Radiant Heating has come to be known as perhaps the most comfortable type of heating available today (next to laying in the sun at least). In-slab heating is a common option to consider for basements, slab floors, and garages. While building codes do not necessarily call for it, there is no question that insulation under the slab will positively impact the performance of any in-slab radiant heating system. Here is what every contractor should know about insulation and in-slab heating.
Insulating Under the Slab
As was noted in our piece Radiant Heating: In Slab Systems – What You Need to Know About In-Slab Systems, the first thing to keep in mind is “when heating a slab floor, the goal is to efficiently heat the slab and direct as much of that heat as possible into the living/working space above it – practically speaking 6-8 foot air space above the slab floor.”
The use of an insulation barrier under the slab is a critical component that will result in much greater efficiencies for the heating system.
Without an insulating barrier, the slab will likely be resting on a bed of sand and/or gravel. Even though sand is not considered to be a good thermal conductor, it is also not a good insulator. It will allow heat to escape in a direction that is not in our target area – the 6-8 foot air space above the slab floor.
Insulation options: There are a few different options to provide an insulation barrier for a slab installation. Three of them are discussed here.
Rigid foam sheets: If the ground is leveled off well, a 1’ thick layer of blue foam is sometimes used as an insulation layer. The cost of material is relatively inexpensive. Because it is typically made of closed cell foam, this can also effectively serve as a vapor barrier, but would need to be taped or sealed somehow where pieces butt together in order not to compromise this functionality. If working outdoors, care must be taken to hold the pieces in place prior to pouring the slab. Pieces can be crushed or broken while being walked upon during the installation phase as well, sacrificing performance. Lastly, while a 1” thick layer of foam will typically provide an R-5 insulation value, the foam itself will still allow some heat from radiant tubing in the slab to pass through it into the ground (i.e. – someplace other than our target area).
Spray foam: This method has many risks with it. The foam itself should be closed-cell foam. If it is not closed cell, it will likely lose its insulation value over time. Spray foam has no inherent vapor barrier capability. This would need to be added perhaps through a plastic layer both below and above the foam. Perhaps the most difficult aspect of using spray foam is the ability to maintain a consistent thickness and density prior to pouring. For all of the above reasons, using spray foam as an insulation barrier under a slab is strongly discouraged.
Insulating Tarp or blanket: An insulating tarp provides a built in vapor barrier above and below an insulating layer of air (much like the bubble-pack used for shipping in packages). The upper layer of the tarp also serves as a reflective layer to help to maximize the amount of heat being directed to the target area above the floor. Insulating tarp is easy to work with. It can simply be unrolled over the desired area. It can be easily cut and taped down to adjacent pieces in only a few minutes. It can be walked on by installers without worry to integrity or performance of the product. It does not take up as much thickness as a rigid foam sheet and best of all, it provides superior insulation performance (typically in the R-6 to R-7 range).
While the per square foot cost of the insulating tarp may be more than that of rigid foam, it’s benefits during installation (quicker and easier) and its superior insulating performance and contribution to toward the increased efficiency of the radiant heating system provides a quick payback for both the installer and the homeowner. Going back to the goal of our in-slab system – to efficiently heat the slab (and not the earth below it) and maximize the amount of heat being directed into the air space 6-8 feet directly above the slab – it is easy to see that the use of an insulating tarp makes sense in any in-slab radiant heating system.