Metal Vented Roof Assembly
crowscroft
| Posted in General Questions on
I’m building in a humid zone 5 mixed climate. I have a vaulted ceiling and want to use blown cellulose insulation in the roof. I’m using metal roofing, and I understand I’ll need a vented roof system. I’m wondering what the simplest assembly is to achieve this?
I’m familiar with roof assemblies that looks like:
Rafters
OSB sheathing
WRB
strapping nailed perpendicular to rafters
metal roofing
Does this assembly pose any condensation issues? Would it leave sufficient air channel to allow for the roof to dry to the outside?
Thanks!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
crowcroft,
The assembly you propose needs two additional things to make it work: A good interior air-barrier, and baffles to ensure the vent channel isn't impeded by the insulation.
If your roof slope is higher than 4/12 you may need to use dense-pack cellulose to avoid settling.
Thanks for your reply Malcom. I will definitely have a good interior air barrier. Does the vent channel need to be between the insulation and the OSB (with baffles as you describe), or can it be on top of the OSB (by creating a vent space with the strapping between the OSB and the metal roofing)?
crowscroft,
Practically it can be above. There isn't much difference between an assembly with OSB sheathing, and one with OSB baffles. However I'm not sure the IRC allows permeable insulation in contact with the sheathing - and there may be some provisions for using vapour diffusion ports - but take all this code talk with a grain of salt. I'm just not that familiar with your codes. Hopefully someone who knows more will chime in.
If you are venting above the sheathing, you will need two layers of strapping. The first one vertical for continuous ventilation from eaves to ridge, and a second horizontal one to attach the metal roof panels to.