Underlayment for Cold Climates and Ice Dam Protection

Heat escaping through the attic melts snow on the upper roof. The meltwater runs down and refreezes when it reaches the colder eave and overhang, which are outside the heated envelope. The ice builds into a ridge, and subsequent meltwater ponds behind it.

That ponded water sits against the shingle course and, because shingles are overlapping rather than sealed at the butt edge, finds its way underneath. What stops it there is the underlayment — and nothing else.

Cold-climate codes typically require a self-adhered ice barrier from the eave edge extending to a point inside the exterior wall line — that is, past the warm envelope boundary where the refreezing happens. The material is specified to ASTM D1970.

• Self-adhered bond — no fastener penetrations in the critical band.
• Nail sealability — where nails do penetrate, the membrane reseals around them.
• Low-temperature flexibility — the sheet must stay pliable when the deck is cold, not crack and debond.
• Near-zero permeance — intended here, because this is a barrier location rather than a drying location.

Outside the eave band, the field underlayment should remain vapour permeable so the deck has a drying path. Pairing a self-adhered eave membrane with a permeable synthetic field sheet is the standard cold-climate assembly — barrier where water ponds, permeability where it does not.

Ventilation and insulation are the upstream fix for ice dams and are outside the underlayment's scope, but they are worth flagging in any specification conversation: no membrane solves a badly vented attic indefinitely.