Unvented Attic, Vented Soffit?

[jar546]

Typically, when an attic is set up as unvented, usually with spray foam insulation against the bottom of the roof deck, the soffits are blocked off. Is there a requirement for the soffit to be vented? Should there be a requirement to vent the soffit? What happens if you don't and you have dead air space?

 

[steveray]

Nope...As it should be the same as outside air, nothing bad should happen....#notlikeBerkley

 

[Joe.B]

Depends on the climate zone, and the type of insulation used. 1202.3

 

[Joe.B]

Eh maybe not...

I was thinking about the permeability of the insulation, which may require a vapor diffusion port at the top and:

5.2.9. Where only air-permeable insulation
is used and is installed directly
below the structural roof sheathing,
air shall be supplied at a flow rate
greater than or equal to 50 cubic feet
per minute (23 .6 Lis) per 1,000
square feet (93 m2) of ceiling.

Don't know if that would be best supplied by a vented soffit.

 

[Joe.B]

R806.5 is the residential version of what I cited above.

 

[mtlogcabin]

What happens if you don't and you have dead air space?

Depends on the climate zone you are in.

[RB]VAPOR DIFFUSION PORT. An assembly constructed or installed within a roof assembly at an opening in the roof deck to convey water vapor from an unvented attic to the outside atmosphere.

IRC

1. 5.2. In Climate Zones 1, 2 and 3, air-permeable insulation installed in unvented attics shall meet the following requirements:
   
   1. 5.2.1. An approved vapor diffusion port shall be installed not more than 12 inches (305 mm) from the highest point of the roof, measured vertically from the highest point of the roof to the lower edge of the port.
   2. 5.2.2.The port area shall be greater than or equal to 1:600 of the ceiling area. Where there are multiple ports in the attic, the sum of the port areas shall be greater than or equal to the area requirement.
   3. 5.2.3.The vapor-permeable membrane in the vapor diffusion port shall have a vapor permeance rating of greater than or equal to 20 perms when tested in accordance with Procedure A of ASTM E96.
   4. 5.2.4.The vapor diffusion port shall serve as an air barrier between the attic and the exterior of the building.
   5. 5.2.5.The vapor diffusion port shall protect the attic against the entrance of rain and snow.
   6. 5.2.6.Framing members and blocking shall not block the free flow of water vapor to the port. Not less than a 2-inch (51 mm) space shall be provided between any blocking and the roof sheathing. Air-permeable insulation shall be permitted within that space.
   7. 5.2.7.The roof slope shall be greater than or equal to 3:12 (vertical/horizontal).
   8. 5.2.8.Where only air-permeable insulation is used, it shall be installed directly below the structural roof sheathing, on top of the attic floor, or on top of the ceiling.
   9. 5.2.9.Air-impermeable insulation, where used in conjunction with air-permeable insulation, shall be directly above or below the structural roof sheathing and is not required to meet the R-value in Table R806.5. Where directly below the structural roof sheathing, there shall be no space between the air-impermeable insulationand air-permeable insulation.
   10. 5.2.10.Where air-permeable insulation is used and is installed directly below the roof structural sheathing, air shall be supplied at a flow rate greater than or equal to 50 CFM (23.6 L/s) per 1,000 square feet (93 m2) of ceiling. The air shall be supplied from ductwork providing supply air to the occupiable space when the conditioning system is operating. Alternatively, the air shall be supplied by a supply fan when the conditioning system is operating.
       
       Exceptions:
       
       1. 1.Where both air-impermeable and air-permeable insulation are used, and the R-value in Table 806.5 is met, air supply to the attic is not required.
       2. 2.Where only air-permeable insulation is used and is installed on top of the attic floor, or on top of the ceiling, air supply to the attic is not required.

 

[steveray]

R806.1​

Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow.

NR.....

 

[steveray]

Depends on the climate zone you are in.

[RB]VAPOR DIFFUSION PORT. An assembly constructed or installed within a roof assembly at an opening in the roof deck to convey water vapor from an unvented attic to the outside atmosphere.

IRC

1. 5.2. In Climate Zones 1, 2 and 3, air-permeable insulation installed in unvented attics shall meet the following requirements:
   1. 5.2.1. An approved vapor diffusion port shall be installed not more than 12 inches (305 mm) from the highest point of the roof, measured vertically from the highest point of the roof to the lower edge of the port.
   2. 5.2.2.The port area shall be greater than or equal to 1:600 of the ceiling area. Where there are multiple ports in the attic, the sum of the port areas shall be greater than or equal to the area requirement.
   3. 5.2.3.The vapor-permeable membrane in the vapor diffusion port shall have a vapor permeance rating of greater than or equal to 20 perms when tested in accordance with Procedure A of ASTM E96.
   4. 5.2.4.The vapor diffusion port shall serve as an air barrier between the attic and the exterior of the building.
   5. 5.2.5.The vapor diffusion port shall protect the attic against the entrance of rain and snow.
   6. 5.2.6.Framing members and blocking shall not block the free flow of water vapor to the port. Not less than a 2-inch (51 mm) space shall be provided between any blocking and the roof sheathing. Air-permeable insulation shall be permitted within that space.
   7. 5.2.7.The roof slope shall be greater than or equal to 3:12 (vertical/horizontal).
   8. 5.2.8.Where only air-permeable insulation is used, it shall be installed directly below the structural roof sheathing, on top of the attic floor, or on top of the ceiling.
   9. 5.2.9.Air-impermeable insulation, where used in conjunction with air-permeable insulation, shall be directly above or below the structural roof sheathing and is not required to meet the R-value in Table R806.5. Where directly below the structural roof sheathing, there shall be no space between the air-impermeable insulationand air-permeable insulation.
   10. 5.2.10.Where air-permeable insulation is used and is installed directly below the roof structural sheathing, air shall be supplied at a flow rate greater than or equal to 50 CFM (23.6 L/s) per 1,000 square feet (93 m2) of ceiling. The air shall be supplied from ductwork providing supply air to the occupiable space when the conditioning system is operating. Alternatively, the air shall be supplied by a supply fan when the conditioning system is operating.
       
       Exceptions:
       1. 1.Where both air-impermeable and air-permeable insulation are used, and the R-value in Table 806.5 is met, air supply to the attic is not required.
       2. 2.Where only air-permeable insulation is used and is installed on top of the attic floor, or on top of the ceiling, air supply to the attic is not required.

And that is mostly based on permeable insulation.....