If you have a vented attic it makes no sense to install rigid foam above the roof sheathing.
Rigid foam on roof unconditioned.
All of the sloped roof assembly from the soffits to the area above the small flat ceiling above your second floor needs to be insulated to r 49.
Foam typically used on roofs can support 15 lb.
In so bearing shouldn t be an issue.
You can use 2 inch thick rigid foam as a ventilation baffle if you want.
Either 1 install an adequately thick layer of rigid foam on the exterior side of the roof sheathing or 2 install an adequately thick layer of closed cell spray.
If you want to create an unvented roof assembly that is an assembly that lacks the ventilation gap between the insulation and the roof sheathing you have only two choices.
Closed cell spray foam can be used in any climate.
Gable end detail overhang cut and then rebuilt to hide the thickness of the over roofing rigid insulation.
Superior air sealing performance eliminates energy wasting leaks at the top of the house.
You might as well put the rigid foam in the branches of a tree in your back yard.
Install rigid insulation board in multiple layers with joints offset vertically and horizontally between adjacent layers.
On small porches as long as you use a 2x plate where the rafters meet the foam it should be fine.
Now comes the really really big point.
The foam layer is waterproof rigid and custom contoured to improve drainage.
Foam roofing bonds around roof penetrations such as pipes vents and edges forming an impenetrable wedge leaving these penetrations high and dry additionally the roof surface is built up in ponding areas to eliminate heavy standing water and improve drainage.
A vented attic is basically outdoors so installing rigid foam above your roof sheathing is a waste of money and a waste of rigid foam.
Rigid foam insulation also known as insulating boards provide high r values.
You can even place it under the foundation slab.
That includes the sloped roof assembly above the small triangular attics behind your kneewalls.
Install it anywhere from the roof to the foundation.
Silverglo rigid foam insulation is durable and waterproof it won t shrink or lose r value over time.
They keep your home at a comfortable temperature without the need to crank your heat.
Rigid insulation is one of the best tools to reduce your energy costs.
Humid or cold climates require either rigid foam insulation above the roof deck to keep its temperature above 45 f throughout the year or spray foam insulation under the roof deck to keep interior moist air from contacting it.
Rigid insulation layers exterior to existing roof sheathing must provide sufficient thermal insulation r value to control condensation risk see climate specific guidance.
The 6 inches of rigid insulation added to the top of the existing roof deck needed to be done in a manner that was invisible.
Install nailbase for cladding.
