The frustrating case
Ice dams are a heat problem rather than a roof problem — meltwater from a warm roof deck running down to a cold overhang and freezing there. We have written that out twice already, in ice dams are an insulation problem and in more detail in why they form and how to stop them. If you are starting from scratch, start there.
This page is for the other call. You already insulated the attic. The ice came back anyway. That is a genuinely common outcome and it is not bad luck. There are four usual reasons, and the uncomfortable part is that three of them mean the insulation went in on top of the actual problem.
1. The insulation went in before the air sealing
This is the big one and it is an order-of-operations failure, not a quality one.
Insulation slows heat that is conducted — heat moving through a material. It does almost nothing about heat that is convected — warm air physically pouring through a gap. And a house loses a great deal of heat the second way: up through the gap around a chimney chase, through the holes drilled for wiring and plumbing, around recessed light cans, through an unsealed attic hatch.
Blow sixteen inches of cellulose over all of that and the gaps are still gaps. Warm air still moves through them at the same rate; it now does so under a thick blanket where nobody can see it, feel it, or reach it without digging the insulation back out.
So the sequence is air seal first, insulate second, and getting it backward is worse than doing nothing, because the second attempt now costs what the first one did plus the cost of undoing it.
How to tell this is what happened
Go into the attic with a flashlight and look at the surface of the insulation. Patches that are gray or darker than the rest are dust filtered out of air that has been moving through it for years. Insulation is an accidental air filter, and the dirty spots mark the leaks precisely. That stain is the single best diagnostic in the building and it costs nothing to look at.
2. A bathroom or kitchen fan vents into the attic
Pleasantly simple, remarkably common, and about the most efficient way there is to make ice dams worse.
A bath fan is a device for moving warm, wet air. If its duct ends in the attic instead of passing through the roof or the wall to a hood outside, it delivers warm wet air directly against the underside of the roof deck every time somebody showers. That is heat on the deck and moisture on the deck, which is ice dams plus the beginnings of a mold problem in the sheathing.
Sometimes the duct was never run outside. Sometimes it was, and it came off the hood, and the flexible duct is now lying in the insulation blowing into the open attic. Both look identical from the bathroom, where the fan sounds like it is working.
Check it from outside. Stand back and find the hood for every fan in the house. No hood means it goes into the attic.
3. The new insulation buried the soffit vents
A vented attic works because cold outside air comes in at the eaves, washes up under the roof deck and leaves at the ridge. That airflow is what keeps the deck cold, and a cold deck is the entire objective.
The eave is also the one place in an attic where there is very little room — the roof is converging on the top of the wall. It is the easiest place in the building to over-fill with insulation, and a crew working fast with a blower will fill it. Now the air cannot get in at the eave, the deck warms up, and you have paid to make the roof warmer at precisely its coldest end.
The fix is baffles: rigid channels stapled to the deck at each rafter bay that hold the air path open from the eave upward, installed before the insulation goes in. From inside the attic you can see whether they are there. If insulation is packed solid into the eaves with no channel above it, the ventilation is blocked.
4. The attic hatch was never sealed
Everybody insulates the attic floor and a great many people leave a two-by-three-foot hole in the middle of it.
A typical hatch is a sheet of plywood resting in a frame with nothing on top of it and no weather stripping around it. That is a direct opening between the warmest air in the house and the attic, and warm air is going up. One uninsulated, unsealed hatch can undo a meaningful fraction of the insulation around it.
It is also the cheapest item on this entire page to put right: weather strip the frame, a latch to pull the hatch down against it, and rigid insulation fixed to the top of the panel.
The two things not to do
Heat cable
Not installed here, and it is worth saying why rather than just declining. Heat cable melts a channel through the ice using your electricity, all winter, every winter, for as long as you own the house. It does nothing whatever about the heat loss causing the dam. It treats the symptom forever instead of the cause once, and on a roof that is already losing heat it is an odd thing to pay a power bill for.
Chipping the ice off
Nobody is sent onto a roof to break ice off. It damages shingles, it is dangerous on a surface that is by definition covered in ice, and it does not stop it happening again next week. Raking snow off from the ground is a reasonable stopgap during a bad winter — it reduces the snow available to melt — but it is not a fix and done carelessly it takes granules off the shingles.
What to do this week
- Photograph the roof edge from the ground and note which direction that side of the house faces. A dam on one elevation only is a strong clue.
- Look for gray patches in the attic insulation. Those are your leaks, marked for you.
- Find every fan hood from outside. Count them against the fans inside.
- Look at the eaves from inside the attic. Packed solid with no baffle means the venting is blocked.
- Put your hand on the attic hatch from the house side on a cold day.
- If water is coming in right now, that is a leak today — catch it, stay off the roof, and deal with the cause when the weather breaks.
Shingle replacement and steep-slope roofing are not taken on here and are referred out. Where a dam has already put water into the structure, the structural question is somebody else's trade as well. What is handled is the attic — the sealing, the depth, the baffles, the fan ducts — and the ceilings and trim the water already ruined.
Book it
Book a free quote · call 517-505-7135 · or read more about ice dam prevention.
In order: air sealing first, then attic insulation. Want it measured rather than guessed? Home energy assessment. How much insulation you have now: measure it yourself. Who pays: energy rebates and grants.
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