Two losses, and the smaller one gets all the attention
Heat leaves a ceiling two ways. Conduction goes through the assembly and is set by area, temperature difference and R-value: 1,400 square feet at R-38 with 55 degrees across it is 1,400 times 55 divided by 38, which is 2,026 BTU an hour. Air leakage carries heat bodily through holes, and the standard arithmetic is 1.08 times the airflow in cfm times the temperature difference. That 1.08 is not arbitrary — it is air at about 0.075 pounds per cubic foot, times a specific heat of 0.24 BTU per pound per degree, times 60 minutes in an hour.
At 120 cfm and the same 55 degrees, leakage is 1.08 times 120 times 55, or 7,128 BTU an hour. That is three and a half times the conduction loss, from a hole area most people would struggle to point at. Total: 9,154 BTU an hour, of which 78 percent is air.
| What you change | Heat off the ceiling | Cut |
|---|---|---|
| As it is — R-38, 120 cfm | 9,154 BTU/hr | baseline |
| Insulation to R-60 | 8,411 BTU/hr | 8% |
| Air sealing to 60 cfm | 5,590 BTU/hr | 39% |
| Both | 4,847 BTU/hr | 47% |
Adding 22 R-values of insulation buys eight percent. Halving the leakage buys thirty nine. That ratio is the reason air sealing is done before insulating rather than after, and it is also the reason a house that got a foot of new blown insulation still has a dam the following February.
From BTU to pounds of snow
Melting ice takes 144 BTU per pound, and that figure does not depend on temperature — it is the latent heat of fusion, the energy needed to break the solid apart at 32 degrees without the temperature moving at all. Snow colder than freezing has to be warmed first, at roughly 0.5 BTU per pound per degree. Snow at 20 degrees therefore costs 144 plus 6, or 150 BTU a pound.
Take half of that 9,154 BTU an hour as reaching the deck — the rest going out the vents — and it is 4,577 an hour, or 109,852 BTU a day. Divide by 150 and you get 732 pounds of snow melted a day, which is 88 gallons of water. It runs down the slope, reaches the overhang where there is no heated space underneath, and freezes. Then the next day does it again behind the ice that formed yesterday.
Spread over the ceiling footprint, 732 pounds is only a tenth of an inch of water equivalent a day, which sounds trivial and is exactly why this happens quietly. Nothing dramatic occurs on any one day. It is a slow tap that runs for weeks.
Ventilation dilutes; it does not fix
The share-reaching-the-deck field is where attic ventilation lives in this model, and it is deliberately a percentage rather than a calculation, because turning vent area into a deck temperature honestly requires wind, stack effect and a real attic geometry. What is worth understanding is the direction: ventilation removes heat that has already got into the attic. It is the third line of defence, after keeping the heat in the house and after keeping the air in the house.
An attic where the soffit vents are stuffed with insulation has no intake, which turns the ridge vent into an exhaust drawing conditioned air out of the house through every ceiling hole it can find. That is a case where adding insulation made the dam worse. Net free area and the intake-to-exhaust balance are worked out on the attic ventilation calculator; what the ceiling assembly is actually worth in R-value is the assembly R-value calculator, and how much material a top-up needs is the attic insulation top-up calculator.
What the ice tells you and what it does not
Ice at the eave is evidence of heat, and it is also evidence that water has already been where it should not be. Water ponded behind a dam does not sit politely on top of the roof covering; it works back under it, and asphalt shingles are shedding surfaces rather than waterproof ones. So the visible ice is a late symptom of two things: a warm roof deck, and an assembly that has been wet.
Not every icicle is a dam and not every dam is heat loss. Sun on a south slope melts snow on a perfectly cold roof, and it refreezes at the eave for exactly the same reason. A dam that appears on one slope only, in the same place every year, over a particular room, is a different story from one that appears everywhere after a sunny cold snap. The one that follows a bathroom or a stairwell is the one this page is about.
The energy side of the same leakage, priced across a whole heating season, is on the air sealing payback calculator, and the whole-house version of the heat loss arithmetic is the heat loss calculator. Fuel use over a real winter rather than a fixed number of days is the heating degree day calculator.
Questions people ask
What causes ice dams?
Heat reaching the underside of the roof deck over heated space, melting snow that then runs to the unheated overhang and freezes there. Two sources feed it: conduction through the ceiling insulation and warm air leaking through holes in the ceiling. On the default figures — 1,400 square feet at R-38, 120 cfm of leakage, 55 degrees of difference — leakage carries 78 percent of the total, which is the usual proportion and the usual surprise.
Will adding attic insulation stop an ice dam?
By itself, usually less than people expect. Going from R-38 to R-60 on the default ceiling cuts total heat off the ceiling by eight percent, because the conduction it improves was only 22 percent of the loss to begin with. Halving air leakage cuts thirty nine percent. Insulation blown over an unsealed ceiling also buries the holes, which makes them harder to find later without making them smaller.
How much snow does attic heat loss actually melt?
On the defaults, about 732 pounds a day, or 88 gallons of water, assuming half the heat leaving the ceiling reaches the deck. Melting takes 144 BTU a pound as latent heat plus about half a BTU per pound per degree to warm snow up to freezing first. Spread over the ceiling footprint that is a tenth of an inch of water a day, which is why it is invisible until there is ice at the gutter.
Does roof ventilation prevent ice dams?
It helps by carrying away heat that has already entered the attic, which makes it the third measure rather than the first. Keeping heat in the house and air in the house come before it. Ventilation also fails silently: soffit intakes packed with insulation leave a ridge vent pulling air out of the house through ceiling penetrations, which is a case where more insulation made the problem worse.
Can I remove an ice dam myself?
This page gives no procedure and does not suggest one. Winter roof work puts ice, a ladder on frozen ground, a steep pitch and no fall protection together, and people are killed at it every year. Chipping at a dam damages the covering underneath while you stand on the part that is already wet, and heat cable is a live circuit on a wet roof. Clearing and steaming are done by contractors who do that work; the electrical side belongs to a licensed electrician.