I have a question that I cant find the answer to and i am sure you guys will help If I build I will be putting radiant in my slab, since the slab is like a big thermal mass would it be feasible/possible to somehow isolate each part of the slab by room. I would kinda be poring 8 separate slabs separated by 2 inch EPS. I am looking at a 1700 sqf 3 bedroom 2 bath house single floor. this way I could have more control of each room/zone. I am looking at 8 zones, 1 for each bedroom, 1 for each bath, 1 for kitchen/dining room, 1 for living room, 1 for mud room. thanks for any help Larry

Yes, you could do that. We isolated our three season sunroom slab from the rest of our house slab in that manner. However, you will find that you will get fairly good temperature isolation in separate rooms by merely having separate heating zones and thermostats. Also keep in mind that you may need separate heating zones and thermostats even in a large room if there are many windows in one section and few or no windows in another section. You need to have a good room by room heat loss analysis (using invisible walls in large rooms) to properly design a HR floor heating system.

Hi Larry,
Yes you are talking about Sub Zoning. This can be done on a manifold with actuators.
With as many zones as you mention pumps become costly.
On a 1,700 sq ft slab we would normally not divide it down room by room.
You would want to look at the rooms that are common to each other for instance if the kitchen, living, dining rooms are open you are better served as one zone (thermostat) area.
Same is true of bedrooms and bath rooms. Slab heat does not end its self well to zone by zone control, More lie flywheel control lots of mass that will radiate for hours. So if you home has say 2 kid bedrooms with a bath serving both, easier to place those rooms on one zone.
Master bedroom and master bath, yes sub zoning there is common, we see clients that prefer a cool bedroom and warm bath, not uncommon.
Another up shot of larger zones is the boiler will run longer as it serve larger areas .Depending on your mechanical set up a longer run can be more efficient.
Just my thoughts for what there worth.
Dan

It depends on location. What you propose would be either be super expensive or a disaster in an area with expansive clays, for example, or on a sloping site that requires grading. In either case, you'd want the slab poured integral with grade beams that reach undisturbed soil. Excavation contractors can tell you what your are up against. In extreme cases you might want a boring sample analyzed. We built a house in Texas on clay soil that rose 6 inches from bone dry to saturated.

In our current house in Pa, we floated the slab inside frost walls, completely separate from the foundation, and isolated the garage and a walkway. The 1,000 SF slab has two zones -- a great room with three loops on one thermostat and a den and bath on a second. The loops can be adjusted independent of stats to put more heat on the perimeter under windows and less in the center. The bedrooms upstairs have individual stats heating transfer plates under hardwood floors. But we draw heat from a 200 gal storage tank. If the bath upstairs was working directly on a boiler, it would fire for a few minutes at best. You should consider a buffer tank if you want micro control.

duplicate post

Yes, structural considerations should always take precedence over everything else. Where we build, we typically do elevated slabs above an engineered pad. Several inches of rolled pad, footings on top of pad, self-compacting pea gravel from top of pad to top of footing, 4 inches of EPS insulation, vapor barrier and 5 inches of concrete slab with #4 rebar 12 inches OC. So the slab floats within the building walls which are usually ICF where we build.

Again, to properly design the HR system, you first need a good room by room heat loss analysis. The large thermal mass of the slab shouldn’t be problematic at all as long as you keep each zone at some given desired temperature for a long duration (e.g., always keep the bedrooms at say 64F, living space at 68F and bathrooms at 75F). You definitely don’t want to use nighttime setback with a slab HR system. If you keep your smallest zone (i.e., the smallest total length of PEX in the zone circuits) sufficiently large relative to lowest heat source output (i.e., modulation level) and properly design the HR system supply temperature and circuit flow rates, you will likely be able to avoid needing a buffer tank. We have a buffer tank design calculator on our website that will enable you to properly evaluate that once you start considering heat sources.