❄️ Ice Dams: Causes, Damage, and How to Prevent Them

CNHInspections
4 days ago
3 min read

When the right conditions exist, ice dams can develop on a home’s roof — leading to fascia and gutter damage, attic staining, moisture intrusion, mold growth and insulation damage.

Ice dams are not just a winter nuisance — they are a symptom of heat and air escaping from the home. Here’s what you need to know.

In the photo above from a previous inspection, the gutter and fascia board were backing out due to an ice dam.

What Is an Ice Dam?

An ice dam forms when:

Snow melts on the warmer upper portion of the roof
Meltwater flows downward toward the colder eaves
Water refreezes at the roof edge, forming a ridge of ice
Meltwater behind that ridge backs up under shingles and leaks into the home

Icicles don’t always mean damage is occurring, but they often indicate heat loss and refreezing at the roof edge.

Why Ice Dams Form

Ice dams occur when the roof deck warms unevenly. Contributing factors include:

Warm indoor air leaking into the attic (attic bypasses) – the most common cause
Insufficient insulation
Freeze–thaw weather cycles
Poor or blocked attic ventilation
Roof designs that trap snow (valleys, shallow eaves)
1½-story homes, which are notoriously difficult to insulate

Attic Bypasses: The Most Common Cause

Insulation contributes, but air leaks are the primary driver behind ice dam formation. Warm indoor air escapes into the attic through:

Chimneys
Attic access doors
Bathroom and kitchen fans
Plumbing vents
Recessed lights
Wall top plates

This warm air heats the roof deck → snow melts → meltwater refreezes at the cold eaves → ice dam.

In the photo above from a previous inspection, an unsealed chimney chase is allowing warm indoor air into the attic, directly contributing to roof warming and ice dam formation.

Ventilation: Helpful, But Not the Fix

Proper attic ventilation helps keep the roof cold, but ventilation alone will not prevent ice dams. If warm air is leaking into the attic or insulation is insufficient, heat will still reach the roof deck.

Once the attic is properly air-sealed and insulated, only modest passive ventilation is needed.

Important Building-Science Note: Insulation Is NOT an Air Sealer

The Building Science Corporation notes that attics should be air-sealed before adding insulation.

Insulation alone does not stop air movement, and without sealing attic bypasses, heat can still reach the roof deck and contribute to ice dam formation.

Insulation Challenges at Eaves

Many older homes simply don’t have enough space at the eaves for proper insulation, which allows warmth to escape and heat the roof.

Modern construction typically incorporates raised-heel trusses, which allow insulation to fully cover this area and help keep the roof cold.

Where eave space is limited, materials with a higher R-value per inch, such as closed-cell spray foam, are most effective. Focusing on the eaves, rather than the entire attic, can significantly improve performance.

Powered Attic Fans Can Make Ice Dams Worse

Powered attic fans exhaust air that must be replaced. If soffits are blocked or limited, that replacement air is pulled from the home through attic bypasses, drawing warm indoor air into the attic and heating the roof deck.

Best practice: Avoid powered attic fans. After air-sealing and improving insulation, rely on passive ventilation (soffits and a ridge vent).

Bathroom & Kitchen Exhaust Fans Venting in the Attic

Fans improperly terminating in the attic worsen conditions by pumping warm, moist air into the space.

In the photos above from previous inspections, exhaust fans are improperly terminating in the attic.

Ice & Water Shield: Helpful, Not Foolproof

“Ice and water shield” is a waterproof membrane installed at roof edges and valleys. It is useful, but:

It does not prevent leaks
It only helps resist leaks
If water backs up far enough, it can still enter the home

It is a first line of defense, not a guarantee.

How Homeowners Can Reduce Ice Dam Risk

Seal attic air leaks — This is the single most effective step in preventing ice dams.

If your home was built before ~1990, significant attic bypasses are likely. Work should be completed by a qualified insulation contractor.

📄 Building Science Air-Sealing Guide

Improve attic insulation

After bypasses are sealed, insulation helps keep heat inside the living space and off the roof deck.

Ensure proper passive ventilation

Cold air should enter through soffits and exit through ridge or gable vents. Ventilation helps, but only after sealing air leaks and addressing insulation.

Final Thoughts

Ice dams often seem like a roofing problem, but the root cause is usually inside the home. Understanding a few basic building-science principles — air sealing, insulation, and ventilation — can go a long way in preventing costly damage. If you’re unsure whether your home is at risk, a professional inspection can help identify problem areas before damage occurs.