Question: How do I determine how much radiant barrier foil I will need for my project?
There are two good methods for getting the measurements you need: measure the space or estimate based on square footage and slope.
Measure. This is the best method if you can easily walk on the roof and measure the different slope or surfaces of the roof. Just multiple the Length x Width for each roof slope, add them up and order the amount of foil needed. Another method is to count the sheets of plywood / decking on the roof from inside the attic. Each full piece of plywood is 32 square ft. Count how many sheets of roof decking and multipy by 32.
Estimate. If you can't measure the roof, then the next best method is to estimate the roof surface area. The key to this method is to determine the "footprint" of the attic. Once the footprint is determined, then you will multiply this number by a Pitch Factor between 1.2 – 1.5 depending on the pitch or steepness of the roof.
1. Determining the Attic Footprint
Single story homes
Typically the square footage of the home plus 400 ft for a 2 car garage (and add any additional square footage of porches).
For example: 1700 sq ft home + 400 sq ft for a standard 2 car garage + a 100 sq ft porch = 2,200 sq ft total.
Two (or more) story homes
Determining the footprint of the attic is the way to go for measuring. If your home is a perfect two story, with the second floor perfectly over the first floor, you can usually measure the foundation of the home to get the approximate total of the attic.
For example: If the foundation measured 50' x 30', the foundation and footprint of the attic will be 50 x 30 =1500 sq ft.
For some homes that are all cut up with lots of slopes, valleys and dormers it can get tricky. If you can determine how many sq ft are on the first floor, this is a pretty close measure of the footprint of the attic; don't forget to add in the garage and porches.
Once you determine the footprint of the attic, you will need to estimate the pitch (or slope) factor if you are staping the foil up. Otherwise, if you are going to lay the foil over the insulation, then the footprint number is the quantity you will need.
2. Pitch Factor Guide (How to Multiply) - used when stapling foil to the rafters
Now that you have the footprint, or square footage of the attic space, multiply the total roof square footage by the applicable pitch factor to complete your estimate for the amount of foil you will need to cover the roof slopes.
Multiply by 1.2 to 1.3: If your roof is under a 5/12 (i.e. easy to walk on and probably under 8' at the highest point for most homes).
Multiply by 1.3 to 1.4: If your roof is a 6-8/12 pitch (i.e. slightly difficult to walk up with a ridge usually over 8" high).
Multiply by 1.4 to 1.5: If your roof is a 10/12 pitch or higher (i.e. very difficult (or impossible) to walk up, this is often considered a very steep or church-style roof).
If you are using the FlatTop Installation method, you an reduce your number by a little since the area covered is a little less than the actual roof surface area.
If you have gable ends that catch sunlight, then add their square footage to your total amount.
If you have an vaulted or cathedral ceilings that are inaccessible , subtract this area from your total.
If you plan on covering over a garage area, a typical 2-car garage is around 400 square feet - add this to your total.
Remember, it is not an exact science. You can always order a single roll to finish up a job or you can easily sell a left-over or partial roll to a neighbor, on Craigslist or donate it to Habitat for Humanity.