Synthetic vs. Felt Underlayment: Which Is Better For You?

Jun 27, 2023

WRITTEN BY:

Moises V.

CATEGORY:

Uncategorized

POSTED ON:

June 27, 2023

UPDATED ON:

December 13, 2023

Got Questions?

Roof underlayment is a material that sits between your roof’s deck and roofing material. It acts as a secondary barrier that protects your roof and home from the elements, especially moisture and related damages.

Primarily, it helps in preventing leaks, mold, rot, and water damage that occur if the elements damage your roofing material. But it also helps to reduce heat transfer into your home and improve the performance of your HVAC system, saving you energy costs.

Common underlayment materials include asphalt-saturated felt and synthetic underlayment. In this article, we’ll compare them to help you make an informed decision in your next roofing project. Ready? Let’s begin.

What Is Felt Underlayment?

Roof frame of wooden beams on walls of hollow foam insulation blocks. Roofing underlayment, water-resistant waterproof barrier material.

Felt underlayment is a thick, heavy roll of material designed to provide a smooth, even surface to layer your roofing material. There are two common types out there—asphalt-saturated felt or felt paper.

It’s generally more cost-effective than synthetic underlayment, and it’s excellent at absorbing moisture and providing slip resistance during installation. However, it’s water-resistant, not waterproof, and is more susceptible to UV rays, tears, and punctures. It’s also heavy to handle during installation. 

It comes in two grades, No. 15 and No. 30. No. 30 is thicker, stronger, and less prone to tears and punctures but weighs more than No. 15. The latter is good enough for most roofing jobs, but for extra protection, No. 30 is more suitable.

Paper or fiberglass mat saturated with asphalt makes up asphalt felt. It provides decent water resistance and smoothness but can be susceptible to damage due to its brittle nature. 

On the other hand, felt paper is made of wood pulp and recycled paper and coated with asphalt or other waterproofing materials. It’s eco-friendlier than asphalt felt but can be less durable and more susceptible to moisture and UV rays damage.

What Is Synthetic Underlayment?

A variety of synthetic materials can make up synthetic underlayment. These include polypropylene, polyester, and fiberglass. Its durability and performance have made it popular among homeowners; it’s more resistant to tears, punctures, water, and UV rays than felt underlayment.

Synthetic underlayment is also light and easy to install, but it does have some problems you should be aware of

For example, it can trap moisture, leading to mold and rot development. It’s slippery and incompatible with a few roofing materials, such as metal roofs and cedar shingles. And, most importantly, it’s more expensive than felt underlayment. Its price point may make you hesitate if you’re budget-conscious!

Synthetic vs. Felt Underlayment

Waterproofing and insulation at construction site.

Below, we’ll compare and contrast both underlayments to give you a better idea of what will suit your situation best.

Cost

Despite coming in much thinner sheets and rolls than felt underlayment, synthetic underlayment is the more modern option with several advantages, justifying its higher price point. It costs from $0.17 to $0.25 per square foot for the material, depending on its brand and composition.

However, this isn’t to say that felt underlayment is terrible. It’s still popular among homeowners and widely used in small construction projects thanks to its cost-effectiveness and fair resistance. It provides a smooth and even surface for your roofing material and comes in thick and heavy rolls that cost $0.05 to $0.10 per square foot.

Labor costs will vary depending on your region and contractor. If you ask us, consulting a professional like Viking Contractors to get a precise quote for your situation is best.

But, generally, synthetic underlayment may be for you if you’re looking for premium material. It’s an excellent option for homeowners who value durability, reliability, and longevity.

Contrastively, if you’re budget-conscious and would rather provide your roof with a basic level of protection for a short time, then felt underlayment is an option.

Installation

Worker installing underlayment sheet on wooden roof of new house. Renovation, improvement for exterior residential by professional builder. Durable build and construction business.

Synthetic underlayment comes in thin sheets that go between your roof deck and roofing material. It’s lightweight and easy to handle on the job site, making its installation much easier than felt underlayment. Your contractor will roll it down on your roof to create a smooth and even surface, then nail it down using special fasteners.

However, synthetic underlayment must cover the edges of your roof and overlap with the roll to prevent any openings for water from leaking through.

Felt underlayment is much thicker and heavier than synthetic underlayment. This makes its installation somewhat more tedious. To install it, contractors first cover the edges of your roof with flashing, then roll the paper or fabric layer onto your roof and attach it with staples or nails. 

Again, it must cover the whole roof and overlap with the next roll, but it may also tear or wrinkle during installation. 

Both materials are relatively easy to install, but the felt underlayment’s thickness and heavy weight make it more challenging to handle than synthetic underlayment. However, since it’s cost-effective, it can be an alternative for homeowners on a budget.

Durability

Durability should be among your main concerns. After all, underlayment serves as a secondary barrier that protects your home from the elements; it must withstand harsh conditions and last long.

In this context, synthetic underlayment performs significantly better than felt underlayment. It’s more resistant to mold and mildew, safer to walk on, and lasts longer than its felt counterpart.

However, synthetic underlayment can be prone to trapping moisture if you don’t correctly install or ventilate it. Moisture is dangerous because it leads to the development of mold and rot. Also, despite being more resistant to UV rays, prolonged periods of exposure can degrade it, impacting its strength and effectiveness.

Other than that, you should mind the typical concerns of any roofing material. Excessive exposure to heat and subpar quality can contribute to delamination and degradation.

Felt underlayment can serve as a temporary roof if you don’t install shingles or tiles immediately. It provides a smooth surface for installation. Despite its weight and thickness, it’s more prone to tearing and puncturing in harsh weather conditions and during installation than synthetic underlayment.

It can also absorb moisture and wrinkle, affecting the performance of your roof. In high temperatures, it can deteriorate, dry out, or leach oils. 

Long story short, durability-wise, synthetic underlayments are better than felt in every aspect.

Energy Efficiency

Synthetic and felt underlayment plays a minor role in the energy efficiency of your roofing system. Their energy efficiency depends on the type, quality, and color of your roofing materials, the climate and weather conditions, and the ventilation and insulation of your attic.

Synthetic underlayment is generally more energy-efficient than felt underlayment. 

First, it’s lighter in color, meaning it can reflect more heat and reduce the temperature of your roof deck. It’s also less prone to absorbing water and can provide a smoother, even surface, reducing air gaps and heat loss. It can also maintain its performance longer thanks to its UV resistance.

But one advantage felt underlayment has over synthetic underlayment is that its thickness and heaviness allow it to provide more insulation and thermal mass to your roof system, meaning it can reduce heat transfer to your deck.

So, synthetic underlayment is better at reflecting heat, while felt underlayment is more suitable for insulation.

However, underlayment by itself doesn’t have a significant impact on the energy efficiency of your roof system. What’s important is that you invest in an energy-efficient roofing material. For example, dark-colored shingles will perform worse than light-colored ones in this aspect because lighter colors are better at reflecting heat.

Lifespan

Synthetic underlayment lasts longer than felt underlayment. Depending on its quality and thanks to its resistance, you can expect it to last between 20 to 40 years. 

Felt underlayment is more susceptible to UV and water exposure and can deteriorate if you don’t account for its weaknesses. On average, it lasts between 10 to 20 years.

Conclusion

Investing in a high-quality underlayment material is the best way to ensure your house is properly protected against the elements. 

The two popular underlayment options include felt and synthetic underlayments. Synthetic underlayments, while expensive, are the more modern option that excels in every aspect, from durability to ease of installation. 

If you’re able to afford it, opt for synthetic underlayments. However, felt underlayment is still ideal for budget-conscious owners or temporary roofing solutions. Call us for a free inspection if you can’t decide; we’ll tell you what suits your situation best for free.

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