Venting and insulating a flat roof with a fiberglass roof deck
I live in Philadelphia where building Rooftop Fiberglass deck is pretty common. How most (and I’ve) always detailed this was to have a sub ceiling on the upper level which sits about 4-6″ below the Structural Roof framing (which is pitched to slope 1/4″ per ft min). This sliver of uninhabitable “attic” as they call it, becomes the venting space and roof vents are installed for airflow. The Fiberglass Batt Insul sits just above the sub clg which is dry walled. Seemed logical until I started researching other ways to go about this for a project I’m doing at a shore point in NJ. I’ve learned a lot already from reading the articles on this site especially about the option for a non-vented roof and the mistakes commonly made regarding penetrating that clg in the vented assembly with hi-hats. I had no idea we were making a huge mistake.
I’m faced with needing a tighter roof framing anyways so the vent- less option seems like a good move. To eliminate the hi-hat issue I would love to have the insulation on top of the sheathing (warm roof) but I’m assuming I can not do that with an occupiable fiberglass roof deck? Am I able to lay plywood on top of the rigid insul to then apply the fiberglass to or is that not a solid enough structure to apply fiberglass on? I would worry about any compression straining the fiberglass.I’ve never worked with Rigid Roof insul so i don’t know. If I do the cold roof method with the closed cell spray poly this doesn’t eliminate an issue with the hi hats correct? Wondering how to still get hi hats on that level without compromising my roof/clg assembly.
Rather then sloping the Structural Framing I had hoped to rip 2s’ and stack on top of my structural members to achieve pitch on the roof. That shouldn’t effect anything right? I would just still spray foam to the underside of the roof sheathing?
Last but no least. Cost? Is using a vent-less option with Closed cell spray insulation cost a lot more then traditional vented and FB Batt?
Lots of questions looped into that.
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Replies
PhiTown,
There are two general groups of questions here:
1. What's the best way to insulate a low-slope residential roof?
2. What type of substrate is needed for a walkable fiberglass roof deck?
Answers to your first question can be found here: Insulating Low-Slope Residential Roofs.
If you need to limit the total thickness of your roof assembly, you probably want to go with an unvented assembly -- and the best way to do that is to install an adequate thickness of rigid foam insulation above the roof sheathing. This rigid foam insulation is usually supplemented with some type of fluffy insulation (fiberglass, cellulose, or mineral wool) installed directly under the roof sheathing (and in contact with the roof sheathing).
Roofers routinely install a second layer of roof sheathing (plywood or OSB) on top of this type of rigid foam. The typical method requires the upper layer of sheathing to be fastened to the rafters with long screws that extend through the rigid foam. For more information, see How to Install Rigid Foam On Top of Roof Sheathing.
Concerning your second question, you may want to consult the technical representative of the company that supplies the fiberglass materials used for your deck. I don't see any reason why you couldn't install fiberglass on a plywood deck that is fastened through rigid foam. If you are worried about deflection, follow the usual rules: 16 inch on center framing is better than 24 inch on center framing, and thicker plywood has less deflection than thinner plywood.
Wish I found this thread sooner. Did you ever end up doing the fiberglass warm roof? We just completed one this past week in Philadelphia and am curious to see where you landed!