15 lb vs 30 lb Felt For Siding: The Critical Choice You Can’t Afford to Get Wrong
You’re about to invest thousands of dollars into new siding. The last thing you want is for a simple, inexpensive component to undermine the entire project. This is the exact problem homeowners face when choosing a weather-resistive barrier, and it often comes down to a deceptively simple question: should you use 15 lb or 30 lb asphalt felt paper?
Making the wrong choice can lead to trapped moisture, wood rot, and a host of problems that negate the value of your beautiful new siding. This guide will break down the critical differences between these two materials, helping you understand which is the right shield for your home’s walls and ensuring your siding investment is protected for decades.
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What is Asphalt Felt and Why Is It Essential for Siding?
Before diving into the “15 vs. 30” debate, it’s crucial to understand what this material is and its role. Asphalt felt paper, also known as tar paper, is a time-tested material made from a base of organic or fiberglass mat saturated with asphalt. It serves as a secondary line of defense—a weather-resistive barrier (WRB)—sandwiched between your home’s structural sheathing (like plywood or OSB) and the exterior siding.
Its primary job is to block any bulk water that might find its way past the siding due to wind-driven rain or small installation imperfections. While your siding is the primary shield, no siding system is perfectly waterproof. Felt paper is the critical backup that ensures your wall structure stays dry, preventing rot and costly structural damage. A properly installed barrier is a key component in preventing moisture issues that can sometimes manifest indoors, so if you’re ever asking yourself “why is there a white haze in my house,” ensuring your exterior walls are protected from water intrusion is a good place to start.
The Core Differences: 15 lb vs. 30 lb Felt Explained
The names “15 lb” and “30 lb” are historical artifacts from a time when a 100-square-foot roll of the material literally weighed 15 or 30 pounds. While modern manufacturing has made them lighter, the naming convention stuck and still represents a clear difference in thickness and durability. The 30 lb felt is roughly twice as thick and heavy as its 15 lb counterpart.
This fundamental difference in thickness translates directly to performance, durability, and cost. Understanding these trade-offs is the key to making an informed decision for your home. Here is a direct comparison of their key attributes.
Key Attribute Comparison
Let’s break down how these two materials stack up against each other in the areas that matter most for a siding project.
| Feature | 15 lb Asphalt Felt | 30 lb Asphalt Felt |
|---|---|---|
| Thickness & Weight | Thinner and lighter, making it more flexible and easier to handle during installation. | Roughly twice as thick and heavy, providing a more substantial physical barrier. |
| Tear Resistance | More susceptible to tearing during installation, especially in windy conditions or on rough sheathing. | Significantly more resistant to tears and punctures. This is a major advantage during the construction process. |
| Water Resistance | Provides adequate water resistance for many applications and climates. | Offers superior water resistance due to its greater asphalt saturation and thickness. |
| Durability & Lifespan | Can degrade more quickly if left exposed to UV light before siding is installed. Shorter overall lifespan. | More robust and can withstand longer exposure times. Offers a longer service life behind the siding. |
| Cost | More economical and budget-friendly. | Commands a higher price, but the increase is often minimal in the context of an entire siding project. |
When is 15 lb Felt the Smart Choice for Siding?
While 30 lb felt offers superior durability, there are specific scenarios where 15 lb felt is a perfectly acceptable and intelligent choice. It meets the minimum requirements of many building codes and can be effective when used correctly. The International Residential Code, for instance, requires a “water-resistant barrier” behind exterior cladding, and 15 lb felt meets this standard.
Consider using 15 lb felt paper when:
- Budget is the primary concern: For projects with an extremely tight budget, the cost savings of 15 lb felt can be appealing.
- You are in a mild, dry climate: In regions that don’t experience frequent heavy rains or high winds, the moderate protection of 15 lb felt is often sufficient.
- Installation will be immediate: If the siding is going to be installed immediately after the felt paper, the risk of tearing from exposure is minimized.
Why You Should Seriously Consider Upgrading to 30 lb Felt
For the vast majority of homeowners, the modest additional cost of 30 lb felt is a worthwhile investment in long-term peace of mind. The enhanced durability provides a much larger margin for error during installation and superior protection over the life of your siding.
Upgrading to 30 lb felt is the superior choice for:
- High-Impact Weather Zones: If your home is in an area prone to hurricanes, strong storms, or wind-driven rain, the enhanced water and tear resistance of 30 lb felt is non-negotiable.
- Long-Term Durability: Think of it as cheap insurance. The heavier felt is far less likely to be damaged during installation, ensuring the integrity of your water-resistive barrier from day one.
- Protecting Premium Siding: If you are investing in expensive siding like fiber cement, cedar, or high-end vinyl, it makes little sense to skimp on the critical protective layer beneath it.
- Delayed Siding Installation: If there’s a chance the underlayment will be exposed to the elements for more than a few days, 30 lb felt will hold up much better against sun and wind.
The Modern Alternative: Felt Paper vs. Synthetic House Wrap
It’s important to acknowledge that asphalt felt is not the only option on the market today. Synthetic house wraps (like Tyvek) have become increasingly popular. These materials are made from woven polyethylene or polypropylene and offer some distinct advantages.
House wraps are significantly lighter, come in much wider and longer rolls (resulting in fewer seams), and often have higher “perm” ratings, meaning they are more breathable. However, traditional asphalt felt has its own strengths. It is a time-tested material that has successfully protected homes for over a century. Furthermore, felt has a unique ability to absorb a small amount of moisture and then release it, a process that some builders argue is beneficial for the wall assembly.
Installation Best Practices: Don’t Ruin Your Siding Project
Choosing the right material is only half the battle; proper installation is what truly makes a weather-resistive barrier effective. Whichever weight you choose, following these steps is critical for a watertight seal.
Always start at the bottom of the wall and work your way up. This ensures that each subsequent layer overlaps the one below it, creating a “shingle” effect that directs water down and out. This fundamental principle is the same one used for roofing, siding, and any exterior water-shedding system. Any home exterior work requires careful planning, much like when you’re trying to figure out how to hang lights on a steep roof; gravity is your best friend when you work from the bottom up.
Ensure horizontal seams are overlapped by at least two inches and vertical seams by at least six inches. Around windows and doors, the felt paper must be carefully integrated with flashing to direct water away from these vulnerable openings. Finally, use plastic-capped nails or staples for fastening, as the caps help seal the holes and prevent tearing.
Common Mistakes to Avoid (And How to Fix Them)
Even with the best materials, simple mistakes can compromise your home’s entire defense system against moisture. Be vigilant and avoid these common pitfalls.
The most catastrophic error is reverse lapping, which is installing the felt from the top down. This creates seams that funnel water into the wall rather than shedding it away. Another frequent issue is insufficient overlap between courses, which can allow wind-driven rain to penetrate the seams. Lastly, failing to repair any rips or tears that occur during installation is a critical oversight. Use flashing tape or a patch of felt paper to seal any damage before the siding goes on.
Frequently Asked Questions
Can I use roofing felt on walls?
Yes, absolutely. The 15 lb and 30 lb felt paper discussed here is often marketed as “roofing felt,” but it is the standard and appropriate material for use as a weather-resistive barrier under siding on walls.
How long can felt paper be exposed to sunlight?
Exposure to UV rays will degrade asphalt felt over time, making it brittle. As a general rule, 15 lb felt should be covered with siding as soon as possible, ideally within a week. The more durable 30 lb felt can typically withstand exposure for a longer period, sometimes up to a few weeks, but it should still be covered promptly for best results.
Is 30 lb felt paper a vapor barrier?
No, asphalt felt paper is considered vapor-permeable, not a vapor barrier. This is a crucial feature, as it allows water vapor that may enter the wall cavity from the home’s interior to escape and dry to the outside. This “breathability” helps prevent moisture from becoming trapped in the wall, which can lead to mold and rot. Trapped moisture inside wall assemblies can sometimes contribute to indoor air quality problems, such as a fog in the house when humidity levels are high.
The Final Verdict: Making the Right Choice for Your Home
When you weigh the evidence, the choice becomes clear. While 15 lb felt is a viable, code-compliant product suitable for budget-driven projects in mild climates, it offers a minimal margin for error. For a relatively small increase in material cost—often just a few hundred dollars on an entire project—30 lb felt provides a significant upgrade in performance.
For the vast majority of siding projects, 30 lb asphalt felt is the superior choice. Its enhanced tear resistance, superior water repellency, and overall durability provide invaluable protection and peace of mind. Investing in new siding is a major financial decision; choosing 30 lb felt is a small, smart insurance policy to protect that investment for years to come.
