Construction Design-Floor Covering and In-floor Heat

In-floor heat has been a very popular choice for heating systems in my area for the past couple decades.  One of my first jobs as an apprentice electrician 25 years ago was plowing electrical heating cable for a basement slab heating system.  (I was the mule pulling the plow.)
These older buried resistive electric cable heating systems are rare today (and many have failed.)  The systems and the way we install them have changed over the years and we have also discovered some floor covering don’t work well with in-floor heat.

Hot water radiant heat in a slab on grade home.
Hot water in-floor heat tubing before the concrete pour.
A wood in-floor heating system called Warmboard.

Working as an energy auditor, I often see poorly installed systems.  Some with not enough or missing insulation either under the slab or at the slab edge, incorrect heating tube spacing, and I’ve also seen floor coverings that limit the heat flow into the living space.  Carpets with the wrong pad is a good example.  Much like a good insulation, some of these carpet pads work well to slow the movement of heat.  Rule number one, make sure the proper carpet pad is used when installing carpet over in-floor heat.  I recommend limiting carpeting in homes with in-floor heating.  Area rugs are a better choice.

A heated slab missing it’s slab-edge insulation.
Hot water tubing with improper spacing creates hot and cold spots across the floor.

I have seen wood floors installed over in-floor heating systems.  They can work over a wood subfloor but if it were my home, I would not use a solid wood floor over concrete.  Concrete subfloors require a vapor barrier between the concrete and wood.  Some are foam pads with an attached vapor barrier and will insulate or slow the heat transfer.  There are some manufacturers that have engineered wood and vinyl floors that can be used, which, in my opinion, would be a better product for this application over all-wood flooring.  If anyone is planning wood flooring over in-floor heat, do your homework and ask questions, preferably from the retailer, manufacturer and installer.  It can expensive when a good product fails because it was installed in a location where it shouldn’t be.

The best choices for good heat transfer are always dense surfaces connected directly to the slab or wood floor.  My area has seen a big increase in vinyl flooring, mostly luxury vinyl tile or LVT.  A common install method is to glue the vinyl directly to the floor.  Depending on the substrate, additional prep work may be required, but heat transfer through this type of flooring works well.

Another good option is tile.  This very dense flooring does nothing to slow heat flow through the product.  There are some issues that need to be addressed when installing tile over a heated floor.  When working with a wood subfloor, I recommend installing an uncoupling membrane.  My first choice is Schluter’s Ditra.  There are other membranes that perform the same task, just be sure to follow the manufacturer’s directions when installing.  Concrete floors, such as slab on grade houses, also need an uncoupling membrane.  Ditra will also work over concrete but there is an alternative method, a roll-on product.   These paint-on products work as both a waterproofing membrane and also to decouple the tile from the subsurface.  Easy to apply and less costly than Ditra, I often use roll-on products when tiling on concrete with heat.

Schluter’s Ditra uncoupling membrane over a wood sub-floor. The banding, or lighter colored strip, makes the installation waterproof.  This installation was over a wood sub-floor.
A roll-on uncoupling membrane installed over concrete with hot water in-floor heat.

Another requirement, when tiling large spaces with in-floor heat it’s best to have an expansion joint.  The concrete or wood surface under the tile moves at a different rate than the tile itself.  Even when uncoupling membranes are used, there can be enough movement in a large area to cause the tile to expand and without an expansion joint, the tile can buckle.  This typically only happens in large areas.  The Tile Council of North America requires expansion joints when the tiles extend 25′ in either direction.  There are exceptions where smaller areas require expansion, consult the TCNA code book for current tiling codes.

I have a story about tiling over a concrete slab on grade with in-floor heat as told to me by a local flooring retailer, I was not involved in this job.  A large, single level vacation property in the area was tiled throughout.  The homeowner was at the property when what sounded like a gun went off inside the home.  The owner later discovered several loose tiles in different areas throughout the home.  The noise was the tiled floor lost it’s bond to the concrete.  There may have been a few different reasons for the loss of connection, I don’t know if any expansion joints were installed in the tile.  There could have also been a concrete sealer installed over the floor without the tile setter realizing, which may have limited the bond between the tile and concrete.  The flooring retailer attributed the failure to the in-floor heat and now requires all installations by their subcontractors to include a roll-on product.  They have had no failures in more than 10 years after the change.

So, what’s the best floor covering for in-floor heat systems?  Well, if it’s over a concrete slab, no floor covering at all.  Bare concrete is the cheapest and most durable floor covering.  The concrete could be stained, polished, stenciled or stamped which would add interest and still have really a good heat transfer.  If installing over a heated wood subfloor, anything that does not require a pad that can slow heat transfer.  Be sure whatever covering is chosen, it’s compatible with an in-floor heating system.  Do some homework and talk with the installers.  They often have the experience in installing, and sometimes removal these products because of failures.

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