Installing Pex Under Floor Options

There are many ways to install PEX under the floor between the floor joists. We will review four of them; heat transfer plates, hung, staple up and joist hung.

To properly install PEX we need to understand what happens to PEX when it is heated. PEX expands at a rate of 1.1 inches for every 10 feet at 100-degree rise in temperature. If the PEX is held tightly it will make noise when it expands and contracts. For under the floor radiant heat applications the temperature should not exceed 160 degrees Fahrenheit with heat transfer plates or staple up. The hung method can be run as high as 180 degrees Fahrenheit as it does not come in contact with the floor directly. This is the maximum water temperature you should use. The water temperatures should be set as low as possible to provide even heating for your comfort level.

First is the Heat Transfer Plates. They are made of aluminum and steel (aluminum is used most often) and provide the most BTU’s per sq ft of the four methods (24 BTU’s per sq ft) and allow for lower water temps for higher efficiencies with condensing boilers.

The extruded aluminum plates (C-Track) are the nicest and the most expensive. They are 4 to 4-1/2 inches wide and 4 feet long and approximately 1/32 inch thick. The extruded part allows the PEX to snap in sideways holding it securely and held in place with screws.

The other type of heat transfer plate is a stamped aluminum sheet. These are made of thin metal that look like flashing material. The sheets typically are 4-1/2 inches wide and 16 to 24 inches long (we sell the 24 inch ones). These plates have a stamped groove in them to hold the PEX up against the bottom of the floor (but not as securely as the C-Track) and are usually staples to the sub floor.

The extruded heat transfer plates are the least noisy of the plate method and only at higher water temperatures. The stamped plates tend to be the nosiest as the PEX rubs against the floor and plate when it expands and contracts. This tends to scratch the oxygen barrier on the PEX surface. Heat transfer plates can lead to uneven floor heating as they concentrate the heat directly above the plates.

The second is the Hung Method. The PEX rests on supports mounted between the joists every 2 1/2 feet and approx 1 5/8 below the floor. The PEX is laid over the top of them and zip tied to the supports. This method provides approx 18 BTU per sq ft.

One important advantage of the Hung Method is the PEX does not touch the floor (1” away) and allows for convective air currents to heat the floor evenly. This also means hard wood-flooring nails will not reach the PEX.

This is why we prefer the hung method (we installed in our own homes) because when the PEX expands it just sags up and down in the joist space causing no noise or damage.

The third method is a staple up using talon, standoff clamps or staples to attach the PEX directly under the sub floor. This is one of the least expensive methods and offers approximately 15 BTU per sq ft, as there is minimal convection around the PEX.

When stapling the PEX directly to the sub floor, the spacing needs to be approx every 2 feet to avoid noise. The staples are only 1/16 inch wide and, in our opinion, this is not enough support to keep the PEX from damage over time.

The forth method (which is not recommended) is to attach the PEX directly on the sides of the floor joists with talon or stand off clamps. The joists act as heat sinks and draw some of the heat away from the floor.

All methods require containment of the heated air between the joists (dead air space) so heated air will not be replaced with colder outside air. And the use of a small length of protection sleeve is to protect the PEX where it passes through the holes drilled in the joists. (It is recommended that you use protection sleeve whenever you pass through floor joists with any under floor installation.)

When heating within the framework of a building you need to caulk all the electrical holes, plumbing and chimney chases, around all of the band and rim joists, and any other area that heated air could escape. This is easiest with modern platform framing as the plywood sub floor caps off the heated air.

For older ballooned framed houses and houses that are sheathed with boards we do not recommend radiant heating within the joist spaces. When houses that are sheathed with planking the gaps in the boards allow the heated air to travel out of the joist spaces easily. This causes a negative air pressure with in the framework and draws in cold outside air.

Under floor radiant installations should have a reflective insulation, which helps contain the heated air. The insulation is positioned 4 inches down from the PEX and stapled against the joists. If the space is heated, the use of a secondary insulation is optional. If it is not heated, (like a basement), we recommend the use of fiberglass or high-density foam board below the foil.

Remember, it is very important to know the heating limits of the floor (25 BTU’s sq ft). The Staple Up method can deliver up to 15 BTU’s per square foot. The hung method can deliver up to 18 BTU’s per square foot and 24 BTU’s with heat transfer plates. If you try to deliver more heat than this you can damage the floor.

When the heat load required for a specific area is larger than the radiant floor can provide, supplemental heat is needed like radiators, wood stoves, or gas direct vent heaters.