Relective radiant barriers in southern climates

[phinsperger]

Can on you in the souther states that use refective radiant barriers in the attic spaces please tell me the reasoning behind using the barriers directly applied the underside of the roof deck?

I understand attaching the barriers to the underside of the rafters as that creates a vented air space. Or even using the double foiled bubble wrap stuff as effective methods.

What I don't understand is something like if foil faced sheathing is used, how is the barrier suposed to reflect radiant heat back out when it is in direct contact with the deck sheathing? Reflective radiant bariers need a air space to be most effective do they not?

[cbottger]

Originally Posted by phinsperger

Can on you in the souther states that use refective radiant barriers in the attic spaces please tell me the reasoning behind using the barriers directly applied the underside of the roof deck?

I understand attaching the barriers to the underside of the rafters as that creates a vented air space. Or even using the double foiled bubble wrap stuff as effective methods.

What I don't understand is something like if foil faced sheathing is used, how is the barrier suposed to reflect radiant heat back out when it is in direct contact with the deck sheathing? Reflective radiant bariers need a air space to be most effective do they not?

Paul I could ask the same question as my thinking is the same as yours. My new house inspection today was my first encounter with the foil lined backing on the roof decking it would be interesting to go back into that attic when it is a 100+ outside ambient.

[Brian E. Kelly]

http://www.radiantbarrier.com/physics_of_foil.htm

Materials whose surfaces do not appreciably reflect infrared rays, i.e.: paper, asphalt, wood, glass and rock, have absorption and emissivity rates ranging from 80% to 93%. Most materials used in building construction -- brick, stone, wood, paper, and so on -- regardless of their color, absorb infrared radiation at about 90%. It is interesting to note that a mirror of glass is an excellent reflector of light but a very poor reflector of infrared radiation. Mirrors have about the same reflectivity for infrared as a heavy coating of black paint.
The surface of aluminum has the ability NOT TO ABSORB, but TO REFLECT 95% of the infrared rays which strike it. Since aluminum foil has such a low mass to air ratio, very little conduction can take place, particularly when only 5% of the rays are absorbed.

Does this help?

[phinsperger]

Not quite Brain. I read that same material already on a different site but I still don't see the answer to my question. How does the reflective barrier work when it is in direct contact (no air space) on the underside of the roof sheathing?

[Brian E. Kelly]

Originally Posted by phinsperger

Can on you in the souther states that use refective radiant barriers in the attic spaces please tell me the reasoning behind using the barriers directly applied the underside of the roof deck?

I understand attaching the barriers to the underside of the rafters as that creates a vented air space. Or even using the double foiled bubble wrap stuff as effective methods.

What I don't understand is something like if foil faced sheathing is used, how is the barrier suposed to reflect radiant heat back out when it is in direct contact with the deck sheathing? Reflective radiant bariers need a air space to be most effective do they not?

Yes to be effective for "radiant" heat there needs to be an airgap.

http://www.radiantbarrier.com/physics_of_foil.htm

Reflection and emissivity by surfaces can ONLY occur in SPACE. The ideal space is any dimension 3/4" or more. Smaller spaces are also effective, but decreasingly so. Where there is no air space, we have conduction through solids. When a reflective surface of a material is attached to a ceiling, floor or wall, that particular surface ceases to have radiant insulation value at the points in contact.

[phinsperger]

Then why is foil faced roof decking being used in the south?

[David A. Andersen]

There are three types of heat transfer; conduction, convection, radiation.
They are three separate distinct means of transfer. Reflective substances reflect radiation. Contact has to do with conduction and is not related.
Airspace is a very good insulator and prevents conduction from happening at radiation barriers. Airspace is not necessary for the reflective device to work, however adding an airspace will improve the overall construction of the wall/ roof /ceiling.

Mirrors have about the same reflectivity for infrared as a heavy coating of black paint.

I don't know what this is about, but a reflective surfaces reflect radiation. It does not have the same absorption factor as black paint! A little more information on this perception would be interesting.

Maybe I'm just tired but I'm not grasping the concept you are communicating.

[phinsperger]

Mirrors have about the same reflectivity for infrared as a heavy coating of black paint.

That is their quote not mine, but I beleive what they are saying is that a mirror reflects the visible light sectrum greatly but only reflects the infared spectrum little. I suspect that is because the glass is in direct contact with the reflecive surface coating.

Originally Posted by dandersen

Airspace is a very good insulator and prevents conduction from happening at radiation barriers.

Right. Now with the foil faced roof sheathing, it is installed with the foil side down facing the interior of the attic.

So solar radiation comes from the sun and hit the shingles and converts to thermal heat in the shingles. That thermal heat is conducted to the roof deck because the shingles are in contact with the roof deck. The underside of the roof deck is face with aluminum reflective foil. Because the decking is in direct contact with the roof deck the foil also gathers thermal heat.

Now here the tricky part. Because the foil has a very low emissivity (5%) the there is very little radiant heat leaving the roof deck into the attic area. So the attic is a bit cooler. Great. but what happened to that heat that would have normally been radiated into the attic? It stays a thermal heat in the roof deck and .... you guessed it. - the shingles. The shingles and roof deck are receiving 95% of the solar radiation hiting them ! That is a lot of energy cooking the shingles!

[David A. Andersen]

Yes, I know it's them talking. Not you.
I just don't know what they're getting at.

Part two: yes that is a lot of energy on the shingles!

[phinsperger]

Thanks David.