For the majority of installations, there are basically three different methods used to install radiant barrier foil in an attic. The first is stapling it to the bottom of the roof rafters, the second is laying it out over the existing attic ceiling insulation and the third is a combination of the first two (called a hybrid method). Do It Yourself Radiant Barrier.
Which Install Method is Best?
We get this question daily. The problem is that it is hard to say since there are several factors to consider when determining the best installation method for a home. Every home is different, so while there are general recommendations for certain climates, sometimes one method is best for a particular home.
The main factor to determine the best method is to decide what your primary goal is and what is inside your attic. As a general rule, many people who are trying to reduce summer cooling expenses choose to staple the foil to the bottom of the roof rafters, while people looking for a winter heat retention benefit opt for laying the foil insulation over the existing insulation. Generally speaking, people who are in mixed climates or those who are looking for year-round benefits, typically opt for either a hybrid method or laying it over the insulation. Again, these are general rules.
The other main criteria is what you use your attic for and/or what is inside of it. For example, if you live in a cold climate but you have an HVAC unit in your attic, a hybrid method of laying the foil down and stapling it up is probably your best method. Another example would be someone who is in the Midwest and they are planning to lay the foil over their attic floor, but they use their attic for storage. In this case they might opt to staple the foil up, or consider a hybrid method as well. The benefit to the hybrid method is that if your attic contains items that need to be cooler for any reason, or if your attic floor is used for storage, then you can still achieve great year round results with the hybrid method. More often than not, customers choose one method or the other, but the hybrid (doing both methods) is becoming more and more popular as weather is shifting and patterns are no longer predictable for certain geographical areas.
Hot Climates: stapling the foil to the bottom of the rafters is usually the first choice, but laying it over the attic insulation is also an excellent method if your attic doesn't contain any temperature specific items.
You will see other websites and many professional installers declare that laying the foil out over the existing insulation is best. There have even been some studies that show that putting the foil insulation over the existing insulation does yield better reduction in heat flow. I'm not arguing with this study and in fact it is probably accurate if you were only measuring heat flow through the ceiling.
The problem is the location of the duct work and/or the HVAC unit. In the South, we do something really stupid: we put the duct work up in the attic! Who decided it was a good idea to run 50ºF air through a poorly insulated (R-4 / R-8 insulation) pipe (that also probably leaks) and then surround it with air that is over 130ºF. This is like deciding to make ice cream in your oven...what are they thinking?!
In most attics, heat flows into your home through two main sources: the insulation and the duct work.
When you staple radiant barrier foil insulation to the bottom of the rafters, you combine the benefit of having a cooler attic by putting your duct work in a less hostile environment with the reduction in radiant heat flow through the insulation. This method creates a greater total reduction in heat flow in warm climates versus laying the product out over the insulation. Period. Furthermore, it's very nice to go into your attic on a hot summer day and actually be able to tolerate it. Invite your friends over, they will be amazed.
Is laying the foil out over the existing insulation easier to install?
Yes, no doubt it is easier to install the foil this way, and I'm not opposed to this method even in predominately hot climates. It may not be the absolute best method to add radiant barrier to a home, but considering how much easier it is to install in general, it is often the best way to go. Additionally, this method uses less material than the other methods.
There are many homes that have an attic space that is tight or crowded, that it's almost physically impossible to get good access to staple to the bottom of the rafters. If you want to lay the foil out over the insulation in hot climates, you need to do the following to ensure maximum benefit:
If your attic is empty (no duct work, no HVAC units, no storage, etc.), then installing it over the insulation on the attic floor is always the way to go.
Cold Climates: laying the radiant barrier foil out over the existing insulation is best for helping keep the heat inside your home in the winter.
In cooler climates, laying foil insulation over the existing insulation is usually the best method because our main concern is to keep the heat in during cold weather. We are always amazed at how many customers from the North order foil after seeing the results of their friends or neighbors. We hear everyday how they are both saving money, and how much the comfort level improves after installing radiant barrier attic foil.
Does't the DOE warn against installing the foil over the floor over existing insulation?
Understandably, and like us, their main goal is to prevent people from installing a radiant barrier improperly. Adding it on the floor can be problematic if the person installing the foil is not aware of the potential problems and if they do not install it correctly. AtticFoil® is perforated, so it is not a vapor barrier. Also, we do suggest some tips for installing in a cold climate to protect you against accruing large amount of moisture that could potentially overwhelm the perforations; the way we see it, it is always better to be overly cautious when moisture accrual is a concern.
Keep in mind that a radiant barrier doesn't cause moisture, moisture causes moisture. Ideally you'd want some sort of ventilation around the installation so moisture can flow freely and evaporate; the basic overall idea is to install the foil in such as if it's not even there (in terms of airflow). For more information on installing the foil with proper gaps for airflow, watch this video: Installing Radiant Barrier with Proper Air Gaps
Remember, if you do the over-the-insulation method in warm/hot climates, then make sure if you have duct work in the attic that it is included in the coverage and that you are satisfied with your current level of insulation. Once you put down foil, you CANNOT blow insulation on top of it.
To be honest, it can be a tough call on which method to choose.
Stapling the foil up will afford you easier access to the attic, provide a cooler attic for your ducts to work more efficiently, keep any stuff stored in the attic cooler, and allow you to add more insulation later, if needed. Laying radiant barrier out over insulation is usually faster and easier, and it requires less material and more wintertime benefits. Either way you decide to go, adding a radiant barrier will have significant benefits to your home's comfort level and energy consumption. Now, choose an install method, and get it done!
Bottom line: if you want to maximize the reduction in summer cooling costs, staple the radiant barrier to the bottom of the rafters. If you are looking for more year-round savings and an easier install method, then lay the foil out over the insulation.