Which is better felt or synthetic underlayment?

When installing new roofing, an underlayment provides an extra layer of protection between the roof deck and the shingles. The two most common types of underlayment are felt and synthetic materials. Both have advantages and disadvantages in terms of cost, durability, and ease of installation. This article examines the key differences between felt and synthetic underlayments to help determine which is the better choice for your roofing project.

What is roofing underlayment?

Roofing underlayment is a layer of material installed on the roof deck before the shingles are laid. It serves several important functions:

• Creates a water-resistant barrier to protect the roof deck from moisture penetration
• Helps prevent wind-driven rain from blowing up under the shingles
• Provides an extra layer of protection in case of damage or leaks in the shingles
• Creates a smooth, uniform surface for easier shingle installation

Underlayment is typically made from asphalt-saturated felt paper, non-asphalt synthetics like polyethylene, or rubberized asphalt membranes. The two most common options for sloped roofs are felt and synthetic underlayments.

What is felt underlayment?

Felt underlayment consists of a fiber mat or scrim that is saturated with asphalt. Traditional versions were made from organic materials like wood fiber or cellulose, but most modern felts use a fiberglass mat. Asphalt saturation makes the felt resistant to moisture.

Felt is available in different weights and grades. Heavier and higher grades provide more durability. Common types include:

• #15 felt – The lightest weight felt at about 18 pounds per 100 square feet
• #30 felt – Heavier and more durable at about 30 pounds per square
• Type A – Heaviest grade, designed for use in critical areas like eaves, valleys and roof edges

Felt underlayment is an affordable option, costing $15-40 per roll. It conforms well to the roof deck and is easy to cut and install using staples, nails or cap nails. However, felt is not very tear- or UV-resistant, so it requires care during installation and may deteriorate more quickly than synthetic alternatives.

What is synthetic underlayment?

Synthetic underlayments are made from woven or non-woven polyethylene, polypropylene or polymer fabrics. The material is coated, laminated or saturated to make it water-resistant. Common brand names include Tyvek, Titanium and RoofTopGuard.

Synthetic underlayment offers several advantages compared to traditional felt:

• Lighter weight for easier installation
• More tear-resistant
• Increased UV resistance and durability
• Can be installed under a wider temperature range
• May meet code requirements as an ice barrier membrane

Synthetic underlayments typically cost $100-200 per roll. They require plastic cap nails or tin tags to prevent tearing during installation. Synthetics may not conform as well to the roof deck as felt.

How do the costs compare?

Felt underlayment is significantly less expensive than synthetic underlayment. Here’s a breakdown of relative costs:

Underlayment Type	Average Cost per 100 sq ft Roll
#15 Asphalt Felt	$15-25
#30 Asphalt Felt	$25-40
Synthetic Polyethylene	$100-130
Synthetic Polypropylene	$130-170
Rubberized Asphalt Membrane	$150-200

For a typical 20 square roof, felt underlayment may cost $300-600 compared to $2000-3400 for a synthetic version. The higher upfront cost of synthetics may be justified for roofs in harsh climates or critical applications, but felt is usually adequate for typical installations.

How do felt and synthetic underlayments compare in terms of durability?

Synthetic underlayments are generally more durable and longer-lasting than traditional felt products. Here are some key comparisons:

• Tear strength – Synthetics made from woven or non-woven fabrics are much more tear-resistant than felts.
• Temperature resistance – Synthetics can withstand hotter summer temperatures (up to 250°F) compared to felt.
• UV resistance – Synthetics are designed to resist ultraviolet degradation for up to 6 months of exposure compared to 30-60 days for felt.
• Rot and mildew – Synthetics resist rot and mildew growth better than organic-based felts.
• Wind uplift – Synthetic underlayments typically meet the same wind uplift ratings as felt when properly installed.
• Foot traffic – The woven structure of synthetics makes them more resistant to damage from foot traffic.

Premium-grade felt designed for durability can still perform well. But overall, synthetics maintain their integrity and water resistance longer when subjected to sun, wind, traffic and other elements.

How do felt and synthetics compare for ice barrier use?

In cold climates with potential for ice dams, underlayment that doubles as an ice barrier helps protect vulnerable roof areas like eaves, valleys and overhangs. Felt alone does not provide reliable ice barrier protection. Some options for ice barriers include:

• Self-adhering rubberized asphalt membrane – Membranes provide the best ice barrier protection and are compatible with both felt and synthetic overlays.
• Synthetic underlayment meeting ASTM D1970 – Some heavy duty synthetic underlayments are rated as ice barrier equivalents when installed properly.
• Modified bitumen strips – These asphalt-saturated strips help but work best when paired with a full membrane or ASTM-rated synthetic.

For roofs prone to ice dams, a full rubberized ice barrier membrane is recommended. But ASTM-rated synthetics can also perform if installed carefully according to code requirements. Standard felt alone is not sufficient as an ice barrier.

How do felt and synthetic underlayments compare for installation?

Good underlayment installation comes down to following the manufacturer’s instructions. But there are some differences in working with felt versus synthetics:

• Nailing – Felt can be installed with standard roofing nails while synthetics require plastic cap nails or metal tin tags to prevent tearing.
• Overlap – More overlap is required between layers of synthetic to shed water properly.
• Conformability – Rigid synthetics may not conform as tightly to the roof deck as flexible felt.
• Sealing – Synthetics may require an additional tape seal at seams, edges, tears, and nails.
• Stretch-in – Felt can be stretched in but synthetics are pre-cut requiring more precision.

Synthetics require more care during installation but are not necessarily harder to work with than traditional felt products. The key is proper training and following the manufacturer’s application methods.

Conclusion

For most roofing jobs, inexpensive #30 asphalt felt provides adequate performance as an underlayment. But synthetic underlayments offer increased durability and may be preferred in regions with extreme climates or installations that demand the highest performance. Synthetics also provide reliable ice barrier protection.

While synthetics have a considerably higher upfront cost, their added longevity may justify the investment on premium homes or in challenging conditions. They provide an extra degree of protection against rain, wind uplift and UV exposure. For critical roof areas like eaves and valleys, a hybrid approach combining rubberized membrane strips with synthetic underlayment offers the best all-around performance.

In the end, regional building codes often dictate the required underlayment standards. Both felt and synthetics remain good options with their own sets of pros and cons. Careful product selection and proper installation is key to success when choosing an underlayment for your next roofing project.