I plan on installing radiant throughout my future house to include the basement.
Before the basement floor is poured I will insulate it with at least 2" of rigid insulation.
What kind is most appropriate? XPS, EPS, Polyiso, other?
also..
How do I keep the insulation from being damaged as we walk on it laying the PEX and then haveing the floor poured on top?
thanks in advance- Joe

I use the XPS. Its really tough, you dont have to worry about walking on it.
First, compact the subgrade really well and get it flat. This is important because you dont want the slab to lock in to the subgrade. With the slab bottom being flat, the slab will be free to expand and contract.
Then, lay on the XPS. I usually use 2 layers of 1". Its a little easier to handle and you can make it so the seams dont overlap. I rip a 3/4" piece and glue it to the wall to use as a screed edge and an expansion joint. Put the vapor barrier over that, and then #10 6X6 wire mesh to tie the tubes to. Lay down the tubes to your design. Have the manifold built already and connect up the tubes and pressurize the system with air.
Then put in the re-bar. Use 1/2" bars at most, 2 ft spacing. For my house I use 1 ft spacing, and I have no cracks (none).

To bring in the concrete make a walkway from planks or plywood that you can roll a wheelbarrow on. The biggest danger to the tubes is when the wheelbarrow is tipped forward and the front piece rests on the floor. If you pinch a tube, it could cut it. If its in a basement, you might not have wheelbarrow access. How big is the slab?

Thanks BillN for the quick and indepth responce!
I have an opportunity to purchase some pre-used Polyiso at a very good price and was hoping that I could make this work for me.
Is there a way to make Polyiso work ok for under the slab?

also...
anyone who needs any help with installing radiant in their house (near SE PA or Central/South NJ) feel free to give me a call.
I dont want paid, I just want to learn as much as I can before I have it installed in my own house.
also I am interested in helping anyone working with SIPs and/or Geothermal.
-Joe
646-226-1227

Joe, why not start with the best underslab insulation and vap retarder all in one? the Barrier. It is available in 3/8" 34" or 1 1/4" thick shipped factory direct. Can fax you info if you want. let me know callie at www.greeninsulationproducts.com 877 904 8424

Just a minor correction. Use 2" insulation for the perimeter, not 3/4".
I am working on a slab install today. We are prepping the subgrade. After a morning rat slab pour on tuesday we are putting the tubing and re-bar in for a radiant slab pour the following day. If you want you can come up. I live about 12 miles west of Newark Airport.

The poly iso will work OK. Its got a little more insulation value than the XPS, but I find that its a little more fragile than the XPS. I think that a loaded wheelbarrow might push the rebar chairs into the Polyiso, but the XPS is really tough.

I have plenty of images from when I did my radiant slabs on my website, www.wdnichols.com  Check it out.

cool website...
I think I will take your advice of using XPS instead.
thanks-Joe

Remember you get what you pay for. R-value underslab is a direct relationship to material thickness. Check out Crete-Heat.com that's all we use.

Underslab insulation is high performance EPS foam and vapor retarder designed to insulate and retard moisture. Moisture is the issue. THE BARRIER is a thermal block against cold and and moisture. the barrierX5.com

Don, when you're posting you should use a sig or note that you are involved with crete-heat. we love your product, but please don't make it sound like you're just some guy who likes crete heat.

Yes Rob, I own Crete-Heat. I also own a company that installs radiant long before Crete-Heat came to market. We've done it the old way and that's why Crete-Heat came about, to save time and labor dollars on installs.  I just hate to see these other contractors keep strugling the old way as we use too. My email is always provided with a post for anyone wanting to reply. Crete-Heat is a real product with real third party verified numbers. Though I feel this isn't the proper forum for us to have this dialog, I felt I better clear the air here. 

 

Yes, I am a contractor that uses Crete-Heat on every install.

it's always better to clear the air in public. thanks Don!

I have found nothing faster than stapling tubing directly to 2" XPS. This is more than enough insulation, easy to install and inexpensive to buy.

MA

foam staples are fast, but not as fast as crete heat.. no jams, no loading, it's really a super blazing fast install. like "holy cow" fast. and I say that as someone who makes more money on foam staples. If you have a big staple clip and a jamless stapler, maybe it's close, but that hasn't been my experience..

around here it' cheaper than foam plus staples too. YMMV depending on staple/foam costs locally too.

I do have a big staple clip! but I would like to use Crete Heat. I will have to check the price again. As I don't install the insulation I don't worry about anything but the spec.

Like Don, I hate to see people struggle with blankets, foil and other such nonsense and doing the wire tie to 6-6, 10-10 thing.

BTW, a vapor barrier is a good thing; makes for better concrete and meets code most everywhere now, but is not significant to heat loss as high ground water is rare and a vapor barrier does not affect heat transfer.