I've got an old 450sqft concrete slab in the basement that I'm thinking of topping with a thin as possible hydronic radiant concrete slab. I'm setting up hydronic radiant on the main floor so I thought I might as well spend the extra bucks to make the basement comfortable with one more zone. The old slab is 3-4" thick with no insulation underneath (get regular -20C to -30C cold spells in the winter). On top of the old slab I was thinking of using a thinner layer of insulation like the Barrier XT then embedding 1/2" PEX in 1.5-2.0" of concrete over the insulation. How much heat will conduct downwards with such a minimal layer of insulation and is it worth it? Are there other types of insulation that you'd recommend? Basement has R24 walls, insulated ceiling, and the thermostat will be kept fairly low most of the time - I'll happily use a portable heater for quick heat. In the back of my mind I know that I can skip the cost of radiant and easily self-install electric baseboards for a few hundred dollars...but I'm not a big fan of baseboards. Oh yeah, the ceiling height with the topped up slab as mentioned above will be around 7' 2" and I'm 6'4" with my shoes on, so I want to go as thin as possible with the insulation. Opinions??

According to their brochure, it's flexible proprietary-process 3/4" EPS that has been certification-tested at R5.7, which is WAY better than zero.

If you can tolerate the height, 2" of rigid EPS without feeling claustrophobic, that will give you R10, and is the gold standard for under-slab insulation around these parts (New England). If your basement slab is 3-4' below the frost line it may not need to be the full R10. On surface slabs some people only insulate the 2' outer perimeter to R10, and run either R5 (1" EPS) or even nothing but sand in the middle. R5.7 in your basement slab-sandwich is probably better insulated than those configurations. Price it out- it may be cheaper to just run with 1" rigid EPS with a polythene sheeting vapor barrier over it with similar thickness & benefit.

Whatever you do, don't let anybody sell you on a radiant-barrier (bubble-pack type or other), which really need a minimum of 1/2" of air above/below to be AT ALL effective.

Assuming your first floor radiant is a staple-up with heat spreaders (and not a lower temp gypcrete slab) with R24 walls and an R5-R5.7 floor, and a radiant floor overhead, unless the basement is really leaky/drafty you probably won't need much heat source to maintain the temp. Have you considered the middle-road, running a hydronic baseboard zone for the basement? It won't be as cushy under foot as a radiant slabbed basement, but it's not rocket-science to design in sufficient length to be more than adequate even running at radiant-floor temps (no need for multi-temp hydronics), and will be much quicker to respond (and a cheaper) than a radiant slab, and cheaper to run than electric baseboard or portable heaters. Run a manual-J type heat loss on it the slantfin.com freebie is probably good enough, but you'll have to fake the floor & wall selections since they don't give you exactly the same options that you've built. The R5/R5.7 slab is roughly equivalent to a floor over an unconditioned basement, which is one of their options. I forget how flexible the Slantfin tool is, but you multiply the baseboard length for a 180F water design by ~1.5-2 it'll still deliver enough heat with staple-up radiant water temps. Going that way you can probably get away with an inch of (wire reinforced) concrete over an inch of EPS/XPS for a mere 2" of total lift and call it a day.

But if all forms of baseboards really bug you (not just the electric type), just insulating the floor and selectively removing insulation from under your first-floor radiant until you achieve the baseline temp you want down there will be the cheapest. Or zoning it with a radiant ceiling (with or without insulating the floor) keeping the between the joists is another option that won't reduce the head clearance. R13-R15 batts between the radiant ceiling for the basement and the radiant floor for the first floor should isolate them sufficiently.

Ok, this is just bad advice.

If you want to heat the basement, heat the basement. Using uncontrolled back loss from another floor's emitter, is a horrible idea. You get fairly wide temperature swings in the basement and you hurt the performance (and possibly comfort) of the floor above. If this is just a freeze protection measure then any number of cheap emitters, including baseboard, would work fine. But if you're considering a retrofit radiant floor down here it's because you care about the space being comfortable. You CAN NOT achieve that with uncontrolled backloss from another floor. All you can do is guarantee "it'll have heat" because heat loss characteristics between a basement and an above ground floor are pretty radically different.

Radiators in conjuction with other radiant; properly sized baseboard with higher temp radiant; radiant ceiling over an insulated floor all make some sense.

But if you want to retrofit over an existing, uninsulated slab, with a minimum height buildup, you want Roth panel. it's foam insulation and aluminum topcoat all in one. You can do 3/4" or 1" thickness. Float a floor over it, call it a day. The only way to beat that for height loss is to go with wall/radiator/baseboard...

I wasn't 'xactly recommendin' it Rob, but thought I'd throw it out as the CHEAPEST option, since he doesn't seem to much care much about what the temp is or general cushiness or comfort control:

"...and the thermostat will be kept fairly low most of the time - I'll happily use a portable heater for quick heat."

Basements usually have very minimal window & door loss, and at R24 heat loss through the foundation wall will also be very low compared to most first-floor construction. Giving it "some heat" with a bit of leakage off the first floor to take the chill off might well be just what he's looking for.  It doesn't sound like heating-as-freeze control would likely ever be needed at those insulation levels, with a a radiant floor above- it reads like he just wants to keep it well above the temperature of sub-basement dirt.

Which is why radiant floor seems like overkill. With an insulated slab and a radiant floor above I'd bet it stays over 50F all year even without local heat or zone control, maybe even over 60F, depending on how thick/thin the insulation on his 1st floor radiant is, and how infiltration-air-leaky his basement is, and how high the exposed foundation is above ground level all the way around the house, etc.  Radiant floor seems like a Mercedes Benz high-precision high value/cost solution to a not-so-precision non-critical problem that could be handled with a Radio Flyer-red-wagon approach.  (I'd do it with a hydronic baseboard zone, myself, but he's "...not a big fan of baseboards..." and doesn't care about precision control.)

I've not encountered Roth panel before- how does it price out per 1000 square feet (or relative to 1" XPS to simply insulate, not heat)?

Fair enough, maybe I overreacted, but I doubt anyone is going to mistake radiant as the cheap option for baseline heat. I would expect the OP was simply commenting on response time not being an issue. of course he can clarify any misunderstanding. But in general, in the vast majority of circumstances, "heating by back-loss" for anything other than base freeze protection or other "I really don't care" situations is a very poor idea. I guess you hit a button ... I get asked that all the time ;)

roth is of course much more expensive than just insulating. with tubing/manifolds etc, you're over $6/sq ft. But, it's competitive for slab radiant retrofits for sure and it's pretty hard to beat the height profile without skipping insulation..

In the good old days when we were blissfully ignorant, we radianted basement slabs (existing and overpour) without the benefit of insulation. Slabs below the frost line have modest back losses, but don't get my friend Rob started on that one.

Dana is right R5, would be plenty.

Here is an alternate budget system, effective in applications with un level slab, where a topping pour is not an option.
pressure treated 2x4 12 inch on center laid flat attached to cement (concrete nails, liquid nails), shim as needed, bubble foil insulation laid between slats, 12x24 inch RHT heat transfer plates span the air space, minimal floor contact, 1/2 pex pipe, 3/4" floating floor on top perpendicular to 2x4 support every 8 inches (no nails), or traditional nail down floor nail 12" pattern. I would suggest an X block off existing water heater as alternate to open loop heat system.
We have seen this done in barns, out buildings, and warehouse space office area's. cost is about 2.50 sq ft or less not including the x block. I understand effective insulation is not as high as 2 inch rigid pink.
Dan

Actually not a bad idea Dan, and I like the use of the Xblock in these applications.

if you have a 2x4 there, why not put 1" rigid in between them?

you only need a hair more than 5/8" for the pipe and plate.

Based on how much I contoured 3/4" Dow blueboard for my crawl space rat slab, I would say no problem! The yellow lines are beads of foam to help keep the concrete from sliding down hill on the slopes!

Great minds think alike!

Bubblepack radiant barrier with an air gap will probably be the CHEAPEST option that has a chance of doing any good at all, but  it won't perform nearly as well as 1" of extruded EPS. (Which is more than flexible enough over curves & uneven surfaces for his needs.)  The thermal short-circuit of the wood is significant- the pressure-treated lumber is only good for about R1-1.5, and is a double-digit percentage of the total surface area.  The bubble pack its would only perform to about R5-6 in a nearly-perfect- installation combined with higher water temperatures.  (Under-basement-floor gaps often becomes a cozy homes/chases for vermin too.)

The design temperature for most basements is a third of the upper floor i.e. lower design water temperatures. I am not a fan of bubble foil (makes better packing material) or short cuts for that matter. But given the application I still like the low profile. It floor is really rough rigid insulation could be a nightmare.

As for thermal short-circuit; there is no heat to short, as the tube would be beside and not on top of the tubing. The standard for retrofit are rightly different than new construction. Trying to apply one to the other can make many reasonable compromises impossible.

It always reminds me that many houses still have no rim joist insulation and their parimeters are not even protected to the frost line (25% of our heat loss around these parts). Of course we have to note we are talking about 450 sq. ft.

There's no direct heat to short from the 40-50Cdegree PEX to the ~10C dirt, but the floor temp to ground temp (probably always ~20C+ deltaTs with staple-up heatspreader type radiant floors) is still significant.

I'm hoping/thinking that if he has R24 basement walls he did due-diligence on sealing & insulating the rim joist & sill(?). Even at Home Depot pricing 450' of R5 XPS just isn't gonna break the bank (about $200USD), and will feel a lot more like "doing it right", eh?

http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&langId=-1&catalogId=10&productId=100320343&N=10000003+90039+527397

I can't see how one would ever justify the additional LABOR (even as a DIY) of running pressure treated sleepers and hanging the bubble pack "just-so". Not to mention, the additional cost of heat-spreaders would probably exceed that of just R5 XPS under an embedded PEX 1.5-2" concrete slab.

Depends on how the floor will be attached. I have no problem with ridgid insulation, but in small spaces below grade, mild climate, 2" XPS won't cost much and won't save much either. The guy is tall, at 5'6" I can't argue ceiling height...hehehe

Let me just say, that in my general radiant-slab design work, XPS represents 98% of my insulation specs., leaving all blankets for beds and foil for cooking. Only extremely rough or broken slabs present a problem.

You will have NO issues with getting 1" XPS to conform to moderately wavey slabs. It's stiffer than bead-board EPS, but it's also more resilient- takes quite an arc to actually crack it. Even if it cracked under the weight of the concrete while pouring (which it won't, in your case) it's no big deal from a heat-loss point of view. If you're intending it to be a perfect vapor/radon barrier it could be somewhat compromised, but it doesn't sound like that's your primary goal. (If that's important to you, sheet polyethylene under the XPS lapping & taping the seams by a foot or more would more reliably form the gas & vapor barrier for less labor than seam-sealing the XPS).

I think you will really regret not doing something about headroom while you are doing this project, 7'2" would really stink. I know it's a little more work (ok maybe a lot more) but I'd breakout the jackhammer, remove the existing slab, dig a little deeper so you can put in 1" or 2" foam board with a 4" concrete slab containing your pex radiant and still get as close to 8' headroom as possible (at least 7'8").

1" XPS it is then! Blankets for beds and foil for cooking...that's a good one. Greg, I know what you're saying about the headroom, and believe me I've spent some time wondering why I didn't raise the house a foot or two while it was gutted. But all of the framing and wiring is done now and I've gotta live with it. Thanks for everyone's help.