A Homeowner’s Guide to Crawl Space Insulation

A Homeowner’s Guide to Crawl Space Introduction: Crawl spaces are often out-of-sight areas beneath a house, but they play a crucial role in your home’s energy efficiency and structural health. Properly insulating a crawl space can lead to warmer floors, lower utility bills, and reduced moisture problems in the living area above. Conversely, poor or damaged insulation in a crawl space may result in cold drafts, high heating costs, and even mold or wood rot issues caused by excess moisture. In this guide, we’ll explore the common types of crawl space insulation, how to install them correctly (especially fiberglass batts), the effects of moisture on each material, and why keeping the crawl space dry through ventilation is so important. We’ll also explain why any damaged or contaminated insulation should be handled by professionals to ensure your crawl space remains a safe, healthy part of your home.

Common Types of Crawl Space Insulation

Homeowners have several insulation options for crawl spaces, each with different characteristics. The most common materials include fiberglass batts, rigid foam boards, and spray foam (open-cell or closed-cell). There are also other less-common materials like mineral wool or cellulose used in some situations. Below is an overview of each insulation type and its typical use in residential crawl spaces:

• Fiberglass Batt Insulation: Fiberglass batts are the traditional choice for vented crawl spaces. They are thick, fluffy blankets of spun glass fibers, often with a paper or foil facing on one side. The batts are sized to fit between floor joists and are usually held in place by friction or wire supports. Fiberglass is popular because it’s inexpensive and easy to install as a DIY project. In a crawl space, fiberglass batts are typically installed between the joists under the subfloor. If the batts are faced (have a kraft paper backing), that facing must go toward the heated living space (upward against the subfloor), which we’ll discuss more below. Fiberglass provides decent thermal insulation when dry, but it does not stop air movement or moisture by itself. Homeowners should be aware that over time in a damp crawl space, fiberglass may sag or fall out if not secured properly.

• Rigid Foam Board Insulation: Rigid foam panels (made of closed-cell foam like extruded polystyrene (XPS), expanded polystyrene (EPS), or polyisocyanurate) are another common crawl space insulation, especially for encapsulated or closed crawl spaces. These boards are cut and fitted against the crawl space foundation walls or along the rim joists. Unlike fiberglass, rigid foam is highly resistant to water and humidity – it does not absorb moisture, so it retains its insulating value even in damp conditions. Rigid foam panels also add a measure of air sealing when joints are taped, and they provide a continuous layer of insulation. In many modern crawl space designs, insulating the walls with foam board (and covering the ground with a vapor barrier) is preferred over insulating between the floor joists. Foam boards come in various thicknesses and are typically attached with construction adhesive or mechanical fasteners. Because they are a closed-cell material, they are impervious to moisture and do not support mold growth, making them well-suited for the often damp crawl space environment. (We’ll discuss moisture effects on rigid foam in more detail later.)

• Spray Foam Insulation (Open-Cell and Closed-Cell): Spray foam is applied as a liquid that expands and cures into a solid, filling gaps and sealing air leaks. There are two types used in crawl spaces:

  • Open-cell spray foam is a lighter, spongy foam (about 0.5 lb/ft³ density) that expands significantly. It provides good insulating R-value and excellent air sealing. However, open-cell foam is not a vapor barrier – its cells are open, so water vapor can penetrate it. In other words, open-cell spray foam will allow moisture to move through it rather than blocking it. Because it can absorb and hold water, open-cell foam is generally not recommended for crawl space walls or areas prone to water, but it might be used on the underside of a floor in certain dry conditions. If used, it must be in conjunction with proper moisture control.

  • Closed-cell spray foam is a denser, rigid foam (about 2 lb/ft³) that expands less but forms an airtight and moisture-tight barrier. Closed-cell foam has a higher R-value per inch and once cured it acts as an effective vapor barrier and moisture barrier. This type of spray foam is ideal for crawl spaces in that it both insulates and seals out air and moisture. It can be applied to crawl space walls, the underside of the subfloor, and rim joists. Closed-cell spray foam essentially encapsulates the surface with a water-resistant layer, protecting wood framing from humid air. Its main drawbacks are higher cost and the need for professional installation, but it provides a permanent solution that won’t sag or fall. In many cases, closed-cell foam is considered the “best in class” insulation for crawl spaces due to its ability to seal and insulate in one step.

• Other Materials (Mineral Wool & Cellulose): Though less common, you may encounter other insulation materials. Mineral wool (rock wool) batts can be used between crawl space joists similarly to fiberglass. Mineral wool is made from stone fibers and is naturally water-resistant and non-combustible. It does not absorb water and does not support mold (since it contains no organic material). However, like fiberglass, it is air-permeable and typically installed unfaced (no vapor barrier attached), so a separate vapor retarder may be needed if used in a crawl space. Cellulose insulation (loose-fill shredded newspaper treated with fire retardant) is rarely used in crawl spaces because it can settle out of place and absorb moisture easily. In an open crawl space, cellulose would soak up dampness and can become a breeding ground for mold unless the space is extremely well-controlled for humidity. In general, cellulose is not recommended for crawl space applications – other materials like the ones above are far better suited.

Newly installed fiberglass batt insulation between the floor joists of a crawl space. The pink fiberglass batts are fit snugly up against the subfloor (notice the white vapor barrier on the ground as well). Fiberglass is a common choice for insulating vented crawl spaces, but it must be installed correctly with the facing towards the living space to work effectively.

Proper Installation of Fiberglass Batt Insulation

When using fiberglass batts in a crawl space, correct installation is critical to ensure the insulation performs well and does not create moisture problems. The most important detail is the orientation of the kraft paper facing (if the batts are faced). The rule is simple: the paper or foil facing should always point toward the heated or cooled living space. In a crawl space application, this means the paper side of a kraft-faced fiberglass batt must be up against the underside of the subfloor (toward the home’s interior), not facing down toward the crawl space. This placement positions the vapor retarder (the kraft paper) on the “warm-in-winter” side of the insulation, which is generally the correct approach for vapor barrier placement.

In practical terms, if you’re insulating the crawl space ceiling with fiberglass rolls, you will press the batts between the joists so that the paper backing is touching the wood subfloor above. The open, fluffy side of the fiberglass should be exposed on the colder side (the side facing the crawl space). This orientation helps control moisture by blocking humid air from migrating upward into the insulation, and it keeps the batts in contact with the subfloor for better thermal performance. If the insulation were installed upside-down (with the paper facing the crawl space), warm moist air from the house could pass into the fiberglass and then hit the cold paper on the far side, trapping moisture in the insulation. Indeed, installing the facing incorrectly can trap moisture and lead to mold growth, wood rot, and reduced insulation effectiveness. Thus, always remember: paper faces the house (the conditioned space).

For unfaced fiberglass batts (which have no paper at all), you don’t have an attached vapor barrier, so you would typically install a separate vapor barrier elsewhere (often on the ground in the crawl space) if needed. Whether using faced or unfaced batts, it’s good practice to secure the insulation so it doesn’t sag or fall. Spring metal insulation rods can be wedged between joists to hold batts in place, or staple wire/twine across the joists. Also, be aware that exposed kraft paper is flammable, so building codes sometimes require covering a paper-faced insulation with an ignition barrier (like drywall) if it’s left exposed. Often in a crawl space, the paper is against the subfloor (not visible), so this isn’t an issue, but it’s something to keep in mind.

In summary, fiberglass insulation can be a cost-effective way to insulate a crawl space, but it must be installed correctly. Always orient vapor-barrier facings toward the living area, fit the batts snugly without gaps, and secure them so they stay dry and in contact with the subfloor. This will maximize their insulating value and prevent the common pitfalls of fallen or ineffective fiberglass in crawl spaces.

How Moisture Impacts Crawl Space Insulation

Moisture is the enemy of most insulation materials, and crawl spaces are notoriously damp environments. Water from the soil, humid outside air, plumbing leaks, or condensation can all end up in a crawl space. If insulation gets wet or even continually exposed to high humidity, its performance can degrade, and mold or mildew may develop. It’s important to understand how each type of insulation copes with moisture:

• Fiberglass Batts: Fiberglass itself is made of glass fibers, which do not absorb water. However, fiberglass insulation can hold water in the tiny air pockets between the fibers, essentially becoming a sponge if exposed to moisture. When fiberglass batts become damp, two bad things happen: (1) the insulation’s R-value drops significantly because the air spaces are filled with water (and water conducts heat much more readily than air); and (2) the added weight causes the batts to sag or fall out of place under the floor. Homeowners often discover crawl space fiberglass hanging down or lying on the ground after a period of wet weather. Wet fiberglass also contributes to mold problems – while the glass fibers won’t mold, the insulation collects dust and organic debris over time, and the typical kraft paper backing is an organic material. When soaked, these become a ripe surface for mold growth, which can then spread to the wooden joists and subfloor. In short, moisture can render fiberglass insulation almost useless and even harmful: as one industry source put it, once fiberglass is wet, it’s likely “doing more harm than good”. Drying it out may restore some function, but often the best course is to remove and replace fiberglass that has been saturated or grown mold.

• Rigid Foam Board: Rigid foam boards are highly resistant to moisture. Closed-cell foam boards like XPS or polyisocyanurate do not readily absorb water – they maintain their structure and insulation value even in humid or damp conditions. If crawl space foam board insulation gets wet on the surface, it will generally dry off without damage, since the water cannot penetrate or be absorbed. Additionally, rigid foam contains no food for mold (it’s plastic), and it doesn’t hold moisture, so mold cannot grow on the foam itself . This stability is a big advantage in crawl spaces. Do note, however, that while the foam panel is waterproof, moisture can still find its way around or behind improperly installed panels. For example, if there are gaps at the edges, humid air could condense on the cooler foundation wall behind the foam. For this reason, seams between boards should be sealed (with tape or spray foam) to prevent moist air from reaching the wall. Overall, rigid foam is unaffected by occasional moisture and will not deteriorate like fiberglass in a crawl space environment. Its insulation performance remains steady even if the crawl space is damp. (One exception: some types of foam, like EPS, can absorb a small amount of moisture over a long time if submerged, but normal crawl space humidity won’t significantly impact foam boards.)

• Open-Cell Spray Foam: Open-cell spray foam is vapor-permeable, meaning water vapor can diffuse through it. It also can absorb liquid water to an extent because of its open-cell, spongy structure. If open-cell foam in a crawl space is exposed to high humidity, the moisture can work through the foam and potentially reach the wood structure behind it. The foam itself doesn’t foster mold (it’s also plastic), but if the wood stays damp, mold can grow on the wood. In a scenario of a plumbing leak or flooding, open-cell foam will soak up water – although it will eventually dry out once the source of moisture is removed. The key issue is that open-cell foam does not act as a moisture barrier. It allows bulk moisture and vapor to pass, so you cannot rely on it alone to keep a crawl space dry. Over time, consistent exposure to moisture can also degrade the foam’s insulating ability (a waterlogged open-cell foam has lots of water where air pockets should be). For these reasons, if open-cell spray foam is used in a crawl space, it’s vital that the space be properly sealed and a vapor barrier or other moisture control (like a dehumidifier) be in place. Many experts favor closed-cell over open-cell for crawl spaces specifically due to moisture concerns.

• Closed-Cell Spray Foam: Closed-cell spray foam is highly resistant to moisture once cured. Its closed-cell structure means water cannot easily penetrate into the material. In fact, closed-cell foam can serve as a combined insulation and vapor barrier. It will not absorb water if there’s a minor leak or condensation; instead, water would typically remain on the surface. Because it’s effectively waterproof and tightly adhered to surfaces, closed-cell foam also prevents moist air from reaching the crawl space walls or the subfloor. This makes it excellent for preventing condensation on those surfaces. Mold cannot grow on closed-cell foam (it’s inorganic and stays dry). However, one thing to be aware of is that if there is a hidden source of water (say, a foundation crack leaking or a plumbing leak above), closed-cell foam can sometimes conceal the issue – the water might get trapped behind the foam against wood. This is not common, but it’s a reason why even with closed-cell, you should maintain good overall moisture management. In general, closed-cell foam is the best performer under wet conditions: it acts as a moisture barrier by itself, retaining full insulating value and protecting the structure from humidity. This robustness is why closed-cell is often recommended in crawl spaces that are prone to moisture or even minor flooding.

• Mineral Wool: Mineral wool insulation is moisture-resistant in that the fibers do not absorb water. In fact, many mineral wool products are engineered to be hydrophobic (water-repellent). If mineral wool batts get damp, they do not mat down or collapse like fiberglass can. They also dry out quickly since water does not stick to the fibers. Importantly, like other inorganic insulations, mineral wool won’t support mold growth. Nonetheless, mineral wool is vapor permeable, meaning water vapor can pass through it freely. So if you use mineral wool in a crawl space, you still need a separate vapor barrier to prevent moisture intrusion. The material itself will be fine if it gets a bit damp, but the wood or other materials around it could still get wet from vapor passing through. In summary, mineral wool handles moisture better than fiberglass (no loss of insulation when wet), but it doesn’t stop moisture movement by itself.

• Cellulose: As noted earlier, cellulose (loose-fill or batt made from recycled paper) is extremely vulnerable to moisture. It readily absorbs water and takes a long time to dry. Wet cellulose will sag and settle, losing all insulating ability, and it absolutely can grow mold since it’s basically paper. A wet, clumpy mess of cellulose can also promote wood rot in adjacent joists. Because of these issues, cellulose is almost never used in crawl spaces unless the area is completely dry and conditioned—which is rarely guaranteed. If you have an old home where someone installed cellulose in a crawl space, be cautious: any sign of moisture damage means it should be removed immediately.

As we can see, moisture affects insulation materials very differently. Fiberglass and cellulose are the most susceptible to moisture damage, often leading to sagging, mold, and loss of R-value when wet. Spray foam and rigid foam are much more moisture-tolerant, with closed-cell spray foam and foam boards being essentially impervious to water and therefore maintaining their performance in a damp environment. These properties should be taken into account when choosing how to insulate a crawl space, especially if your crawl space has had past moisture issues. No matter what insulation you use, controlling moisture is key to a successful outcome.

A technician applies closed-cell spray foam insulation to the walls of a crawl space. Closed-cell foam expands to form an airtight, rigid layer that not only insulates well but also acts as a moisture barrier, sealing out humid air. This helps protect the crawl space from condensation and mold. Spray foam must be installed by professionals with proper equipment and safety gear, as shown here.

The Role of Ventilation in Preventing Moisture Accumulation

Because moisture is such a threat to crawl spaces, one of the most important considerations is crawl space ventilation and overall moisture control. In a traditional vented crawl space (one with openings in the foundation walls), ventilation is meant to allow damp air to escape and bring in drier outside air. Properly designed ventilation can help to carry away moisture and keep the relative humidity in the crawl space lower. In fact, providing adequate air flow can help keep a crawl space dry under the right conditions. The idea is that by moving air through the space, water vapor won’t stagnate and condense on surfaces. Proper ventilation is crucial for preventing moisture buildup that could lead to mold growth or wood decay.

However, it’s worth noting that the effectiveness of crawl space ventilation can depend on your climate. In very humid climates or seasons (e.g. hot, moist summers), simply venting might actually introduce more moisture into the crawl space than it removes. Warm outside air can carry a lot of moisture; when that air enters a cooler crawl space, the moisture can condense on cool surfaces (like metal pipes or the floor joists), causing the very problems we’re trying to avoid. This is why some modern building strategies advocate for crawl space encapsulation (sealing off vents and using dehumidifiers). But encapsulation is a complex topic on its own.

For the scope of this discussion, assuming a typical vented crawl space design, the key takeaway is: don’t let your crawl space become a stagnant moisture trap. Ensure that existing crawl vents are not blocked by debris or insulation, and that there is some cross-ventilation happening (air entering from one side of the foundation and exiting through vents on the opposite side). The airflow will help expel water vapor that emerges from the soil or from occasional leaks. It essentially dilutes the humidity. During colder, drier times of year, you might actually close vents to avoid heat loss – but when the weather is mild or if humidity is building up, open those vents to flush the space with outside air.

In an encapsulated crawl space (with no open vents), ventilation is achieved differently – usually by conditioning the crawl space air or running a dehumidifier. The goal is the same: control the humidity. Builders who seal crawl spaces will lay down thick ground vapor barriers and often insulate the walls with rigid or spray foam (as discussed above), then actively manage moisture with mechanical means. Encapsulation can keep a crawl space very dry and is often recommended in regions with high groundwater or humidity. But even then, you might have a vent to the interior or a small fan to circulate air.

In summary, proper ventilation (or air exchange) is important to prevent moisture accumulation in a crawl space. Without it, any water vapor that enters has nowhere to go and will eventually condense on the coolest surfaces – often your insulation or wood framing. By allowing air to move, either through vents or mechanical systems, you reduce the likelihood of condensation and keep the crawl space environment more stable. This protects your insulation from becoming damp and maintains the overall health of that space under your home.

Handling Damaged or Contaminated Insulation

Crawl spaces can be harsh on insulation materials. Over the years, you might encounter insulation that has become damaged, water-soaked, moldy, or contaminated with animal droppings (since crawl spaces sometimes attract pests). It’s very important to address such insulation promptly. Any insulation that is water-damaged, has mold growth, or is fouled by rodents should be removed and replaced, because it can impair your indoor air quality and the insulation itself is likely no longer effective.

If you discover moldy or pest-contaminated insulation in your crawl space, it’s strongly recommended to hire a qualified professional to remove it. Damaged insulation can be hazardous: moldy insulation can release spores that are dangerous to breathe, and disturbance of soiled insulation could expose you to animal waste, which may carry diseases. Professional insulation contractors or remediation specialists have the proper safety gear (respirators, suits, gloves) and know-how to safely dispose of the old insulation. They can also assess whether mold has spread to any wood members that need treatment. As one expert notes, if insulation is contaminated with mold or pests, professional removal is advised to ensure safe handling and disposal.

For homeowners, trying to DIY remove nasty crawl space insulation can pose health risks. There may also be local regulations on disposing of moldy materials or insulation with certain contaminants (like asbestos in older homes). Professionals will handle all that for you, leaving the crawl clean and ready for new insulation if appropriate.

After removing damaged insulation, fix the underlying cause before installing new material. If a leak or high humidity caused the damage, address that moisture issue (repair plumbing, improve drainage or ventilation, etc.). Simply replacing insulation without solving the moisture source will lead to a repeat problem. Once the crawl space is dry and any mold is remediated, you can then install fresh insulation – preferably a type that will better withstand the conditions or a better moisture control strategy.

Conclusion

Crawl space insulation might not be the first thing on a homeowner’s mind, but it has a significant impact on your home’s comfort, energy efficiency, and even air quality. By choosing the right insulation material and installing it correctly, you can enjoy warmer floors in winter, cooler floors in summer, and potentially lower heating and cooling costs. We’ve seen that fiberglass batts, rigid foam boards, and spray foam each have their own pros and cons in the crawl space environment. Fiberglass is economical and easy, but must be installed with the vapor barrier facing upward and kept dry. Foam boards and closed-cell spray foam offer superior moisture resistance and can double as air/vapor barriers, making them excellent for damp areas.

Equally important is managing moisture in the crawl space. Ventilation or encapsulation strategies should be used to prevent humid air and water from lingering under your house. A dry crawl space means your insulation will perform as intended and you won’t have issues with mold or rot. Always remember, if insulation does get wet or contaminated, take it seriously: have it removed and replaced as needed, and let professionals handle any hazardous cleanup to keep yourself.