Square Foot Calculator
How To Calculate Amount of Radiant Barrier Foil For Your Attic

Measure: The best method to determine how much radiant barrier foil to staple to the bottom of the roof rafters is to walk on the roof and measure the different slope surfaces directly. Just multiply the length x width for each roof slope surface, add them up, and order that amount of foil. The next best method is to count the sheets of roof plywood from inside the attic. Since each full piece of plywood is 32 sq-ft, multiply the total number of sheets by 32.

Estimate: The indirect method is to estimate the roof surface area. The key to this method is determining the Footprint of the attic and multiply by the pitch or "steepness" of the roof, the Pitch Factor, a number between 1.2 and 1.5.
How To Determine Your Attic Footprint

The footprint for a one-story house is basically the square footage of the home plus 400 sq-ft for a 2-car garage plus the sq-ft of any porches. Example: 1700 sq-ft home + 400 sq-ft for 2 car garage + 100 sq-ft porch = 2200 sq-ft footprint.

For a perfect two story house, with the second floor placed precisely over the first floor, the footprint of the attic floor is usually equivalent to the measure of the foundation. Example: For a foundation measuring 50 ft x 30 ft the foundation and footprint of the attic is 50x30=1500 sq-ft. For roofs with numerous slopes, valleys, and dormers, the amount of sq-ft on the first floor can be used as a rough estimate of the footprint of the attic. Don't forget to add in the garage and porches.

How To Determine Your Attic Pitch Factor

For foil laid directly over the insulation, the footprint is all you'll need. Otherwise you will need to multiply by a pitch factor.

Under 5/12 - Easy to walk on - Probably under 8' at the highest point - Multiply by 1.2 to 1.3

Between 6 to 8/12 - Slightly difficult to walk up - Ridge usually over 8" high - Multiply 1.3 to 1.4

Over 10/12 - Difficult or impossible to walk up - Very high pitch, church style roof - Multiply by 1.4 to 1.5

For Flattop reduce this by a small amount since the area covered is less than the actual roof surface area. For Gable Ends add this amount to your total. For Vaulted Ceilings that are inaccessible subtract this area from your total.