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Polyisocyanurate foam, or polyiso for short, has the highest R-value per square inch: 6.5 to 6.8. Polyiso is a rigid foam that comes with a reflective foil on both sides to serve as a radiant barrier. Given its high R-value, rigid foam is usually meant for foundations and basement walls.

Yes, it is possible to have too much insulation, which can have detrimental effects. While more insulation might seem better for energy efficiency, a home needs to "breathe" or ventilate to release moisture that accumulates inside. Over-insulating can seal a house too tightly, trapping water vapor and leading to several problems, including mold, mildew, and condensation. Homes in warm, humid climates are particularly susceptible to mold, while those in colder climates can experience condensation from temperature fluctuations. Additionally, adding more insulation than the recommended amount for your climate may not provide recoverable energy savings. To avoid these issues, consult a professional insulation contractor who can determine the optimal amount of insulation for your home.

The best type of insulation for your home depends on your specific needs, climate, and the area you are insulating. For new construction, structural insulated panels are a highly energy-efficient option that is easy to install during the building process. In very warm climates, reflective insulation can be a cost-effective choice. For cold climates, fiberglass is an excellent option due to its high R-value and resistance to settling over time. Spray foam works well in humid environments because of its moisture-resistant properties. For standard attics, loose-fill, spray foam, or blown-in insulation are all suitable options.

The most affordable way to insulate behind unfinished walls is to install fiberglass batts or rolls. These total around $0.75 per linear foot, with the width matching the gaps between standard studs perfectly. However, it’s important to consider the R-value of your insulation and the climate in your area, as fiberglass insulation isn’t the best in terms of preventing heat transfer. Plus, the most affordable option may cost more on electricity bills over time. The most affordable way to insulate behind finished walls is to leave them up and use blown-in insulation to avoid drywall replacement.