The first part in installing a radiant barrier is figuring out how much barrier you will need.

This is all about roof square footage rather than house square footage. You may have a 2000sqft house, but the square footage of your roof is quite a bit more. You need to know this so you do not run out of material.

Rather than measuring every inch of roof surface, I prefer to estimate. This is obviously not perfect, but it saves an immense amount of time and comes really, really close (which is fine by me). We need to get some tools for this:

1. level (at least 12 inches long)

2. ruler (12 inches long)

3. calculator (because I am lazy at math)

Very few roofs combine different pitches. Most roofs are a single pitch for the entire surface. This makes our calculation much easier. Crawl up into your attic with your tools. Take the level and put one side of it horizontally touching the plywood of your roof. Make sure the level, is leveled out. While holding your level in one hand, take the ruler and place it vertically at the 12 inch mark on your level. Holding the ruler at 90 degrees to the level, find out how many inches the plywood is from the level.

We are trying to find out what the pitch of the roof is. This is rise over run for those math wizards out there. My roof is 7 inches vertical for every 12 inches horizontal. Here is a chart to look at once you find out the pitch.

Pitch Multiply by:

4 in 12 1.06

5 in 12 1.08

6 in 12 1.12

7 in 12 1.16

8 in 12 1.20

9 in 12 1.25

10 in 12 1.30

11 in 12 1.36

12 in 12 1.42

Take the square footage of the bottom floor of your house plus garage. An example could be a 1513sqft house with a 453sqft garage. You can find these exact figures by going to www.taxnetusa.com and finding the house you are about to install a barrier in. This site will give you exact figures, so you are not guessing.

Take the horizontal square footage (1513+453=1966) and find your pitch in the chart above (1966 x 1.16 = 2280). Now you have a more accurate estimate of the material you will need. But wait, there's more. Your roof actually hangs over your foundation a little bit and there is usually a porch in the front or back of the house that may or may not contribute its roof heat to the main structure. Remember to count that extra space into your equations.

The above method is great for a house that is shaped like a box, but what happens when your house has an abundance of angles and extra little aesthetic attic windows? Well, you need to grab your tape measure and start adding up all of the extra spaces using the same above method. This is more math than I care to do. I simply add an extra 10% to the square footage I had already calculated (2280 x 1.1 = 2508)

Since the radiant barrier foil comes in preset boxes, you will need to always get the amount just past what you need. In my above example I had 2508sqft needed, so it would be worth it to buy the 2500sqft of foil and then buy the next increment (usually either 250sqft or 500sqft). The foil is never really wasted, since anything extra once you are done with your roof will get put to use wrapping A/C cold lines, stapled to the sides of walls in the attic if your house is multi-level, or in creative spaces that need to reflect heat. Ok, well, that last one usually results in my wife reminding me that the foil is only allowed in places that are not visible inside the house (I tend to get too creative).