Install Foil Vertical vs Horizontal

This week someone asked me to help them understand their radiant barrier quote. They had questions about some of the things they had been told about the installation process. He said the salesman told him that it didn't really matter how the barrier was installed. Since I always install the foil vertically on the rafters, I took exception to that. It DOES matter. Now, how much it matters is up to serious debate. More importantly, the idea that you wouldn't let the homeowner make the call is bad juju all around.

Here's the scoop about why vertical or horizontal.

Horizontal Pros
1. Installing radiant barrier horizontally is easy to install. All you are doing is laying it down like a blanket over the top of the insulation. It's fast and simple, so it makes sense that a company would want to do it this way. After all, they can charge you for installation and it only takes them a third of the time to do it.

Horizontal Cons
2. Dust. This is a problem. This will always be a problem. I have never been in a crystal clean attic. It just doesn't happen. Some companies may measure for dust accumulation or may decide that the loss of reflection is within acceptable parameters. I, on the other hand, want a solution that lasts for the life of the house.

Radiant barrier foil is meant to reflect radiant heat. it does absolutely nothing against conduction. Aluminum is a rather good conductor of heat (they make cooking pots out of it). When dust gets on the foil, then the dust gets heated up and passes that heat directly to the foil.

Yes, I know that the foil doesn't emit out the other side much at all (3%), but if your foil is laying on the ground, then it is touching the insulation and heating it up just fine. All of the manufacturers I have spoken to repeat "air gap, air gap, air gap".

Between the dust and the direct contact to the insulation, you have just spent a lot of money buying something that you are not getting the full use of. Its like tying cinder blocks to your ankles right before a marathon. The dust will eventually diminish the usefulness of your barrier, its just a matter of "how long".

Vertical Pros
1. Installing radiant barrier vertically provides an air gap. By stapling the foil to the rafters, you achieve the air gap on both sides (which is exactly what the manufacturers tell you is the optimum way to install it. This is like reading the instructions on the Xmas toy BEFORE you assemble it. It always seems to work better when you follow the directions.

Vertical Cons
1. More time. It takes many, many times longer to staple the foil to the rafters than it does to just lay it down. Its hot in attics, so the longer the time you spend in them, the better laying down the foil starts to look.

Remember, you are in this for the long term. your savings start when you put it up, but there really is not an ending. If you install the good stuff vertically, it will probably outlive your roof.

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