A follow-up thought...
It might help to understand the nature of the heat itself. Your idea
regarding thickness of insulation, suggests you don't know understand
how radiant heat does its thing relative to conductive heat. In radiant
heat transfer, heat is is carried in waves, ie the sun radiating the
earth. Objects do not have to be touching the heat source in order for
the heat to be transferred. The wave can travel through a vacuum. In
conductive transfer, heat is transferred by adjacent atom to atom
effects. In conductive transfer, thickness of the insulation layer
matters, because it makes it difficult for adjacent atoms to effect
other adjacent atoms.
Non-radiant heating systems, like the ones we are used to seeing in
buildings, hot air heat, baseboard heat, heat by transferring heat by
conduction, to air at very high temps, then distributing that air
aggressively over the house. The comfort of the home is created by the
temp of the air, which transfers heat, by conduction, to everything it
touches. It requires huge amounts of air movement and convection. The
air being heated also lowers the RH of the air agressively and then
distributed it all over the building.
Radiant systems require no air flow. Though some convection or air
occurs, and some conduction to that air, it is not the primary or even a
significant avenue of heat transfer. Radiant system heat objects,
including people and pianos...they mostly do not heat air directly. The
air is warmed by conduction, meaning it is warmed by objects touching
the air. The air is heated at very low temps, relative to conductive
heat systems. This means the air's RH, relative to Non-radiant systems,
is effected less by the heat source.
DC rods are, by the way, radiant heat sources. Their point is to heat
the surfaces of the piano with as little air movement as possible.
In the radiant system, if you block the transfer of radiant energy in
(wave form) you stop the majority of the transfer. Thickness of the
barrier is not necessarily effective, but material of the barrier is
effective...tin foil will do it. Where a wood EMC stability issue may
occur with radiant systems is when the temp of the piano's wood is
elevated by the radiant floor, while dry air from the outside of the
building (high air changes/leaky buildings) is continually introduced
and convected in and around the piano. Stop or reduce the introduction
of excessive dry outside air, and the EMC will remain reasonable, even
though the temp of the piano itself is very slightly elevated.
Running the floor at high temps not only heats the piano to a higher
temp, it is run at high temps because too much air is coming in from the
outside. So you get a double-bad whammy...a combination of more elevated
temps for the piano wood combined with huge amounts of dry outside air
contacting that warmer piano wood. If there is outside air continually
entering into the space, the pianos EMC% will drop. The bottom line is,
controlling the air changes, ie the drafty-ness, will control EMC%
problems.
Original Message------
How new is the home? Where is it in the country? Are the clients senior
citizens, and need the house kept excessively warm? Do they run the
system intermittently or continually?
Your problem may be one of excessive air movement in and through the
home (air changes), causing them to run the system at too high a temp. I
have no stability problems with the pianos I serve with underfloor heat
(DC undercover), but they are all in appropriately constructed
buildings, regarding air changes. Addressing the overall heat settings,
or the zoned heat settings, or adding a separate zone in the piano room,
offer the possibility of addressing this in an elegant fashion, while
saving them some heating bucks in the long term. The water supplied to
the floor in that room should be no higher than 100 -110 f, even lower,
it the system runs continually.
Stability issues come from RH/temp swings, not from consistent RH/temps,
even if the consistent temps are elevated. If there is a stability
problem look for the source of the swings, which is often one of
excessive air changes, meaning the home is excessively drafty. A simple
rug with a foil underneath should seriously cut radiant heat underneath
the piano. If this doesn't work check overall building conditions.
Chaces are chances are, this is the chronic culprit.