I started this discussion last month on the importance of proper insulation for radiant heating systems. Don’t let the builder labor under the mistaken impression that heat always rises. It does in forced-air systems, but it needs to be directed, by insulation, in radiant systems.
Slab on or below grade: First and most importantly there’s the moisture factor that you must address. If there is ground water within 5 ft. of the surface, there must be a heavy-duty moisture/vapor barrier put down prior to putting down insulation. If not, there’s no amount of reasonable insulation that’s going to keep the moving groundwater from cooling your slab. Make sure you have the water/vapor barrier specified in your contract.
Slab floors are typically exposed to a lesser degree of differential temperature. With that being said, I typically use a 1-in. minimum eps foam throughout the slab. Don’t forget to insulate the edge of the slab where it comes into contact with the wall/footer assembly.
I saw a neat idea recently. Cut a 4-in. tall piece of 1-in. thick insulation at a 45° angle. This allows a good clean finish line at the finished floor surface but gives 1-in. of edge insulation to provide a good thermal break on the outer edges of the slab. (See drawing, top right.)
It’s virtually useless to use a foil-faced foam on slab applications. Without a dead air space on both sides of a reflective surface, the spectral reflectivity is zero.
Suspended floor staple down: In this system, the floor is suspended above grade in the basement. This is generally done because of expansive soils. Suspended floors can be either metal or wood in construction. You would insulate these the same way as framed upper floors as I discussed in last month’s column. I would not suggest that anybody attempt either a staple-up or a suspended tube installation on a metal decked suspended floor. I have many reasons.
First, it would be nearly impossible to properly attach the tube to the metal decking. Secondly, if you got the tube suspended, proper installation of the insulation would present a severe challenge.
If tube is put down on top of the metal floor and poured-in concrete, make sure the general contractor puts insulation on the crawl space side of the decking. This can and should typically be done at the time the decking is installed using sharp metallic wires that are spot welded to the bottom of the panel. Kraft-faced insulation is pushed onto the spikes and special washers are installed to keep it from sliding off.
Radiant wall panels
If you’re doing radiant walls and the heat source is on an outside wall, you want as much resistance value in the wall as is technically possible, preferably with a reflective surface within the insulation assembly.
One recommendation would be to use two sheets of foil-covered polyisocyanute board insulation. The first sheet would be installed approximately 1/2-in. from the inside wall. The second would be installed within 1/2-in. of the face of the stud on the outside wall.
What you end up with is an actual R-value of 10 for the wall insulation assembly. More importantly, you get three dead air spaces with four reflective surfaces. This has been shown to reflect almost all heat transmission back to its source. Kind of like having heat mirrors built into the wall.
Although this method of insulation is obviously more labor and money intensive than conventional methods, it will work well during the summer, lowering air conditioning costs, as well as during the winter.
In general, make sure that some method of insulation is spelled out in your contract, because when it doesn’t get done, it becomes your problem. After all, it is your heating system that’s not working, isn’t it?
Steel panel radiators: If you are installing steel panel radiators on an exterior wall, the heat mirror recommendation would be a good idea where possible. In the case of retrofit, I recommend using double foil blister-pack insulation directly behind the radiator.
On interior walls adjoining heated space, no insulation is necessary unless you are worried about uncontrolled migration of heat from one room to another. If this is an issue, a minimum R-value of 7 is advisable.
On interior walls connected to unconditioned space (garage, storage etc.), a minimum of R-7 is recommended.
The bottom line is this: If you are not directly responsible for the installation of required insulation components, make sure that the insulation installation specifications are a part of your contract with both the homeowner and the general contractor. This way, if the insulation is not installed or is improperly installed, you are protected from a liability standpoint.