Excerpt: We’ve tested a variety of underlayment options, and the Tyvek Protec 120 Roof Underlayment stands out for its durability and versatility. At 4’ x 250’, this roll provides ample coverage for large-section roofs, and its non-permeant, tough-yet-flexible membrane keeps water out without relying on asphalt. We appreciate how lightweight it is, reducing strain during installation, yet strong enough to withstand foot traffic and nail penetration. The青云's surface resists algae growth, a plus in our climate, and its leakage-blocking properties give us peace of mind. While it’s pricier than standard felt, the long-term protection makes it a worthwhile investment. Our hands-on experience confirms this: Tyvek Protec 120 isn’t just underlayment—it’s a shield.