Can any of the radiant people tell me what the normal amount of insulation is under a basement slab to be heated radiantly.  What I've seen so far (and used) is the Foamular 250 in a 2" thickness.  Seems thin considering the ICS walls boast R-26 and then the slab ends up on 2" of pink (R-10).  The slab at 100 degrees and the earth at 50ish makes 2" seem weak.  If it was 70 in the living room and 20 outside would we want less than R-19 for exterior walls? Is this a matter of economics?  Thanx in advance!

From Wikipedia: http://en.wikipedia.org/wiki/R-value_%28insulation%29

"The R value or R-value is a measure of thermal resistance used in the building and construction industry. Under uniform conditions it is the ratio of the temperature difference across an insulator and the heat flux (heat flow per unit area, ) through it or . The bigger the number, the better the building insulation's effectiveness. R-value is the reciprocal of U-value."

"The U-value (or U-factor), more correctly called the overall heat transfer coefficient, describes how well a building element conducts heat. It measures the rate of heat transfer through a building element over a given area, under standardized conditions. The usual standard is at a temperature gradient of 24 ^oC, at 50% humidity with no wind (a smaller U-value is better).  U is the inverse of R with SI units of W/(m²K) and US units of BTU/(h °F ft²)"

I created a really quick graph (below) to visualize the relationship of increasing R-value on overall heat transfer coefficient (U-value).  What is shows you is that ANY insulation is a good thing.  Heat transfer initially drops dramatically.  However, the relationship is not linear.  For example, R20 is NOT twice as good as R10.

You can use this information to create your own cost/benefit analysis.  You need to know how many square feet are you insulating, the temperature difference (delta T) from one side to the other, and the cost the insulating material, and the cost of heating.  For temperatures, I usually use design temp for the outside temp and the - desired - inside temp (say 70F).

If you want to get a good idea of how much heat costs to generate based on your fuel type and appliance efficiency, here is a good spreadsheet calculator from the US Department of Energy: www.eia.doe.gov/neic/experts/heatcalc.xls.

I hope this doesn't muddy the water.

Regards,
Ed

2" rigid is standard in most areas. some go up to 4" but the savings at that point are generally not cost effective.

Thanx Ed and Rob for taking the time to answer this rather mundane question.  The graph really helps.  I realized that the relationship was not linear but I am still not sure which way to go on this one.  We are heating homes with solar thermal panels via basement slab.  For the formula there is no cost of heat or relatively little (electricity for pump).  We simply allow the slab to heat the home not unlike many homes up here in the Pine Tree State that have a wood stove in the basement that allows the heat to flow under the floor joist.  We simply put 2 inches down because that's what the homeowner could afford.  We have high limits on the system but since the heat is free the customer tends to "let her go".  As the temp gets higher and delta T widens, I started to think of increasing the insulation commensurate with the walls on the next one which is coming up.  Since solar heat in the winter is precious heat we want to establish a standard to work with that make good thermal and economical cents particularly where insulation cannot be added later. 

Regards,

Flathead

Even with an airtight equation, there's no answer that will get you to the "correct" spot 100% of the time.

The soil underneath the slab will, to a larger or smaller or non-existent extent, act as a flywheel, or a capacitor, or a sink, for the heat that's transmitted into the ground. For example, consider the two soils - "sand", and "wet sand". Sand is an excellent insulator as far as soils go, and if you are lucky enough to have dry sand under your slab, you can add a few free R's to the amount of insulation that you install. Wet sand, on the other hand, or worse, sand with moving water, is a terrible insulator, and will wick heat away as fast as you can get it through the insulation that you install. 2 or 3 R's may not make that much of a difference when you are already adding 2 or 3 inches of insulation, but if you are adding only one inch, it could matter.

In the same manner, bedrock, if you are unlucky, is also a terrible insulator for the same reason that concrete makes a great radiant panel - both materials transmit heat well. Of course, with bedrock you won't have the water issue, but that issue would be the least of your worries if you are building on solid rock.

BTW, there are some excellent physics/thermodynamics experiments that you can do - once your slab is installed - to determine what the "effective R-value" is. For example, if you are building a passive wine cellar with no insulation under the floor, but heavily insulated everywhere else, you can increase the R-value of the floor by laying a piece of plastic on it. If you measure the temperature variation, from "before plastic" to "after plastic", you can determine the R-values of the insulation-less floor, which is in essence the R-value of the soil. Fascinating stuff. (BTW, if you are building a passive wine cellar, having sandy soil under the slab is a bad thing...)

Jeff

I highly doubt you can effectively provide 100% of heating needs with solar here in maine. Solar is not capable of the kinds of outputs wood stoves have, and if you are asking this question then the envelope is not being as tightly engineered as it would have to be to make 100% solar fractions feasible. Not that this is a simple question, just that to make such an envelope is very difficult and requires rigorous attention to glazing orientation and sizes, minute framing details, etc. and usually it would require someone who would be capable of answering this slab insulation question with real calcs.

PassiveHouse, for example, I think recommends 12" of foam under slabs. I think that's ridiculous, but if this is the level you're trying to get to, that's the kind of thing it takes.

Then should come the question of IAQ.

Jeff,

You couldn't be more right on (IMHO)!  I speak from no special education but just what we've learned so far.  Last year we did a slab heat on a 5000 sq. ft. Morton metal building.  It was put up ten years ago and the tech was to put only 2" of insulation for the outer 2 feet of the perimeter.  The entire center of the slab lays on the ground.  As you mentioned, dry sand good....wet sand no good but we weren't sure what we had.  The entire valley sits on an aquifer and the surface well water level appeared to be 4 feet down but that changes.  We have developed ratios of floor area to collector space but we really weren't sure with this one.  The boiler died in December and we worked in the cold till we got the system on line and by the end of Feb. the building got to 60 degrees which was the target temp.  It helped that the period Jan-Feb was the 5th sunniest on record in the N.E.  Point is we did not do a lot of calculating and we, fortunately, came out o.k.

"there's no answer that will get you to the "correct" spot 100% of the time."  This is so true from what we are learning!  Where we are this week the builder used sill-seal on all horizontal sills but nothing on the vertical step areas.  Not only can you feel cold air but you can see daylight!  The Generac guy drilled 1" holes for 1/2 inch copper gas line and the barometric damper goes horizontal with the air backing in the louvered  dryer vent.  How do you calculate all that stuff!

I like the b4 and after plastic test.  If I ever get a clear basement floor I'll experiment.

Thanx again for your insights!

Flathead

Rob,

You had me checking my original post.  Can't find where I made any statements of achieving 100% of heating needs with solar, nor likening the heat output of solar to a wood stove, nor attempting 100% solar fraction.  However, I'd be remissed if I didn't apologize for leading you in that direction being that I am a newbie and you are a veteran poster.  I want you to know I appreciate your comments however critical they might be.

Flathead

Hi Flathead,

this was the comment I may have misinterpreted:

"We are heating homes with solar thermal panels via basement slab. For the formula there is no cost of heat or relatively little (electricity for pump). We simply allow the slab to heat the home not unlike many homes up here in the Pine Tree State that have a wood stove in the basement that allows the heat to flow under the floor joist."

it sounded like that was the only heat in the house. My apologies if I got that wrong. If you're not going for 100% solar fraction then you probably don't need 12" of foam under your slab ;)