Turning up the heat

Well, not quite yet, but we are getting ready for it. I am talking about the hydronic heating system in the basement floor, also known as radiant floor heat.

The material

To get to the radiant floor heat, we have to install ½ inch cross-linked polyethylene tubing (commonly known as PEX tubing) that will be encapsulated in the basement concrete floor.

The material properties of PEX tubing make it ideal for radiant floor heating systems. It also begins to replace copper tubing in domestic plumbing systems, local building codes permitting.

PEX has an incredible resilience, with two exceptions. First, it degrades under the exposure of UV light. Most product information advises to keep the exposure of the tubing to daylight to less than 90 days. It is needless to say that the less it is exposed to any light, the better.

Secondly, it degrades on the exposure of radical oxygen molecules, which are occasionally found in water. Because of that it is recommended to use PEX tubing for hydronic heating systems with an oxygen barrier.

We would like the tubing to last as long as the concrete floor!

The installation

The radiant floor heat in the basement is organized into several zones. Each zone serves different heating or temperature needs.


All the tubing originates in the utility room, where will have the hot water source for heating, and returns to the utility room. Eventually we will connect the ends to a manifold.

The first two lines along the floor edges are spaced 6 inches apart, delivering extra heat along the foundation walls, which mitigates their potential cooling power. All other lines are then spaced 12 inches on center. The spacing is easy as we can use the six by six inch grid of the welded wire mesh as a guide.

We attached the tubing to the welded wire mesh with four inch zip ties at the recommended spacing of every two feet.

To control cracks in the concrete floor, we have planned expansion joints at various locations. At these locations, we run the PEX tubing through a PVC sleeve. The sleeve is the sacrificial lamb, protecting the PEX tubing from stresses caused by any cracking.


It is also recommended to protect the PEX tubing with PVC sleeves wherever it transitions in or out of the thermal mass, i.e. the concrete floor.


This is the case in the utility room where the lines originate and terminate (see image above) and where the lines transition out of the main basement into the porch area (see image below).


The pressure test

Very good! The PEX tubing is in place. Tomorrow, there will be a bunch of guys running around with equipment pouring the concrete over the tubing. But what if we damage a line and cause a leak during the concrete pour?

If that happens, we want to know about it right away, not after the concrete has cured!

Solution: connect all loops with compression fittings, set a pressure gauge on the last line, put the system under pressure and monitor the pressure during the floor installation.

pex-05 pex-06

Done! I got the bicycle pump out, put 30 psi on the system, and we are ready for the concrete the next morning. Except – when I showed up, two hours before the concrete arrived, the pressure had dropped to 10 psi. I wasn’t sure what to make of it and call Mariusz, our plumber, in a panic.

He pointed out that it is very unlikely that I have a leak in the PEX tubing and recommended that I check the compression fittings for a leak.

Our neighbor, who was up early, supplied me with soapy water in a spray bottle. Before long I got a big soap bubble around the culprit fitting and had it fixed. I pumped the pressure back up to 20 psi, and this time it was holding for good.

Actually, after the concrete pour, the pressure rose to 22 psi due to the heat in the concrete from the curing process. Let’s call that airtight!

About Marcus de la fleur

Marcus is a Registered Landscape Architect with a horticultural degree from the School of Horticulture at the Royal Botanic Gardens, Kew, and a Masters in Landscape Architecture from the University of Sheffield, UK. He developed a landscape based sustainable pilot project at 168 Elm Ave. in 2002, and has expanded his skill set to building science. Starting in 2009, Marcus applied the newly acquired expertise to the deep energy retrofit of his 100+ year old home in Chicago.

8 thoughts on “Turning up the heat

  1. Congrats on finding that leak and getting the concrete poured. It must feel great to reach such a big milestone in this long process.

  2. In a later post, you point out that it was “exciting” to shoot the wall sill plates into the slab. If you had it to do over, would you consider spray-painting your wall layout onto the mesh/vapor barrier, and running your tubing loops to avoid being under the future wall sill plates? The tubing looks “malleable” enough to do it, but organizing the loop layouts could be a lot more complicated…

  3. That is a strategy that I definitely recommend! Some of the PEX has however to cross underneath the walls somewhere into the next room. But that could also be documented and mapped and help avoiding the tubing when drilling.

    Even with all those measures, it is still nerve wrecking when drilling and one still feels sort of blind – with the tubing hidden in the concrete.

  4. Hey Marcus – Your site has been a great resource for me over the last few months while I’ve been doing the same project on my basement. I had a question regarding the PVC sleeves which protect the pex when traveling under a planned expansion joint. I cant seem to find straight 3/4″ sleeves for 1/2″ pex – They’re all 90 degree bend supports. Did you make these yourself out of a straight piece of 3/4 PVC, and just put a slit down the middle? Thanks – Andrew

  5. Andrew, glad you find the blog helpful! I used simple 3/4″ PVC pipe for the sleeves (I think it was 3/4″ – if a 1/2″ PEX tube fits into a 3/4″ PVC pipe, than it was what I was using…). I cut the PVC pipe into 12″ long sleeves, pushed them over the PEX tubing, and located the sleeves along the planned expansion joints.

    I can’t quite recall, but I may have had a couple of sleeved that I did indeed slit down the middle to bend them open and slip over the PEX tubing (I can’t remember if it was an idea that I abandoned, or if I was indeed able to bend open the PVC sleeve without breaking it). If I did slit the sleeve, I did make sure that once installed, the slit was pointing down.

    Hope this helps.

  6. I love this blog! I do have question about the Rigid Foam insulation you have around the perimeter of the basement. Are there any tricks to installing it? Is laid down first then the rigid foam on the basement floor? Does it have to be the same you used on the floor or can it be thinner?

    My basement will be dug down several feet in the coming months and we are planning on doing what you have done to prep the slab. I am just unsure about how the finished floor will look with the insulation visible. My house is a row house in DC and we are going to polish the concrete when done so I am concerned about the aesthetics of the insulation.

    Do you have any images of what it looks like before you closed up the gap?

  7. You find more images of the bond break in the following posts:

    The last post has an image of the bond break after the concrete floor is installed.

    We put a thin layer (1/8 inch) of concrete patch material over the bond break. This gives the appearance of the concrete floor extending all the way to the foundation wall, while minimizing the thermal bridging.

    Installation: We set the bond break on the first layer of insulation, and then installed the 2nd layer. This way the 2nd layer holds the bond break upright. It is critical that the top edge of the bond break is level all the way around, because your concrete contractor will use it as an elevation guide during the pour.

    Our bond break is 2 inches, because I used the scraps from the 2 inch XPS boards that were left over. Some people go with a 1 inch bond break. I wouldn’t recommend less. In the end it depends on your performance goals.

    Hope this helps!

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