Where is the best placement for a radiant barrier? Radiant barrier, cellulose, home, energy, fiberglass, radiant barrier, attic insulation, energy conservation, green building

Fact: There is more bad information about radiant barriers than there is good. 

Fact: Many sites are outdated. (#1 hit site on google was last updated June 27, 2001)

Fact: Radiant barriers are not rated using the R factor

Fact: Radiant barrier was not invented by NASA, it was invented by Sir James Dewar, a Scottish physicist who invented the Thermos Bottle among other things at the turn of the century.

Fact: Lowering the emissivity of a surface (which is one aspect of a radiant barrier), can have negative consequences, as in making your home use more power and become more uncomfortable if placed in the wrong area.

(Note to reader: The information on this page only applies to buildings in hot climates). 

Under the Roof Decking:

1. This application has absolutely no benefit during the winter, despite how mild your winters are.  Would it not make sense to purchase and install a product to control heat gain and heat loss?  This way your investment pays off all year and increases comfort all year.
2. Lowering the emissivity of a roof decking the attic with silver paint, laminated foil, or stapling a radiant barrier up under the roof decking raises the temperature of the surface above it, that's just basic physics and common sense.  If radiant heat is restricted from emitting from a surface (as in the bottom of your roof decking, a chrome car bumper or a chrome socket in your toolbox), heated by the sun, where does this heat go?  With lower emissivity it's going to increase the roof temperature and have to radiate back out...one problem: the sun is beating on it at the same time.  Is that going make your roof hotter?  Of course it is.  Is this going to extend the life of your roofing system?  (shingles or on tile roofs, the underlayment?).  I'm not going to leave myself open telling you my experiences so let me tell you what the largest shingle manufacturer told me during a recent phone call: (question) "Yes, I have a question.  Will installing a radiant barrier under the roof affect the warranty of your product?"  (answer) "Our 25 year warranty covers manufacturers defects".   When pressed for an answer regarding the installation of a radiant barrier and elevated roof temperatures I was again told: "Our warranty only covers manufacturers defects, that's all I'm going to say, goodbye".  It seems I'm not the only one afraid to touch on this topic so that leaves the reader on his/her own to decide if increasing the roof temperature is going to lessen the life of the roofing system by making it hotter and drying it out.  Really a problem with concrete tile roofs that rely on a thin piece of bitumen based (tar), felt (paper), to keep the ceiling dry.  Think twice before spraying paint under your roof, buying foil backed OSB or stapling a reflective membrane up under the roof deck.  
3. If increasing the roof deck temperature when lowering the emissivity under the roof increases roof temperature, how is this going to affect the air in the attic that is in direct contact with this hotter roof?  Stands to reason if the roof deck is hotter, the attic air is going to get hotter from the increased convective air movement from the hotter roof deck.  If the air in the attic is getting hotter via convection from the hotter roof, wont this in turn make the insulation hotter and the ducting in the attic hotter?  Hotter than a house without RBS on the bottom of roof deck?  Basic physics and the laws of thermodynamics guarantees it will and the graph below illustrates it.  Note: This is not true in flat roof homes, only in homes with a pitched roof.  In flat roof homes installing a radiant barrier under the deck is a perfect application.   
4.  Next question: If the attic insulation gets hotter with the radiant barrier than an attic with bare wood, what happens when the sun goes down and the roof cools off?  What is this superheated insulation now facing?  A radiant barrier.  A radiant barrier which now effectively holds the heat in the attic for hours after the sun goes down. (again, see the graph below).  We would rather see the RBS right above the insulation, keeping heat out during the summer and heat in during the winter, with multiple layers to insure a lifetime of performance.  This also does not affect roof temperature as the emissivity of the roof has stayed the same.  Covering the ducting in the attic with RBS is also another significant energy conservation method.  Click here to see our duct wrap page.  NOTE: Installing a radiant barrier in the attic must also be accompanied with increasing the attic ventilation.  The combination of a radiant barrier and increased attic venting will have a significant impact on energy use and home comfort.

Before you spend your energy conservation dollars on anything, especially a radiant barrier product, do some research, follow the logical path and ask a lot of questions.  Specifically, ask for a lifetime performance warranty and compare samples.  
 

RBS placement in the attic is very important

Depending on the climate your home is located, RBS placement is critical not only to performance, but to prevent damaging condensation problems within your walls and insulation.

UNDER THE ROOF DECKING

Placing a radiant barrier directly under the roof decking as illustrated above raises the attic temperature (and energy consumption), between 2pm and 8pm.  The reason is simple, when the inside roof deck  surface emissivity is lowered from .95 (bare wood), to a lower value the result is a hotter substrate hence the logic behind the radiant barrier.  This is a perfect application for a flat or nearly flat roof deck.  The reason being, the increased temperature of the roof deck will not create convective movement since the roof is not pitched.  The steeper the roof, the more convection there will be which in turn heats up the attic insulation and once the sun goes down, the elevated attic insulation is now facing the radiant barrier on the bottom of the roof.  Again, this only applies to pitched roofs.   Spraying a roof silver underneath the roof deck or using a plywood decking product with a radiant barrier laminated to it as can be seen above is an improper installation, especially if the attic is vented to standard venting code. 

Solution: In hot climates our Thermal Control Membrane product is best. RBS Chips are perfect for cold climates where moisture egress is a consideration. 

 Thermal Control Membrane installed between the trusses in the attic.  Easy to install with simple tools, no staples.

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