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
These systems use thin aluminum heat transfer plates that are stapled up with radiant heat tubing under your subfloor. 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. Simply make certain that each heat transfer plate is stapled up in the center of the floor joists. When you staple up the plates, staples towards 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 Transfer Plates
When you forgo the heat transfer place, make sure that the proper staples are used to hold the tubing in place. You do not want the tubing to directly contact the sub floor, otherwise, the tubing will make noise as it expands and contracts and rubs against the sub-floor.
Depending on what your design calls for, each tubing run should be placed around 8" away from each other, This will ensure that you obtain a much more even heating pattern in the floor above.
Hanging in Joists Space
This system suspends the tubing several inches beneath the subfloor 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.
