Insulation needed for a proper radiant heating system is typically one of the most overlooked areas of radiant heat installation.

If we as good and competent radiant contractors install our systems as best we can but depend on others to install the insulation, we may (and usually do) find ourselves called onto the carpet to explain why it is that our radiant heating system is not keeping their house warm. Many times, the complaint of a general lack of warmth upstairs is accompanied by a similar complaint that the living spaces one level below are extremely warm. The consumer doesn’t usually make the connection.

The unsuspecting building contractor turns up the heating system and the problem gets a little better upstairs, but the basement gets even hotter. Upon further investigation, we find that the general contractor didn’t install insulation in the joist bay areas below the heat source because he “didn’t think it was necessary. That part of the house is over a heated space, and besides, heat rises doesn’t it?”

Don’t let yourself get caught in this trap. If you are not actually responsible for the physical installation of the insulation, make sure that you specify in your contract to the homeowner and the general contractor what type, how much and how the insulation is to be installed.

In general, you should follow the recommendations of the heating system manufacturer. If the supplier has not made any recommendations, then the following information can be used as general guidelines.

How much: Generally, if the radiant floor is over a heated space you need at a minimum two times the R-value below the heat source as you have above it. For example, if there is 11/2 in. (approximate R-value = 1.5) of wood on top of the radiant-floor heating source, you’d want a minimum of R-3 underneath the heat source.

Obviously, R-3 insulation is a little hard to come by. I’ll typically double the given R-value above the heat source because you never know when Ms. Homeowner is going to pile on the woven throw rugs (more R-value) right on top of your radiant heat source. In any case, R-7 should be considered a minimum in framed construction. Some codes dictate a higher R-value for floors over unheated (crawl space) floors.

What type: In standard framing applications, unfaced fiberglass, kraft paper-faced or foil-faced fiberglass insulation will work, depending on the application.

Staple down: If the radiant floor heating is done via staple down on top of the plywood subfloor or deck and the tube is poured in a cementitious material, you can use unfaced insulation. Push the insulation tightly against the bottom of the subfloor. Don’t compress the insulation or its net R-value will be affected. In all cases, the outer rim joist must be insulated as well as the rest of the subfloor.

Staple up: Staple-up tubing is attached to the underside of the plywood subfloor every 6 in. or so. Methods of attachment vary, but in any case, the tube is kept in semi-continuous contact with the plywood floor’s bottom. This method of heat transfer depends primarily on the tube-to-wood contact to conduct heat from the tube through the floor and into the room. When insulation is applied, it can be unfaced and should be pushed tightly against the tube and subfloor assembly.

Heat-transfer plate: This method uses any one of numerous metallic heat-transmission plates that are installed below the plywood subfloor. This system depends solely on conductance for its heat-transfer method and the insulation should be unfaced and should be pushed tightly against the heat-transfer surfaces and the plywood subfloor assembly.

Suspended tube: In this application, the tube is generally hung in the air within a few inches of the plywood subfloor. This method depends on conductance and convection for its heat-transfer capability. This method requires the use of a foil-faced insulation with a minimum air gap of 2 in. and a maximum of 4 in. between the foil and the subfloor.

This is usually the area where most trouble begins. Make sure that the air gap requirements are met. I’d suggest that, if all possible, you show up during the insulation process and make sure the insulators understand the importance of the air gap. Remember, they don’t understand the dynamics of a radiant system.