Radiant Barriers in attics?

[Patrick Coulter]

What is the general opinion on radiant barriers and how they may affect the structure and the roof covering? Thanks.

[David C. Macy]

Radiant barriers reflect heat radiation coming toward them. They also impeded the heat flow by being poor emitters of heat radiation.

Radiant barriers must face an air space with thier shiny surface to be effective.

Air spaces bordered by radiant barriers have higher R values than those bordered by common building materials.

For the radiant barrier to be an effective insulator, it must face a calm air space -one without air convection. (It is difficult to achieve dead air space)

Expect a 2 to 10 percent savings depending on climate and insulation levels.

Radiant barriers are fastened to the bottom of rafters or roof sheathing.

[Brian A. MacNeish]

Originally Posted by dmacy

Radiant barriers reflect heat radiation coming toward them. They also impeded the heat flow by being poor emitters of heat radiation.

Radiant barriers must face an air space with thier shiny surface to be effective.

Air spaces bordered by radiant barriers have higher R values than those bordered by common building materials.

For the radiant barrier to be an effective insulator, it must face a calm air space -one without air convection. (It is difficult to achieve dead air space)

Expect a 2 to 10 percent savings depending on climate and insulation levels.

Radiant barriers are fastened to the bottom of rafters or roof sheathing.

So radiant barriers reflect incoming heat radiation back at the roof sheathing....... Wouldn't this lead to premature shingle failure as the sheathing would stay very hot, raising shingle temperatures, thus lessening the shingle life??

[jrivera]

Ha, sounds like HERS talk to me!!!

Mic

[rspermo]

Radiant barrier effectiveness in order:
1. Original roof decking made of Tech Shield, Solar Ply, etc
2. Paint roof decking (inside) with Heat Block 75
3. Hang solar barriers to rafters - collects dust and becomes less effective over time

[David C. Macy]

Originally Posted by Brian A. MacNeish

So radiant barriers reflect incoming heat radiation back at the roof sheathing....... Wouldn't this lead to premature shingle failure as the shaething would stay very hot, raising shingle temeratures, thus lessening the shingle life??

It depends on a lot of variables.

Color of the shingles.
Shading from trees.
Exposer to sun.

But yes it could reflect the life of the shingles and may even void the manufacturer's warranty.

I would think the 2-10% savings of installing a radiant barrier would be lost due to shortened roof life.

I am still new to this and yes it is HERS talk as I am studying to be a Certified rater. But I want to di it right.

Just going into a home with just an IR camera is not the correct way to do a thourough energy audit. In my opinion.

[Patrick Coulter]

Thanks, that was helpful information. I was aware of how they worked but didn't know how good they were for the life of the roof and structural concerns.

[Kenton H. Shepard, CMI]

Originally Posted by pcoulter

What is the general opinion on radiant barriers and how they may affect the structure and the roof covering? Thanks.

Patrick, it's a trade-off.

• If the attic space is effectively ventilated and the attic floor is well-insulated the radiant barrier may have little effect on the amount of heat radiated from the roof to the living space, but may contribute significantly to premature shingle failure by retaining heat next to the roof deck.
• The reverse is also true.
• And then there are all the conditions in between.

It really depends on the climate zone, the home design and combination of materials installed. There isn't always a simple answer. Probably the best answer is to take the time to learn about building science- especially conduction, convection and radiation- and become familiar with the properties of common building materials.

[Patrick Coulter]

Originally Posted by kshepard

Patrick, it's a trade-off.

• If the attic space is effectively ventilated and the attic floor is well-insulated the radiant barrier may have little effect on the amount of heat radiated from the roof to the living space, but may contribute significantly to premature shingle failure by retaining heat next to the roof deck.
• The reverse is also true.
• And then there are all the conditions in between.

It really depends on the climate zone, the home design and combination of materials installed. There isn't always a simple answer. Probably the best answer is to take the time to learn about building science- especially conduction, convection and radiation- and become familiar with the properties of common building materials.

Thanks, Ken. That's what I had suspected but needed to confirm. Much appreciated!!

[Barry Adair]

http://www.ornl.gov/sci/roofs%2bwall...ant/rb_03.html

Please provide doc that shows 2-10% savings

[rspermo]

Kenton,

I disagree with "contribute significantly with premature shingle failure". Numerous neutral studies have pointed out that a radiant barrier only raises the temperature of the shingle 2-5 degrees. When you are talking 140-160 degrees the increase is miniscule. Shingle manufacturers try to void warranties (which are pretty much meaningless anyways) by saying it leads to premature failure.
http://www.austinenergy.com/energy%2...ding/Source...

[Kenton H. Shepard, CMI]

Originally Posted by rspermo

Kenton,

I disagree with "contribute significantly with premature shingle failure". Numerous neutral studies have pointed out that a radiant barrier only raises the temperature of the shingle 2-5 degrees. When you are talking 140-160 degrees the increase is miniscule. Shingle manufacturers try to void warranties (which are pretty much meaningless anyways) by saying it leads to premature failure.
http://www.austinenergy.com/energy%2...ding/Source...

What studies, Robert? (that link was dead)

When I researched radiant barriers about a year ago, manufacturers were making big claims about the large percentage of reduction in heat radiating down into the attic (and sometimes the living space) from the roof. That heat is going to go somewhere and if it's radiating down into the rafter bay and then being reflected by the radiant barrier stapled to the bottom of the rafters, how is it not going to heat up the shingles a significant amount?

I don't agree that manufacturer's warrantees are meaningless. I also don't believe that they try to void warrantees, but they don't want to pay for problems that were created by policyholders who created conditions that cause shingles to fail prematurely. That would include installing a product that makes and keeps shingles hotter.

[rspermo]

GAF and CertainTeed will offer the same warranty if Tech Shield is used or not used. They do not change their warranty if TechShield is used. Just Google CertainTeed Tech Shield you will find numerous references to this.

[Kenton H. Shepard, CMI]

That's a pretty good indicator.

[rspermo]

Kenton,

It took me awhile to find that these companies would still warranty their shingles if Tech Shield is used!!