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How to properly insulate a non vented roof

J

Jake the Dog

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Joined
Aug 19, 2018
Messages
2
Location
New Jersey
I am enclosing what was a screen room off the back of my house and making it living space. The rafters are double 2x8. I was planning on using an R21 batt to insulate so I would have space for airflow. My drywall installer (also a GC) said I do not need to leave space since there is actually no air flow at all since the roof has no peak.
The inspector wants to see a higher r value. R30 would completely stuff the cavity and compress down to a value of r21 any way.
Is it better to stuff the cavity or leave the air space?
It should be noted also that the roof is covered in solar panels.
 
Not into insulation, Lower the ceiling a little so the r 30 is not smashed??

Maybe your best route to go all the way around is the spray in
 
Ask the nice inspector where in the book is he getting the requirement::

The inspector wants to see a higher r value.

So you can research it!!!!!!!!!!!!

And post the section cited here
 
The objective of the vented ceiling space is to keep your roof sheathing below freezing. Hot roofs in heating climates must be designed well in order to operate correctly. Completed incorrectly, a hot roof will cause ice damming and can cause water to enter into the living space.

Generally, we look for at least an R50.
 
Can you pack down the ceiling from inside to achieve the R Value? If so will it affect the required minimum ceiling height required by code?

We handle all of these on a case by case basis, there are times when a ceiling can't be insulated to a target R Value without significant cost, so we note the reasoning and move on to what can be achieved.
 
I don't know about your code but in CA the code is very specific about unvented insulated ceilings...it spells it out 3 or four options clearly. I often use a combination of spray (non-permeable) and batt (permeable) as mtlogcabin mentioned, since spray is pricey, but another option is tight fitted rigid insulation sheets (like styrofoam with no permeability), cut to fit the joist bays...seal the perimeters. This would not require ventilation and is cheaper then spray. Batt, if used by itself, needs ventilation. When the warm air moves through batt into the cold air, it condenses, causing moisture, causing all kinds of problems.
 
Rick18071 said:
Are you allowed to use batts in cathedral ceilings in CA?
Click to expand...
Yes, but needs to be vented above the batts. Typically the depth needed for the batt to meet energy requirements plus the required 1 1/2" airspace above the batts will mean very deep rafters, so I usually do spray or a combination of spray and batt (as long as the spray is applied to the underside of the roof sheathing).
 
Your real issue in an unventilated ceiling assembly is making sure no moisture vapor reaches the sheathing as it will condensate and cause rotting roof syndrome.. the only thing that might help is to have a few inches of hdf foam sprayed (r6.2 per inch) covering the edges as well.. HDF is a vapor barrier, and will keep your framing and sheathing dry... don't use ldf as its vapor permiable and dont put up plastic behind the rock, keep away from the double vapor barrier) you need the assembly to be able to dry in.
Without knowing your climate zone, hdd or cdd, im assuming your in an area that needs more heating than cooling...
 
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