Ice Dams and a Freezing Upstairs? Look at Your Knee Walls

Wed, 28 Oct 2026 14:50:43 GMT

If you live in a Cape Cod, a Philly-area twin with a finished third floor, or any house where the upstairs bedrooms tuck under the roof, you probably know the drill. Those rooms bake in July and freeze in January. And after the first good snow, you might see a row of big icicles hanging off the gutters while the neighbor's roof looks fine.

Those two problems usually have the same cause. Here's what's going on, what you can check yourself, and where spray foam actually makes a difference.

What a knee wall is (and why it matters)

In a story-and-a-half house, the upstairs rooms sit partly under the slope of the roof. The short wall on the side of the room, usually three to five feet tall, is the knee wall. Behind it is a little triangle of attic space, often with a small access door.

In a lot of older homes around Philadelphia and New Jersey, that's where the insulation story falls apart. Typically there's a thin fiberglass batt stapled to the back of the knee wall, sometimes falling off. The floor of that little attic space is open to the joists below, so cold air runs right under the bedroom floor. And the sloped ceiling above the room has a few inches of old batts squeezed in behind the drywall.

How that turns into ice dams

Ice dams aren't really a gutter problem. They start when warm air from the house leaks into the roof area and warms the roof deck. Snow melts on the upper part of the roof, the water runs down to the colder edge over the eaves, and it freezes again. Do that for a few days and you get a ridge of ice that holds back water. That water can work its way under shingles and show up as stains on your ceiling or wet spots in the wall.

Knee wall attics and sloped ceilings are a prime spot for this, because warm air leaks through them easily and there's very little room for insulation.

What you can check this weekend

Open the knee wall access door. Look for batts that have fallen down, gaps at the top and bottom, and an open path from that space into the floor framing.
Feel for drafts. On a windy day, hold your hand near outlets, baseboards, and the bottom of the knee wall in the upstairs rooms.
Watch the roof after a snow. If the snow melts in stripes or patches on the upper roof while it stays on the neighbors' roofs, heat is getting out.

Where spray foam helps

There are a couple of ways to handle a story-and-a-half house, and the right one depends on the house.

Option 1: Seal and insulate the roofline. We spray the underside of the roof deck in those side attics and, where we can get to it, the sloped sections. That brings the whole upstairs inside the insulated part of the house. It also seals off those open joist bays under the bedroom floor in the same step.

Option 2: Seal the knee walls and the attic floor behind them. Sometimes it makes more sense to foam the back of the knee walls and block off the floor joists behind them, then deal with the attic above separately.

For tight spaces like these, closed cell foam is usually our pick. We use PolyCon PC 2000 closed cell at R-7.1 per inch , so you get a lot of R-value where there isn't much depth to work with, and it blocks air in the same pass. Where there's more room, PolyCon PC 450 open cell at R-3.9 per inch can be a good, more budget-friendly option. We'll walk through it with you and tell you honestly which one fits.

A few things to know before you call

Fix active roof leaks first. If water is already coming in, get the roof repaired. Foam isn't a patch for a bad roof.
Access matters. Knee wall doors make the job easy. If the sloped ceilings are closed up with plaster or drywall, we'll talk about what can be reached and what might need to wait for a remodel.
Ventilation for the house. When you tighten up a home, it's smart to have your heating contractor check that the furnace, boiler, and water heater are venting the way they should.

Why now

Ice dams show up after the first real snow, and by then it's too late to do much this season except knock icicles down. Late fall is the time to get the upstairs sealed up, so you go into winter with warmer bedrooms and a roof that sheds snow the way it's supposed to.

About us

Sunrise Valley Spray Foam is an owner-operated company based in Morgantown, PA. An owner is on every job, so the people quoting your project are the same people doing the work. We travel regularly to the Philadelphia area and New Jersey for homes, barns, and commercial buildings.

If your upstairs is freezing or you've had ice dams in past winters, call or text us at 717-892-8212 , or hit the Free Quote button. We'll take a look and give you a straight answer.

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