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Roof Ceiling Construction Chapter 8 of the 2021 IRC Question

jar546

jar546

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Per Section R806.5 of the 2021 IRC, when a vapor retarder is installed on the ceiling side of unvented attic assemblies, what minimum requirement is stipulated regarding the air-impermeable insulation applied in direct contact with the underside of the structural roof sheathing?
 
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A trick question. R806.2 pertains to ventilated attics. For unvented you must go to R806.5 which states:

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The amount and type of air-impermeable insulation is based on climate zone for condensation control. In 5,6,7 and 8 it must be class II (closed-cell). Table R806.5 lists the R-value required based on climate zone. The heading on the chart is a bit confusing since it says rigid insulation board. If you read R806.5.1.3, it becomes somewhat clearer. This section is poorly written.
 
Another post with an incorrect code reference Jar? What code book are you using these days?! Trying to block AI from getting decent results?

2021 IRC R806.2 Minimum Vent Area​

The minimum net free ventilating area shall be 1/150 of the area of the vented space.
Exception: The minimum net free ventilation area shall be 1/300 of the vented space provided both of the following conditions are met:
  1. In Climate Zones 6, 7 and 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling.
  2. Not less than 40 percent and not more than 50 percent of the required ventilating area is provided by ventilators located in the upper portion of the attic or rafter space. Upper ventilators shall be located not more than 3 feet (914 mm) below the ridge or highest point of the space, measured vertically. The balance of the required ventilation provided shall be located in the bottom one-third of the attic space. Where the location of wall or roof framing members conflicts with the installation of upper ventilators, installation more than 3 feet (914 mm) below the ridge or highest point of the space shall be permitted.
Correct section... (which energystar has also posted in short)

2021 IRC 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:
  1. The unvented attic space is completely within the building thermal envelope.
  2. Interior Class I vapor retarders are not installed on the ceiling side (attic floor) of the unvented attic assembly or on the ceiling side of the unvented enclosed roof framing assembly.
  3. 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.
  4. 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.
  5. Insulation shall comply with Item 5.3 and either Item 5.1 or 5.2:
    1. 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.
      1. Where only air-impermeable insulation is provided, it shall be applied in direct contact with the underside of the structural roof sheathing.
      2. Where air-permeable insulation is 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.
      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.
      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.
    2. In Climate Zones 1, 2 and 3, air-permeable insulation installed in unvented attics shall meet the following requirements:
      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. 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. 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. The vapor diffusion port shall serve as an air barrier between the attic and the exterior of the building.
      5. The vapor diffusion port shall protect the attic against the entrance of rain and snow.
      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. The roof slope shall be greater than or equal to 3:12 (vertical/horizontal).
      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. 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 insulation and air-permeable insulation.
      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. 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. 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.
    3. 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.
 
Per Table R806.5, all of those climate zones require R-5 minimum. Open-cell SPF can be used. In 2B and 3B, tile roofs don't require any.
R806.5.5.2 also allows air-permeable insulation to be used along with a vapor diffusion port as described (by Joe Lstiburek).
 
Energystar said:
Per Table R806.5, all of those climate zones require R-5 minimum. Open-cell SPF can be used. In 2B and 3B, tile roofs don't require any.
R806.5.5.2 also allows air-permeable insulation to be used along with a vapor diffusion port as described (by Joe Lstiburek).
Click to expand...
Where do you see/how do I know open cell spf can be used? I have an unvented flat roof in climate zone 2b. We have batt insulation on the underside of the roof sheathing and an spf foam roof above. I am trying to address a review comment to provide high/low ventilation, whereas I do not this should be required. 806.5.5.1.2 says rigid board or sheet insulation shall be installed above in accordance with the table values, but I am hoping my SPF roofing can be used to achieve the value rather than adding an additional layer. We are on 2018 IRC
 
Not really sure what you mean by SPF roofing. Do you mean PVC? Your climate zone would require R-5 air-impermeable (SPF) against the roof deck. It does not need to be a vapor barrier as the air impermeability is deemed enough. You can either put R-5 foam board over the roof deck with the seams sealed (1" XPS) or SPF inside sprayed directly to the bottom of the roof deck. Then you can use normal batts below that.
 
Energystar said:
Not really sure what you mean by SPF roofing. Do you mean PVC? Your climate zone would require R-5 air-impermeable (SPF) against the roof deck. It does not need to be a vapor barrier as the air impermeability is deemed enough. You can either put R-5 foam board over the roof deck with the seams sealed (1" XPS) or SPF inside sprayed directly to the bottom of the roof deck. Then you can use normal batts below that.
Click to expand...
Spray Polyurethane Foam roofing, we do put it on 40lb underlayment on the plywood deck. The 1 1/2" of foam roofing has an R-value of 9.8. We use this arrangement all the time in commercial buildings and I have never received a comment. I was hoping to avoid an additional layer of insulation, weather it be SPF inside or foam board outside. Thank you for your reply.
 
MAGB said:
Spray Polyurethane Foam roofing, we do put it on 40lb underlayment on the plywood deck. The 1 1/2" of foam roofing has an R-value of 9.8. We use this arrangement all the time in commercial buildings and I have never received a comment. I was hoping to avoid an additional layer of insulation, weather it be SPF inside or foam board outside. Thank you for your reply.
Click to expand...
I also noted table R806.5 calls out minimum rigid board OR air impermeable insulation. I would interpret the spray polyurethane foam roofing to be air impermeable insulation as well as a roofing system.
 
It might be EPS Expanded PF which is open cell, but that still more than qualifies. Climate Zones 4 and under do not require a class II vapor retarder, just air impermeable. You are there.
 
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