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Tuesday, February 7, 2012

Radiant Barrier Done Wrong

Shouting from the rooftops I do declare that a lot of homeowners have been had, badly. The repeatedly made stupid mistakes never cease to amaze me. First and foremost is the failure to follow the 300 Rule of Balanced ventilation. Roof failure will be imminent and there is no warranty. Besides your sick kids (heat+humidity+organic material= spores, or equation #2, negative attic pressure can result in radon or other gases infiltrating through the foundation), high utility bills, peeling paint, premature deck failure, compressed insulation, and general chaos and mayhem.

But my stomach churns the most lately when I inspect an attic and see radiant barrier decking nailed directly to lathe. Here's the run down. 1) Wood shingles have a low density and reduce conductivity.
                                              2) Solid decking transmits a lot more heat.
                                              3) Energy efficient decking installed over lathe makes your attic even hotter.

The inventors of Techshield, formerly called KoolPly, warned that aluminum is a great conductor. Now the brochures leave out this quality and only mention that aluminum does not emit and reflects. But it doesn't when in contact with wood. Otherwise we would just put the aluminum side upwards.

The current manufacturers tell us to remove every other lathe and a 305 reduction of efficiency will occur 9 because there is more contact with wood than when placed onto rafters spaced every two feet. They forget to mention that both plywood and OSB are strong only in one direction so that if there is about a 15" gap on the 50% removal technique, the decking will warp just like when the eight foot side is the vertical rather than horizontal side. When you remove half the lathes the decking thinks that the lathe is the rafter.

So more contact means more heat. People pay extra for a hotter attic. They pay everyone. Build more dams, power plants, Chernobyl, Three Mile Island because that is what your attic will feel like:MELTDOWN! You are better off with normal decking or wood shingles under your roof that with misapplied radiant barrier decking.

Many times you can't remove the lathe because the rafters are not on two foot centers. Why? Because when you install lathe or plank decks, they don't have to be. What carpenter would take the time for a half inch difference if it made no difference. Larry Sample said rather wisely, "What's a half inch amongst friends?"
He also said that h his house had a perfectly good ventilation system until the previous owner put in turbines. I almost fell down while I inquired as how the hell did he know that. He's a mechanical HVAC guy and understands that roofing is rocket science: Venturi, Bernoulli, negative pressure, fluid dynamics (in a simple kind of way) and String Theory. Well not actually the last one but all the others apply.

Back at the inferno, the sun shines on the roof, the heat permeates the roofing, then the deck, and is now super conducted into the lathe by the highly conductive aluminum, which as turned on you like your tonsils. I love that old Bill Cosby record.

You want vent ridge? Sure we'll do it. and we'll put in some turbines and some power vents, and whatever you want... No education going on here. Just a whole lot of baking going on.






Jon Alan Wright
Jon Wright Roofing, Siding, and Windows
1915 Peters Rd., Suite 310
Irving, TX 75061
972.251.1818 Office
214.718.3748 Cell
972.554.8090 Fax
    Follow jwrightroofing on Twitter

4 comments:

Hampry said...

Thank you very much to bring out this issue before all of us. This is really a very informative one and the discussion and the points you have raised is just wonderful. I have already gone through the whole and just loved this.

nat said...

Are you ever going to explain the "wright" way to install new decking when encountering lathe?

Kuldeep said...

Thanks, I have been searching for details about this topic for ages and yours is the best I have discovered so far.

Radiant Barrier

Anonymous said...

run the 8' run of OSB perpendicular to the lathe and leave a 1/4'' gap so the roof can"breathe " please