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I can’t decide whether , for new construction, I want RFH in slab, or above ground, on joists, maybe using Warmboard.
With above ground, I can insulate under floor really well, but it’s going to cost considerably more. But I always have access to underfloor for utilities. However, I keep reading that, with minimal peripheral insulation under slab, there’s “no significant” heat loss into ground.
Has anyone seen a real study about heat loss and slabs, and whether/if more foam insulation under slab actually would be a good idea? This is going to be a small, retirement home, and as I’ll be on fixed income in a few years, I want to be as energy-efficient as possible.
Replies
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Jim:
Regarding the heat loss into the ground, I believe the standard rule is to use a minimum of R-10 (2" of rigid foam) under the slab as well as perimeter. Heat travels first through any conductive path before radiating much. If you have a solid connection to the earth under the slab, you will heat the ground first.
Visit http://www.heatinghelp.com/ discussion board called "The Wall" and solicit the pro's advice. They will tell about the great lessons learned from Levittown, NY, where even in the dead of winter, momma can grow posies in her front yard. Un-insulated slabs....
Barry LaDuke
[email protected]
*Barry,Ironic that you, the Warmboard guy, are the first to respond w/ slab info. Thanks. I must say, though, that while heatinghelp.com had some amusing stories, there was not much there--that I could find--in the way of specific information.Here's the sort of thing I've been finding:from: http://www.radiantheat.net"Insulation below a Radiant Heated Slab: Rigid Polystyrene or Polyurethane with a recommended 2" and a minimum of 1". For colder weather climates, the proper insulation under a radiant slab will greatly increase the efficiency and responsiveness of the system. The "how and where" of adding the insulation is based on the required system designed and the gained cost/benefit relationship. 'b' In milder climates, the use of insulation under the slab is generally not required nor recommended. " (My emphasis. Is Oregon coast a "mild climate"? And, why would insulation be "not recommended"?? Intuition sez to always insulate, but could there be a negative efect, somehow, in some cases?)"The perimeter of a radiant slab accounts for the majority of the slab heat loss. "John Siegenthaler, who is apparently one of THE experts, also says 2" at perimeter and min. 1" for interior."But, when you compare that amount of insulation, under slab, to a raised floor with, say, R-19 under it, what might be the comparative heating costs? Maybe I'm splitting hairs, but it would be nice to see some studies, instead of a lot of prescriptive and/or anecdotal advice.I don't think energy costs are going to get any cheaper, and propane, which will probably be my fuel of choice, is fluctuating like crazy. Heck, w/ that cheap hydro power up there (.05KWH, last I heard), I might ought to consider ELECTRIC water heater for boiler.
*And here's another slant, from http://www.low-e.com/Product_Line/Slab_Shield/slab_shield.html:"Slab-Shield™ insulation was designed specifically for under slab applications. Manufactured using two separate layers of 1/4" polyethylene foam with a pure aluminum center. This product is available in 4' x 63' rolls for easy application. It is simply unrolled and taped together ( this is necessary for a complete vapor barrier to be achieved). Slab-Shield provides excellent thermal performance, and a vapor barrier all in one easy to use product. With Slab-Shield there's no time wasted installing 4' x 8' foam boards. With a puncture resistance of 92.9psi you can work and walk on top of it without it crumbling apart. One major difference between Slab-Shield and the foam board insulations is the pure aluminum center-leaf of the Slab-Shield. It is important to know that in most applications the ground temperature that the insulation is going to be directly exposed too is a constant 55 degrees or lower all year round. Mass insulations ( foam boards/extruded polystyrene) even though they may have high R-values( depending on thickness) will eventually stabilize at that constant ground temperature of 55 degrees or lower depending on climate. Therefore the mass insulation provides a constant drain on the radiant system and the slab. This is where the pure aluminum center-leaf comes into effect. It provides an effective thermal-break between the foam layers and prevents the slab or heating system from seeking the cold ground below it, thereby allowing the slab to reach temperature sooner and help the system perform more efficiently. "According to thses guys, foam by itself has a negative effect.Malarky, or not?
*>Heck, w/ that cheap hydro power up there (.05KWH, last I heard), I might ought to consider ELECTRIC water heater for boiler.Alternatives are geothermal and http://www.aquachillers.com/ and probably others. Do a cost analysis. From an elec water heater I get Also, do a search on warmboard. There was a discussion in this folder w/ Barry a couple months ago.
*Just how many BTUs do you get from a Kilowatt of electricity with a geothermal heatpump...to radiant floor system?Here, I pay right now...... .11/kw electric... $1. oil $1.30 propane. That makes oil least expensive during the coldest 4 months averaging 32 degrees...nights at 20 degrees. propane costs twice oil..and electric resistance 4 times oil. My heat pump does semi OK for spring and fall...maybe as good as propane.near the 34 degree stream,ajto answer my own question...It seems like Geo here might equal $1.00 oil.
*Total malarky in my opinion. Put your hand on a piece of foam that is outside in cold weather. Your hand will feel warm foam almost instantly.near the stream,ajAnd as to the water chiller company...I think they are blowing smok too. They say a Michigan home went from $400 midwinter bills /month to $45 bills for electric heat pump heat..... Bull. they say they heat their 15,000ft building for $119/month.
*Barry....What does your board cost?near the stream,ajAlso....your name has me thinking of some Southern Supremissist guy that ran for Congress....?:
*Hi aj! Long time, no chat. Getting out on the skis much? Here yesterday I was out in shirt sleeves cutting trees and clearing brush![The bad dude is David Duke, not Barry Duke.]You might have to check my math be/c some of this is from memory and the website I used for conversions is no longer active.Ground-source heat pump. Mine draws 13.9 amps. It's wired 220, so that's just over 3000 watts, right? For that, it produces approx 56,000 btu/h, possibly a bit higher, as measured by temperature differentials and flow rates and other stuff I don't remember. The water heater is rated at 4500 watts, and so is drawing 20 amps. I don't remember the exact conversion of watts to btu/h, but I seem to recall the water heater would add about 1 ton, or 12,000 btu/h capacity to my system. We left the element in the hot storage tank to give a boost to the system if needed, but when I ran the numbers and saw that the heat pump would give me the first 56K btu/h for roughly 10 cents/hr, while the water heater added only 12K btu/h for 14 cents/hr, I locked the power to the heating element in the off position.We pay 3.1 cents/kw some hours and 4.5 others. That's a time-of-use structure. Regular is 7.1 or 8.1, I think.Does that all hang together?JimOn the ps, if the water chiller is malarky, then I won't mention it again. I was just passing on a company name I got from an HVAC contractor who had not used it. Caveat emptor.
*I am building a log home for the first time and am trying to figure out the "best" most economical way to do the roof. The roof will have purlins with 2x6 tongue and groove decking on top. I have had three ways recommended to finish the roof.1.Put down another set of rafters 16" with fiberglass and plywood etc.This is what I am comfortable with. 2. Use R Panels, but if the roof is complicated, which it is, the labor involved is not worth the high cost of the panels. 3. Build a rafter system 2" on center divided into smaller "boxes" and fill with insulating foam. Then apply your plywood on top of that. This has been recommended to be the best way with complicated roof lines. Anyone have any ideas or experience with these methods? Thanks Mark D in Colorado
*Uh, Mark, you need to "Add A Discussion," not post a question in this thread.And, guys, my discussion has gotten side-tracked into this fuel thing, but I guess I opened the door. And I hadn't thought of fuel oil before; it seems like a Northeast/Midwest thing; I don't think I've ever seen a fuel oil fired water heater.......but I guess they exist? (I've seen ads for fuel oil fired boilers that claim to burn cleaner than anything.AJ, what you said about putting your hand on a piece of foam when it's cold outside. Well, you can put MacDonald-hot coffee in a styro cup that's about 1/16" thick, and not burn your hand, so why can't you insulate a house with, say, 1/4" styro?I found a cute fuel cost calculator at:http://www.warmair.net/html/fuel_cost_comparisons.htm
*Jim:As you saw in aj's post, there is some common sense that can easily substitute for the current lack of empirical data. Also, the aluminum core in Slab-Shield will CONDUCT heat away from the nearest source (the foam).The aluminum shield must have some value for the infra-red radiation energy that many knowledgable people claim is part of the equation for heat transfer from the heated slab to the ground. A reflective layer would turn that energy back, supposedly. My belief is that the primary heat transfer method in play here is conductive (through solid objects) and the more solid, the more conductive. The aluminum is extremely conductive, so using it under a slab (even between foam layers) seems counter-intuitive.Heat transfer will always follow the path of least resistence, so the simple rule will always insure that if you have more resistence in the direction you don't want heat to travel, it will go where you want. More resistence under than over the slab. Simple as that. If you have R-10 under, and an R-3 or 4 carpet/pad assembly above, guess which way the heat will want to go?Mark Eatherton, Denver RPA chapter prez and radiant teacher at a community college there, says the rule of thumb is 2X the resistence in the opposite direction you want the heat to go. Others have offered 3X as a better rule. I guess the 3X people are also the ones who typically over-engineer everything to MAKE SURE they get what they want from thier design (and avoid lawsuits in the process).The "thermal break" principle has worked in nearly every energy-related building product I've ever worked with, including windows.If I had a slab, or any other substrate, generating heat with my hard-earned dollars to comfort my family from the cold, I would make darn sure that it was thermally-broken from the earth.BarryBTW: I checked your website, Hidden Cloud. First time ever I saw such a unique structure. I loved your literary reference (particularly because it was so obscure to me before then).
*We should take the cost discussion offline. E-mail me directly.Regarding supremist references, I am color-blind. Up here in Oregon, after you've been here a while it doesn't matter what color you started with, you'll turn to rust color by spring. You can always tell the lazy ones, though. They turn moss-green on their north side. ;-)[email protected]
*Insulate under the slab. I wouldn't advise using the insulation with the Al embedded in it. Barry is correct, it will act as a conductor. It can only act as a radiant surface if there is a continuous film of air on its facing. If the Al is in contact with anything, including a foam layer, it essentially loses it's ability to radiate and turns into a conductor. We can thermodynamic this thing to death, but in a nutshell, that's my opinion.As to the RFBI having "zero" R-value becuase it has assumed the temp of its environment...well, if it's 55 degrees below the insulation (in the earth) and 55 degrees above the insulation, yes, the RFBI will assume a temp of 55 degrees throughout its thickness. It will have zero resistance to heat, because no heat is attempting to flow through it. If the earth is 55 degrees and above the RFBI it is 56 degrees, the RFBI will now reduce the flow of that delta-t through it at its stated R-value. Insulation can only insulate when there's a delta-t across it's thickness.I'm from the "3x the R-value below the RFH as is above" school. Reason? It does "force" more heat upwards, resulting in less loss downwards. However, as soon as the homeowner throws down an area rug with pads...all that extra "R" above the floor may force you back down to the 2x rule. And if you started with 2x...Minor point, but I have seen a room "on the thermal edge" become too chilly due to too much of the floor being covered with "stuff." Easy to fix by tweaking the gain to that room, but again, a minor point.Warmboard seems to be a decent product. Good points, bad points, though both seem somewhat petty. I can give you feedback on a house it's going to be going in up north, up on Lake Champlain (framing now). Only real gripe I've heard, is the damage that can occur to the boards (before the finish flooring goes down) while the trades are working in the house.
*Mongo, Barry, and others,This is great stuff, thanks.Regarding rugs and such, I've always wondered: If you have wooden floors, rugs, or anything else that heat does not readily "pass through", then what happens to the heat? Does it travel laterally, assuming that's the path of least R?I've been assuming, by the way, that if I went with slab, I'd have a stained concrete floor, w/ NO rugs. If Warmboard, then tile, w/ NO rugs.
*Jim, Rugs will increase the R-value of the floor. Makes sense. All the rug will do is "slow" the heat as it moves up into the living space. With the greater resistance upward, more heat may also go downward. Not a real big deal, actually. The heating heats the floor, the floor heats the rug, the rug radiates heat through the room. However, the next time you're in a RFH home that has an area rug, and the heat is on, slide your toes under the edge of the rug. You'll easily note the temp differential.Floor rugs can come into play when designing a RFH setup. Still, it's a fairly minor factor that will not cause problems in a typical house in a typical climate with a typical required heating load.The only time rugs really come into play in a negative fashion is with glued hardwood floors. With a rug over hardwood that's been glued down, the R-value of the rug causes the rug, essentially, to act like an insulating blanket over the hardwood. It can increase the temp of the hardwood, and thus the temp of the glue...possibly...and I stress possibly...causing the glue to prematurely crystallize, become brittle, and fail. This failure can occur in the glue between plies of an engineered wood product as well, though it's not nearly as likely. With gluedowns, as with engineered wood products, the manufacturer slides max allowable temperatures into the scheme of things. Usually the glue is the limiting factor in determining that temp.Regardless, feel free to toss down an occasional area rug on your RFH floors. Even with glue-downs, the rugs typically won't be a problem...unless you couple it with a big, thick (high R-value) carpet pad.
*Other than Warmboard, has anyone used Thermofin on top of the sub floor? You must use sleepers between the aluminum channels so it may be a bit more labor intensive. At $4.25-$4.50 per sq. ft. for the Warmboard panels it seems to be fairly expensive.
*Jim...This is what I have learned from you and all....That my electric cost is 2-4 times higher than many...That Geo is 2-3 times more efficient than an air sourced heat pump...sounds like the best system here is either Geo...or oil...as I have no gas line nearby.I really want a stirling powered combination electric and heat producing Geo attached system...or fuel cell...Oh and use a bit of solar and pull heat out of waste water...put fridges on outside walls with a door that opens to cool with outside air during winter...near the whacky stream,ajSo...with super low cost electric...it may be possible that the $119/mth story is true...
*Below...someone mentions $4.5/sqft....adds $4/sqft to the cost of the home.near the stream,aj
*I talked to the slab shield people today....Nice people...I still am scratching my head on this though...I am getting samples...I made a test today...sandwiched al foil in a towel against a cold window...one hand with al...one without...no difference...But still much better than bare hand to glass.I have heard that eventually the ground under a slab warms...so that insulation is really only needed toward the perimeter...Like if you had a Home Depot...no need to insulate the middle 1000' of concrete! My point then is...that if nuttin works, then anything works...so...lets ski!near the cool stream warming by the fire,ajand skiing!
*AJ,I confirmed at several places yesterday that with KWH at .05, and propane at $1.65 (down from a high of $2.20 last winter), I can heat water a LOT cheaper w/ electric. Propane would have to drop below $1 for it to compete, and that's without tank install, rental, etc. And I love the idea of being able to stick a little low boy electric just about anywhere, like in the wasted corner of a lower cabinet in kitchen.I'm assuming, also, that since my little retirement cabin is only 500sf, even if electric costs escalate significantly down the road, I'll be OK. (One side question for any techies out there: w/ 2" foam under slab, R-19/R38 above, 2 zones, will a WH w/ 32 gal first hour recovery be big enough?)Haven't confirmed, but some claim oil fired water heaters are finicky, w/ burners clogging; and oil prices (I think) are about on par w/ propane, though there are more BTUs per gallon. I admit I haven't done any research on oil prices in the area, though.Correct me if I'm wrong, but ain't Geo damned expensive to set up? And I've PROBABLY got bedrock at 3-4', though we haven't done any excavation yet.
*I've been told that it's most important to insulate the perimeter of a slab as far in as the footer is deep. Beyond that, there are diminishing returns. I don't know if this is true or not.$4+ sounds expensive to me, also. Of course the gypcrete was also $$$, but not near that much. And I wouldn't have saved the cost of a subfloor--though most people would--if I had used warmboard. We needed a goodly chunk of plyform for forming the main floor, and we reused it as the subfloor for the gypcrete floor.>That my electric cost is 2-4 times higher than many...That Geo is 2-3 times more efficient than an air sourced heat pumpThat sounds ballpark. Gotta admit that our utility has a real nice time-of-use structure--saves me about $80 a month, with minor, minor inconvenience. Most utilities don't make TOU as convenient.My contractor tried to talk me into some solar. Aside from the issue of covenants requiring approval of the neighbor who hates me, the economics didn't support it. When the sun's out, we get such good winter solar gain that we wouldn't need the solar panel btu's. When we'd need them is on the overcast days. Now throw up a few windmills, and we've got something to talk about!
*Jim...For your numbers....Propane is out.Oil would cost about the same as air heat pump. Air heat pumps are very cheap to install. Maybe $3000 for your cabin. The aquachillers would cost maybe $7000 or more so they say at company...I have no component costs...Geo would cost $10000 or more. Oil would cost $6000 Electric...If you want radiant floor electric...they make it...no water needed.near the stream,ajOh.,..I installed an oil fired water heater back 5 years ago...I do not maintain it...so have not heard of any problems. Like any boiler, it has a standard gun. Servicing needs to be done annually at least so as to maintain them. All the horror stories I have heard were when serving annually was skipped.
*Jim,There is no question that geo is expensive to install. We fell into it by having a well already drilled, so that money was already spent.For a 500' cabin, and this is only a guess, your monthly savings from geo would never in your lifetime pay back the higher installation costs. For convenience and economy, I'd stick with electric and just be careful to size the water heater for my btu/h heat load. Easier maintenance out in the boonies, if nothing else.If you insulate your 500' as well as David Thomas insulated his 1700' (or whatever) place in Alaska, then a tea cup warmer should be sufficient to heat! :)aj, good point about the electric floor radiant. I'd forgotten about that, but for only 500', could be realistic.........
*So Jim...your best fuel costs...Geo/electric by far the bestelectric heat pump or oil boiler nextelectric resistance nextpropane last and rediculous.Best...10 Xs better than worst (GEO/el)then...5 Xs better for Hpump or oilthen 2+ Xs better for resistancenear the calculating stream,aj
*Exactly...insulate it well and put a monitor heater on a wall for $1500 dollars plus $300 for a 275 gal tank set up and be done with it.The monitors get like 90+% efficiency and are very quiet while running too.That's it Jim...look no farther...near the end of the stream,ajMonitor also makes a tankless water heater...Hook it to a tank...and you have hot water..and radiant floor....and 90+% on Kerosene which will be half the fuel cost of electric for water heating.
*Jim, Go with the slab on grade with a vapor barrier an 2" of xps foam under the slab and along the slab edges, down a foot. The slab has a lot more thermal mass that the warmboard so it makes for more even heating. My recommendation would be to run the whole setup using continuous circulation with outdoor reset and an indirect hot water heater for DHW. TekmarControls makes a good control for this setup (Tekmar 363). Take a look at using a condensing gas boiler like the new Munchkin - it's 91% efficient and fairly compact.Did the research, installed my own RF heat system and will never ever, ever go back to living in a scorched air home.
*Thor....Good post except Jim's gas cost is sky high like mine. He needs to heat with oil or electric. I think the monitor would be great..or a Toyo boiler.near the never gonna freeze this year stream ,aj
*AJ,I hadn't thought, either, of putting electric resistance heating directly in slab; how do you do it? I guess I've seen something like that for under bath tiles. H'mmmmmm......Yeah, Thor, I put a cheapo home-rigged hydronic system undet a little studio I build for my wife in the backyard, and that's where we both want to be in the winter. Don't know what you mean, though, by "outdoor reset", and what's the Tekmar control? My little system is just a pump activated by a simple line voltage thermo.Thor, or anyone,Have you seen any good clips for attaching PEX to foam? Or just go w/ hogwire? I've decided that since it wouldn't cost that much anyway I'll put 2" foam all the way under slab.Other issues, re foam under slab. I've read that termites will burrow in them to make houses, so get borate-treated foam. And, since there are always gophers, should one go this way, from bottom up: roadbase to deter gophers, sand for easy leveling, 6mil poly moisture barrier, 2"foam, hogwire, PEX, concrete? It would be nice, though, if there were some sort of push in clips that would hold PEX up a bit from foam.
*>Have you seen any good clips for attaching PEX to foam? Or just go w/ hogwire?We used 6 x 6 construction wire in addition to the specified rebar. The nice thing about the wire is that we could keep the PEX on perfect lines and with perfect curves. Makes for a neat and organized layout.
*Yeah, I used that in my studio job (we call it hogwire), and tied PEX to it w/ cable ties, but it didn't want to lie particularly flat. I was just going to be filling the space w/ gravel for mass, so I put brick pieces on it to hold it down, but I wouldn't want to do that in a slab.
*Look what I just found.http://www.radiant-tech.com/heat/catalog/pexacc2.htmScroll about 2/3rds of the way down the page to the screw clips.
*I must respond in Warmboard's defense, if only to keep the facts straight about "even" heating and Warmboard's comparison to thermal mass.Thor said: "The slab has a lot more thermal mass that the warmboard so it makes for more even heating." Warmboard tests at 1 - 3 degrees delta T at cold start, and evens to better than that within 1/2 hour. That is with a 12" spacing! Slabs usually run 7 - 9 degrees during warm-up, eventually evening-out as Thor said. Warmboard has no disadvantage regarding even heat--in fact it obtains a greater degree of even heat at the weakest point in the cycle.
*It just dawned on me why people are so adamant about perimeter insulation as being more important than underslab... right now it is just a theory on my part, but it has basis in thermodynamic law.At has to do with the rate of heat transfer. Heat will always transfer at a greater rate with a greater Delta-T (difference in temp). If it is -10 degrees air temp, and ground is assumed to be about 55 degrees, the greater delta-t is outside--near the perimeter. Heat will want to go there faster than going to the ground.It's kinda like a stream--yeah, that's it--with eddies and currents. Some water moves faster and some moves to the eddie. Some heat rushes to the perimeter on the days significantly colder than 55, and some heat meanders to the ground. Not much gets to the house until the delta-t between the slab and the ground is narrower than the T between the slab and the inside of the house.Better to insulate it all. Perimeter and under.Maybe, just maybe, one would rethink the whole slab thing and frame the floor above the ground. Just think of the possibilities then....Just thinking out loud. Does my theory make sense?Barry
*Barry,If SOMEONE would build 2 test houses, one with insulated slab (say 2" foam all the way under), and one with raised floor with moderate insulation, say R-13, use same hydronic systems, and then monitor costs to heat.........we might get somewhere in this discussion.Course, that still doesn't help me roll my table saw out on the carport during nice summer weather.
*So far, the only electric resistance mat website I can find that supplies prices up front (warmlyyours.com) quotes about $9 SF for the stuff. Seems way over-priced to me for resistance wire fastened to a big weave FG mat. But I'm going to keep looking around; this is an idea worth pursuing.
*Jimhttp://www.floorwarm.com/products.htmlhttp://muextension.missouri.edu/xplor/agguides/agengin/g01408.htmI did a google search on "electricl heat cable concrete" and these where the first two that came up.
*BarryYou are partially right. The ground in the center is had both a thermo conductivity and a thermal mass. There thermo conductivity is fairly low. But you have a large mass of dirt that you are heating up.I don't have any number to plug in, but it would not be that hard to calculate for the steady state conditions. I think that I have seen the number years ago. I am sure that they are published. Someone in your tech support department should know them.
*Bill,I found plenty of places, through google, that sold electric resistance mats, but, as I said, the only one furnishing prices was exorbitant. Your U Missouri farm brochure had an interesting statement, though:"The cost of pre-assembled heat cable mats with a 40 watt per square foot density, but without accessories or installation, may vary from 15 to 22 cents per square foot of heated floor area."Maybe if you say you're heating a barn floor, instead of a, um, Fine Home, you can get the mats reasonably.
*Here's an interesting overview, from a reputable source:http://www.oikos.com/esb/48/radiantheat.htmlBack to google......
*Bill in message #38 has it right, Barry. (With the exception of there not really being a steady-state, the earth is essentially an infinite heat sink to your house. But in a year, you'd be really close to steady state - only very small changes to follow.)Not insulating under a slab definitely effect response time. Imagine pushing a Yugo around. Versus a Ford Exploder. The small mass can be moved quickly to where you want. The large mass takes longer to get moving and, if it has speed as it passes your target, it will keep going (overshoot).Not insulating always loses heat to the earth (middle of the slab) and the environment (perimeter). The loses to the earth diminish over time as the earth under house heats up. Loses to the perimeter continue.TWIMC: Barry is right about the advantage of aluminum versus concrete. Aluminum is 160 to 230 times more conductive (per cross sectional area) than concrete. So 0.01" of aluminum can spread as much heat, as evenly as 2" of concrete.Thermal mass is good to carry the heat of the day into the night. And it is good if you have a power failure (my house stays warm for 36 hours). But otherwise, it makes it harder to control the heating system for steady temperatures, harder to drop temps at night when you sleep, and makes the house less safe in an earthquake. -David
*One unanswered question: If I go w/ electric water heater for hydronic radiant in slab, is a 30 gal unit w/ 32 gal first hour recovery sufficient? (500sf house, 2" foam under, R-19/R-38 above.)One system I've seen mentioned several places on web: electric resistance cable buried in sand under slab. Presumably w/ insulation under it. But I'm still trying to get reliable costs somewhere between the $9 sf and .25 sf mentioned above, for just cable alone.
*I'm trying to imagine trying to fix an electric blanket.... With that blanket buried in or under a concrete slab.
*Luka,I'm starting a new thread on resistance radiant floor, since this one has gotten so far afield. Hoping someone will respond here to my question above, though.
*Just moved my thoughts on electric slabs to Jim's new thread: "Anyone Using Electric Resistance Floor Heat?"
*I'm a lurker here that has just installed radiant floor heat in my house. I did the staple up technique between the joists. Couple of comments, lot of work snaking 300' lengths of 1/2" tube back & forth by myself. I don't think the Al fins really do all that much. Pretty good heat, however there are hot & cold spots.The reason you only need perimeter insulation in slab heating is that dirt has its' some thermal resistance. When the heat is first turned on you begin warming up the dirt. Say your water temp is 90 deg. in the tubing, 6" below the tubing the dirt will only get to 80 deg. (guess) because of thermal resistance. This temp gadient keeps going down as you get further below the slab until finally the temp decreases to the surrounding earth temp. When this point is reached there is no further conduction (energy loss) because there is no temp. differential.The perimter insulation prevents heat being leaked outside to the cold environemnt, not the earth.
*From #22:>My contractor tried to talk me into some solar. Aside from the issue of covenants requiring approval of the neighbor who hates me, the economics didn't support it. When the sun's out, we get such good winter solar gain that we wouldn't need the solar panel btu's. When we'd need them is on the overcast days. Now throw up a few windmills, and we've got something to talk about!If you spend a little extra now to make the whole house really well insulted, then solar should be an excellent match with RFH. The up front cost for the insulation & collectors would mean lower costs down the road when you are on the fixed income. The thermal collectors do well at producing water ~120F which is just what the tubing needs. With a large (>200 gal?) tank, you'd store up the heat for those cloudy days. I suspect windmills (connecting to batteries, invertors, and electric RF mats) would be much more costly.As for the neighbor, what if the collectors were integrated into the roof rather than the dorky panel-on-legs that are more common? At least over in the UK they sell thermal collectors that look and mount just like a roof window. [see new thread]I'm currently working on my first RF system, but am already convinced that I'll never go back. And next time I'll plan it in before I start building so it will be easy to do!
*Well I woulda been paying about $10K for about 12K btu/h with requirements of 60K btu/h @ 20 degrees, or something like that. At best, even with a large tank, it woulda provided only auxilliary heat, and it's too much money for that. My contractor, who's done numerous solar and wind apps, was good-naturedly trying to talk me into both! Luckily, our geothermal is sufficient.You gotta know this neighbor (developer of development) to know why cooperation from him was outa the question. He tipped my prota-pot, cut my phone and cable, etc. Think he's gonna approve anything optional?!?! It became moot once we got our Wirsbo manifolds reconfigured.Agreed--RFH rocks!
*>You gotta know this neighbor...Good grief! One of my neighbors doesn't exactly like me, but this gives a whole new perspective to what could be. Now back to RF planning.
*Sounds like another Neighbor-From-Hell story. Ours isn't quite that bad, but he makes us glad we're moving soon.
*Makes me curious...if you stick a tube....ummm...errr...the hot air from a bad neighbor--what's the btu/h capacity?
*Yes, the methane quotient should be quite high.......
*Jim, outdoor reset is way of controlling the comfort in a radiant floor heating system. The control has an outdoor sensor that monitors the outdoor temperature as well as the indoor temperature. Since the system uses continuous circulation, the system tracks the out door temperature and increases or decreases the circulating system water temperature based on the out door temperature and the buildings heatloss reset ratio. What this means in plain english is that as the ouside temperature goes down the system automatically compensates for the added heatloss of the house so that the house is always the right temperature and your always comfortable.