Looking for some guidance on designing a suspended slab over a basement with hydronic radiant heat. My wife has a vision of a finished cement floor (no additional coverings such as tile or engineered hardwood) that would be suspended over a basement and contain hydronic radiant heating. Unfortunately, we're unable to find information on the internet regarding this style of flooring. Any suggestions/recommendations/guidance would be greatly appreciated. Thanks, Trevor

How big of area? Remember concrete cracks.

You might want to do an inch and a half thick slab pour and separate it into ten foot or smaller box areas with say a 2x4 between them laid flat.

1 1/2" concrete will crack,
usually runs from the wall outside corners,
increasing thickness to 2" will help some what.
Joist structure under is critical, engineering for the load 16 lbs ft sq more or less for 1 1/2" .
Once poured cutting a pattern of 4'x4' squares and acid washing the concrete looks good.
Next question is why not get the basement?
core cost is the boiler side.
Dan

Thanks for the reply. The area would be the entire downstairs: one large area comprising of the great room/kitchen; and the other area comprising of the master bedroom/bath/laundry. And yes, the cracking is a concern... the last thing we want is a crack through the middle of the floor, and hence why we're looking for guidance. Specifically, we're looking for suggested slab thickness and corresponding framing.

Dan, Thanks for the reply. Cracking is certainly a concern. I appreciate your input with regards to the framing and cement cut pattern. To further alleviate the concern for cracking, should there be a consideration of installing wire mesh above the pex lines to help tie together the slab? As it stands now, we are hoping to pour the entire first floor which would include two sections: 1) a great room and kitchen; 2) master bed/bath, utility/laundry room, half bath. If we can emulate the photos we see online of acid washed finished cement floors, it'll look great. Cleary the big concern is with the cracking and more importantly, the weight. With regards to your question, I'm unclear as to which aspect of the basement you're inquiring about. Our hope is to have a full basement (versus a slab on grade) with its own 4" slab (with two layers of rigid insulation and compacted gravel underneath) and poured between the basement walls. Given the depth of the excavation (well below the frost level, which is roughly 38"+ here in Upstate NY), we expect the ambient temp to remain in the 60's...even without heat....though we are considerating a wood stove as a secondary heat source that would be installed in the basement. Thanks again for your input. Trevor

A 12' pattern in the basement stapled to the foam is a good solution to keeping that area comfortable. The primary cost is already factored in, foam and boiler system. Pipe and related pump is minimal.
fiberglass strand is typical in concrete topping pours. 1 bag per yard. drawback is that it can present small bumps. If you use wire mat you have to use flat sheets 8'x20' and be mindful of the sharp edges not to place them over the pipe. 3/8" rebar placed on top works to help minimize cracks but you will have them.
Dan

Gotcha. Thank you

Suspended reinforced concrete slabs are frequently done in residential construction these days. However, you will need to have your architect or engineer actually engineer it. You really don’t want to be soliciting this off of the Internet.

Minimizing undesirable cracking is accomplished by having adequate control joints with most of them being placed under the walls. However, there is always risk of having some minor undesirable cracking...that’s just the reality of concrete. The concrete can be polished or colored/stamped to look like stone or even wood. Having cracks in a stamped concrete floor that looks like stone looks fine.

If you are pouring concrete slab floors, we always recommend putting the PEX in even if you don’t think you will immediately have HR heated floors. This is a relatively low cost provision to eventually have the highest efficiency HR emitter possible in the future.

And please be sure to accomplish a proper room-by-room heat loss analysis before properly designing the HR floor heating system. There is lots of info on our website about this subject and John Siegenthaler's "Modern Hydronic Heating" is the best reference book on this subject.

First off, Thank You for the detailed reply. Secondly, I have every intention of involving an engineer to ensure all bases are covered. My solicitation for information was solely based on my own limited knowledge with suspended slabs coupled with the lack of information available online. Also, I appreciate the recommendation of including PEX in the basement slab. Sounds like we'd be foolish to not include it given the minimal cost. Thanks again for the input. Trevor

Look into the various ICF flooring systems (like QuadDeck/BuildDeck/LiteDeck/InsulDeck).

Instead of a solid slab, it creates an insulated T-beam structure with integrated metal supports and utility chases built in. The slab on top is usually 3-5 inches thick.
This leads to a lighter flooring structure that's still strong enough to span significant distances unsupported by anything other than the forming itself. Note: DURING the pour, you'll need supports under the forms, but they can be removed afterward.

The depth of the form you use will determine your rebar and wire mesh schedule.
You place your hydronic before the pour.

Look at fiber or Helix reinforced concrete (possibly with chemical additives) for crack control.
And on pour day, go over your delivery with a fine toothed comb to make sure that what's in the truck is EXACTLY what you ordered.
I've seen a couple job sites where orders where the concrete that arrives is missing the fiber/helix additive and they have to send someone running back to the depot to get it.

+1 on QuadDeck. However, I wouldn't recommend Helix or fiber for a polished or stamped concrete slab as this may show in the finished floor and be problematic. I would definitely recommend Helix for ICF wall pours to reduce the amount of rebar needed or to just increase the strength of the wall for seismic or other reasons. Helix is still relatively new and not all engineers have started designing with it. We use 5000 psi concrete for our floor slabs.

Thank you for the suggestions...greatly appreciated.

Great! Thank you for the input.

If you're going to avoid fiber reinforcement for crack control, then definitely talk with your concrete supplier about crack control via:

Moisture and thermal control.
Chemical admisxtures.
Concrete joints.

Composit deck might work for you, I have one engineered for an ICF that I’m building. I’m using W2 18 gauge metal, comes in 3’ wide and any length. Panels are overlapped and in my case crimped. Composite decking supports are on 11’6” centers ran perpendicular to the decking. 4” concrete over pour with control joints over the supporting walls and 12’ centers parallel with the deck. A engineer probably could design something to fit your needs fairly reasonable. Many different metal decking styles over pour thickness and spans. I’m going to start another thread on this about slab heating in the radiant heat Column, don’t want to hijack yours.

I designed my house with a 7" thick slab to use as a heat battery. It sits on R24 of foam. I haven't yet used the PEX to heat up the slab and I have a question on the maximum temperature changes which might be safe.

The slab is typically 11C which is the ground temperature here. It gets up to 22C when we have a few days of really strong sun. If I use the PEX to heat it up further, how high can I go before there is a risk of cracking?

I thought maybe 30C might be the top end from a practical point of view and maybe 25C much more common with my heating system. With repeated cycling between say 11C and 30C would I be asking for trouble?

If the slab is 24' wide, how much would it expand at the temperature range of 11C to 30C?