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

 

 

Where Should Radiant Barrier Be Installed?

Fact: There is more bad information about radiant barriers than there is good.  Many sites are outdated.

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. 

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.

Reality: Radiant heat is not just from the sun, radiant heat is everywhere and is the fastest way heat enters your home during the summer and contributes to large losses during the winter.  Many radiant barrier systems (RBS), are designed to be placed at the roof to either reflect radiant heat or lower the emissivity from bare wood at .95 to something lower.  Many things happen:

  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.  When this dries out and curls up, all the tile has to come off, all the wood strips, all the underlayment, and be replaced.  Homes around here are having to have it done at the tune of $8,000 which is mostly labor.  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. 
  4.  Next question: If the attic insulation gets hotter 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.  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.
  5. Fact: Dust is a major problem with RBS rolled out over the attic insulation.  This is why our RBS Chips and Thermal Control Membrane have multiple layers to deal with this problem.  It's also proven, patented and once installed, will change the way your home feels forever.   
  6. More facts:  Insulation has a tremendous potential for storing heat.  Insulation resists heat loss (R factor).  Here's a good question: If you are outside in January at night, would you put a sweater on?  Question #2:  Do you wear sweaters in August when you working out in the yard? In the sun???   And your home has walls and ceilings packed with sweaters?  To keep the heat out????    We have developed innovative, proven methods that far exceed the performance of conventional insulation products.  Read on and become enlightened and once aware of what the alternatives are, you can make better decisions on obtaining comfort for less cost. 

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

In the summer of 1993 an Arizona electric utility began testing the performance of three types of Radiant Barrier Systems (RBS) in three different placements in four identical unoccupied homes. One home was a control house with no RBS.  Data collection included time, temperature and measurement of power usage. All power was shut off in these homes except for the heat pump. The results of this testing brought to light some interesting data as shown below.  Our product, RBS Chips,  is outlined in green
  
 

The graph above illustrates some interesting points:

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: Our RBS Chip product works everyday, all year keeping heat out during the summer and heat in during the winter.  RBS Chips are perfect for cold climates where moisture egress is a consideration.  In hot climates our Thermal Control Membrane product is best.

WALLS: Heating Dominant Climates (cold winters)
RBS sheeting should be placed between the interior sheet rock and wall insulation. This will act as a vapor barrier as well as a radiant barrier, all seams must be taped. IMPORTANT!  Do not place a non permeable membrane of any type between the wall insulation and the outside wall. Water may condense on the film creating all kinds of problems including but not limited to: wood rot, mildew, ruined insulation and unwanted critters.

WALLS: Cooling Dominant Climates (mild winters, hot summers)

In climates where the night temperatures rarely gets below freezing, our Thermal Control Membrane  can be placed on the outside of the exterior sheathing and finished wall system (stucco, brick, siding, plywood).  Remember to create an airspace to the exterior finished wall does not contact the RBS sheeting, this will create a path of conduction and negate the performance of the RBS. Some products that work well for this are:

TCM applied in a hot climate on an exterior wall which will be re-stuccoed.  In cold climates, install on the inside just
behind the sheet rock and tape the seams.

Building a new home? Adding a room?  Drop us email anytime if you have questions.

 Thermal Control Membrane installed between the trusses in the attic.  Easy to install with simple tools, click here.

 


RBS Chips, the most efficient attic insulation in the world


 

We accept all major credit cards, money orders and checks as payment

More information click here
mail address  Click here

Back to Top

Back to Frequently Asked Questions Page

Back to Main Page


Web page design and maintained by B Rad Design Group
Copyright 1997- 2009 Horizon Energy Systems. All rights reserved.
Information in this document is subject to change without notice.
Other products and companies referred to herein are trademarks or
registered trademarks of their respective companies or mark holders.