• Welcome to The Building Code Forum

    Your premier resource for building code knowledge.

    This forum remains free to the public thanks to the generous support of our Sawhorse Members and Corporate Sponsors. Their contributions help keep this community thriving and accessible.

    Want enhanced access to expert discussions and exclusive features? Learn more about the benefits here.

    Ready to upgrade? Log in and upgrade now.

Rafter Ventilation Code Requirement

R

reo73

Member
Joined
Jun 24, 2013
Messages
2
Location
United States
The IBC Section 1203.2 requires a 1" air gap btwn the roof deck and the insulation for ventilation of enclosed rafter spaces. Is this true if you use rigid insulation or closed cell foam insulation to completely fill in the rafter space. The code section does not have exceptions listed so I am assuming a ventilated 1" air gap is required no matter what.

Project is in California so technically it falls under the CBC.
 
2009 IRC contains the following exception/permission for unvented attic space.

I know nothing about CBC, but there are several here who will.

R806.4 Unvented attic assemblies. Unvented attic assemblies (spaces between the ceiling joists of the top story and the roof rafters) shall be permitted if all the following conditions are met:

1. The unvented attic space is completely contained within the building thermal envelope.

2. No interior vapor retarders are installed on the ceiling side (attic floor) of the unvented attic assembly.

3. Where wood shingles or shakes are used, a minimum 1/4 inch (6 mm) vented air space 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 vapor retarder, or shall have a vapor retarder coating or covering in direct contact with the underside of the insulation.

5. Either Items 5.1, 5.2 or 5.3 shall be met, depending on the air permeability of the insulation directly under the structural roof sheathing.

5.1. Air-impermeable insulation only. Insulation shall be applied in direct contact with the underside of the structural roof sheathing.

5.2. Air-permeable insulation only. In addition to the air-permeable installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing as specified in Table R806.4 for condensation control.

5.3. Air-impermeable and air-permeable insulation. The air-impermeable insulation shall be applied in direct contact with the underside of the structural roof sheathing as specified in Table R806.4 for condensation control. The air-permeable insulation shall be installed directly under the air-impermeable insulation.

TABLE R806.4 INSULATION FOR CONDENSATION CONTROL

CLIMATE ZONEMINIMUM RIGID BOARD ON AIR-IMPERMEABLE INSULATION R-VALUEa
2B and 3B tile roof only0 (none required)
1, 2A, 2B, 3A, 3B, 3CR-5
4CR-10
4A, 4BR-15
5R-20
6R-25
7R-30
8R-35

[TABLE=width: 100%]

[TR]

[TD]a. Contributes to but does not supersede Chapter 11 energy requirements.[/TD]

[/TR]

[/TABLE]
 
You must log in or register to reply here.
Back
Top