When insulating your attic, blown-in fiberglass is a popular choice due to its ability to fill gaps and provide consistent thermal performance. However, one of the most common questions homeowners and contractors ask is: “How many bags of blown-in fiberglass do I need per square foot?” The answer depends on the desired R-value, the type of machine used, and the specific product’s coverage chart.

Blown-in fiberglass attic coverage is typically measured in bags per square foot at a given thickness to achieve a specific R-value. For example, a standard bag of blown-in fiberglass may cover approximately 40 to 50 square feet at a thickness of 10 inches to achieve an R-value of 30. However, coverage varies by manufacturer. Always consult the product’s label or the manufacturer’s online coverage calculator for precise numbers.

To estimate the number of bags needed, follow these steps:

1. Measure your attic’s total square footage.

2. Determine your target R-value. The U.S. Department of Energy recommends R-49 for most northern climates and R-38 for southern regions.

3. Check the product’s coverage chart. For instance, if a bag covers 44 sq ft at R-38, divide your total attic area by 44 to get the number of bags.

4. Add 10-15% extra to account for settling, waste, or irregular areas like eaves and obstacles.

Keep in mind that blown-in fiberglass settles over time. The initial installation thickness is usually higher than the settled thickness listed on the bag. Professional installers use a depth gauge to ensure uniform coverage. Additionally, proper ventilation and air sealing should be completed before blowing insulation to maximize efficiency.

In summary, calculating blown-in fiberglass attic coverage bags per sq ft requires careful planning. Always prioritize the R-value goal, use manufacturer data, and consider local building codes. With the right calculation, you can achieve a comfortable, energy-efficient attic space that reduces heating and cooling costs for years to come.