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TOPIC: Ventilation

Metal Roof Ventilation

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We are trying to decide on the correct approach for our new cathedral roof that we are designing. We want very high R-values in this home, and thus don't want to be limited by the standard approach: leave a 1" gap in the joist cavity and ventilate it. Instead, we had planned to fill the cavity, and even add a layer of rigid on top of the sheathing - as highly advised by many places I've read on the web. Problem is, no one explains how to ventilate with this situation. The sheathing is generally what you want to ventilate, if I understand correctly - but now it is sandwiched. Seems silly to vent between the insulation as it just negates the top layer. If Do we ventilate above the rigid, using strapping like I've read about here? Does this do much good? I thought the goal was to dry out the sheathing that is likely to get wet from vapor condensation. Hmmm... Any advice or alternate approaches would be very much appreciated.
Guest User

7/31/2009
I would suggest a vapor barrier behind your ceilings to prevent moisture migration into the roof assembly. Still, I would say that building codes call for a minimum 1/2" vented airspace in your roof assembly and I would encourage you to do so. You could vent on top of the sheathing with a cross grid of vertical and horizontal battens followed by roofing. Todd Miller
Todd Miller
Isaiah Industries, Inc.

8/1/2009
Thanks Todd, We will certainly put a vapor barrier behind the ceiling. Do you mean to vent with battens: - on top of the sheathing, and under the rigid insulation (does this negate the insulative effect?) - on top of the rigid insulation (& water barrier), and under metal roof
Guest User

8/3/2009
I would do this venting on top of the insulation. Todd Miller
Todd Miller
Isaiah Industries, Inc.

8/7/2009