R806.5 Unvented attic and unvented enclosed rafter assemblies when thermal envelope rafter cavities are small.
When you are working with unvented or have a sealed roof design their are a few set rules in the IRC 2015 that need to be reviewed. After your code is covered we monitor our actual heat and humidity levels to prevent roof shingle and deck damage.

By Murray Wennerlund published 1-15-2023 updated 1-15-2023

Disclaimer: This is a review of the full house envelope insulation practice without foams or sealers that attach to the roof decking. This article is a living document and as tests are completed updates will be made. This is by no means any standard that you should follow if you are not 100% positive it will create a better living environment for your household. 

The build will follow the ENERGY STAR Single-Family New Homes National Program Requirements, Version 3.1 (Rev. 12) and will identify issues between real-world construction, labor, training and materials to achieve an energy efficient home.

To review R806.5 in the IRC-2015 you have to be willing to say your climate zone is perfect for your application. Otherwise you are going to create a long term reoccurring issue with materials above your rafters. 

Here in Louisiana I have monitored several different roof designs and find that when a home is insulated using the industry standard of attic floor or what is otherwise your first floor celling in a single floor home. We look to place R-30 to R-38 in Zone 2 areas to keep our homes cooler in the summer. The winters aren't really a major issue if you can get over a few days of hard freeze temperatures and a few ice storms every 2 to 3 years. 

Working data:

  • Roof pitch 7/12
  • Asphalt Shingles Color Slate
  • Grip Rite underlayment
  • 5/8" Exp. 1 OSB
  • BCI-6500s 11 7/8" rafters
  • Radiant Barrier attached to top inside flange creating 1 1/8" air pocket
  • Hardie Plank soffit vents
  • no roof or gable vents
  • Insulation installed creating thermal envelope modeled to 2012 IECC Section R402 levels and Grade I installation per ANSI / RESNET / ICC 301
  • Thermal Envelope (Rafter insulation will extend over top plates externally to eaves beyond the external wall and top plate.) 
  • Unfaced insulation throughout thermal envelope. External Walls R-23, Raised floor R-23, Rafters R-30.
  • No ceiling or attic floor insulation
  • All HVAC and ducting located within thermal envelope.
  • Attic provides 2 6" vents covering 1,365/sf to stabilize equipment temperatures to no more than 85 degrees July, August. 
  • Radiant Barrier has a 1 1/8" air gap. Item 5 of R806.5 will not apply based on manufacturers requirement to have air gap between roof deck and radiant barrier. Insulation should be to the underside of the radiant barrier that is not sealed to the bottom flanges but rather stapled every 8 to 12 inches.
  • IRC-2015
  • 2012 International Energy Conservation Code (IECC) 

Snip from IRC2015 code:

R806.5 Unvented attic and unvented enclosed rafter assemblies.

Unvented attics and unvented enclosed roof framing assemblies created by ceilings that are applied directly to the underside of the roof framing members and structural roof sheathing applied directly to the top of the roof framing members/rafters, shall be permitted where all the following conditions are met:

  • The unvented attic space is completely within the building thermal envelope.
  • No interior Class I vapor retarders are installed on the ceiling side (attic floor) of the unvented attic assembly or on the ceiling side of the unvented enclosed roof framing assembly.
  • Where wood shingles or shakes are used, a minimum 1/4-inch (6.4 mm) vented airspace separates the shingles or shakes and the roofing underlayment above the structural sheathing.
  • In Climate Zones 5, 6, 7 and 8, any air-impermeable insulation shall be a Class II vapor retarder, or shall have a Class II vapor retarder coating or covering in direct contact with the underside of the insulation.
  • Insulation shall be located in accordance with the following:
    • Item 5.1.1, 5.1.2, 5.1.3 or 5.1.4 shall be met, depending on the air permeability of the insulation directly under the structural roof sheathing.
    • 5.1.1. Where only air-impermeable insulation is provided, it shall be applied in direct contact with the underside of the structural roof sheathing.
    • 5.1.2. Where air-permeable insulation is provided inside the building thermal envelope, it shall be installed in accordance with Section 5.1. In addition to the air-permeable insulation installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing in accordance with the R-values in Table R806.5 for condensation control.
    • 5.1.3. Where both air-impermeable and air-permeable insulation are provided, the air-impermeable insulation shall be applied in direct contact with the underside of the structural roof sheathing in accordance with Item 5.1.1 and shall be in accordance with the R-values in Table R806.5 for condensation control. The air-permeable insulation shall be installed directly under the air-impermeable insulation.
    • 5.1.4. Alternatively, sufficient rigid board or sheet insulation shall be installed directly above the structural roof sheathing to maintain the monthly average temperature of the underside of the structural roof sheathing above 45°F (7°C). For calculation purposes, an interior air temperature of 68°F (20°C) is assumed and the exterior air temperature is assumed to be the monthly average outside air temperature of the three coldest months.
  • 5.2. Where preformed insulation board is used as the air-impermeable insulation layer, it shall be sealed at the perimeter of each individual sheet interior surface to form a continuous layer. 

In Table R402.1.1 Insulation and Fenestration requirements by component for Zone 2 Baton Rouge Louisiana the ceiling R-Value is listed to be 38. We are building with 34' BCI-6500s rafters which are 11 7/8". Most R-38 comes in 12" thicknesses. We could actually press the insulation in but because of the air gap of 1.125" and the radiant barrier for the hot sunny days we'll given only one real option. R-38 HD which some of you can find in 10" thicknesses. We can not find a supplier here in Louisiana so we'll work with R-30 and follow the code requirements when it's not possible to use the required insulation R value. 

The IECC states that the R-values are the minimums. Then explains when you attempt to install insulation in a cavity not designed for the insulation thickness you can not compress to fit because that will change the R factor. 

R402.2.1 Ceilings with attic spaces.
When Section R402.1.1 would require R-38 in the ceiling, R-30 shall be deemed to satisfy the requirement for R-38 whenever the full height of uncompressed R-30 insulation extends over the wall top plate at the eaves.