As I said yesterday, the reflective radiant barrier inhibits heat transfer by thermal radiation. It does not necessarily protect against heat transfer by conduction or convection. Why do you need to know about reflective radiant barriers? It could mean thousands of dollars saved over the years for heating/cooling, in what you choose for roofing materials, and what lies beneath them. It might help for you to read yesterday’s blog for background on how I got to this point.
For installing a reflective radiant barrier under a pole building steel roof, the reflective radiant barrier may be applied directly by draping the reflective radiant barrier over the roof purlins. Even more effective is to install the reflective radiant barrier over the purlins, install 2×4 furring strips on top of the insulation, and then the roof steel. The furring strips ensure the reflective radiant barrier faces into a sufficient air space to be effective. If an air space is not present or is too small, heat may be able to conduct through the reflective radiant barrier. Since the metal in the reflective radiant barrier is highly conductive, the heat transfer would all be through conduction and the heat would not be blocked.
I did this on my huge 3 story gambrel style accessory building I built in South Dakota about 7 years ago. I put 2×4’s over the reflective radiant barrier, and then applied the roof steel. My wife chose black steel, and so I added in the 2×4’s both to counteract her color choice and get the most out of the insulating value of the reflective radiant barrier. Our heating/cooling bills are phenomenally small. In fact, this 84’ x 60’ x 20’ building is easily a third less expensive to heat/cool than our 30’ x 60’ single story home across the road from it!
For shingled roofs, the reflective radiant barrier may be applied over the rafters or trusses and under the roof decking (usually osb or plywood). This application method has the reflective radiant barrier sheets draped over the trusses of rafters, creating a small air space above with the reflective radiant barrier facing into the entire interior attic space below.
I even used this method in re-roofing our house two summers ago. The shingles had seen their better use, so we applied 2×4’s right over the shingles, put down the reflective radiant barrier and then applied white roof steel for a “cool roof” solution. It’s noticeably cooler in summer and much warmer in winter, along with lower energy bills.
Another method of applying a reflective radiant barrier to the roof in new construction would be to use a reflective radiant barrier which is pre-laminated to OSB or roof sheathing. While manufacturers of this installation method often tout the savings in labor costs in using a product which serves as roof decking and a reflective radiant barrier in one, these products are generally considered by most to be “pricey”.
One common misconception regarding reflective radiant barriers is the heat reflecting off the reflective radiant barrier back out the roof has the potential to increase the roof temperature and possibly damage shingles. This is simply not the case. Performance testing by Florida Solar Energy Center conclusively proved the increase in temperature at the hottest part of the day was no more than about 5 degrees F. In fact, this study showed the reflective radiant barrier had the potential to decrease the roof temperature once the sun went down because it was preventing the heat loss through the roof. RIMA (Roofing Insulation Manufacturers Association) International wrote a technical paper on the subject, where they collected statements from the largest roofing manufacturers, and none said a reflective radiant barrier would in any way affect the warranty of the shingles
Wrapping a building’s walls with a reflective radiant barrier can result in a 10% to 20% reduction in the tonnage air conditioning system requirement, and save both energy and construction costs.
Reflective radiant barriers are also quite effective in floor systems above unheated basements and crawl spaces. The reflective radiant barrier may be either stapled below the floor joists, creating a single reflective air space, or between the joists, followed by some type of sheathing. Reflective radiant barriers work extremely well in this application for two reasons. First, a reflective radiant barrier which is not perforated for breathability acts as an excellent vapor barrier. This means ground moisture will not be able to pass through the reflective radiant barrier and enter the living space. Secondly, the floor is the only part of the building where the heat flow is always down, unlike a roof where the heat would be coming down during the summer and rising to escape in the winter. When the heat flow is down, 93% of the heat is radiant heat, which is exactly what the reflective radiant barrier is designed to block.
A reflective radiant barrier actually IS insulation and it DOES have actual R values in tested systems. It IS highly directional, meaning it is better against heat gain, than heat loss. In tests, it rates as high as R-14, depending upon the installation.
However reflective radiant barrier are not the “end all” of insulation. My preference is to use it for what it is great as – an insulated vapor barrier.
There are many commercially available vapor barriers. These include Tyvek® (a registered trademark of the DuPont Corporation) and other similar housewraps. Even polyethylene plastic sheeting (think of British Polythene Industries Limited’s product Visqueen) is a vapor barrier.
While fine vapor barriers, none of these products have insulation bonded to them, so are not an effective method of condensation control. In order to control condensation, a thermal break must be created. It is the layer of air cells sandwiched between the facings of the reflective radiant barrier (most commonly a white inner facing and an aluminum exterior facing) which creates the necessary thermal break.
If the only intention for heating a pole building is to take the chill off for a few hours with light heat (like throwing a space heater on), then the reflective radiant barrier can be an affordable option. For buildings which are to be climate controlled, other options are more practical.
For more information on reflective radiant barriers, I’d recommend visiting https://www.buyreflectiveinsulation.com, which also features a handy calculator to determine the required amounts of insulation for gable roofed buildings.