Ice Dam Risk Calculator
Work out whether your attic is warm enough to melt the snow on your roof, how fast, and which of air sealing, insulation or ventilation would actually stop it.
How to use this calculator
- 1Estimate the ceiling R-value from insulation depth: roughly R-3 per inch for blown fibreglass or cellulose.
- 2Be pessimistic about airtightness unless someone has actually sealed the ceiling. Recessed lights, plumbing stacks and open top plates all count, and they dominate the answer.
- 3Use an outdoor temperature in the 15 to 30 F band - that is where dams form. Below about 10 F almost nothing melts, and above freezing it all runs off.
- 4Read the fixes table rather than the headline. The useful output is which change moves the deck temperature on your roof.
How the calculation works
k_in = ceiling area / R_ceiling + 1.08 x CFM_leak
k_out = roof area / (R_deck + R_snow) + 1.08 x CFM_vent
T_attic = (k_in x T_in + k_out x T_out) / (k_in + k_out)
T_deck = T_out + (T_attic - T_out) x R_snow / (R_deck + R_snow)
Melt rate = (T_attic - 32) / R_deck / 143.4 lb per hour per square foot- k_in
- How readily heat gets from the house into the attic, in BTU per hour per degree F - conduction plus air leakage
- k_out
- How readily the attic sheds that heat - through the roof and its snow cover, plus ventilation air
- 1.08
- BTU per hour carried by one CFM of air per degree F of temperature difference
- R_snow
- About 1 per inch for settled snow. Deeper snow insulates the deck more and makes melting worse
- 143.4
- Latent heat of fusion of ice in BTU per pound - the energy that has to arrive to melt it
This is a steady-state balance: heat into the attic equals heat out of it, which fixes the attic temperature. Real attics lag and swing with sun and wind, but the steady-state answer is what decides whether dams form over a cold spell.
Once the deck reaches 32 F the temperature stops rising and the arriving heat goes into melting instead, which is why the melt rate uses the attic-to-freezing difference rather than the attic-to-outdoor one.
Air leakage is expressed as CFM per 1,000 square feet of ceiling because that is the form blower door results and air sealing guidance come in. A typical unsealed ceiling runs around 50; a well sealed one under 20.
Worked example
A typical 1970s house with R-30 and an unsealed ceiling
- 1.1,200 sq ft of ceiling at R-30 conducts 40 BTU/hr per degree; 60 CFM of leakage carries another 65 - so leakage is already the larger term.
- 2.The roof is 1,342 sq ft at a 6/12 pitch, and 8 in of snow adds R-8 on top of the R-1 deck, so the roof path sheds 149 BTU/hr per degree; partial ventilation adds 78.
- 3.Balancing those puts the attic at 35.2 F with 20 F outside, and the deck under the snow at 33.5 F.
- 4.Just above freezing - which is all it takes. The snow melts slowly and steadily, and the water refreezes at the overhang.
Result: Deck just above freezing - dams will form
The same house after air sealing
- 1.Nothing changes except the leakage: from 60 CFM down to 18.
- 2.That drops the heat entering the attic from 3,441 to 2,261 BTU an hour - a third less, without touching the insulation.
- 3.The attic falls to 30.0 F and the deck to 28.9 F - three degrees below freezing.
- 4.Air sealing alone fixed it, on a house where adding insulation over the leaks would have cost more and achieved less.
Result: Deck three degrees below freezing - sealing alone was enough
Why a deep snowpack makes dams worse
- 1.Identical to the first case except there is now 24 in of snow rather than 8.
- 2.The snow is roughly R-1 per inch, so the roof path goes from R-9 to R-25 - it sheds far less heat.
- 3.The attic gets warmer because it has lost its main escape route, and the deck warms with it.
- 4.This is why dams appear during a snowy spell rather than after one storm, and why raking the lower few feet of roof helps within hours.
Result: A deeper pack traps more heat against the deck and melts faster
The mechanism, and why the overhang is where it happens
An ice dam needs three things at once: snow on the roof, a roof surface above freezing over the heated part of the house, and an outdoor temperature below freezing. Take away any one and there is no dam.
Heat escaping into the attic warms the roof deck from beneath. Snow sitting on that deck melts at the interface, and the water runs down the roof under the remaining snowpack. When it reaches the overhang - the eave, which projects past the exterior wall and has no heated space under it - the deck there is at outdoor temperature, and the water freezes. The next water runs onto that ice and freezes further up. The dam builds uphill until it is high enough that meltwater ponds behind it, and standing water finds its way under shingles, which are designed to shed running water and not to hold back a pond.
The damage is rarely the ice itself. It is the water that gets behind it: soaked insulation, stained ceilings, rotted sheathing, and mould in the wall cavity below. Icicles hanging off a gutter are the visible symptom and are largely harmless in themselves; what they indicate is that meltwater is arriving at a freezing eave, which means the roof above is warm.
Why air sealing beats insulation
Insulation slows conduction. Air sealing stops convection. In most attics the second carries more heat than the first, and the difference is not close.
A ceiling with unsealed top plates, a few recessed lights, a plumbing stack and an uninsulated attic hatch can pass fifty or a hundred cubic feet of heated air a minute into the attic. That air arrives at room temperature and gives up its heat directly to the attic - the insulation is irrelevant to it, because the air is going around the insulation rather than through it. Worse, loose-fill insulation blown over an unsealed ceiling is air-permeable, so it hides the holes without closing them.
This is why a great many attic insulation upgrades disappoint. The homeowner adds a foot of cellulose, the ice dams continue, and the conclusion drawn is that the house is simply prone to them. The correct order of work is air seal, then insulate, then ventilate - and the first step is usually cheap, unglamorous and done on hands and knees with a caulk gun and a case of expanding foam.
- Attic hatch — Very often the single largest hole in the ceiling, and the easiest to fix with weatherstrip and a rigid insulated cover.
- Recessed lights — Old non-IC cans are deliberately ventilated into the attic. Sealed IC-rated LED retrofits close them.
- Top plates — The gap between drywall and framing runs the length of every interior wall. Caulk or foam, from the attic side.
- Chases and stacks — Plumbing vents, flues and wiring chases are often open straight to the basement. These are the ones that move real air.
- Bath fans ducted into the attic — Not a leak so much as a pump. Illegal nearly everywhere and still common.
The measures that manage rather than fix
Heat cable laid in a zigzag along the eaves does not prevent ice dams; it melts a channel through them so meltwater has somewhere to go. That is a genuine benefit on a roof that cannot easily be fixed - a cathedral ceiling with no attic to seal, a complicated valley, a north-facing wing that never sees sun. It costs real money to run through a winter, and it fails silently.
Roof raking removes the snow from the lower few feet of roof, which removes the insulating blanket over the eave and takes away the water supply. It works, it is cheap, and it should always be done from the ground - climbing onto a snow-covered roof is how people are seriously hurt.
Ice and water shield - a self-adhering membrane under the shingles at the eaves - does not stop dams either. It stops the water that backs up behind them from reaching the sheathing. Codes require it in cold regions for exactly that reason, and it is the reason a modern house with an ice dam often suffers no damage at all while a 1950s house with the same dam has a stained ceiling by February.
What this assumes, and where it stops
Assumptions
- Steady state: the attic has settled to a constant temperature for the outdoor conditions given. Real attics swing with sun, wind and cloud.
- Snow is taken at about R-1 per inch and 15 lb per cubic foot, which is settled snow. Fresh powder insulates more per inch and weighs less.
- The roof deck assembly with its air films is taken at about R-1, which is typical for sheathing plus asphalt shingles.
- Air leakage rates per 1,000 square feet of ceiling are representative bands rather than a measurement of your house. A blower door test with the attic isolated is the real answer.
Limitations
- It models a simple vented attic over a flat ceiling. Cathedral ceilings, knee walls, dormers and finished attics all behave differently and are commonly worse.
- It cannot see the specific faults that dominate many real cases: a bath fan discharging into the attic, uninsulated heating ducts, a warm chimney chase, or recessed lights in a vaulted section.
- Solar gain is ignored. A dark roof in February sun can melt snow with no heat loss at all, which is why south-facing slopes sometimes dam while north-facing ones do not.
- It assumes the snow cover is uniform. Drifts, valleys and the shade of a dormer all create local conditions the average does not describe.
- Melt rate assumes all the heat arriving at the interface goes into phase change, which is the right assumption once melting is established and an over-estimate at the margins.
Common questions
Why do I get ice dams when my neighbour does not?
Almost always heat loss rather than weather. The two houses see the same snow and the same temperatures; what differs is how much heat reaches the roof deck. Recessed lights, an unsealed attic hatch, ducts in the attic, or a finished room in the roof will each do it. Occasionally it is orientation - a south-facing slope can melt from sunlight alone in February.
Will more insulation stop ice dams?
Sometimes, and less often than people expect. In most attics air leakage carries more heat than conduction does, and insulation does nothing about a hole. Air seal the ceiling plane first - top plates, penetrations, the hatch, the light fittings - then insulate. Doing it in the other order buries the leaks and disappoints.
Do heat cables work?
They melt channels through the ice so the water can drain, which prevents the ponding that causes damage. They do not prevent the dam, they cost real money to run through a winter, and they eventually fail. They are the right answer on a roof that genuinely cannot be fixed - a cathedral ceiling, a complex valley - and the wrong first answer on an ordinary vented attic.
Is it safe to knock the ice off?
No, on two counts. Chipping at ice with anything hard damages the shingles underneath, and you will not know until it leaks in April. And doing it from a ladder set against an ice-loaded gutter, in winter, is genuinely dangerous. Raking snow off the lower roof from the ground is the safe intervention; the ice itself is best left alone unless a professional removes it with steam.
Why did the dams start after I had the roof redone?
Usually ventilation. A new ridge vent without matching soffit intake makes the attic draw warm air from the house, which puts more heat against the deck than before. Blocked soffits from an insulation top-up at the same time will do it too. It is worth checking that the intake and exhaust actually balance.
Sources
- Ice dams - causes, prevention and repair — University of Minnesota Extension
- Air sealing the attic - guidance and priorities — US Department of Energy
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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