Hydronic (hot water) radiant floor heating is mostly associated to thick concrete slabs. But radiant floor heating can also involve thin slabs and metal plates for wood floors.
Three types of Hot-water radiant floor heating
In fact, modern hot water radiant heating systems can be of three types: the common and traditional 1) slab-on-grade system, based on thick concrete slabs, but also the 2) thin-slab system and the 3) plate-type system (based on heat-transfer aluminum plates).
All rely on tubing circuits laid underneath the floor covering; and all receive hot water from a boiler, which circulates through the tubing circuits before returning to the boiler to reheat.
1) The slab-on-grade floor system is the most common and the most economical radiant heating system; whenever a concrete is already planned, the cost of floor heating is basically the cost of installing the tubing in the concrete slab and, of course, the cost of slab insulation (insulation is critical for the performance of the system, including electric radiant systems).
2) Plate-type radiant heating is the best choice for retrofit situations.
These systems rely on aluminum plates to transfer heat into the floor; the plates (and the tubing) can be installed below (the more common option) or above plywood subflooring. This system requires a closer network of tubing and higher water temperatures than slab-type systems.
3) Thin-slab radiant floor-heating systems are a good alternative to the more common slab-on-grade systems in wood frame floors.
The most common approach to install this system is to fasten tubing to the subfloor and to cover it with a thin layer (up to 1,5 inches/2,5 cm) of poured underlayment. Poured gypsum-based underlayments are a very common option. Prices per square foot are somewhere between those of slab-on-grade and plate-type radiant floor (often above $5 per square foot).
radiant floor heating sucess depends on Insulation and other details
Radiant floor heating tubing
Copper tubing is a thing of the past, and a poor choice compared to new PEX (polyethylene), PEX composite and polybutylene tubing.
Though simple in their basics, the success of any radiant heating system relies heavily on details. Whatever your choice, proper planning and attention to details are crucial.
The tubing circuits and the manifold stations, where each tubing circuit begins and ends, should be properly designed, ideally on a room-by-room basis.
Insulation to control back losses in slab-on-Grade floor heating
The role of insulation in a radiant floor heating is obvious: to reduce "back losses", that is, the heat that goes in the wrong directions.
Even reasonably insulated floors can lose significants amounts of heat to the ground or to basements and crawlspaces.
Whether the radiant floor heating system is electric or hydronic, you should install enough insulation beneath it, and block as much as possible heat loss through the adjacent walls.
In the slab-on-grade system the sub-slab insulation is typically provided by extruded-polystyrene, and involves not only the areas under the slab but also any exposed edge.
Builders and manufacturers often recommend 1”/25 mm of XPS insulation, which is too little. The ideal varies with climate, but some energy experts recommend up to 4”/100 mm in cold climates. XPS is widely used, but you can aksi use other high density foams.
Insulation of Thin-slab and plate-type radiant floor heating systems (Wood-frame floors)
In thin-slab and plate-type systems (wood-frame floors), also consider using rigid foam insulation.
Some people and builders use to install 3 inches (75 mm) of fiberglass of insulation between the joists and stapled plastic (polyethylene) to cover the insulation. But that's not the ideal choice.
For those who want to install an air barrier under the joists, rigid foam foil-faced polyiso with taped seams is a better option; and foremost, consider a higher amount of insulation (the double or more), especially in colder climates, above unheated basements or crawlspaces.
Radiant floor Covering Insulation issues
Radiant heating works great with ceramic tile flooring, but not as well with other types of flooring.