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Building Science 4 Dummies: Radiant Barriers

A Radiant Barrier (RB) is a product designed to only block radiant heat transfer from happening and is rated based on its reflectivity & emissivity values per ASTM C 1313. In order to qualify as a Radiant Barrier it must have a reflectivity rating of .9 or more (the higher the better). A radiant barrier DOES NOT have an R-Value (though bubble faced styles may claim the air pocket as an R 1), nor does it improve another materials performance or R-Value though it can reduce the amount of heat said material has to deal with.

Reflectivity & Emissivity:

Reflectivity is probably the easiest part of the barrier to understand as it simply refers to how much radiant heat it reflects away from it (remember radiant heat only applies to the transfer across air or a vacuum).  Emissivity (ɛ) on the other hand deals with how much heat the barrier allows to radiate out. For example this iron pictured below shows how this principle works.

The Low-ɛ side is only radiating or emitting at 130° whereas the high- ɛ side is radiating out a much higher temperature. If we were to turn the iron off & place a candle in front of it then we would be able check out the reflectance or how much heat is being absorbed instead of reflected back at the source.

Now in order to remember that a radiant barrier needs an air space to be effective just consider what would happen if you put your hand on said iron while it is on… Yeah, you would get burnt big time as the heat no longer is being radiated out but conducting straight through. The same principle applies, if the barrier is touching anything the heat will conduct straight through it.

As for the Hmmm part – well that is a good example of what happens if the coating is missing, damaged, or gets dirty.

Electricity & Lightning:

Lately a few people have stumbled upon a “study” where some “insurance inspectors” applied current to a certain brand of radiant barrier & managed to create fire. Unfortunately it appears that brains are not a requirement to publish a study, nor is pointing out that installing said materials in such a fashion is not only incorrect but would have happened if anything conductive touched said loose connectors. Granted I could see them using this as a way to promote installers to follow manufacturer’s directions / ASTM guidelines (Residential – C 1743 or Commercial – C 1744) but that is not the way they decided to go.

Amazingly they then go on trying to scare people about lightning strikes, by using bogus experiments, pictures, and diagrams that are not only laughable, but show a complete lack of understanding about lightning and it’s power. In case you didn’t know, lightning strikes can contain up to 20,000 amps of power, with temperatures reaching 50,000° F and fry almost anything in its path (which includes shooting nails out from fascia boards). If you have a radiant barrier or sheathing installed & are worried about lightening, check out lightening arrestors & rods.   (Shoot that is a pretty good idea to check out, even if you don’t have a radiant barrier)

Radiant Barriers & Codes: While certain organizations & jurisdictions (California Title 24, Austin, ENERGY STAR®, etc…) mandate the use of radiant barriers, it currently is not listed in the Building or Energy Codes for many places. Even when it might be mandated, their use is generally restricted to specific conditions applying like this one from ENERGY STARHot Climate Zones 1 – 3; If more than 10 linear feet of ductwork are located in an unconditioned attic, a radiant barrier or ENERGY STAR qualified roof product shall be installed.

Does it work?

The answer like many is; it depends. If you live in a shady area the answer would probably be no. If you live in a very sunny hot area the answer would probably be yes. Having been in some attics with them installed I can say sure say it beats broiling.

Speaking in generalities anything from Climate Zones 1 to 3 with a typical attic assembly would see some savings, whereas everyone north of that would probably not see much if any savings. A few other major factors that apply is where it is installed at & is it installed properly. The best practice to prevent unwanted radiant heat gain is still the oldest – proper siting of your house with appropriate shading. This is quickly followed up by properly air sealing & insulating your attic space or investigating the hot roof option.

No matter which climate you live in, if you wish to block the summer sun the best location would be to start with the roof. Not only do they make roof coatings but there also “cool roof” products like shingles, metal, tiles, etc…

If you are building new the radiant barrier sheathing (shiny side down) will help prevent the heat from radiating into the attic. Now if you are going with an encapsulated or hot roof system, don’t waste your time or money on this (remember the iron example about the air gap).

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  • http://www.RadiantGUARD.com/ RadiantGUARD

    Great article! So encouraging to see others taking the time to properly explain how a radiant barrier works without making outrageous claims. We, at RadiantGUARD .com have always been committed to being factual information regarding reflective insulation technologies to help debunk the many fraudulent claims in the marketplace. Kudos to you for staying true!

  • johndpoole

    Nice post, bud. I like the iron/hand analogy a lot, BTW. It makes clear the difference between radiation and conduction in a manner easily understood. I’ll have to remember that one.