What is The Best Way To Install Radiant Barrier

What is the best way to install Radiant Barrier... Truthfully, I don't even know why there is a debate.

In Fla, the only accepted way to install Radiant Barrier is to Staple it up to the Rafters, and cover the rafters too.  

According to the FSEC website (Florida Solar Energy Center) they did an extensive test a few years back on a number of homes here in Florida.

 The homes that did the best had up to a 19% difference in electric consumption during the peak summer days. One home had a kilowatt hour reduction of over 10.6 per day, or over 360 kilowatt hours per month.

One thing that is strange, is the home that did the best, was actually disqualified because they installed attic fans along with the radiant barrier. ​Funny thing is, we always insist in installing at least one attic fan with every radiant barrier job. 

Since one of the sponsors in this "test" was a major utility, I believe the test was rigged against Radiant Barrier from the beginning, and the homes still showed improvements. Every one.​   I also know that anyone in the business knows you need to install attic fans for ventilation improvements to remove the convective heat buildup, so it makes sense that the big utility company knew that too. 

                Here is an excerpt from the report, which is about 30 pages long...​                                                   


Figure 5. Site #199 attic temperature history before and after retrofit on June 22, 2000 (Julian day 174).

Using periods with very similar average temperatures in 1999 and 2000, the RBS showed a reduction in cooling energy use of 19.7% (10.6 kWh/day). The average daily peak attic air temperature was reduced by 12.2oF. The reduction in peak cooling demand was 19% or 1.10 kW.

It is noteworthy that the occupants mentioned that interior comfort had been considerably improved which may have lead to a change in thermostat preference. Site #199 maintained a one degree cooler average temperature after the RBS retrofit. The alteration of the load profile is shown in Figure 6. The long-term weather-matched time periods extended from June 15 - October 16 of 1999 and June 25 - September 26 of 2000.

Also noteworthy is that the FSEC testing showed the best results were from stapling up to cover the rafters... not from laying it on the insulation. I do not see where they even considered installing between the rafters... It does not make sense then, does not make sense now.

Executive Summary

The FPC Monitoring project has evaluated radiant barrier systems (RBS) as a new potential DSM program. The objective was to examine how the retrofit of attic radiant barriers can be expected to alter FPC residential space conditioning loads. An RBS consists of a layer of aluminum foil fastened to roof decking or roof trusses to block radiant heat transfer between the hot roof surface and the attic below. The radiant barrier can significantly lower summer heat transfer to the attic insulation and to the cooling duct system. Both of these mechanisms have strong potential impacts on cooling energy use as illustrated in Figures E-1 and E-2.

e1

Figure E-1. Heat transfer mechanisms for standard vented attic.

e2

Figure E-2. Altered attic heat transfer mechanisms due to RBS.

The same type of testing was done by the Dept of Energy. They also show the best results with the staple up method. Here is an excerpt from the article.. 

Most common insulation materials work by slowing conductive heat flow and -- to a lesser extent -- convective heat flow. Radiant barriers and reflective insulation systems work by reducing radiant heat gain. To be effective, the reflective surface must face an air space. Dust accumulation on the reflective surface will reduce its reflective capability. The radiant barrier should be installed in a manner to minimize dust accumulation on the reflective surface.

When the sun heats a roof, it's primarily the sun's radiant energy that makes the roof hot. Much of this heat travels by conduction through the roofing materials to the attic side of the roof. The hot roof material then radiates its gained heat energy onto the cooler attic surfaces, including the air ducts and the attic floor. A radiant barrier reduces the radiant heat transfer from the underside of the roof to the other surfaces in the attic.

A radiant barrier works best when it is perpendicular to the radiant energy striking it. Also, the greater the temperature difference between the sides of the radiant barrier material, the greater the benefits a radiant barrier can offer.

Radiant barriers are more effective in hot climates than in cool climates, especially when cooling air ducts are located in the attic. Some studies show that radiant barriers can reduce cooling costs 5% to 10% when used in a warm, sunny climate. The reduced heat gain may even allow for a smaller air conditioning system. In cool climates, however, it's usually more cost-effective to install more thermal insulation than to add a radiant barrier.

​Location 4 In Location 5, the radiant barrier is laid out on the attic floor over the top of existing attic insulation. (See the discussion for important information about performance problems associated with dust for this option.) With either Location 2, 3 or 4, the space between the roof sheathing and the radiant barrier provides a channel through which warm air can move freely. For any of the radiant barrier locations (and for attics without radiant barriers), ideal venting would provide equal areas on intake (soffit) and exhaust (gable or ridge) vents.

For proper air flow, with a roof-mounted radiant barrier, a gap of approximately 3 inches should be left between the radiant barrier and the insulation, and a gap of at least 6 inches should be left near the ridge. Both of these gaps should be left for attics with either ridge or gable venting. 12 Even though some testing shows that a brand new application in location 5, the attic floor, will work better than the roof applications, there are several drawbacks to this location.

The attic floor application is most susceptible to accumulation of dust, while downward facing reflective surfaces used with many roof applications are not likely to become dusty. After a long enough period of time, a dusty attic floor application will lose much of its effectiveness. Predictive modeling results, based on testing, suggest that a dusty attic floor application will lose about half of its effectiveness after about one to ten years. For this location, moisture (see below) issues must also be considered.

The floor location is not appropriate when a large part of the attic is used for storage, since the radiant barrier surface must be exposed to the attic space. Also, kitchen and bathroom vents and recessed lights should not be covered with the radiant barrier. If a one-sided radiant barrier is laid on top of the insulation with the reflective side facing down and touching the insulation, the radiant barrier will lose most of its effectiveness in reducing heating and cooling loads.

OK   Hate to say I told you so, but this is now backed up by two unbiased experts. But anyone in the business for very long knows that the staple up method works best. But some of the other people that install any other way are just lazy, and don't want to use the extra material.

But again, that's just my opinion... and that of the Dept of Energy, and the Fla Solar Energy Center.​

​If You are considering installing Radiant Barrier, then make sure to get it installed correctly. And make sure the job includes at least one solar attic fan, or the results won't be nearly as good as they should be.

If you have questions and would like to talk to someone about Radiant Barrier...

Call (352) 388-1124​