I am going to staple down the pex to my 1600sq ft upstairs apartment renovation.  Then lay down sleepers and fill the voids with clean dry sand (8 lbs/ sq foot. Then cover the whole thing with snap together cork flooring panels (12"x36") .  Basically same as light concrete or qypcrete but loose.  Am I crazy?  Good heat retention I suspect. 

The sand idea, I have done that in an 500 sq ft addition of my home 15 years back, bad idea. The sand works like an insulator, I thought it would work just fine, but I actually have to run that zone at a higher temp. The room still heat up and all but the best solution is to mix you sand with Portland cement (like mortar) or some other binder, then it will work fine at a lower temp H2O. Your floating floor can still bear on the sleepers and the mass will hold and emit the heat much more effectively. You might consider the RHT floor panel system, same 3/4 thickness but all wood and heat transfer plates. Installed cost is about 2.50 pipe, plates, manifold, turns and local purchased 3/4 ply.
Good Luck,
Dan

To give you an idea how good an insulator sand is. In central Idaho there is an old town restoration/museum project. In it is an old blacksmith shop. The forge, for burning coal to heat metal for forming, is a wood box with a bed of sand! The fire was on the bed of sand. The box shows no, or very little, signs of scorching or burning!!

dry sand = .2k
mortar = 1.7k

What does that mean????

It's a measure of thermal conductance. Not sure at the moment what the units are, but it indicates that mortar conducts heat 8 1/2 times better than dry sand.

Loose dry sand is lousy because of all the air gaps, bad thermal conductivity. Think about it, when you're insulating your walls you want a certain amount of air gap because it adds to the insulating property by reducing conductive contact. Adding a cement creates a solid mass that will better conduct the heat.

Hi,

I also though of the sand thing. I read it there (http://www.radiantcompany.com/details/suspended.shtml) From what I see here is a bad idea to use it. But it still a good idea to use sleeper  ?

I though using sleeper with 4 part of sand for 1 part of portlan ciment. Is this a good mix ?

Jonathan

I got some iron oxide from mining waste and will be testing the thermal conductivity effect on mixing it with cement.

Jonathan,
your question about mix,
all that is needed is to bind the material, not structural.
6 to one mix would be fine 6 sand 1 portland cement, water to stiff slurry.
The sleepers I assume are for nail down flooring? If you chose to go as thin as possible you can use 1x4 flat, fasten to platform with liquid nails and ring shank nails. Athach your 1/2 pex with large romex nails, these have a thin head and the combination of 5/8 od on pex plus brad thickness of the staple is a hare less than the 3/4" of a 1x4,
In fill with your mix as needed.
Dan

In my area it is a MAJOR expense to have a company come and pour 1 1/2" gypcrete over my 1500 sq ft project.  Like about $4500!  Are you saying I could do this myself with some quailfied flat workers and some portland cement/sand mix?  What is this "gypcrete" anyway.  Is it a LIGHTER product?  Are there any other additives in a DIY project?  Is there like plaster of paris added or something like that?  
Has anyone poured this themselves over a wood floor with radiant pex or is this too hard to get level and right?

 Gypcrete is made from gypsum, the same stuff they use for making sheet rock. My concern is, what happens if you get water on it for and extended period of time, say a small leak, wouldn’t it turn back into mud?

A lumber overlay is a good alternative to gypcrete, and you can do it yourself. With one of the lumber overlay methods, you can cut 3/4” plywood to fit between the pex with only a minimal gap, then router out the bends. This allows you to put a new hardwood directly on top, or a 1/4” plywood cap with carpet on top.

 

Hi Dan,

If we reduce the sleeper from 1 1/2 to 3/4 inches, does this affect the thermal resistance of the floor ? is it better to have only 3/4 or the better will be te 1 1/2 sleeper ?

regards

Jonathan

Hi Jonathan,
If you have 3/4 mortar in fill you will have less heat mass, but the transfer will be the same or faster. site work and cost on the fill side is minimized by 50% less fill. thermal resistance, haven't done the math, I am simply relaying a system that works and a mix that is right regarding sand and portland cement.
Barron, you may want to look at out RHT floor panel system, this is a straight forward site built application that runs about $2.50 sq ft complete with pipe, heat transfer plates, plywood fill and turns.
http://www.blueridgecompany.com/image/view/527 Keep in mind there are several other systems like this as well, ours is not the only.
Dan

The problem with my project is that the pex is already stapled down.  So I guess I am committed to some kind of crete  huh?

staples are not permanent. That said, 1.5 inches of any crete product is an excellent medium. gypcrete is lighter, somewhat more conductive and self levelling, it doesn't shrink and I believe cracks are less prevalent.

$3 a foot plus tubing isn't that much more than a sandwich method. Do it, and be glad you did. Sandwich is cheaper and good, but gyp is undeniably better.

best if you can avoid sleepers and nailing, but if not, so be it.

So is gypcrete a brand name?   Are there different "kinds" of crete?  Is one of them something different than portland + sand?   Just curious so I can ask the right questions to a potential contractor.

gypcrete is a class of cementitious flooring product. It is gypsum concrete, not portland cement. It comes in different brand names, such as Thermaxx.

regular concrete shrinks, cracks, is heavier, and is less conductive. that said, it too works well.

I see concrete as about twice the thermal conductivity of gyp-crete:

concrete k = 1.44 (10.0)
gyp-crete  .6840 W/[m · ºC]  (4.75 Btu/(h · ft² · ºF)

concrete varies widely, and that which is typically used in thin pours I believe is less conductive (more air to make it lighter) than structural concrete.