The summary is based on residences that are built to Chapter 11 of the 2006 IRC or to the 2006 International Energy Conservation Code. If the residences meet the energy code, they are classified as an unusually tight construction. With the building thermal envelope being on the underside of the roof by applying spray foam insulation and putting it on the rafters and roof decking, the attic is now conditioned space with no ventilation due to the fact it is just like being on the inside of your residence. If you bring in fresh air into the attic, it would just like you leave the front door of your residence open at all times.

With an unusually tight construction residence, all combustion air is to come from the outside and is not to communicate with the inside of the residence. Therefore, if fuel gas equipment, i.e. water heaters and mechanical furnace, a closet must be built around them with the door weatherstripped and with a self closer so the equipment can not communicate with the remainder of the attic.

Depending on what type of spray foam insulation is used, it may need to be covered up as listed below. From our research, all of the spray foam insulation I have found must either have the thermal barrier or ignition barrier if the attic is used for servicing of utilities. If any storage is done in the attic, a thermal barrier will be required.

Another item you need to look at is the roof. Some roof shingles manufacturers will not warrant their shingles if they are installed over the spray foam insulation. It will be the builders responsibility to provide the information as to whether or not the manufacturer will warrant the shingles. Also, it is required that a vapor retarder of 1 perm or less is required to be installed before the shingles installed. From my research roofing felt does not have a rating of 1 perm or less.

BUILDING THERMAL ENVELOPE. The basement walls, exterior walls, floor, roof and any other building element that enclose conditioned spaces.

CONDITIONED SPACE. For energy purposes, space within a building that is provided with heating and/or cooling equipment or systems capable of maintaining, through design or heat loss/gain, 50°F (10°C) during the heating season and 85°F (29°C) during the cooling season, or communicates directly with a conditioned space. For mechanical purposes, an area, room or space being heated or cooled by any equipment or appliance.

UNUSUALLY TIGHT CONSTRUCTION. Construction in which:

1. Walls and ceilings comprising the building thermal envelope have a continuous water vapor retarder with a rating of 1 perm (5.7 10-11 kg/Pa s m2) or less with openings therein gasketed or sealed.

2. Storm windows or weatherstripping is applied around the threshold and jambs of opaque doors and openable windows.

3. Caulking or sealants are applied to areas such as joints around window and door frames between sole plates and floors, between wall-ceiling joints, between wall panels, at penetrations for plumbing, electrical and gas lines, and at other openings.

M1701.1.1 Buildings of unusually tight construction. In buildings of unusually tight construction, combustion air shall be obtained from outside the sealed thermal envelope. In buildings of ordinary tightness, insofar as infiltration is concerned, all or a portion of the combustion air for fuel-burning appliances may be obtained from infiltration when the room or space has a volume of 50 cubic feet per 1,000 Btu/h (4.83 L/W) input.

M1702.3 Unusually tight construction. Where the space is of adequate volume in accordance with Section M1702.1 or Section M1702.2, but is within a building sealed so tightly that infiltration air is not adequate for combustion, combustion air shall be obtained from outdoors or from spaces freely communicating with the outdoors in accordance with Section M1703.

R806.4 Conditioned attic assemblies. Unvented conditioned attic assemblies (spaces between the ceiling joists of the top story and the roof rafters) are permitted under the following conditions:

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

2. An air-impermeable insulation is applied in direct contact to the underside/interior of the structural roof deck. "Air-impermeable" shall be defined by ASTM E 283.

Exception: In Zones 2B and 3B, insulation is not required to be air impermeable.

3. In the warm humid locations as defined in Section N1101.2.1:2

3.1. For asphalt roofing shingles: A 1-perm (5.7 × 10-11 kg/s m2 Pa) or less vapor retarder (determined using Procedure B of ASTM E 96) is placed to the exterior of the structural roof deck; that is, just above the roof structural sheathing.

3.2. For wood shingles and shakes: a minimum continuous 1/4-inch (6 mm) vented air space separates the shingles/shakes and the roofing felt placed over the structural sheathing.

4. In Zones 3 through 8 as defined in Section N1101.2, sufficient insulation is installed to maintain the monthly average temperature of the condensing surface above 45°F (7°C). The condensing surface is defined as either the structural roof deck or the interior surface of an air-impermeable insulation applied in direct contact with the underside/interior of the structural roof deck. "Air-impermeable" is quantitatively defined by ASTM E 283. For calculation purposes, an interior temperature of 68°F (20°C) is assumed. The exterior temperature is assumed to be the monthly average outside temperature.

 R314.4 Thermal barrier. Unless otherwise allowed in Section R314.5 or Section R314.6, foam plastic shall be separated from the interior of a building by an approved thermal barrier of minimum 0.5 inch (12.7 mm) gypsum wallboard or an approved finish material equivalent to a thermal barrier material that will limit the average temperature rise of the unexposed surface to no more than 250°F (139°C) after 15 minutes of fire exposure complying with the ASTM E 119 standard time temperature curve. The thermal barrier shall be installed in such a manner that it will remain in place for 15 minutes based on NFPA 286 with the acceptance criteria of Section R315.4, FM 4880, UL 1040 or UL 1715.

R314.5.3 Attics. The thermal barrier specified in Section 314.4 is not required where attic access is required by Section R807.1 and where the space is entered only for service of utilities and when the foam plastic insulation is protected against ignition using one of the following ignition barrier materials:

1. 1.5-inch-thick (38 mm) mineral fiber insulation;

2. 0.25-inch-thick (6.4 mm) wood structural panels;

3. 0.375-inch (9.5 mm) particleboard;

4. 0.25-inch (6.4 mm) hardboard;

5. 0.375-inch (9.5 mm) gypsum board; or

6. Corrosion-resistant steel having a base metal thickness of 0.016 inch (0.406 mm).

The above ignition barrier is not required where the foam plastic insulation has been tested in accordance with Section R314.6.

R314.6 Specific approval. Foam plastic not meeting the requirements of Sections R314.3 through R314.5 shall be specifically approved on the basis of one of the following approved tests: NFPA 286 with the acceptance criteria of Section R315.4, FM4880,UL1040 orUL1715, or fire tests related to actual end-use configurations. The specific approval shall be based on the actual end use configuration and shall be performed on the finished foam plastic assembly in the maximum thickness intended for use. Assemblies tested shall include seams, joints and other typical details used in the installation of the assembly and shall be tested in the manner intended for use.

So as you can see, it is not simply stating that you are going to use spray foam insulation.  It takes up a world of its own.