Is it efficient to use radiant heat to heat two spaces, both over and under, at once?
I recently built a loft (30' x 10') with a dormer within an existing large room (30' x 20') with vaulted ceilings. The long end of the loft (30') is open to the larger space via a balcony. The loft floor, is now also the ceiling over half of the ground floor of the large space. The other half of the large space still has a vaulted ceiling. The entire space is currently heated with radiant over a slab installed in the ground floor.
I would like to install radiant tubing within the loft floor/ceiling and have the radiant effect work both over (radiating up through the floor into the loft) and under (radiating down from the ceiling toward the space below). Unlike in an underfloor joist installation (in a basement for example) where the heat is isolated from the space below and is directed upward toward the floor above, this entire space is open to itself, and, in effect, the radiated heat will be shared (both up and down) within the entire large space.
Will this be effective, efficient and comfortable? What is the best way to radiate the heat in both directions? Would aluminum heat emission plates help with distributing the heat more efficiently?
Whether you apply the plate to the ceiling/floor/wall is a matter of which structural mass the heat is transferred to, giving it a preferred direction, but you'll still need at least R10 to make that direction well-defined. In an open loft zone in an otherwise slab-heated space it probably makes more sense to apply it under the floor (above the subfloor, if you want to work well at the same low water temps as the slab, for maximum efficiency on the boiler.) If your goal is to work as radiant ceiling as well, don't insulate the joist cavities.
It's difficult/impossible to perfectly balance it as one zone, since the heat loss of the loft space is guaranteed to be dramatically different (and likely higher) from that below, which is why it probably make most sense to plate the floor, not the ceiling, and place the thermostat in the loft.