batten positioning & condensation


  #1  
Old 02-26-06, 06:11 AM
Y
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Question batten positioning & condensation

I am ready for a roof on a domestic extension. The roof material is a metal tilesheet stamped to resemble individual tiles. Accepted practice is to apply a membrane to the roof joists, then the battens perpendicular to the joists and finally the roofing material. Roof pitch is 12/1, the minimum required for this material. Condensate will appear on the underside of the metal, insulation will be placed under the sarking felt/memebrane. Would it not be better then to place the sarking/membrane over the battens instead of under. Reason tells me there would be a better path for the condensate run-off, also a more efficient ventilation gap between the roof and the membrane. I do not wish to question accepted practice here, but I need someone to explain the error in my logic.
 
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Old 03-01-06, 11:37 AM
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We installed the roof today adopting standard practice/methodology. This forum displays a very sound knowledge base by the regular contributors so I thought you might have some imput to share. You obviously take pride in your work and that is an absolute pleasure to see. Regards.
 
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Old 03-01-06, 04:26 PM
P
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Condensation

Hi yrret,

In North America, the accepted practice is to have a vapour barrier on the warm side of the insulation, preventing moist air from reaching the cold space (attic), and effective ventilation of the cold space to dissipate any residual moisture (soffit intake, ridge-level exhaust). With this method, you should not have any condensate appearing on the underside of the metal.

It is not clear to me what you mean by "apply membrane to the roof joists". What exactly is the membrane, and on what side of the joists is it applied? What is the purpose of this membrane in the standard practice as you know it? Given that the roof pitch is very low slope, our practice would be to have the panel joints waterproofed either by caulking or similar material. As well, we usually look for a minimum height of air channel of 4 inches which would allow some air circulation by convection.
 
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Old 03-03-06, 05:22 AM
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I agree w/Pgriz.....except that I think 2" air space is good enough.
Jim
 
 

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