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Wednesday, February 10, 2010

Does Radiant Barrier Decking Make Roofing Hotter?

No! Well a little, come to find out after after a little foray into Google-land.
My father taught me that heat goes where heat isn't and him being an electrical engineer gave him much more credibility than me, the political philosopher. You see as a warm area transfers the excitement of its atoms to the cold area's atoms an equilibrium is achieved. That frozen meat you submerge in the sink begins to thaw quickly but the water gets colder too. As the two zones approach each other in atomic excitement the transfer slows. The urgency is gone but the last remaining part of the job needs to be completed. The process continues until the two are the same in temperature.
The same with the hot Texas highway. As the road gets firecracker hot you begin to see water where there is none. That is a mirage caused by heat distorting the light. This is heat bouncing back from the asphalt because it has reached a saturation point. It is not getting hotter. The only way for it to get hotter is for the sun to emit more heating rays or for the surrounding air mass to heat up, which it has if there is a mirage.
Here's the process: The sun starts to heat up the roofing. The heat transfers into the attic or is mostly reflected by radiant barrier or insulation beneath the deck. The roof gets hot either way and refuses to accept more heat as it is as hot as the sun can get it.
The radiant barrier caused the roof to heat up a lot faster but only a little bit hotter ,3 to 5 degrees, as I now know. The heat just can't enter the roof anymore because the roofing is burning hot. But if the attic is properly vented nearly the same process happens to the attic. The attic will get pretty hot but not as hot and not as quickly. Proper ventilation works because it transfers cooler ambient air from the soffits and out the roof exhaust almost as fast as the attic heats until it does it as fast as the attic heats. It is saturated to the level that the ventilation allows.
Radiant barrier applied under the deck slows heat transference so much that ventilation is able to do a better job of reducing attic temperatures because the lower loading of heat is easily handled by the ventilation. Wallah. Your attic is near the temperature of the outside ambient air. Yet you don't ventilate foamed attics.
The biggest foreseeable problem with foamed attics is what do you do when the decking eventually fails. Repeated replacement of roofs over time due to age, hail, defective materials or bad workmanship makes this daunting task inevitable. The decking will be nearly impossible to remove. The rafters might be damaged in the removal of the deck and foam. Now that foam debris is everywhere you have to go back with a similar system because the HVAC and ceiling construction is for a closed attic system. Now reblow the foam and wait for "Disaster II" to greet you in old age.
Bathing, boiling, burning gas and breathing, the four "B's" of internally generated humidity, must be contained. The by-product of burning natural gas is heat and water. Thus all homes with closed attics should be all electric. Better yet if the inhabitants don't cook, bath, or breathe.
Almost all roofing manufacturers accept radiant barrier as outlined in this technical bulletin and only concern themselves with ventilation, something the closed attic system does not allow. My advice, after being a contractor for over 30 years, is don't be a guinea pig. Wait and see. Your marriage may be at stake. Worse, your kid's health is at risk. How would you like to find out when you're 80 that you have to raise your grandchildren because you killed your kids with COPD causing fumes.
But what do I know. I'm lust a roofer.

Jon Wright
www.jonwrightroofing.com

1 comment:

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