Felt vs synthetic with radiant barrier
Roger_S39| Posted inGeneral Questionson
Hi all,
If the roof sheathing has radiant barrier on it, is there a preference for choosing either felt or synthetic underlayment?
The question is asked with the concern one option may be better suited for sheathing with radiant barrier.
Ie: permeability for example: does the radiant barrier, even with it perforated aspect limit the sheathing drying to the inside if the deck got wet and then one really needs to rely on the underlayment to allow for drying to the outside etc.
Trying to decide on something like 30 felt vs Certainteed Diamondback on a 12/12 pitch roof with possibly roof sheathing with the radiant barrier.
Thank you.
Replies
Hi Roger,
What type of roofing are you planning to install? If it is an asphalt shingle roof, there is some (but little) outward drying potential. A permeable underlayment will support what outward drying the roof has. If it is metal roofing on furring strips, a permeable membrane would be even more helpful for outward drying.
You should be able to find the perm rating of the sheathing product, which will give you a better idea of what you are dealing with.
All that said, water management is the most important thing to pay attention to. Install the roofing underlayment and roofing when the sheathing is dry and install all aspects of the assembly (ice and water membranes, drip edges, flashing, etc) really well to minimize potential for wet sheathing in the first place.
Here an article on roofing underlayments that you may find helpful:Choosing Metal Roofing
>"Ie: permeability for example: does the radiant barrier, even with it perforated aspect limit the sheathing drying to the inside if the deck got wet and then one really needs to rely on the underlayment to allow for drying to the outside etc."
Unless it's a tile roof or metal roofing on purlins there is almost no such thing as a roof deck "...drying to the outside..."
RB-clad roof decking is sufficiently vapor permeable to the interior to be protective as long as there it's facing a vented air gap. (Without the air gap the RB has zero thermal benefit anyway.) In an UNvented insulated roof the insulation needs to be at least semi vapor permeable to provide any moisture resilience to the roof deck.