Insulating the concrete slab before you build your home is one of the best decisions you can make for energy efficiency. Many building codes require a vapor barrier and rigid foam insulation under a heated slab.
Our experts recommend Prodex, a new-generation radiant floor insulation that serves as a concrete vapor barrier and foam board insulation. It reduces heat loss and protects against condensation, mildew, mold, soil gases like radon and unwelcome creatures.
Under slab insulation acts as a strong barrier against everything that can enter your home through the concrete slab, including toxic gases, unwanted creatures and cold temperatures. It also blocks moisture, condensation, mold and mildew from entering your home, and it serves as an effective radon barrier.
Regardless of climate zone, the code requires slabs to be insulated under footings, pile caps, grade beams and foundation walls, as well as beneath slab edges. This prevents direct contact between the ground and heated or unheated floors, which will lose heat to the ground surface like a chimney drafting up smoke.
EcoFoil Ultra CBF under slab insulation features a layer of metalized foil centered between a layer of bubble insulation laminated to each side with a permanent bond. This encapsulates the reflective foil sheet, preventing it from being exposed to the concrete and ensuring the product’s longevity. It’s also engineered to withstand the pressure and corrosive nature of concrete and provides the highest level of reflectivity on the market.
The slab foundation is the most significant area of heat loss for both heated and unheated slabs. In heated homes, the slab sucks radiant heat down and back to the ground, where it is lost. An insulated slab eliminates this heat flow and greatly reduces air-conditioning costs.
Expanded polystyrene, or EPS, is the type of foam most commonly used under slab insulation. It comes in a variety of thicknesses and compressive strengths, with R-values up to 3.6 per inch. Unlike reflective foil products, EPS retains its R-value over time.
XPS and polyiso are both rigid foam board insulations that offer high R-values, excellent moisture resistance, and fire resistance. Polyiso has the added benefit of water vapor permanence, which helps prevent condensation and mold problems. It also has a higher R-value than both XPS and EPS. It’s important to choose the right insulation for your project. Contact the insulation experts at Rmax to help you select the best option for your home or business.
While a number of slab-on-grade homes are insulated in one or the other of two ways, builders should consider adding horizontal insulation under the entire slab (see figure below). This will reduce heat loss and condensation of vapor in the summer, while saving energy in winter by reducing the amount of natural gas required to heat that slab.
Having insulation under the slab also reduces temperature swings, which are a cause of condensation and mold problems. Insulation also responds much quicker to new changes in control inputs.
Spray foam is a great choice for under slab insulation because it has a built-in vapor barrier. XPS is another good option, but it is not as vapor resistant as polyiso and can lose R-value over time. Spray foam also fills the tiny cracks in the slab, which prevents bugs and other pests from entering the house. In addition, it provides a perfect seal against soil gases like Radon.
In climate zones 4 or more, a full 2″ of rigid foam insulation under the slab is money well spent. This reduces downward loss in winter and prevents condensation of humid air on the floor during summer.
Rigid foam insulation like XPS or EPS provides much higher thermal efficiency than concrete or other common building materials. It also has a lower water absorption rate, which is important for under slab insulation applications.
If your home is going to be built on a slab on grade, the concrete slab should always be insulated. If the slab is not insulated, it will act as a thermal bridge between the heated house and the cold ground or to the outside air. Slab edge insulation is especially critical, as it is a major source of heat loss. Generally, a 4′ wide band of R-10 insulation is adequate in most cases. A thicker band or continuous insulation is better, but it’s usually more expensive.