Tar paper, also known as building felt, asphalt-saturated felt, or rosin paper, has been used as an underlayment beneath wood flooring for over a century. While modern synthetic underlayments have gained popularity, tar paper remains a cost-effective and proven material for reducing moisture transmission, cushioning the floor, minimizing squeaks, and providing a slip layer that allows wood to move during seasonal humidity changes. Understanding when and how to use tar paper under wood flooring is essential for achieving a long-lasting, stable installation that performs well for decades.
The Purpose of Underlayment Beneath Wood Floors
Underlayment materials serve several critical functions in wood floor installations. They provide a moisture barrier between the subfloor and the wood flooring, cushion minor subfloor imperfections, reduce friction that causes squeaks, and can provide a small amount of thermal and acoustic insulation. Tar paper has been the traditional material of choice for these purposes, particularly in nail-down installations over plywood or OSB subfloors. The National Wood Flooring Association (NWFA) includes building felt as an acceptable underlayment in its installation guidelines for solid hardwood flooring.
| Function | How Tar Paper Performs | Performance Rating |
|---|---|---|
| Moisture vapor barrier | Asphalt-saturated felt blocks vapor migration from subfloor | Moderate — 5-8 perms |
| Friction reduction (slip layer) | Smooth surface allows wood to expand/contract without binding | Good — reduces squeaks by 60-70% |
| Cushioning / gap bridging | Minor compression bridges subfloor voids up to 1/16 in | Low — not a substitute for flat subfloor |
| Thermal break | Thin air layer provides minimal R-value | Negligible — R-0.02 approx |
| Sound dampening | Reduces impact transmission slightly | Low — STC improvement of 1-2 points |
Types of Tar Paper and Their Ratings
Building felt is classified by its weight per square (100 square feet). The most common grades for flooring applications are 15-pound and 30-pound felt. The weight designation is a historical reference to the approximate weight per 100 square feet; modern materials may differ slightly due to manufacturing process changes.
| Grade | Weight per Square | Thickness | Perm Rating | Best Use Case |
|---|---|---|---|---|
| #15 felt (ASTM D226 Type I) | ~15 lb | ~0.030 in (30 mil) | 8 perms | Standard wood floor underlayment over plywood |
| #30 felt (ASTM D226 Type II) | ~30 lb | ~0.060 in (60 mil) | 5 perms | High-moisture areas, concrete subfloors (over vapor barrier) |
| Rosin paper (uncoated) | ~5 lb | ~0.010 in | 20+ perms | Primarily slip layer, minimal moisture protection |
| Rubberized asphalt membrane | N/A (roll) | ~0.040 in | 0.5 perms | Highest moisture protection, premium applications |
Installation Over Plywood Subfloors
For nail-down solid hardwood flooring over plywood or OSB subflooring, the installation sequence is straightforward but requires attention to detail. Start by ensuring the subfloor is clean, dry, and flat (within 1/4 inch over 10 feet, and 1/8 inch over 6 feet). Any high spots should be sanded down and low spots filled with leveling compound before laying the underlayment.
Roll out the tar paper parallel to the direction of the finished flooring. Overlap adjacent rows by 2 to 4 inches. Use a staple gun (1/4-inch or 3/8-inch staples) or cap nails to fasten the paper every 12 to 18 inches along the edges and at the center of each sheet. Avoid excessive fastening that might create bumps under the flooring. The paper should lay smooth and flat without wrinkles, which would telegraph through the finished floor.
The paper should extend to within 1/2 inch of walls and obstructions. It should not be wrapped up the walls, as this would interfere with baseboard installation and might trap moisture against the wall framing. For glue-down installations, tar paper is not used because the adhesive must bond directly to the subfloor.
Installation Over Concrete Slabs
When installing wood flooring over a concrete slab, the moisture protection requirements are more stringent. A 6-mil polyethylene vapor barrier is generally required between the slab and the wood subfloor or sleeper system. Tar paper alone is insufficient as a primary vapor barrier over concrete because its 5-to-8-perm rating allows moisture vapor to pass through at rates that can damage wood flooring over time.
The proper sequence for concrete slab installations is: clean and seal the slab, install the polyethylene vapor barrier (6-mil minimum, lapped 6 inches at seams), then install the subfloor or sleeper system, and finally the tar paper and wood flooring. Concrete slabs must be at least 60 days old and must test below 3 pounds of moisture per 1,000 square feet per 24 hours using a calcium chloride test (ASTM F1869) before any wood flooring is installed.
Moisture Performance Data
The moisture vapor transmission rate of underlayment materials is measured in perms (grains per hour per square foot per inch of mercury). Comparative testing has shown that 15-pound asphalt-saturated felt transmits approximately 5 to 8 perms of moisture vapor. By contrast, 6-mil polyethylene transmits less than 1 perm, and rubberized asphalt membranes transmit less than 0.5 perms.
For climate-controlled buildings in most regions, the perm rating of #15 felt is adequate to prevent moisture-related issues when installed over a wood subfloor. The key is to maintain the building’s relative humidity between 30 and 50 percent, which keeps the wood flooring within its stable moisture content range of 6 to 9 percent. In regions with high groundwater or crawlspace humidity, an additional vapor barrier below the subfloor is essential.
Alternatives to Tar Paper
| Material | Cost per sq ft | Moisture Barrier | Acoustic (STC) | Best For |
|---|---|---|---|---|
| #15 tar paper | $0.05 – $0.10 | Moderate (5-8 perms) | 45 | Nail-down over plywood subfloor |
| Foam underlayment (1/8 in) | $0.15 – $0.30 | Low (10+ perms) | 55-60 | Floating floors, condos |
| Rubber membrane | $0.50 – $1.00 | Excellent (0.5-2 perms) | 60-65 | Concrete subfloors, premium |
| 6-mil polyethylene | $0.03 – $0.08 | Excellent (0.1 perm) | 40 | Direct vapor barrier under subfloor |
| Cork underlayment | $0.50 – $0.80 | Low (15 perms) | 55-60 | Acoustic isolation, green building |
Common Mistakes to Avoid
One of the most common mistakes is using tar paper that is too thick or too thin for the application. Using #30 felt when #15 would suffice creates a surface that is too soft, allowing the flooring nails to lose grip over time as the felt compresses under foot traffic. Conversely, using rosin paper (which is merely a slip layer with negligible moisture protection) in a basement or over a crawlspace can lead to moisture damage.
Another frequent error is failing to overlap the seams adequately or leaving gaps that allow moisture vapor to bypass the barrier. A 2-inch overlap is the minimum; 4 inches is better in areas with higher moisture risk. Installing tar paper over a damp subfloor is a critical mistake. The paper traps the moisture against the wood flooring, leading to cupping, warping, and mold growth within months. Always verify that the subfloor moisture content is below 12 percent using a pin-type moisture meter before laying the underlayment.
Tar paper remains a reliable and time-tested component in wood flooring installations. When selected correctly for the specific subfloor type and environmental conditions, and installed with proper overlap and fastening, it provides the moisture protection and slip-reduction necessary for a durable, squeak-free wood floor.