
This assumes you can maintain a 66F slab, and you neglect the extra heat coming in from the soil below. Heat into the slab from the soil will require more water evaporated into the air, then the upward spiral of the sensible heat of even more triple digit make up air and the amount of ADDITIONAL water needed to compensate for it.
A swamp cooler will move less air and use less water to maintain a lower temperature. A swamp cooler will have a dry floor and the outside air has been filtered. Ever go camping and get your feet cold and wet? Try and get a good nights sleep? Lol, just make them wear golashes inside with wool socks when it is 106 outside.
Just seems so much easier and so much more plausible to simply cool the outside air first and then blow it into the house, displace air to avoid the high humidity buildup rather than make people live inside of an evaporative cooler that does not work so good.
This is all on sea level, Pheonix is about 1100 ft but, making the home itself into a swamp cooler wastes water, creates discomfort, and will grow mold. Hot wind convecting heat directly to the occupants or a 74 degree breeze. You will be using extra energy for ceiling fans. If they could have done it with a fart fan and a dehumidistat they would of already.
Well lets just stick to your plan with the 21K grainger fan and see what it does.
Too bad you are going to be needing a wind tunnel in there.
Well what can I say, swamp coolers can work although posters from the SW already point out the maintenance problems and their limitations. This is the proven system, you keep proposing a revolutionary system that cuts down on the air flow,but you have not proven anything yet.
Well yes lets stick to numbers, calculate the temperature that the slab will equalize at. So far you have been assigning arbitrary values to everything. To spare you the double iterations, take the room air as an average temperature of 80F, averaged from 6 inches above floor, middle of 9' high room and 6 inches below ceiling. Or work out the convection coefficient and keep in mind that water will evaporate at its wet bulb temperature.
Sq lit's link shows that soil is easily is 80F 20 inches down. The house is slab on grade, you may have to use a flux plot .
You can ignore the moitsture migration then, assume conductivity of dry compacted sand.
Well do some research on ground source heat pumps then, in particular in a cold climate. In a horizontal loop configuration,in the heating mode the pipes get cold, even making frost below the frost line. Mositure in the soil will migrate towrds the pipes.
These type of systems extract a lot of latent heat from the ground, and can you can use less pipe per nominal ton than places in warmer climates with dryer soils and no snow cover.
In air conditioning mode, the pipes become warm and moisture tends to migrate away from the pipes.
Perhaps search "moisture migration" & "ground source heat pumps" and avoid the atricles that deal with buildings having moisture migration problems through the building envelope. It is a difficult concept to properly allow for in the calculations, a lot of papers out there discuss the difficulties in adequately allowing for mositure migration to the cold pipes.
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