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RE: Roof flashing leak at ridge of cathedral ceiling (Follow-Up #1)

posted by: manhattan42 on 03.08.2008 at 09:51 am in Home Repair Forum

I'm not sure what 'literature' you are referring to, but ventilating rafter spaces in a cathedral ceiling is mandatory under building codes.

If you do not vent the rafter spaces, you will trap water vapor from the room below. If you do not vent the rafter spaces, your roof will be hotter than it should be.

The result of an unventilated cathedral ceiling will be the complete failure of the entire roof system and premature shingle failure.

The solution without ventilation, could result in needing the entire roof system demolished and rebuilt...I have seen it happen.

So find some way to maintain your ventilation while solving your leak problems.

NOTES:

Sorry for getting here 4 years late :) Present (2010) IRC Building Code (NY State) allows spray foam in an unvented rafter bay. See below:

SECTION R806 ROOF VENTILATION

R806.1 Ventilation required. 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. Ventilation openings shall have a least dimension of 1/16 inch (1.6 mm) minimum and 1/4 inch (6.4 mm) maximum. Ventilation openings having a dimension greater than 1/4 inch (6.4 mm) shall be provided with corrosion-resistant wire cloth screening, hardware cloth, or similar material with openings having a minimum dimension of 1/16 inch (1.6 mm) minimum and 1/4 inch (6.4 mm) maximum. Openings in roof framing members shall conform to the requirements of Section R802.7.

R806.2 Minimum area. The total net free ventilating area shall not be less than 1/150 of the area of the space ventilated except that reduction of the total area to 1/300 is permitted provided that at least 50 percent and not more than 80 percent of the required ventilating area is provided by ventilators located in the upper portion of the space to be ventilated at least 3 feet (914 mm) above the eave or cornice vents with the balance of the required ventilation provided by eave or cornice vents. As an alternative, the net free cross-ventilation area may be reduced to 1/300 when a Class I or II vapor barrier is installed on the warm-in-winter side of the ceiling.

R806.3 Vent and insulation clearance. Where eave or cornice vents are installed, insulation shall not block the free flow of air. A minimum of a 1-inch (25 mm) space shall be provided between the insulation and the roof sheathing and at the location of the vent.

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 (Class I or II) 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 and 6 any air-impermeable (eg.- spray foam) insulation shall be a Class III 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 (eg. spray foam) insulation only. Insulation shall be applied in direct contact with the underside of the structural roof sheathing.

5.2. Air-permeable (eg. fiberglass or cellulose) insulation only. In addition to the air-permeable insulation installed directly below the structural sheathing, rigid board 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 (eg. spray foam) 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 (eg. fiberglass or cellulose) insulation shall be installed directly under the air-impermeable (eg. spray foam) insulation.

TABLE 806.4 INSULATION FOR CONDENSATION CONTROL

CLIMATE ZONE MINIMUM RIGID BOARD OR AIR-IMPERMEABLE INSULATION R-VALUEa
4 R-15
5 R-20
6 R-25

clipped on: 08.22.2012 at 12:58 pm    last updated on: 08.22.2012 at 01:06 pm