I am not so sure, A retired engineering buddy of mine once said that there was a huge diminishing return on R value after the first few R's so I realize we are talking about a glorified vapor barrier, but it may be worth the math exercise to see.
Dan

I would think it's more about what you get for your money( yes me the consumer speaking). What is the cost/R value ratio of a sheet of plastic/blue board combo compared to the product such as the concrete foil.

the testing has been done. nothing robust enough to provide a real R-1 under a slab is cheaper than, say, a small amount of EPS or XPS which could easily provide twice the R-value at a similar or better price. If you used cheaper stuff it would probably collapse entirely be not even be worth an R1.

here's one http://www.healthyheating.com/downloads/HRAI_Insulation.pdf

but in short, this topic has been studied to death. No study ever done has shown bubble foil products under slabs to either be good insulation, or even cost effective when compared to other options.

In very short, the stuff is junk and a total waste of money.

In my view, a radiant slab should have XPS under it, unless you have access to inexpensive btus like a woodlot. Natural gas will return to $18/mcf and higher, and it's not like you'll get a second chance. In Wis, Minn. and Canada, code requires full underslab coverage of R10, R5 and R12 respectively. But you should recognize that underground heat loss operates by different rules. Dunno how many heated slabs out there have insulation under them but nothing between the slab and the perimeter wall, where a thermal break matters a great deal.

My Slab is Insulated with spay foam. Just so you know if its done right it works wonders.
I have been a heating contractor for 19 years and this way seems to be the best for me any how.

I have 2.5" of spay on the bottom (closed cell) and on top of that i have solargaurd (slab shield) that runs up and over the foundation. At footings I have 2" foam board and that slab sheild runs up and over the top of the footings.

Its the way I have been doing all Big Buildings. Have been doing it that way for over 5 years.

It does cost more but its worth the money in the end.

Also the next trade show I atend You all are welcome to attend also. I have slab test sites Mobile with heat sensors under the slab in the dirt level with foam board on top of it. and also a unit the way I have mine with spary foam. You would be amazed at the diffrence in heat loss.

Also DAN, The spay foam your talking about is not closed cell. I have a sample of that also. Its been in a 5 gallon bucket of water for 5 years and it still is the same shape and weight evan just like it has never been in water at all..

So for a location such as Markdale, Ontario, Canada, what would the Code requirement and the recommended amount of closed cell XPS for under the slab in R Values?

Slab on grade.

I can't believe anyone is still buying into the notion that radiant barrier under slabs has any thermal value.  Solarguard as insulation is a waste of money- sheet poly works fine for a ground vapor retarder & gas barrier.

Use of spray foam under slabs can work, but density matters.  IIRC the Building Science Corp folks have recently moved away from using it under slabs after running into issues after initially being pretty keen on it.  (I don't have insight into what the issues were, but they changed the spec mid-stream on a project they were consulting on near me reverting to XPS.)

XPS works, so does EPS (must be type II or type III EPS, to meet code in Canada, low density Type-I not 'llowed).  XPS retains staples for holding tubing better than EPS, but you can go high-R with EPS and still get a net lifecycle reduction in greenhouse gas, since EPS is blown with pentane, which has a fraction of the greenhouse potential of HFC245fa use for both closed cell polyurethane and XPS.  With HFC245fa blown materials anything beyond ~R10 or so introduces more greenhouse gas the lifetime CO2 emissions offset in lowered fuel use of fossil-fired heating systems. Pentane blown goods have only ~1% of the greenhouse emissions of equivalent R x area of HFC245fa blown goods.

A guide to insulating slab-on-grade foundation to code in Canada can be found here:

ftp://ftp.cmhc-schl.gc.ca/chic-...28W%29.pdf

See also table 12.3.4.2.B  in this document:

http://www.mah.gov.on.ca/AssetFactory.aspx?did=8823

R10 is the generic code min for heated portions of  across all Canadian climates. (For unheated slabs it's ~R20 if only insulated on the first 600mm perimeter, bu ~R10 if  it's 1200meter perimeter.)  But the wing-R frost protection varies with local climate.

With radiant slabs adding R5 to the recommended center-slab values would be economic for most heating fuels, and even adding R8-R10 (for a total of R18-R20) may make long-term sense if propane or heating oil is your fuel.  Even if not strictly economic at today's fuel pricing, the costs of upgrading it now are a tiny fraction of what it would be later, should fuel inflation take off.  An additional R10 of type-II EPS adds ~$1.00 per square foot or ~$11/square meter to an R10 XPS-only solution.  Whether that's a cheap enough insurance hedge against future energy costs depends on where you think that's headed, but there's no going back once the concrete is poured.

I believe it. Snake oil has always sold.

As crows often line their nests with shiny objects, the ignorant believe that "reflective" insulation a.k.a. bubble-foil and "blankets" are the best sub-slab insulation. You can tell them that professional radiant floor contractors don't use it, but they "know" better.

If they refuse to hire a professional to design or install their radiant floor heating system and to follow the manufacturer's installation instructions well, there it is, as the say in Britain.

We use R-10 XPX for its resistance to heat transfer and its structural integrity during construction. As Dana suggests, with a year-round ground temperature near 50°F, the temperature differential is just not great enough to justify more. As with many building strategies, the law of diminishing returns should not be ignored.

We have designed many commercial building without under-slab insulation save at the perimeter where any heated structure must be insulated. When inside design temperatures are close to ground temperature, there is no return. It is all about the inside design temperature vs. outside. Guess or do the Manual J heat load, that's it.

Since price of natural gas has gone down for 6 consecutive years, a hedge against higher fuel bills (unless you burn electricity) should likely be a thin one.

I have two buildings close to the same size one with foam board insulation under slab and up the footings. And the other Our new idea that has close to the same idea but under the slab its down 6 foot and up the foundation then filled with sand and a 6 inch slab on top of that. I burn less fuel in the last setup then the older one. Also when we had a power outage for 48 hours the one with 6 foot of sand kept the temp and the other building cooled down over 10 degrees over the first night. that was under 0F durring the day and around or below -15F at night. Note there is slab shield on top of the foam board also in both setups. also both building are running the same boiler as the other set at 120F.

So I guess there is many ways of doing things. The best way we will only have to find out I guess.

Happy Holidays to you all!

Foil is for cooking.

The answer is: 4" of XPS capping the outer edge of the slab and extending in 4' with the "field covered with 2" of XPS.

Two things to consider; one is the year round ground temperature and two the temperature differential. Since the ground temperature will vary according to atmospheric temperature one has to assume extreme variation at the parameter but as you move to the center of any insulated slab the ground temperature will be very constant depending on ground water mostly. It pays then to find the frost line for your particular area and add insulation to overcome this extra load. Whether you cap the end of the radiant slab or follow the footer down is of no consequence. Doing both is simple redundant.

A proper radiant floor specific Manual 'J' design is the only reliable way to model heat loads for a building and determine specific heat loss to a given slab. Note, that it is not only the difference in ambient room temperature but the design water temperature of the radiant panel itself that will determine the heat loss to the ground below. Weather sensitive controls will also effect the extra load presented by the ground temperature in you area.

The focus for insulation is usually and properly focused on attics, walls and basements/slab-on-grade loads. The last thing I will leave you with is the philosophy of insulation in which one can say insulation is a hedge against higher fuel cost, generally makes you more comfortable and cannot be taxed once installed. Oh yes, slab sheild... http//www.ftc.gov/opa/2010/06/enviromate.shtm http//www.ftc.gov/opa/predawn/F93/permarpro7.htm www.healthyheating.com/Page%2055/Page_55_o6_bldg_sys.htm

I believe that part of the missing equation is the soil moisture and the nearness of the ground water levels to the slab. You would probably not need as much R value over dry soils as you would over running water (close surface aquifers)