Ice dams in Central Ohio: a fall attic air-sealing checklist

Late September is the right time to think about ice dams. The attic is still comfortable to work in, and nothing done after the first real snowfall will help this winter. Here is a checklist built around the way Central Ohio houses leak.

How an ice dam forms

Heat escaping from the house warms the roof sheathing. Snow sitting on that warm section melts even when the air outside is below freezing, and the meltwater runs down to the eaves, which overhang unheated space and stay cold. There it freezes. Each melt-and-freeze cycle adds to the ridge of ice, and water pools behind it and works under the shingles. Building Science Corporation's ice dam digest names air leakage into the attic as the most common and most damaging cause, ahead of thin insulation, with sunshine on dark shingles adding to the melt.

Why Ohio builds for it

The Residential Code of Ohio records a history of damage from ice damming in every jurisdiction in the state and requires an ice barrier under shingle roofs: two layers of underlayment cemented together, or a self-adhering membrane, running from the lowest edge of the roof to a point at least 24 inches inside the exterior wall line. That membrane limits the damage when water backs up. It does nothing to keep the dam from forming.

Stopping ice dams is an attic job, not a roofing job.

The houses that ice up most around Columbus

A few house types show up again and again. Story-and-a-half Cape Cods and bungalows with finished upstairs rooms, common in Clintonville, Whitehall, and Grandview Heights, have knee walls and sloped ceilings that leak heat straight to the roof. Houses with complicated roofs, where valleys and dormers trap snow, melt unevenly. Vaulted great rooms and cathedral ceilings from the 1990s have shallow rafter bays with little room for insulation. And additions built on a lower, flatter roof often sit under a warmer part of the house. If yours fits one of these descriptions, start the checklist at the spot that matches.

The checklist

1. Find last winter's warm spots

Photos of the roof after a snowfall show where snow melted first. Bare patches over a bathroom, a hallway, or a room full of recessed lights mark the leaks below. Icicles bunched over one section tell the same story. If you have no photos, take some after the first snow this year and use them next fall.

2. Follow every bath fan duct

Go up with a flashlight and trace each bath fan duct. One that ends in the attic, lies loose on the insulation, or blows toward a soffit vent is pumping warm, wet air into the coldest part of the attic. Each fan should run through a sealed, insulated duct to a roof or wall cap with a damper.

3. Seal the chimney chase and the stacks

The gap around a masonry chimney or a metal flue where it passes through the ceiling is often wide open. It needs sheet metal and fire-rated sealant, never foam pressed against the flue. Plumbing vent stacks and wires passing through the top plates get sealed with foam.

4. Deal with recessed lights

Can lights in a top-floor ceiling behave like small chimneys. Fixtures rated for insulation contact and air tightness can be sealed and covered; older ones should be replaced with sealed fixtures or boxed in by someone qualified.

5. Weatherstrip and insulate the hatch

The Ohio code expects access doors from living space into an attic to be weatherstripped and insulated to the level of the surrounding surfaces. A rigid insulated cover over the hatch or pull-down stairs, with a gasket, closes one of the largest single leaks in many houses.

6. Protect the eaves

Baffles at every rafter bay keep insulation from blocking the soffit vents and let it reach over the top plate, which is where heat most easily reaches the cold eave. On a low-slope roof this is the hardest spot to get depth, and the code lets R-38 count as R-49 only where its full height extends over the wall top plate.

7. Then add depth

Only after the sealing is done, bring the attic floor up toward the R-49 the code sets for new Zone 5 attics. Loose fill over unsealed leaks keeps the leaks working.

8. Or move the insulation to the roofline

If the attic holds ductwork or an air handler, as it often does above the vaulted great rooms of 1990s houses in Gahanna and Westerville, moving the insulation to the underside of the roof deck takes that heat source out of the attic entirely. The roof deck or attic floor FAQ explains the choice.

What not to rely on

Heat cable melts channels through a dam and draws electricity every winter; treat it as a stopgap. Raking snow off the lower roof reduces the fuel for a dam but has to be repeated after every storm. Bigger gutters change nothing, because the dam forms above them. Adding attic vents without sealing the leaks can pull even more warm air out of the house.

If a dam is already forming

Keep downspouts clear, use a roof rake from the ground if it is safe to do so, and watch ceilings and the tops of exterior walls for stains. Do not chip ice off shingles. Then put the attic on the list for spring or next fall, when the checklist can be worked properly. The ice dam FAQ covers the physics, and the attic service page describes the work in detail.