Crawl Space Insulation Types: A Material Guide
Not all crawl space insulation is created equal, and picking the wrong material is one of the most common and expensive mistakes homeowners make below the floor. A batt that performs beautifully in a dry attic can slump, absorb water, and grow mold in a damp crawl space within a few seasons. That is why so many contractors now reach for foam board insulation for crawl space walls instead of the fluffy fiberglass that once dominated. This guide breaks down every major material by R-value per inch, moisture resistance, cost, and the situations where each one earns its place, so the choice fits the space rather than fighting it.
What are the main crawl space insulation types?
Five materials cover almost every crawl space job: fiberglass batts, rigid foam board (in XPS, EPS, and polyiso forms), closed-cell spray foam, open-cell spray foam, and mineral wool. Each behaves differently once moisture, air, and time enter the picture. The short version is that the fluffy, fibrous products are cheaper and easier to handle but vulnerable to water, while the foams cost more yet shrug off the damp conditions that define most crawl spaces.
The right pick depends heavily on one upstream decision: whether the crawl space stays vented or gets sealed and encapsulated. In a vented crawl space, insulation goes between the floor joists overhead, and it has to tolerate humid, unconditioned air. In an encapsulated crawl space, insulation goes on the perimeter walls, the space becomes part of the conditioned building envelope, and moisture is controlled by a full vapor barrier. The pillar overview on crawl space insulation strategy walks through that fork in more depth, and it is worth settling before buying a single sheet of anything.
R-value is the headline number, but it is not the whole story. A material’s resistance to water, its air-sealing ability, its fire behavior, and its cost per installed square foot all matter just as much below grade. The tables and comparisons below weigh those factors side by side.
Fiberglass batts: cheap, familiar, and easily ruined
Fiberglass is the material most homeowners already recognize: pink or yellow rolls of spun glass fiber, sold faced or unfaced. It carries roughly R-3.1 to R-3.4 per inch, so a standard R-13 batt runs about three and a half inches thick and slides neatly between joists.
The appeal is price and availability. Fiberglass is the least expensive option per square foot, widely stocked, and simple to cut with a utility knife. In a genuinely dry, well-ventilated crawl space, batts installed tightly against the subfloor can perform acceptably for years.
The weakness is water. Fiberglass does not resist moisture; it holds it. When humid crawl space air condenses on cold fibers, or a plumbing drip soaks a batt, the insulation compresses, loses most of its R-value, and provides a damp bed for mold. Batts installed in floor joists also tend to sag and fall over time as the facing staples fail and gravity wins. Because fiberglass offers essentially no air-sealing, drafts move right through it.
Best use case for fiberglass
Fiberglass makes sense in a dry, vented crawl space on a tight budget, installed overhead in the floor joists with proper support wires and a facing oriented correctly for the climate. It is a poor choice for perimeter walls, for any space with a history of moisture, or for encapsulation projects. If a crawl space has ever flooded or shown condensation, fiberglass will disappoint. The material also does nothing to deter pests, and rodents readily nest in it, which is another strike against leaving it exposed in an open, vented space over the long term.
Rigid foam board: the workhorse for crawl space walls
Rigid foam board comes in large panels that are cut to fit and fastened against the crawl space perimeter walls or the rim joist. It is the go-to for encapsulation because it resists moisture, adds meaningful R-value in a thin profile, and creates a clean, continuous thermal layer. There are three common chemistries, and they are not interchangeable.
XPS (extruded polystyrene)
XPS is the familiar pink, blue, or green board. It offers about R-5 per inch and strong moisture resistance, making it a durable pick for below-grade walls and ground contact. It is more water-resistant than EPS and holds its R-value well in damp conditions. XPS sits in the middle of the price range and is arguably the most popular rigid board for crawl space walls.
EPS (expanded polystyrene)
EPS is the white, beaded board. It delivers roughly R-3.6 to R-4.2 per inch, slightly less than XPS, and costs less. EPS is more vapor-open, which can actually be an advantage below grade because it lets a wet wall dry rather than trapping moisture. It is the budget-friendly rigid option and performs well when detailed correctly.
Polyiso (polyisocyanurate)
Polyiso boasts the highest R-value of the three, around R-6 to R-6.5 per inch, and usually ships with a foil facing. The catch is that polyiso can lose R-value at cold temperatures and its facing can be vulnerable to prolonged moisture, so many builders keep it off ground contact and damp locations. It shines on the rim joist and in above-grade applications.
Across all three, the strength of foam board insulation for crawl space walls is the combination of moisture tolerance, a semi-rigid air barrier when seams are taped, and a thin profile that preserves precious crawl space clearance. Boards are typically sealed at the seams and to the top of the wall with foam or tape, and building codes usually require a thermal or ignition barrier over foam in habitable-adjacent spaces. For a deeper look at how these panels stack up against everything else, see the companion guide to the best insulation for a crawl space.
Spray foam: air-sealing and insulation in one step
Spray polyurethane foam is applied wet and expands to fill cavities, gaps, and irregular surfaces, hardening into a continuous layer that insulates and air-seals at the same time. It is the premium approach, and it comes in two very different forms.
Closed-cell spray foam
Closed-cell foam is dense and rigid once cured, delivering roughly R-6 to R-7 per inch, the highest of any common crawl space material. Critically, at sufficient thickness it also acts as a vapor retarder and a water-resistant air barrier, which is why it is a favorite for encapsulated crawl space walls and rim joists. It adds structural rigidity and resists water intrusion. The trade-off is cost: closed-cell is the most expensive option per square foot and generally requires professional installation.
Open-cell spray foam
Open-cell foam is lighter, softer, and cheaper, at about R-3.5 to R-3.8 per inch. It air-seals well but is vapor-open and will absorb water, so most professionals avoid it on below-grade crawl space walls and in any location prone to bulk moisture. It has its place in dry framing cavities but is a weaker fit for the damp reality of most crawl spaces.
Because spray foam is a specialized, equipment-intensive job, homeowners weighing it should read the dedicated breakdown of spray foam insulation for a crawl space before committing, since the cost and the code details differ sharply from board or batt work.
Mineral wool (Rockwool): fire and moisture resilience
Mineral wool, often sold under the Rockwool brand, is spun from stone and slag into dense batts. It offers about R-4 to R-4.3 per inch, higher than fiberglass, and brings two standout traits: it is naturally fire-resistant to very high temperatures, and it is water-repellent and vapor-open, so it sheds bulk water and dries readily rather than staying saturated.
Those properties make mineral wool a resilient choice for floor-joist insulation in a vented crawl space, where fiberglass would fail. It holds its shape better than fiberglass, resists mold and pests, and does not slump as quickly. The downsides are a higher price than fiberglass and a heavier, itchier install. Mineral wool does not air-seal on its own, so it pairs best with separate air-sealing at the rim and penetrations.
Best use case for mineral wool
Choose mineral wool for floor-joist insulation in a vented crawl space where fire performance or moisture resilience matters and a rigid-board wall system is not being installed. It is a sensible upgrade over fiberglass for anyone who wants a fibrous batt that tolerates damp air.
R-value, moisture, and cost compared
Reading the materials side by side makes the trade-offs obvious. In R-value per inch, closed-cell spray foam and polyiso lead at roughly R-6 to R-7, XPS follows near R-5, mineral wool and EPS land around R-4, and fiberglass and open-cell foam trail near R-3.1 to R-3.8. Thin clearance favors the high-R foams; generous clearance makes the cheaper, thicker batts viable.
Moisture resistance flips some of that ranking. Closed-cell foam and XPS resist water strongly, EPS and mineral wool tolerate damp and dry out well, polyiso is moisture-sensitive at ground contact, and fiberglass and open-cell foam are the most vulnerable. In a crawl space, moisture behavior often matters more than raw R-value, because a wet R-13 batt performs worse than a dry R-8 board.
Cost runs roughly inverse to performance. Fiberglass is cheapest, EPS and mineral wool sit in the middle, XPS is a step up, polyiso is comparable to XPS, and spray foam, especially closed-cell, is the most expensive and usually the only one that truly requires a pro. Any older crawl space being re-insulated should also be checked for hazards before demolition; loose, grayish, or vermiculite-style fill can contain asbestos, and the safe move is to review how to identify asbestos rather than tearing it out blind.
How to match the material to your crawl space
The cleanest way to decide is to start from the crawl space condition, not the product. For a dry, vented crawl space on a budget, floor-joist mineral wool or well-supported fiberglass works. For an encapsulated, sealed crawl space, rigid foam board on the walls, or closed-cell spray foam where budget allows, is the durable answer. For a damp or previously flooded space, avoid fiberglass and open-cell foam entirely and lean on closed-cell foam or XPS that can handle water.
Clearance, code, and moisture control shape the final call. Low crawl spaces reward high-R foams that do more in less thickness. Codes generally require an ignition or thermal barrier over exposed foam, and any wall-insulation approach needs a proper ground vapor barrier underneath to succeed. When in doubt, an assessment from a crawl space professional prevents paying for a material that the space will destroy.
Matching R-value to your climate zone
The material is only half the decision; the total R-value has to suit the climate. The Department of Energy divides the country into zones, and the recommended crawl space R-value climbs as winters get harsher. A home in a warm southern zone may target something modest, while a cold northern zone calls for considerably more, and that target shapes both the material and the thickness.
This is where high-R foams earn their premium in tight spaces. Consider three scenarios. In a mild climate with a generous crawl space, R-13 to R-19 is often enough, and thick, inexpensive mineral wool or fiberglass batts in the floor can reach it cheaply. In a cold climate with a low, encapsulated crawl space, hitting an R-15 to R-19 wall target in just two or three inches of thickness practically demands closed-cell spray foam or a stacked rigid-board assembly, because there is no room for a thick batt. In a mixed, humid climate, the priority shifts toward moisture-tolerant materials like XPS or closed-cell foam even at a moderate R-value, since a lower dry R-value outperforms a higher wet one.
The takeaway is that no single material wins everywhere. Cold plus tight clearance favors the dense foams, mild plus open clearance opens the door to cheaper batts, and humid conditions push the choice toward whatever shrugs off water. Confirming the DOE-recommended R-value for the specific zip code, then working backward to the material that reaches it in the available depth, is the most reliable way to choose.
Installation, code, and thickness considerations
Beyond R-value and moisture, a few practical factors separate a lasting install from a callback. Foam plastics, including rigid board and spray foam, are combustible and generally must be covered with a code-approved ignition or thermal barrier in crawl spaces adjacent to living areas, so budget for that layer where required. Skipping it is both a code violation and a fire risk.
Thickness and clearance drive the real-world choice more than any spec sheet. A crawl space with thirty inches of headroom can accept thick batts; one with sixteen inches cannot, and there the thin, high-R foams are the only way to reach the target without losing the ability to service pipes and ducts. Fastening also differs: rigid board is mechanically fastened and sealed at the seams, batts are friction-fit and supported with wires overhead, and spray foam bonds directly to the substrate.
Air-sealing is the quiet multiplier. Fiberglass and mineral wool do nothing to stop air movement, so they should be paired with separate sealing at the rim joist, penetrations, and the top of the wall. The foams handle sealing and insulating in one pass, which is a large part of why they outperform batts of the same nominal R-value in a leaky crawl space. Whichever material is chosen, the ground vapor barrier underneath and the sealing details around it decide whether the insulation performs for decades or fails in a few damp seasons. For homeowners who want the numbers behind each choice, the guide to crawl space vapor barrier and insulation connects the material to the moisture system it depends on.
References
- Types of Insulation — U.S. Department of Energy, Energy Saver
- Insulation Basics and R-Value — U.S. Department of Energy, Energy Saver
- Moisture Control Guidance — U.S. Environmental Protection Agency
Choosing between board, batt, and spray foam is easier with a set of eyes on the actual space and its moisture history. If you want a straight answer for your home, connect with a local crawl-space and insulation pro through our contact page and get a recommendation matched to your conditions.