Venting under metal

Guest User
10/29/2015
I'm am in the process of working with designer on out home,it will be built in zone 3,Oklahoma. It is a 2 story with a story and a half truss on the 3 rd floor,so we will insulate roof line(cathedral ceeling)so it will be conditioned with some ducting in the knee walls.We need r38 in ceeling so we will do around r25 or so under touching roof deck and remaining 12-15 above with polyiso.On top of the poly we will have another layer of osb than some underpayment than standing seam. My question is concerning the venting,some say we don't need any under the metal and some say we do,maybe in the form of 1x4 strips running from the eave to ridge and than a ridge cap,with some kind of soffit vent(coravent strip)or maybe a vented nailbase to save on some labor. Do we need venting ?
Info @windowsonwashington.net
An informed customer is our best customer.
10/29/2015
You don't need venting if you treat it like a hot/insulated roof deck. If you are going through all that design considerations, an above deck roof vent in conjunction with all the insulation would work great in your area.
Guest User
10/29/2015
Have any thoughts on john manville 3 inch vented nailbase ? Also on cor-a-vent soffit vent ? Thanks
Info @windowsonwashington.net
An informed customer is our best customer.
10/30/2015
Nailbase is fine but you need the vent to be on the underside of the metal. No point in have two layers of sheathing in this case doubled up. While I wish there was an easy 1 stop shop solution, the fact is that you must really build what you need in these cases
Guest User
10/30/2015
So your saying osb sheathing,polyiso,1x4furring strips than metal? Does the standing seam fasten down to the furring strips(24 inch on center)? Isn't vented nail base made for this,sheathing,polyiso, vented gap,sheathing,membrane,metal? Thanks
Info @windowsonwashington.net
An informed customer is our best customer.
10/31/2015
I prefer to go over a solid deck (i.e. plywood) as compared to 2x4 purlins. Vented nailbase works fine but the vents need to be on the underside of the roof.
Guest User
10/31/2015
That's not how vented nail works,the metal is fastened to the nailbase,the vented gap is under that layer of sheathing,than the plot and than the sheathing layer next to the framing ? Am I missing something or just not understand?
Info @windowsonwashington.net
An informed customer is our best customer.
10/31/2015
That's my point. If you are doing a metal roof, the vent should be directly underneath the metal to allow for convective air movement and the insulation should be tight (air gap free) to the roof framing/decking. http://www.danperkinsroof.com/venting-and-insulation/

If you would like to reply to this thread, please log in. If you do not have an Ask the Experts forum user account, create one here.

Find a Contractor

Get Started Today

Take the first step to increasing the value of your home with a great looking, durable, fire resistant and energy efficient metal roof. Browse our list of qualified MRA Member Roofing Contractors in your area for a free consultation and estimate.