Impermeable membranes effectively keep out wind-driven rain and water backed up behind ice dams, but also trap moisture that’s trying to escape from unvented attic spaces. Apply semi-permeable underlayments on inadequately vented roofs or provide ventilation before drying-in the roof.
Impermeable membranes effectively keep out wind-driven rain and water backed up behind ice dams, but also trap moisture that’s trying to escape from unvented attic spaces. Apply semi-permeable underlayments on inadequately vented roofs or provide ventilation before drying-in the roof.

Do Synthetic Underlayments Make for Better Roofs? Synthetic Roofing Underlayments

Asphalt-impregnated felt — aka tar paper — enjoyed a 100-year run as the preeminent roofing and siding underlayment. First made by soaking rag fibers in tar, the basis later became wood fibers mixed with asbestos and fiberglass for greater strength and economy. In response to oil scarcity in the 1970s, the oil content changed — and so did...

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