Felt vs Synthetic vs Breathable Underlayment: Which Roof Underlayment Is Best for Your Home

Roof Underlayment Guide: Felt vs Synthetic vs Breathable

The underlayment is the layer between the roof decking and the shingles. It is the second line of defense against water after the shingles themselves. When shingles blow off in a storm or water drives under the shingle edges during heavy wind and rain, the underlayment is what keeps the water out of the attic. Artem Guskov of Foremost Exteriors explains the three categories of underlayment available today, what each one does well, and where each one falls short. The choice of underlayment affects the roof's ability to handle wind-driven rain, trapped moisture from inside the home, and the physical stress of installation before the shingles go on.

Foremost Exteriors owner Artem Guskov breaks down the three types of roof underlayment, felt paper, synthetic, and premium breathable systems, comparing tear strength, moisture handling, nail hold, and cost so homeowners understand what is going under their shingles and why it matters.

The reality Artem leads with is that the old standard, felt paper, is being phased out by better materials. The question for most homeowners is not whether to use felt but whether the upgrade from standard synthetic to a breathable premium underlayment is worth the additional cost. The answer depends on the climate, the attic ventilation, and how much moisture the home generates from the inside.

Felt Paper: The Old Standard and Why It Is Fading

Felt paper, also called tar paper, is asphalt-impregnated paper that comes in two main grades, 15-pound and 30-pound. It has been used under shingles for generations and is still sold at every roofing supply house. The advantage of felt is that it is inexpensive. The disadvantages are everything else. Felt tears easily, especially in wind during installation. It is heavy to carry up a ladder in rolls. It wrinkles when it gets wet. Its nail holding strength is the weakest of the three underlayment types, which matters because the underlayment is held in place by fasteners, and fasteners that pull through the material create gaps.

Artem describes felt as the paper towel option, meaning it provides a basic barrier but not much else. Most professional roofing contractors have moved away from felt as the standard underlayment for new roofs because synthetic products do everything felt does at a comparable price with none of the handling and durability problems. Felt is still used on budget installations where the homeowner wants the lowest upfront cost, but Artem generally does not recommend it given how much better the alternatives are for a modest price difference. The guide to comparing roofing estimates covers how to spot when a roofer is quoting felt underlayment versus synthetic and what the cost difference should be.

Synthetic Underlayments: Stronger, Lighter, but Not Breathable

Synthetic underlayments are made from engineered polymers, essentially a plastic-based material that is far stronger and lighter than felt. Synthetics resist tearing during installation and in high wind. They do not wrinkle when they get wet, which means the underlayment lays flat and stays flat before the shingles are installed. They provide better walkability for the roofing crew, which is a safety consideration during installation on a steep roof. Most synthetic underlayments carry longer exposure ratings, meaning the roof can be left with just the underlayment exposed to weather for a longer period before shingles are installed without the material degrading.

The downside Artem points out is that standard synthetic underlayments are not breathable. Most have a perm rating of 0.5 or less, which effectively makes them a vapor barrier. Moisture from inside the home, from cooking, showers, and daily humidity that rises into the attic, can become trapped under a synthetic underlayment. If the attic ventilation is inadequate or the roof deck has residual moisture at the time of installation, trapping that moisture under a non-breathable underlayment can lead to mold and rot in the roof deck over time. This is not a problem that shows up in the first year. It shows up years later when the decking is inspected during a reroof and the wood has been sitting in trapped moisture.

Premium Breathable Underlayments: The Best of Both

Premium breathable underlayments, which Artem refers to by common brand names like Deck Armor and Deck Defense, are synthetic underlayments engineered with breathability built into the material. They block water from getting in from the outside, the same way a standard synthetic does, but they allow moisture vapor from inside the attic to pass through and escape. They are over 300 times more breathable than a standard synthetic underlayment based on perm rating testing. The tear strength is approximately 600 times stronger than felt paper. The nail pull resistance, meaning how much force it takes to pull a fastener through the material, is over 200 percent higher than felt.

The practical benefit of a breathable underlayment is that the roof assembly can dry in both directions. Water from outside is blocked by the underlayment. Water vapor from inside can migrate up through the decking and out through the underlayment rather than being trapped against the underside of the decking. This is particularly important in climates with cold winters where warm humid indoor air meets a cold roof deck and condensation forms on the underside of the decking. A breathable underlayment lets that moisture escape. A non-breathable synthetic traps it. The roof ventilation types guide covers how attic ventilation and underlayment choice work together to keep the roof deck dry.

Fasteners Matter Regardless of Underlayment Choice

Artem mentions a detail that applies no matter which underlayment is chosen. Staples can tear through underlayment, creating hundreds of small holes across the roof surface. Each staple hole is a potential water entry point if water gets under the shingles. Plastic cap nails solve this problem by spreading the holding force across a wider area, preventing the fastener from tearing through the material. The cap nail itself also seals against the underlayment surface. On a roof with thousands of fasteners, the difference between staples and cap nails is the difference between thousands of small potential leak points and a properly sealed underlayment. Artem says his next video covers cap nails versus staples in more detail, and the cap nails vs staples roofing guide covers the same comparison for homeowners who want to understand the fastener decision before their roof goes on. Homeowners in the Savage, Minnesota area can reach Foremost Exteriors at foremostexteriors.com or 952-495-5545.

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