I'm putting a small extension on our home and wood like to use radiant
heat in a floating slab and need some good directions on how to proceed.
I have seen some jobs when they were finished, but need to know more
about the insulation going under/in the slab and anything else helpful.
Are there any good sites with pictures detailing the process?
I keep seeing this expression floating slab. It's a term not
familiar to me. Do you mean a reinforced suspended slab or maybe
one on the ground and subject to raising and falling an inch or so
each season? Confess I have never seen concrete float.
I'll probably get corrected, but it's a reinforced slab set on a well
drained gravel base generally above the surrounding ground level without
a frost wall. As a former (now retired) concrete form builder, I believe
that with a proper amount of rebar & 6" x 6" mesh wire, good 'dry' pour
of cement, and a good cure, a slab that won't crack can be made. Works
best on free-standing buildings such as a garage. My extension has been
prepared by digging down a foot into the clay, and then back filled with
screened rocks. Drainage pipe runs around the perimeter. I like at least
a 5.5" pour.
I hope to incorporate a passive solar water heating system into the
radiant system which should keep severe frost problems at bay if the
building is unattended in the winter. Granted, it's a little experimental.
I concur with much of what you said about trying to make a slab on grade
as crack free as possible, but I have a few suggestions.
Eliminate the 6x6 mesh. It will do nothing to prevent cracks. Here's why:
in order for the steel to pick up any stress it must stretch a certain
amount (strain). The strain required for the steel mesh to pick any
significant load is enough to allow the concrete to crack (they must move
together). The only way around this is to put in a fairly large amount of
steel so that only a small strain is required for the steel to pick up the
Using less water in the mix is the best idea. Use of water reducing
agents (plasticizers) can help workability. I once specified a printing
plant slab mix with 7 sacks per cubic yard and only 3 gallons of water per
sack of cement instead of the more usual 6 gallons. In the mix was a high
range water reducer. I recommended that the contractor use double his
normal finish crew because the concrete was going to set up fast. Worked
like a charm! We got a glass smooth slab with no cracks.
Bob Morrison, PE, SE
R L Morrison Engineering Co
Thanks Bob. Most contractors around here don't use any rebar and might
use 6"x6" mesh, but generally pour a 3.5" slab. I've had the concrete
trucks arrive with too much water to start, thinking they were helping
by making it 'flow' better.
I use 3/8" rebar 2' OC supported with 2" blocks of concrete with wire
(called them something like 'dolby blocks.) I hear what you're saying
about the 6x6 mesh, that would seem like it's only use would be in small
slabs probably with expansion joints.
I'm wondering how much I need to modify my design to include the radiant
system. I've considered two pours since it seems to me that foam
insulation underneath the initial pour would compress and/or breakdown
If you use the proper foam made for underslab installation then I don't
think you need worry about it breaking down. The enemy of most of this
stuff is ultraviolet light and there's not much of that under a slab
<grin>. As for compressing, most of this stuff has a compression strength
in the 80-100 psi range. Allowable soil pressures are usually in the
2000-4000 PSF (14-28 psi) range.
Many radiant heat manufacturers provide a plastic grid to tie your tubing
to. Others simply recommend something like #3 @ 16" to 18" o/c.
Bob Morrison, PE, SE
R L Morrison Engineering Co
I've seen a lot of 6-6-6-6 (most is 6-6-10-10) used over
core-a-form(sp) around here and on the ground floor slabs too in
commericial buildings I've supervised although that was probably
45-50 years ago.
"floating slab" must be a regional term. It's a slab with a deepened
and reinforced perimeter but which does not have frost footings.
Back to the original question, we lay down a layer of 6 mil poly, then
a layer of 2" extruded polystyrene. Then the tubes get stapled to the
foam with special staples. We don't usually use rebar in the field of
a residential slab--since it is usually covered with finish flooring
of some kind, a few cracks which don't involve substantial
displacement are no big deal. In this case, we put the perimeter
rebar in, and then the tubes which can be tied to the perimeter
To answer your original question (which none of the replies have), here are
some links. I put tubing in my 30 x 40 and used (5) 300 foot circuits.
Thanks Steve - especially that first link, it's a real primer. (sorry
for the direct post)
I must add in that mine has 1/2" rebar in both directions on 2' centers and
was poured very wet. This is because it was poured last INSIDE the already
constructed building and I DO have one crack already. It was poured the day
after thanksgiving '06. I do feel, however, if I had been on top of things,
and had my heat source ready to heat the floor, it wouldn't have cracked.
It is cracked across one corner about 3' in each direction from the corner.
I believe this is 100% due to frost heave and wouldn't have happened, like I
said, if it had been heated. If winter is coming when you pour, be ready
with the rest of your system, and I don't believe you'll ever crack
regardless of the concrete consistency when poured.
Just my observations and opinions.
You might take a look @ this site, http://www.tileheater.net/index.html It
'looks' impressive as all get out......I would surmise you might get
additional factors with this product if you build your slab like the cold
storage/controlled atmosphere buildings here in the Yakima valley. IE:
Drainage underneath, vapor barrier, insulation, sand, cooling coils, sand,
mesh, concrete....(May not be the right order of install)...8" styrene (I
believe) under the slab. It's always been the blue rigid stuff. (4'x8')
Prevents heat rising from the ground & cold going down.
Did one a few? years back @ Washington Beef in Toppenish, Wa. Bldg. was
refrigerated (50 degrees?) with a freezer (25 degrees) & blast freezer (-45
degrees) incorporated into the same structure with partition walls. ALL
walls were foam panels. Did a similar bldg. @ the cheese plant in Sunnyside,
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.