I am laying 1/2" pex pipe above 3/4" diagonal T&G and a 1/2" subfloor.The overall thickness above the sub floor is a height issue, so I cannot use a gypcrete product to cover the PEX ( 1.1/2" minimum pour ). Floor below is heated. So, 1/2" pex laid between sleepers with 1/4" backer-board above and then Ceramic Tiled. So what think you of using a Wal-Mart Space Blanket (cheap), reflective side up to reduce any downward migration of heat. For finished height I do not want to exceed 1" overall above sub floor.

waste of time that will do very little without a real airspace. you should insulate in the joist cavity below, or you're going to have to deal with the fact that you are making a downward radiant ceiling and upward radiant floor at the same time. that means higher water temps at least and if your loads are very high there is a possibility it can compromise the ability of the system to maintain room temp. that's a big if but it's a possibility.

If you are going to put 1/4 backer board over it, I'm not aware of a problem with using only 1/2" of concrete. If you don't insulate the joist cavities, then I would expect ~1/4 of your heat to go downward.

OK, so if I used enough cement to just cover the PEX for thermal mass and then use 1/4" backer board with ceramic tile over would that work using 6" on center loops.??? That will bring me to 1" thickness. Insulating the joists is not a problem, although the heat on that floor will be similar to the above.

the room temps may be similar, but the tubing/water temps are not, and will force downward heat migration via radiant transfer. Insulation under radiant floor is almost always a good idea.

people put tubing in mud beds for tile regularly. However the TCNA requires 3/4" gypsum over the top of the tubes in that case. whether they are too conservative or not I couldn't say, but that's what they spec (TCNA Handbook, 2006, RH122-05 installation detail). That has now exhausted my knowledge of tile.

What about using self levelling cement rather than gypcrete.???

I've read a lot of negative comments about using gypcrete products etc, any ideas on which of the two would be better.