Should roof underlayment be seam taped?
I have a vaulted ceiling with tongue and groove decking overlaid with osb for shear. There is lapped vb armour under layment layer over that osb. R30 polystyrene panels ( edge seam sealed) with exterior osb are going over that, followed by a waterproof layer then standing seam metal roofing.
Should i tape the lapped (4 inch overlap) underlayment to prevent moisture currents/ diffusion into the insulation panels or for air tightness or is that over kill? The 8 inch nail base insulation panels will be caulk sealed at the sides between the panels
As a further question, should the layer under the metal roofing be vapor permeable to allow that osb nail base to dry? Thanks
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Replies
chemosabe,
The main function of roof underlayments is to act as a secondary defense against water intrusion through the roofing. So it is better to rely on laps and avoid tape which can hold moisture. Rigid insulation does not suffer from wind-washing, so there is no problem with air movement if any occurs.
A permeable underlayment will certainly do no harm, and may afford a small amount of drying for the sheathing, but I don't think it matters much which type you use.
After seeing this demonstration, I would encourage you to tape anything on the roof that is currently not-taped. An ounce of prevention is worth a pound of cure. https://www.youtube.com/watch?v=tx2VjvSm9zo&t=21m40s
Scott,
Does it always make sense to construct assemblies for extreme conditions, even if those conditions don't exist where you are building?
I don't live in a hurricane zone, but have looked at the recommendations from agencies like Fema out of curiosity. They, like the video you linked to, recommend taping the roof sheathing not the underlay, and relying on either an aggressive fastener schedule or a self- adhered underlayment.