Quick Answer
Yes, you can use rigid foam underneath a concrete slab for insulation purposes, but it's essential to choose the right type and thickness to ensure the desired R-value and prevent moisture issues.
Choosing the Right Rigid Foam
When selecting rigid foam for insulation underneath a concrete slab, look for products with a high compressive strength, a low water absorption rate, and a suitable R-value for your climate. For example, a 1-inch (2.54 cm) thick extruded polystyrene (XPS) foam board has an R-value of around 5-6 per inch, while a 1-inch (2.54 cm) thick expanded polystyrene (EPS) foam board has an R-value of around 3-4 per inch. Consider using a 2-inch (5.08 cm) thick XPS foam board for a higher R-value, which is around 10-12 for the entire thickness.
Installing Rigid Foam Under a Concrete Slab
To install rigid foam underneath a concrete slab, follow these steps: Lay a vapor barrier, such as a 6-mil (0.15 mm) thick polyethylene sheet, on top of the compacted gravel base. Cut the rigid foam to the desired size and shape, leaving a 1-inch (2.54 cm) gap around the edges for a smooth concrete finish. Install the foam board on top of the vapor barrier, ensuring it’s centered and level. Secure the foam board using foam board clips or by nailing it to the subgrade.
Moisture and Vapor Barrier Considerations
When using rigid foam under a concrete slab, it’s crucial to prevent moisture from entering the system. Choose a vapor barrier that’s compatible with the rigid foam and the concrete, such as a polyethylene sheet. Ensure the vapor barrier is tight and wrinkle-free to prevent moisture intrusion. Consider adding a drainage layer, such as a 1/2-inch (1.27 cm) thick drain board, under the rigid foam to direct water away from the slab. This will help prevent moisture accumulation and ensure a dry, insulated, and durable concrete slab.
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