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How to properly insulate a non vented roof

Discussion in 'Residential Energy Codes' started by Jake the Dog, Aug 19, 2018.

  1. Jake the Dog

    Jake the Dog Registered User

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    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.
     
  2. Jake the Dog

    Jake the Dog Registered User

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    That's some good information. Unfortunately the roof is already up and covered in solar panels. Adding foam under the sheathing isn't really an option unless I am cutting to fit the cavities. I got a quote for spray foam but it's a lot of money (about $2200 for r30 equivalent). Any other options?
     
  3. cda

    cda Sawhorse

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    Welcome
     
  4. cda

    cda Sawhorse

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    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
     
  5. cda

    cda Sawhorse

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    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
     
  6. tmurray

    tmurray Sawhorse

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    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.
     
  7. mtlogcabin

    mtlogcabin Sawhorse

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    Compressing fiberglass insulation reduces the R-value. Ask the spray foam contractor for a pricefor applying an inch or two of spray foam and then install the R-21
     
  8. mark handler

    mark handler Sawhorse

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  9. Keystone

    Keystone Sawhorse

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    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.
     
  10. rogerpa

    rogerpa Silver Member

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  11. rogerpa

    rogerpa Silver Member

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    Oops. All of NewJersey requires R-49. I misread the table.
     
  12. Robert

    Robert Sawhorse

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    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.
     
  13. Rick18071

    Rick18071 Sawhorse

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    Are you allowed to use batts in cathedral ceilings in CA?
     
  14. Robert

    Robert Sawhorse

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    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).
     

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