Replies

1. 
   WayneL5 | Dec 06, 2002 01:31am | #1

   I'm not an expert, but I see that no one else has answered.  Another magazine recently did an article on radiant floor heat.  They said industry standard is 2" beneath the slab, but the author recommended 4".   You won't crush the foam because a 4" slab weighs only 0.35 pounds per square inch.  Your foam can support it.  You should use extruded foam, not the kind with little beads.

   Have the concrete mixed on the dry side, because the water won't be sucked out of it like it would on sand, and therefore it will take quite a bit longer setting time before it stiffens up enough to work the surface.

   If you're only heating the garage to keep it above freezing, say 40°F, then you don't really need a lot of insulation.

   You can tie the tubing to the wire mesh or rebar with anything that is strong enough to hold during the pour, and not so skinny it will cut the tubing (like a garbage bag wire tie).  Electrical ties work well.

1. 
   user-457547 | Sep 24, 2003 04:42am | #4

   How about putting a couple of inches of sand on top of the foam insulation to help with the concrete curing?

1. 
   WayneL5 | Sep 24, 2003 05:07pm | #5

   The article didn't say.  But, I can't see why an inch of sand would hurt.  Being careful to stay on the stiff side rather than the wet side helps, too.  Otherwise, like you imply, you'll be there well into the night waiting for the concrete to set because the insulation won't draw any moisture from the mix.

1. 
   CombatRescue | Sep 25, 2003 05:37pm | #6

   The inch of sand is unneccesary.  "Dryer" concrete cures at almost the same rate as "wetter" concrete.  You want to have the concrete as dry as possible not to speed curing time, but to increase strength.  In general, the wetter the concrete is when poured, the less strength it will have once cured, and it will have a greater chance of cracking.

2. 
   SamT | Sep 25, 2003 08:20pm | #8

   Sand on top of that foam will help dry out the concrete, which will impair it's curing, not help, if the proper amount of water was used.

   Concrete needs water to "cure", which is a process of forming crystals. This is why a  concrete mix needs moisture and time (14 days) to reach rated strength.

   Too much water creates water filled voids that the crystals can't bridge.

   The more you mix concrete after it is uniformly wetted, the shorter the crystals can become since they start forming as soon as water contacts the cement powder, and mixing breaks the crystals.

   Stronger concrete has more and longer crystals and fewer voids in the crystal matrix, thefore; more cement (up to a limit), less water, and less mixing, for a given aggregate, all improve strength.

   Since vibrating also breaks the xtals, and liquifies the mud, you should always run the vibrator down into the form as fast as possible and bring it out as fast as the larger (medium sized) bubbles are rising.

   If you are vibrating correctly, you will see several medium bubbles rise 2-3" before the vibrator surfaces. If too slow, 1 or 2 large bubbles 3-6" before, if too fast, a few small ones just as the vibrator comes out of the mud.

   SamT

   "You will do me the justice to remember that I have always strenuously supported the right of every man to his opinion, however different that opinion may be to mine. He who denies to another this right, makes a slave of himself to his present opinion, because he precludes himself the right of changing it."   Thomas Paine

1. 
   caseyr | Sep 25, 2003 09:02pm | #9

   Thanks for the explanation.  Some years ago I worked for a couple of summers on a concrete crew building a factory and a small dam.  I realize from your description that none of us really knew what we were doing back then...

2. 
   stikineriver | Dec 09, 2002 04:11am | #2

   I have poured several heated slabs. I use extruded polystyrene (Dow Blue Board) below the slab. I always install asphalt expansion strip between the new slab and stem wall as a direct connection will suck heat from the slab. I space the PEX 2'-3' apart. They sell metal angles to hold the PEX at 90 degrees for where your hose exists the slab. I tie the PEX with plastic cable ties or metal quick ties. I pull the wire and tube up into the slab as I pour. Just be carefull not to get the tube exposed. Goos Luck.

1. 
   RussellAssoc | Mar 12, 2003 04:17pm | #3

     I agree, and these details mentioned are the keys to success.  I would add, in the northern regions where we build our garage radiant is always filled with glycol.  If you have a power failure or other interruption there's the potential for disaster.

3. 
   johnnyd | Sep 25, 2003 05:46pm | #7

   The way they do it around here (SE MN) is put the foam down, then staple the tubing to the foam...there's even a special tool that allows you to staple standing up...then lay the rebar or mesh, then pour.  Make sure you run the tubing to a manifold with a pressure guage on it and keep an eye on it during the pour. 

   Amazingly tough stuff, that tubing.

4. 
   sphaugh | Sep 25, 2003 09:45pm | #10

   a couple links of interest:

   http://www.dow.com/webapps/lit/litorder.asp?objid=09002f13800e3011&filepath=/noreg

   http://www.buildingscience.com/resources/foundations/sand_layer_under_slab.htm