Radiant Heat From Radiant Floor Company The Slab on Grade Installation

Radiant Heat From Radiant Floor Company The Slab on Grade Installation

The Slab on Grade Installation

A slab on grade is defined as any concrete slab poured over excavated soil. From a radiant heating perspective, it doesn’t matter if the slab is actually at grade or is poured several feet below grade as part of a full foundation. Check out our video How to Install Radiant Floor Heat Tubing in a Slab On Grade. and read this page for a full description.

The fact remains that installing radiant tubing within a concrete slab is probably the easiest, most cost effective, and highest performance application of the science. The thermal benefits are unsurpassed. Virtually any concrete pour should contain radiant tubing. even if you have no immediate plans to heat the space. After all, you may change your mind later and regret your lost opportunity. For most applications, the tubing and manifold are relatively inexpensive and the mechanical components can be installed even years later.

Of course, there are always exceptions to the rule. A woodshed or an outside storage shed with a concrete floor might be a waste of tubing. But even then you should think long and hard about the possibilities of converting these areas into heated space at some future date. I say this because often we work with people faced with the task of pouring a new slab, with tubing, over an already existing slab. and they poured their existing slab only a few years before. How much easier it would have been to install the tubing in the original slab!

But, if you’re fortunate enough to be planning an original pour, the procedure is simple. In fact, the basics of a standard pour remain the same. The compacted aggregate base is first, followed by a 6 mil polyethylene vapor barrier, then insulation, then rebar or wire mesh, or both.

The insulation phase is crucial for a radiant floor. Mainly, heated slabs radiate outward rather than downward, so insulation on the edges of the slab is most important. Remember that your slab will be about 75 degrees F. Any cooler surface in contact with the slab will try to steal its heat. If you’re pouring up against your foundation walls, insulate between the slab and the walls. For a cleaner looking installation, cut the top edge of the foam board at a 45-degree angle so the concrete will flow all the way to the foundation wall and hide the foam.

How you insulate under the slab depends upon the severity of your winters. In lower, warmer latitudes, the 1″ XPS Foam (Extruded Polystyrene foam, i.e. pink or blue board) works fine. In colder regions, use 2 XPS.

Note the vertical insulation on the edges of the foundation. Heated slabs lose heat outward as well as downward.

There are many approaches to insulating a radiant slab, but the above detail shows a frequently used method. Since the slab will be approximately 5 degrees warmer than the room temperature, a 75 degree slab is quite common. Obviously, any cooler surface in direct contact with the slab will try to steal its heat, so a thermal break greatly reduces this heat transfer.

Of course, in many situations a downward heat flow is desired as a means of creating a heat sink to protect the space in the event of a severe power outage or mechanical failure. A slab with such a heat sink could take days to fully cool down.

Note: Many of our customers ask us about alternate slab insulation materials like radiant foils, bubble-type insulation, and thin foams of various kinds coated with vapor barriers. Admittedly, these alternate materials have two distinct advantages over blue or pink board, i.e. the extruded polystyrene mentioned above—they are cheaper and easier to install than multiple sheets of rigid foam.

Unfortunately, customers report dissatisfaction with the performance of bubble wrap and thin foam insulation when used under slabs.

For the record, Radiant Floor Company does not sell under-slab insulation of any kind. Our opinion is based on customer feedback and our own experience. We recommend Extruded Polystyrene .

So, once you’ve insulated to suit your situation, install the rebar and/or wire mesh and use rebar ties to fasten your radiant tubing to the mesh. If like most slabs you require more than one circuit of tubing, you’ll need to install a slab manifold at some convenient spot along the perimeter of the pour. The slab manifold is shipped in a plywood box that doubles as the form you pour the concrete around. Make sure the manifold box is installed plumb. Later, when the pour is complete and you unsweat the pressure test kit from the top of the manifold, you’ll want your supply and return pipes sticking up nice and straight. Install the slab manifold very near your heat source, if possible, to keep the supply and return lines from your heat source short and easy.

Depending upon which size tubing you’re using (7/8 PEX or PEX) you’ll space the tubing either 16 on center, or 8 on center respectively. Keep in mind that while you’re looping the tubing back and forth, up and down the slab and so forth, you won’t be trying to make a 16 bend in the tubing. The actual bend will probably be closer to a 24 radius. depending upon whether you’re installing the tubing on a warm summer day, or a cool fall evening. In other words, warmth equals flexibility. But whatever the temperature, just allow the tubing to conform to its natural bend. You may want to experiment with a 4 ft. piece of tubing before you start. Slowly start bending until you reach the kink point. That will give you some idea of how tight your bends can be. Then later, while laying out your circuits, and after your wide, comfortable bend, you can begin spacing the tubing roughly 16 on center on the straight-away’s (8 on center for 1/2 PEX).

The Slab on Grade Installation

A slab on grade is defined as any concrete slab poured over excavated soil. From a radiant heating perspective, it doesn’t matter if the slab is actually at grade or is poured several feet below grade as part of a full foundation. Check out our video How to Install Radiant Floor Heat Tubing in a Slab On Grade. and read this page for a full description.

The fact remains that installing radiant tubing within a concrete slab is probably the easiest, most cost effective, and highest performance application of the science. The thermal benefits are unsurpassed. Virtually any concrete pour should contain radiant tubing. even if you have no immediate plans to heat the space. After all, you may change your mind later and regret your lost opportunity. For most applications, the tubing and manifold are relatively inexpensive and the mechanical components can be installed even years later.

Of course, there are always exceptions to the rule. A woodshed or an outside storage shed with a concrete floor might be a waste of tubing. But even then you should think long and hard about the possibilities of converting these areas into heated space at some future date. I say this because often we work with people faced with the task of pouring a new slab, with tubing, over an already existing slab. and they poured their existing slab only a few years before. How much easier it would have been to install the tubing in the original slab!

But, if you’re fortunate enough to be planning an original pour, the procedure is simple. In fact, the basics of a standard pour remain the same. The compacted aggregate base is first, followed by a 6 mil polyethylene vapor barrier, then insulation, then rebar or wire mesh, or both.

Radiant Heat From Radiant Floor Company The Slab on Grade Installation

The insulation phase is crucial for a radiant floor. Mainly, heated slabs radiate outward rather than downward, so insulation on the edges of the slab is most important. Remember that your slab will be about 75 degrees F. Any cooler surface in contact with the slab will try to steal its heat. If you’re pouring up against your foundation walls, insulate between the slab and the walls. For a cleaner looking installation, cut the top edge of the foam board at a 45-degree angle so the concrete will flow all the way to the foundation wall and hide the foam.

How you insulate under the slab depends upon the severity of your winters. In lower, warmer latitudes, the 1″ XPS Foam (Extruded Polystyrene foam, i.e. pink or blue board) works fine. In colder regions, use 2 XPS.

Note the vertical insulation on the edges of the foundation. Heated slabs lose heat outward as well as downward.

There are many approaches to insulating a radiant slab, but the above detail shows a frequently used method. Since the slab will be approximately 5 degrees warmer than the room temperature, a 75 degree slab is quite common. Obviously, any cooler surface in direct contact with the slab will try to steal its heat, so a thermal break greatly reduces this heat transfer.

Of course, in many situations a downward heat flow is desired as a means of creating a heat sink to protect the space in the event of a severe power outage or mechanical failure. A slab with such a heat sink could take days to fully cool down.

Note: Many of our customers ask us about alternate slab insulation materials like radiant foils, bubble-type insulation, and thin foams of various kinds coated with vapor barriers. Admittedly, these alternate materials have two distinct advantages over blue or pink board, i.e. the extruded polystyrene mentioned above—they are cheaper and easier to install than multiple sheets of rigid foam.

Unfortunately, customers report dissatisfaction with the performance of bubble wrap and thin foam insulation when used under slabs.

For the record, Radiant Floor Company does not sell under-slab insulation of any kind. Our opinion is based on customer feedback and our own experience. We recommend Extruded Polystyrene .

So, once you’ve insulated to suit your situation, install the rebar and/or wire mesh and use rebar ties to fasten your radiant tubing to the mesh. If like most slabs you require more than one circuit of tubing, you’ll need to install a slab manifold at some convenient spot along the perimeter of the pour. The slab manifold is shipped in a plywood box that doubles as the form you pour the concrete around. Make sure the manifold box is installed plumb. Later, when the pour is complete and you unsweat the pressure test kit from the top of the manifold, you’ll want your supply and return pipes sticking up nice and straight. Install the slab manifold very near your heat source, if possible, to keep the supply and return lines from your heat source short and easy.

Depending upon which size tubing you’re using (7/8 PEX or PEX) you’ll space the tubing either 16 on center, or 8 on center respectively. Keep in mind that while you’re looping the tubing back and forth, up and down the slab and so forth, you won’t be trying to make a 16 bend in the tubing. The actual bend will probably be closer to a 24 radius. depending upon whether you’re installing the tubing on a warm summer day, or a cool fall evening. In other words, warmth equals flexibility. But whatever the temperature, just allow the tubing to conform to its natural bend. You may want to experiment with a 4 ft. piece of tubing before you start. Slowly start bending until you reach the kink point. That will give you some idea of how tight your bends can be. Then later, while laying out your circuits, and after your wide, comfortable bend, you can begin spacing the tubing roughly 16 on center on the straight-away’s (8 on center for 1/2 PEX).


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