Best Radiant Barrier Installation Method?
There are basically two different methods used to install radiant barrier attic foil. Either stapled to the bottom of the roof rafters or laid out over the attic ceiling insulation.
Which Install Method is Best?
It really depends on a couple of things. The main factor to determine the best method is to decide what is your primary goal. If reducing Summer Cooling expenses is your goal, then stapling to the bottom of the rafters is probably your best method. If reducing Winter Heating expenses is your main goal, then laying the foil insulation over the existing insulation is probably the best method. If you are in a "Mixed" climate area, then laying the radiant barrier foil over the insulation is also your best method. The other main criteria is whether your ductwork is located in the attic.
Hot Climates - Stapling To Bottom of Rafters is Usually Best, But Laying Over Attic Insulation is A Excellent Method.
You will see on other sites and many "professional" installers say that laying out the foil over the existing insulation is best. There have even been some studies that show that putting the foil insulation OVER the existing insulation yielded 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 in the ductwork. In the South, we do something REALLY STUPID. We put the ductwork IN THE ATTIC!! Let's run 50º degree air through a poorly insulated (R-4 / R-8 insulation) pipe that probably leaks and surround it with air that is over 130º. This is like making Ice Cream in your OVEN?? What are we thinking??
Heat is flowing into your home through two main sources in most attics: 1) Through the insulation 2) Through the ductwork.
When you staple to the bottom of the rafters, you COMBINE the benefit of having a cooler attic by putting your ductwork in a less hostile environment AND the reduction in Radiant Heat flow through the insulation. This method creates a greater TOTAL reduction in heat flow in warm climates than laying out over the insulation. Period.
Plus, it's very cool 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.
Isn't laying The Foil Out OVER The Existing Insulation Easier To Install?
YES, No doubt it is easier. And, I'm not opposed to this method EVEN in Hot Climates. It may not be the absolute BEST method, but considering how much easier it is to install on many homes, it is often the best way to go. Plus, the this method uses LESS material. There are many homes that the attic is so tight or crowded 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 insure maximum benefit.
Do you have enough attic insulation? - Foil is NOT a substitute for regular insulation. Foil is great, but it works in combination with regular insulation to make the regular insulation more effective. Check with the DOE (Department of Energy) for the recommended level in your area.
Check or air leaks in the ceiling. Although perforated radiant barrier foil is NOT a vapor barrier (it has TINY pinholes that allow moisture to pass through while maintaining the full effectiveness), there is a potential for moisture to condensate if too much warm moist air is leaking out through the ceiling. Can lights, fixtures and attic doors are the main culprits of air leakage. These leaks should be sealed up BEFORE installing radiant barrier attic foil directly over attic insulaiton.
Check Attic Ventilation - By increasing the ventilation you can reduce the AIR TEMPERATURE This puts the ductwork in a less hostile environment and will reduce heat flow INTO the ducts as described earlier. Heat is flowing into the ducts by Conduction NOT Radiation. So, NO you cannot just lay foil over the ductwork and get the same effect. Attic ventilation is real simple: Holes in the top of the roof (exhaust vents), and holes in the bottom (soffits or eves) . The more you've got, the closer to ambient temperature your attic will be. From being in thousands of homes, MOST homes do not have enough holes in the bottom of the attic to allow air INTO the attic. Good ventilation will put the ducts in a cooler environment. Even with BAD attic ventilation radiant barrier will reduce heat flow into the home, but for MAXIMUM benefit be sure you have some decent attic ventilation.
If you DO NOT have ducts in the attic, then the over-insulation (Attic Floor) method is always the way to go. This is often found in Mixed-climate areas like the Midwest.
Here is the Bottom Line. If you want to maximize the reduction in Summer cooling costs, staple to the bottom of the rafters. If you are looking for more year-round savings and an easier install method, then lay out over the insulation. IF you do the Over-Insulation Method in warm/hot climates, then make sure to check your attic ventilation if you have ductwork in the attic AND 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 what to do. Stapling will allow better 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 if needed. Laying radiant barrier out over insulation is usually faster, and easier and uses less material and will provide more benefit when it's cold. Either way, radiant barrier will have significant benefits. Now, choose an install method, and get it done.
Cold Climates - Laying Out Over Existing Insulation is Best.
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. Being a Texas guy, I honestly don't have a lot of first hand experience with this install method. However, I'm always amazed at how many customers from the North order foil after seeing the results of their friends or neighbors. I hear everyday how they are both saving money and how much the comfort level improves after installing radiant barrier attic foil.
 |
 |
How it Helps in Cold Climates:
It's called the "R-Value Myth" - Look it up. Basically, it says that the R-Value of insulation is tested in a lab and air movement is zero. We know that air flows quite nicely through fiberglass. Ever looked at most AC filters? Fiberglass right? In walls, for fiberglass to be fully effective, it must be surrounded on all six sides to minimize air infiltration. Now, look in your attic. The top of the insulation is wide open and exposed to air currents. This is why jackets have an outer shell that is "windproof". Take off the shell on a jacket and walk outside on a cold windy day. You will freeze your backside off. The "R-Value" of the jacket will virtually disappear.
Inside the attic, there can be a huge differential between the temperature inside the building and the attic. For example, if the attic is 20º and the living space is 70º you have a 50º difference. You get what is called "Convective Looping". This is when cold, dense and heavy air falls through the insulation and displaces the warm air against the living space. Installing foil on top of the insulation acts like the windproof shell on a jacket. It helps minimize the convective looping within the insulation. However, Perforated Radiant Barrier Attic Foil is NOT a vapor barrier. So, it WILL allow moisture in it's vapor form to pass through.
The other reason radiant barrier foil helps in cold climates is the reduction of radiant heat loss from the living space. It's like when you wrap a potato with foil to keep it warm. The potato will not "emit" as much radiant heat. This is called "Emissivity" or the ability NOT to release heat. By laying the foil on top or your insulation, you help keep the heat "IN" by not releasing it in the form of radiant heat. Radiant Barrier Attic Foil will has an emissivity rating of .03 meaning it will ONLY release 3% of the heat by radiant energy. This is why so many customers report a noticeable difference in comfort and less problems with rooms being hot and cold.
In Cold Climates follow these warnings:
Check or air leaks in the ceiling. Although the radiant barrier foil is NOT a vapor barrier, there is a potential for moisture to condensate if too much warm moist air is leaking out through the ceiling. In extended areas of cold weather, this can NOT BE EMPHASIZED ENOUGH!! Can lights, fixtures and attic doors are the main culprits of air leakage. These leaks should be sealed up BEFORE installing foil. Too much moisture and the foil will NOT be able to keep up and allow the insulation to dry out. Foil does not CAUSE moisture. Moisture causes Moisture. The trick is to eliminate the SOURCE of the moisture.
Do NOT compress the insulation when installing radiant barrier attic foil on top. This will reduce the drying potential for the insulation and reduce it's effectiveness. Lay the Radiant Barrier Foil LOOSELY over the attic insulation. Do not press into insulation. Your insulation will have some peaks and valleys. This is OK. In fact, if you can get a 1-2 inch gap between the foil and the top of the insulation and only have about 10-20% surface contact with the foil is ideal.