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Under-floor radiant heat is ideal for homeowners that want to take advantage of the
clean, even heat of a radiant heat system, but without the additional cost and labor of
replacing your entire floor.

Under-floor radiant heat systems are installed underneath the current sub floor
between the floor joists. This kind of installation is not as responsive as the aluminum
base method but is more responsive than the thermal mass method.

Under-floor radiant heat needs to heat the wood sub-floor and floor covering above so
it can radiate heat into your home. This makes it the most unobtrusive radiant heat to
install in an existing home. It will not add any additional floor height or disrupt your
existing floor coverings.

There are a few common under-floor installation methods: staple-up with heat transfer
plates, staple-up without heat transfer plates, and hanging tubing in the joist space.
This is how they differ.

Staple-Up With Heat Transfer Plates
This type of system uses thin aluminum heat transfer plates that are stapled up with
radiant  tubing under the sub-floor. The plates are highly conductive and provide a
large surface area that will absorb heat more quickly and keep it warm much longer.
Using heat transfer plates will disburse heat more evenly throughout the floor than
the other under-floor methods.

Most manufactures will make heat transfer plates to accept different sizes of tubing.
So you need to match the width of your heat transfer plates to the size of your tubing.
Heat transfer plates also range anywhere from 5 to 12 inches wide depending on the
brand you choose.

You will want to get the best plate coverage possible. If using narrow plates, you
should run two rows between each floor joist. By separating the tubing runs by
around 8" you will obtain a much more even heating pattern in the floor above.

Check each heat transfer plate before you insert them into the tubing. Be aware of
any sharp edges on the transfer plates as they might cut or damage the tubing.
Installation is easy, simply snap the plate around the tubing and fasten it to the
underside of the sub-floor.

If you are installing 12" wide plate, you will usually find pre-made grooves for two
tubes. Make certain that each heat transfer plate is stapled up in the center of the
floor joists. Staple the outside edges of the plate as well as one row of staples down
the middle.

By placing the staples on the outside edges of the plates you allow the tubing to be
away from the sub floor and will help prevent any noises caused by expansion and
contraction of the tubing.

Staple-Up Without Heat Transfer Plates
When you forgo the heat transfer plate, "stand off clips" can be used to staple the
tubing to the joists an inch or two below the subfloor.  

Hanging in Joists Space
This system suspends the tubing several inches beneath the sub floor in the joist
space. When using this method you will purchase joist heating hangers that are
designed to be flexible in length, making them easy to install between the floor joists.

These hangers usually come in a couple of different lengths to accommodate joists
that are 16" or 24" on center. Install your hangers about 2" below the sub floor and
space them out about every 3 feet. This will give you plenty of room to lay the tubing
in place. You will just place the tubing on top of the Joist Heating Hangers and allow it
to float on the braces.
Alternative-Heating-Info.com
Types of Between the Joists Staple-Up
Radiant Heating Systems
Single tube run between
the joists with continuous
transfer plate coverage.
Double run of tubing with
staggered transfer plate
spacing.
Stand off clips stapled
to the joists
.