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How and Where Radiant Barriers Work

Foil blocks heat transfer best if it stays clean, is adjacent to an airspace, and the delta is large enough. 

August 24, 2018

A radiant barrier, such as that on foil-faced rigid insulation, can reduce heat transfer across the airspace between the strapping and metal roofing and siding. Image courtesy Matt Risinger 

 

Radiant heat transfer occurs between objects of different temperatures, and it always moves from hot to cold. That’s one reason why you feel cold when you stand next to a window in winter—you’re radiating heat toward the colder window. It’s also why you move from that cold window toward the woodstove—the radiant heat from the stove will warm you back up. 

Technically speaking, what’s actually radiating is electromagnetic energy, which is invisible and has no temperature. Electrons buzzing around inside objects generate that electromagnetic radiation, which raises or lowers the temperature of people and objects, depending on which way the energy is flowing. The temperature difference between the two objects determines the intensity of the net radiative transfer. 

If you were to use a radiant barrier, such as a sheet of foil, between you and the window or woodstove, you would not feel as cold or as hot standing next to them. The foil’s reflective surface prevents heat from being absorbed and prevents any heat that is absorbed from being emitted as radiation. But radiant barriers need an adjacent airspace to reduce radiative heat flow. If they are put in contact with another material, heat transfers by conduction.

Why It Matters

In the past, a common location for a radiant barrier was on top of the attic insulation in a ventilated attic. This is actually a bad location, partly because it gets dusty and dirty in most horizontal applications, making the surface less reflective. More important in cold climates, however, is the fact that most radiant barriers are also vapor barriers, which stop the flow of water vapor. If a radiant barrier cools below the dew point, moisture can’t escape into the vented attic, so it condenses on the cold underside of the radiant barrier and can wet the insulation. So it’s important to place a radiant barrier in a location where it will stay warm enough that it is unlikely to reach the dew point temperature.

How to Do It Right

In hot, humid climates, condensation in the ceiling insulation is typically less of an issue because the enclosure materials are vapor open, and moisture that reaches the ceiling plane from the attic easily diffuses in the interior space. In the southern U.S., a better location for a radiant barrier is on the underside of the roof sheathing, especially when HVAC equipment is located in the attic. The radiant barrier can reduce the amount of heat transferred to the attic from the roofing, which increases the efficiency of the HVAC equipment and can result in small to modest cost savings, depending on many construction and enclosure variables.

The radiant barrier can be installed directly against the sheathing, but installing the radiant barrier on the interior of the roof framing will perform better at reducing heat transfer because it has an airspace on both sides. In cold climates, the potential for condensation-related issues on the surface of a radiant barrier adhered to the underside of roof sheathing rule out this option.

Interestingly, there have been studies and reports about the interactions of radiant barriers below the roof sheathing and lightning strikes to the roof assembly, because the radiant barrier is typically a foil film that has high electrical conductivity. 

One practical example of a radiant barrier that makes sense in both hot and cold climates is when a metal roof is installed on strapping, which is installed on top of foil-faced polyiso rigid insulation or on top of a radiant barrier placed over a different type of insulation such as XPS or stone wool. In either case, you can actually reduce the heat transfer from the roofing to the insulation during the day and heat transfer from the insulation to the roofing at night by minimizing the radiative heat transfer across the airspace. 

A radiant barrier works in walls, too. With a ventilated airspace behind the cladding, as in a rainscreen, if the next layer on the other side of that airspace is a radiant barrier, it could reduce the heat flow. Again, though, you have to be careful, particularly in cold climates where you don’t necessarily want a vapor barrier on the cold side of the wall assembly. Perforated radiant barrier material is available, but it can still be vapor impermeable enough to cause condensation.  

How much you reduce heat flow across the enclosure with a radiant barrier depends on how large the temperature differences are. As we move toward enclosure assemblies with higher R-values, radiant barriers don’t make as big a difference as they did when we had smaller amounts of insulation and a larger temperature difference. 

Finally, remember that a radiant barrier only works when it is clean and shiny, which preserves its reflectivity. If it gets dirty or dusty during construction or during operation, it stops working as a radiant barrier and works more like a typical surface.

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About the Author


About the Author


Jonathan Smegal is senior project manager at RDH Building Science Laboratories, in Waterloo, Ontario. This article was adapted from “Vapor Permeance: 7 Minutes of BS,” a building science podcast series at ProTradeCraft.com.

Comments

Comments

I'm curious what your comments are with regard to the product called Energy shield that is based on NASA technology.

And one must always remember that there must be an assembly and an air gap on at least one side. In the 1950's there was a 5 layer reflective barrier designed to fit into wall cavities. This actually was better than mass insulation, but fiberglass manufacturers had more money and easier installation in some situations. They took the market.
Remember that it is about an assembly and or installation for purpose, not about the material being insulation in itself. Never listen to those that say there is no R value to the material, remember that the R value for barrier is always as an assembly and as installed for purpose (actually mass insulation's total effectiveness is also as an assembly). Example, installed under the rafters, I have seen Texas attics reduce temperature from as high as 160+ degrees to only a few degrees over ambient temperatures.
In areas of attics that have knee walls that are walls to the interior, use the maximum mass insulation then staple thermal barrier OVER that insulation as well, this helps slow convection as well as stop radiant heat both directions (from house and to house).
Under the rafters, or under the roof deck and under the rafters is the FIRST and foremost installation in tropical to hot summer climates to stop thermal radiation into the attic. Where winter temperatures are COLD on top of mass insulation stops thermal radiation from leaving the insulation! In Dallas as an example, that may mean both installation methods and I have done this MANY times very effectively.
In instances where an enclosed space (unvented) can be made, especially with thermal barrier and a gap and thermal barrier again it can be VERY effective in radiant heat transmission in both directions. (Think of a west or south facing wall in Texas summers or in vaulted ceilings with only rock wool insulation and no attic). In installations such as this I have seen entire walls/ceilings that were well over 130 degrees in the day time reduced to ambient values with thermal loads on AC systems reduced to proper functioning. In several cases loads were reduced to the point that replacement AC system size was reduced by multiple tons of AC.
There are many instances where a room or area is uncomfortable and generally unusable, hot or cold. Thermal barrier has proven to make those areas comfortable and usable when installed properly for the circumstances.
Lastly, in attic mounted Air handlers and duct systems, creating a thermal barrier "Shade" over those areas results in lowered energy use and more effective distribution by blocking radiant heat. Remember most duct is thermal barrier covered today, and adding another cover with an air gap between lowers radiant heat transmission even more., even if it is under the rafters!
Remember that as dust and dirt collects on the barrier it reduces effectiveness. Therefore it is always best to consider where this will happen and insure that ONE SIDE remains dust free with an air gap. Hope this helps

My subject line goes along to the children's song, "Do your ears hang low?".

My silly humor aside, I'd like to move along to my next "my" comment with the subject at hand.

My understanding of the best way to install reflective barrier in Houston Texas climate is conflicting.

A) best attic location is under sheathing.
B) best installed facing out to avoid reflecting heat back to the house
C) the need for adequate air space and flow for it to work with
D) must remain clean

I'm seeing all kinds of problems popping up in my mind with these requirements.

To not deviate with listing all my popping thoughts of problems, may I simply ask if there's a reason why reflective barrier faced foam boards couldn't be suspended from the rafters instead? If the reason why not is less problematic than my popping thoughts on the above requirements, well then, I just might proceed accordingly.

Thanks a bunch!

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