After the I-joists were hung from the mudsill and inspected we got to try working with a material that we've been hearing about for a long time but until now had not had an opportunity to try. Warmboard is a subflooring material that is 1 1/8" thick and has grooves for 1/2" PEX tubing routed into it in different patterns corresponding to different ways you might wish to lay out PEX to heat a floor. In terms of raw cost per sq. ft. it cannot compete with a radiant slab but it has some interesting properties that make it far superior to all other staple up radiant options and in some jobs superior to radiant slab a well. This was one of those projects where it really did make sense.

This project will have 2,300 square feet of heated space so we needed 83 four by eight panels which, including shipping, cost $17,325.04 delivered to the site. By comparison 83 pieces of advantech sub flooring with staple up tubing would have cost us much less but would not have given us the advantage of being able to use oak flooring with antique rugs and still heat the house with our choice of solar, wood, or propane derived BTUs.

If we had used regular plywood under the oak and stapled up ½" pipe underneath we would have had much slower heat transfer to the home than the tight contact aluminum sheet we get with the Warmboard. To compensate for this we would have had to kick the heat in the pipes up to 140 degrees which would have meant that the "cooled" water returning to the heat exchanger from the floor would still be at 120 degrees. This return temp is too warm to effectively heat with solar and even presents efficiency issues with a Rinnai demand water heater (it's harder to heat water that is already hot). We expect the cooled water returning from the Warmboard to be barely warmer than room temp which makes it ideal for use with solar panels and optimizes efficiency with the wood fired boiler and Rinnai demand water heater that will be our back-up energy sources after the solar.

The Warmboard folks provided right and left turn full and half sheets, straights, fillers, and an elaborate schematic showing exactly how each board fit in the scheme. When we were finished we had used every piece and had one straight and one filler left over. If we had come up short it would have been a week or more back order from their distribution facility in so I was pretty content to have a couple sheets left over even if they did cost about $207 each. It took us a day and a half to install all the sheets and another half day to rout out all the special grooves.

The next project was to install the pipe and again the Warmboard folks had provided a lay-out and calculated the length of each run and identified the locations of the manifolds. We used 3,000 feet of pipe for this project which added $840.00 to the cost of the system. We glued the pipe into the grooves with silicone to keep it from making noise as the warmth hits the system and rolled it in with a lawn roller as we set it out. This took a little over a day with all the constant vigilance to keep the grooves clear of dust and debris.

Backing $10,000 worth of Warmboard into the job site.

Warmboard cuts just like regular plywood but throws up aluminum chips that you really don't want to get in your eyes so safety glasses and a cap are not optional.

The folks at Warmboard provide an installation plan and, at $207.00 a sheet, you can't afford to make any mistakes. We had to pull up one sheet at the outset and decided to assign one person to be in charge of the plan. At the end of the job we had two sheets left over...

The stuff goes down like regular ply except that it weighs over 100 lbs per sheet. I thought it was smart that they marked the pipe locations on the underside of the sheets so if we need to cut or drill in it after the finish floor is down we have at least a chance of missing the pipes.

Toilets and other items that would necessitate re-location of pipe are located and grooves get routed in ahead of time to allow the pipe to dodge the hazard.

Additional grooves get added to link floor sections and accommodate for all the bays in this design.

We made a spool out of left over plywood and 2" electrical conduit to allow the 1000 foot coils of PEX to roll out evenly from between a couple of ladders. Vacuuming out the grooves was critical and tedious.

A very thin bead of silicone is laid into the grooves and a lawn roller filled with water embeds the pipe into it very quickly. On the tight turns we screwed nail plates down over the pipe to keep it from popping up. The manifolds will be in walls 36" above the floor to allow air bubbles to accumulate and be expelled from the system through air vents.

The final project was to pull all the manifold tails back under the floor and pressurize all the pipe to 100 psf before the walls went up. Since the walls were table-framed off site there was little assembly done on the deck and the walls were up in a day and a half so wear and tear on the pipe was minimized.


Warmboard is generally installed with PEX-AL-PEX tubing, a type of tubing that was originally developed to provide an oxygen diffusion barrier so radiant heat installations could be used with old-fashioned cast iron boilers and pumps. This tubing combines a thin, flexible aluminum core with an inner liner of PEX and an outer wear layer of PEX and a waxy lubricant. It has doesn't have the annoying "memory" that PEX has which makes that product want to spring back into the roll shape it came in. You can shape it to any shape you like and it will pretty much hold that shape. It also has a far lesser coefficient of expansion than pure PEX does which makes it less likely to wiggle and squeak when a slug of hot water hits it in a radiant floor application.

But PEX-AL-PEX gets stiff in cold weather and doesn't feed easily through holes and over obstructions. And it's significantly more expensive than plain PEX. (For this project it would have been $1,400 more.) PEX-AL-PEX pipe uses proprietary sleeve and ring fittings and any scrap shorter than 200 feet generally ends up in the landfill as we don't use this type of pipe in potable water supply piping. PEX pipe is the same pipe we use with potable water installations so it uses the same crimp fittings we already have and 100% the scrap pipe can be immediately recycled into the same house.

When plain PEX is installed in a radiant floor application such as Warmboard it must be continuously embedded in a 1/16" to 3/16" bead of 100% silicone caulk in order to bond it to the aluminum to prevent it from slipping and "popping". The pipe is pressed into the silicone with a heavy weighted roller so that it displaces the air between it and the aluminum which enhances heat transfer. This process has the potential to be messy and prone to operator error in that too much silicone will keep the pipe from sinking below the surface of the board and too little can lead to a noisy floor. Still, it really is no more difficult or time consuming that wrestling with the stiffer pipe and I chose to use this system based on the enhanced contact between the pipe and the aluminum combined with the cost savings and my prejudice against the stiffness and proprietary fittings of the PEX-AL-PEX.

We'll install the Rinnai water heater and the flat plate heat exchanger and get the system up and running to dry out the Warmboard before we start installing the hardwood and tile flooring. At this point we have the system pumped up to 100 lbs so we can rest assured that there is no hidden damage to the pipes.

Get the details from the Warmboard installation manual for PEX tubing.


  1. Daniel said...
    This was great information thanks for putting it up!
    Grabber Green said...
    What an excellent project presentation. Thank you for all your details. I learned a lot!
    Nenad Grubor said...
    So very useful - great job explaining. Can you re-post about your satisfaction with the heating system efficiency and performance?
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