12.2 Balanced Airflow, Vent Installation Practice & Preventing Ice Dams

How Attic Airflow Works

Attic ventilation depends on wind and the stack effect. Cooler outside air enters low, at the eave (intake) vents, absorbs heat and moisture, rises, and leaves through high (exhaust) vents near the ridge. For this to work:

• Intake at the bottom and exhaust at the top, on each separate attic space (R806.1 requires cross ventilation for each separate space).
• Enough intake: R806.2's balanced option caps upper vents at 50 percent of the required area. Many manufacturers recommend at least as much intake as exhaust. If exhaust is larger than intake, the exhaust vents may pull air from the house through ceiling leaks.
• An open air path: insulation, blocking, or a closed soffit must not cut off the eave vents.

Don't Mix Exhaust Types

When two kinds of exhaust vents serve the same attic, such as a ridge vent and gable louvers or box vents, the higher or stronger one can pull its air through the other exhaust vent instead of from the soffits. Air then short-circuits across the top of the attic and leaves the lower attic unventilated. Weather can also enter through the vent acting as intake. Most shingle and vent manufacturers therefore say to use one type of exhaust per attic space. When adding a ridge vent, close off gable louvers and box vents on that attic space, as the manufacturer directs. This is industry practice rather than a written code rule, but R806.4 requires installation per the manufacturer's instructions.

Installing the Vents

Soffit and Eave Intake

• Remove paint, blocking, or insulation that plugs existing soffit vents.
• Install eave baffles (rafter vents) in every rafter bay with a vent. N1102.2.3 requires a baffle in vented attics with air-permeable insulation (such as blown fiberglass or cellulose) at the eave vents. It must keep an opening at least as large as the vent and extend over the top of the insulation. R806.3 requires at least a 1-inch air space between insulation and sheathing at the vent.
• Where there is no soffit, use a vented drip edge or a roof-deck intake vent installed per its instructions, and confirm that the NFVA is enough.

Ridge Vents

1. Mark and cut the ridge slot to the width the manufacturer specifies on each side of the ridge. Stop the slot short of the gable ends and of hip or chimney intersections as directed. Do not cut rafters or the ridge board.
2. Install the vent, with its external baffle or filter, over the slot, and nail it with the longer nails the maker specifies to reach through the vent into the deck.
3. Cap it with ridge cap shingles made for that purpose, using the specified nail length.
4. In Michigan, choose a ridge vent with an external baffle or filter rated for wind-driven snow and rain. Snow infiltration is a common complaint with poorly baffled vents.

Static and Turbine Vents

• Place them within 3 feet of the ridge so they count as upper vents.
• Flash them like any penetration. The lower flange goes over the shingles below, and the shingles above cover the upper flange (R806.4 and R903).

Powered, Solar, and Whole-House Fans

• Powered attic ventilators (PAVs) are not required by the MRC. A PAV lowers the attic pressure. If the ceiling leaks, it pulls conditioned air and moisture from the house into the attic, which raises energy use. In a tight or leaky house, that suction can also contribute to backdrafting of chimneys and atmospherically vented appliances. Many building scientists and shingle manufacturers discourage PAVs, and some shingle warranties say not to combine them with ridge vents. Seal the ceiling first if a PAV is used.
• Solar attic fans have the same issues on a smaller scale.
• Whole-house fans pull house air into the attic to cool the home. M1501.1 allows whole-house ventilation attic fans to discharge into the attic of a dwelling with a private attic. They need large attic vent area, often several times the normal NFVA, sized by the fan maker.
• Wiring for fans is electrical work for a licensed electrical contractor, unless an exemption applies.

Ice Dams: Cause and Prevention

An ice dam forms when heat from the house warms the upper roof, snow melts, and the water refreezes at the colder eave overhang. Water then pools behind the ice and can back up under the shingles. Michigan's heavy snow makes ice dams common. Prevention works in this order:

1. Air seal the ceiling plane. Most attic heat and moisture arrive by air leakage through recessed lights, top plates, chimney and duct chases, plumbing and wiring holes, and attic hatches. N1102.2.4 requires attic hatches to be weatherstripped and insulated.
2. Insulate to at least R-38 in zone 5A and R-49 in zones 6A and 7 (Table N1102.1.1), keeping full depth over the wall plates where possible.
3. Ventilate with continuous soffit intake and ridge exhaust, so the roof deck stays close to outdoor temperature.
4. Exhaust fans must discharge outdoors, never into the attic (M1501.1 and M1507.2). Bath exhaust into an attic is a leading cause of attic frost.
5. Protect the roof anyway. The ice barrier required at Michigan eaves (Section 8.3) keeps backed-up water out of the building if an ice dam forms. It does not prevent the ice dam itself.

Dealing With an Existing Ice Dam

• Don't chip at ice with axes or shovels, because that damages shingles. Professional removal uses low-pressure steam.
• Roof rakes from the ground remove snow near the eaves.
• Heat cables are a symptom treatment. Install listed cables per the maker, and leave the electrical work to the right trade.
• Diagnose the cause in the attic, and recommend air sealing and insulation to the owner in writing.