How To Insulate a Concrete Slab using Under Slab InsulationUnder slab insulation is a very important yet often overlooked step in building any home or commercial structure. Even in a home with properly insulated walls and attic space, there is always a certain amount of heat loss, which is why it is imperative that every step is taken to minimize this loss. One area that is often misunderstood is that of slab insulation. For example, in a typical 2,000 square ft. home a slab without proper insulation can be responsible for 15% of that home’s heat loss. While you are only required to insulate the slab if the above space is conditioned or the slab is less than 12” below grade, it is a great way to improve the energy efficiency of your home. If the slab is not properly insulated using concrete slab insulation, your home or building's heating consumption can increase by 8%.
When preparing to insulate the slab it is important to check the minimum insulation requirements for your area. Click here to find the R-value requirements for your climate zone.
For the purpose of demonstration let’s say the home you are building is in a climate zone 5. In this zone the insulation requirement for slab construction is R-10 at a distance of 2ft (horizontally or vertically). If you are planning on installing a heated slab using a radiant floor heating system you need to add R-5 to your R value.
Some common rigid insulation under slab materials used for under slab insulation are: expanded polystyrene, extruded polystyrene, and polyisocyanurate. For calculation purposes we will use extruded polystyrene (XPS) which has an R-5 value per inch. To achieve the R-10 requirement we would need to have 2” of XPS (3” for R15).
You can increase the R value by using a radiant foil barrier such as EcoFoil under concrete insulation. In addition to adding almost R-4 to your existing insulation EcoFoil insulation under concrete slab reflects 57% of radiant heat back up to the surface so this heat does not escape into the ground. EcoFoil slab insulation also functions as a concrete slab vapor barrier and a barrier for methane, radon, moisture, termites and other pests. At this time it is important to bring up the most important detail in insulating the slab: The insulation must extend downward from the TOP of the slab in order to fully cover the slab’s edge. The slab’s edge is responsible for 60% of the slab’s total heat loss so slab edge insulation cannot be ignored.
Perimeter slab insulation is tricky. The 2-3” of foam around the slab perimeter required to achieve the R-10 value causes a problem when installing carpet tack strips. To solve this problem the IECC allows the top edge of the insulation to be cut at a 45 degree angle so as to provide a more suitable base. This solution creates a problem of its own because the concrete will be very thin and brittle at the edge, rendering it useless for anchoring purposes.
Another solution is to install only ½’ of insulation around the perimeter. This lowers the R value but can be compensated by increasing R values somewhere else. At a thickness of 5/16” EcoFoil bubble insulation under concrete slab is an excellent addition to raise the R value. The overall total R value of the home is what matters and computer software programs are available to estimate a home’s total energy cost if you know the R-value of your insulation. Our concrete slab insulation not only provides additional R-value without the thickness of foam board insulation - our reflective insulation product is the only type of concrete slab floor insulation which will reflect heat backup towards the floor of your home or building. Another option to insulate the slab is to place slab insulation on the exterior walls of the foundation, which is the only way to insulate a monolithic slab. Just as with internal insulation the exterior insulation must start at the top of the slab. The under slab insulation can even extend to the top of the foundation wall, which creates a complete thermal envelope.