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? TIA
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.
Buddy:
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 load.
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.
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 over time.
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.
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 . 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.
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.
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.
YMMV.
{G}
"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 rebar.
This is one area where pre or post-tensioned concrete really shines.
Matt
Matt:
You are correct. PT slabs on grade aren't used much here in the Pacific NW. Elevated PT slabs however are used all the time.
You might take a look @ this site,
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, Wa.
Dan
Thanks Steve - especially that first link, it's a real primer. (sorry for the direct post)
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