High mass radiant is an outdated technology. The secret to performance, comfort and energy savings is low mass, high conductivity.
For decades, the term "thermal mass" has been used when describing radiant floor heating systems. Used in this context, it refers to the ability of a high mass radiant floor assembly to store heat.
This concept went mainstream in the 1960s and 70s when "passive solar homes" were growing in popularity. A dark colored, high mass slab made of Portland cement (or gypsum concrete), would sit under south-facing windows and soak up the heat from the sun all day long. When the sun went down, the slab would emit its stored heat to the habitable space for the next several hours.
While this method worked in some locations in the house, it was not an adequate heating method for an entire home. So tubing was installed and covered with concrete in other parts of the home. These homes were therefore a combination of radiant floor heating and passive solar. Because this combined system was common during this era, the term "thermal mass" and "radiant heat" are often thought to be synonymous, when in fact, they are not.
However, this same thermal mass which is so essential to a passive solar home is also the cause of the most common complaint with radiant heat – it's slow. In every home, the heat load changes – up or down – can change quickly making it impossible for high mass systems to respond. It is common for these homes to be too cold in the morning and too hot in the afternoon (overshoot and undershoot). If the house has been unoccupied for a stretch of time, homeowners may have to wait over 24 hours for the system to adequately heat the home.
While the history of passive solar with radiant heat is interesting, given a blank sheet of paper, no one would design a heat system that acted primarily as a storage device. Any heating system, whether forced are or radiant floor heating, has the sole purpose of transferring heat into a specific space. Homes lose heat because heat flows from the warm interior to the cold exterior. A low mass radiant heating system with more conductivity allows heat to be produced at the same rate at which it is leaving the house, allowing the interior to maintain a constant level of comfort.
Conductivity, as we know, is the property of a material that allows heat to flow through it. Concrete is inherently a mediocre conductor, making it a poor choice for conducting heat. But aluminum is 240x more conductive than concrete. Because the amount of heat that must be supplied by a radiant panel is constantly changing due to weather changes, occupancy or just personal preference, the ideal heating system is able to quickly or decrease output in order to maintain optimum comfort.
With a high-mass system, the conditioned space is inconsistent and slow to respond to the needs of the occupant. On the other hand, highly-conductive, low mass radiant floor systems provide greater comfort through fast response.
Remember – Conductivity, not mass, is king.