Insulation for Crawl Space: Best Types, Cost & How to Install

Crawl Space Insulation

There are right types of crawl space insulation and right ways to install them. It takes getting both parts of the equation right to have a well-insulated crawl space beneath your home. This article on insulation for crawl space – best types, cost and instructions on how to install crawl space insulation is your guide to a successful project whether you DIY some or all of it or let a pro tackle all the work.

Did you know? Crawl space can also be correctly written as one word. In fact, many insulation contractors prefer the single-word name. But it’s right either way.

The Cost of Crawl Space Insulation

Expect to pay a pro $3.25 to about $5.00 per square foot to fully insulate a crawlspace using two or three types of insulation. An average price of crawlspace insulation is about $4.30 per square foot. When a vapor barrier is part of the job, cost will be higher.

We explain all these costs including which insulation types cost what, so you’ll know where your money is going to be spent.

The Wrong Way to Insulate a Crawl Space – Just Fiberglass Batts on the Crawlspace Ceiling

If you already know that installing fiberglass batts or rolls on the underside of the floor – beneath the bottom of the floor joists – is a bad idea, then this section might not teach you anything new.

We’re frankly amazed to see some sites still recommend fiberglass as the “only insulation type needed in a crawl space.”

Well, spend a day with an insulation contractor pulling bags of moist, moldy fiberglass batt and roll insulation out of crawlspace, and you’ll understand that fiberglass alone is a poor option in a crawl space. This is especially true in a vented crawl space. One well-known site reports, “Fiberglass insulation batts or rolls are the most economical and easiest DIY choice for insulating between the floor joist in a crawl space…Install the insulation with the paper vapor barrier facing up toward the heated living space, and make sure the insulation fits tightly between the joists…”

OK, let’s be fair. If you want insulation that might do the job for 3-5 years before losing effectiveness due to moisture and taking on a musty smell due to mold, then this method might be OK. Or if you live in a very arid area – think desert Southwest or High Mountain desert of the Interior West, this might work a little longer.

Even if it doesn’t lose effectiveness or become moldy, it still isn’t the best insulation for a crawl space! That’s because the R-11 R-value is pretty good, but you can get better insulation value with the types and methods we recommend below. And they give you better control over moisture, a huge factor in crawlspaces in humid climates. In other words, you’ll have a healthier crawlspace and therefore a healthier home due to reduced dampness and mold.

Best Crawl Space Insulation Types and Costs

The best crawl space insulation types are rigid foam insulation and closed cell spray foam insulation. No question about it. We like them in different locations – Rigid foam for use against the concrete walls of the crawl space and spray foam insulation, often called SPF insulation, in the rim joists of the crawlspace. Both types also work well on the crawl space ceiling, i.e., between or covering the floor joists overhead in the crawl space. Their uses and best locations are explained in this guide.

Yes, they both cost more than fiberglass roll and batt insulation, but you get so much more for your money.

Best Crawl Space Insulation Cost

Before we discuss these two types, here are their relative costs for the R-values commonly sought after in crawl spaces. Costs are per square foot based on the thickness necessary to achieve the stated R-value.

R-Value Installed Cost

R-6 to R-7 Cost

R-12 to R-14

R-18 to R-21

Foam Board Insulation

$1.85 - $2.40

$3.30 - $3.85

$4.00 - $4.65

Closed Cell SPF

$2.75 - $3.35

$4.15 - $4.50

$4.85 - $5.35

Rigid Foam Insulation and Use

This is also called foam board insulation, because that is exactly what it is: Sheets of foam insulation cut to various sizes or which can be easily cut with a utility knife.

Some call it the “perfect” insulation, and they might not be far off. The only “negative” is the slightly higher cost than fiberglass insulation.

The pros of rigid foam board insulation have a lot to do with its closed-cell construction. The cells are fully encapsulated. Therefore:

  1. Rigidity: Closed cells create the rigidity you don’t get with open cell foam insulation. This makes it easy to work with and fit into place against walls, in rim joist cavities and yes, even in overhead floor joist cavities when you want to go all out on insulating your crawl space.
  2. It is super light. Roughly 2% of its mass is plastic; 98% is air.
  3. Great insulator: The lightweight, closed cell construction gives it excellent R-value of between R-4 and R-6 or a little higher per inch of thickness depending on the specific type of rigid foam insulation (more on types below).
  4. Stops moisture and air: The closed cell construction means neither moisture nor vapor pass through it. Foam board insulation helps you keep out unwanted dampness from the crawlspace and it seals the space too from air leaks. However, it must be installed properly to achieve an air seal, so see the section on How to Properly Insulate a Crawl Space.

Pro Tip about Air LeaksStopping air leaks in your crawl space is just as important as having insulation of a sufficient R-value to do the job, as this video shows. This is essential in the attic, as we discuss in the Pick HVAC Attic Insulation Guide, but can have an impact anywhere in your home including the crawl space.

Think about it. What if your car had more and better insulation in the doors and roof than most other vehicles. You think you’d be able to stay warm and run the heater less, right? But not if you keep the windows open a crack or more. The seal is lost, and so is the heat. Badly sealed insulated areas waste energy.

Did you Know: There are multiple types of rigid foam insulation. They vary a little in R-value and cost. Polyisocyanurate, also called Polyiso and Iso by contractors, has the best R-value of about R-6 depending on the specific product. However, it loses effectiveness below about 50F. If your crawl space is below grade, this likely won’t be a problem. But check with your insulation contractor or retailer for the best type in your conditions.

And see our Foam Board Ultimate Guide and Prices post for lots more detail. Each type with its pros and cons plus cost are discussed.

Closed Cell Spray Foam Insulation

The foam insulation is a two-part substance mixed by the applicator and can be sprayed onto most any surface. It sticks well to almost any clean, relatively dry surface. The name of the most common type is spray polyurethane foam insulation; the short name is SPF.

R-Value: Depending on the specific formula, closed cell spray foam has an excellent R-value of 6.5 to 7.0!

For Clarification: There’s a thing called open cell spray foam insulation, aka 1/2lb foam insulation. We mention that just so you buy the right product if you choose spray foam. Stick with the closed cell foam that, like closed cell rigid board insulation, can act as a vapor and moisture barrier. It is commonly called 2lb foam insulation, because one board foot, 12”x12”x1”, weighs roughly two pounds.

Worth Comparing to Foam Boards: When applied properly, usually by a pro, closed cell spray foam insulation takes a lot less time to install. That makes it competitively priced with rigid foam board insulation that requires taping all seams and using small cans of spray foam around pipes, vents and other “cuts” in the exterior of your home.

Another Benefit: It seals air leaks and gaps better than any other insulation type. If you have a drafty crawl space, we strongly encourage you to use closed cell spray foam insulation as one of your crawl space insulation types. We discuss details in the final section on proper crawl space insulation techniques and tips.

See the Pick HVAC Spray Foam Insulation Guide and Costs for comprehensive details, prices and application tips!

Comparing Foam Board Insulation to Closed Spray Foam Insulation

Closed cell spray foam insulation has all the benefits of closed cell rigid foam board insulation (all boards are closed cell). It costs more for the material than rigid foam insulation boards. True.

But spray foam installation takes less time, reducing labor costs. And you cannot beat its ability to stop air leaks, keeping your crawl space warmer in winter and cooler in summer, just the way it should be.

How to Properly Insulate a Crawl Space

These are the essential techniques and tips to properly insulating a crawl space.

1. Seal Vents. Some old homes have crawl spaces with vents, supposedly to let out excess moisture. Bob Villa calls crawl space vents, “a monumental design flaw,” because all they do is let in moisture that doesn’t easily escape.

Insulation Crawl Space Seal Vents

Pro Tip: Villa suggests buying the right size vent covers, or using plywood to make your own and using caulk (or we’d suggest construction glue as an alternative) to hold them in place.

Do the same for a hatchway, if your crawl space has one.

2. Stop Foundation/Crawl Space Leaks. Keep water away from the foundation by cleaning and maintaining gutters, running downspouts away from the house and making sure all concrete or asphalt outside (driveway, sidewalks) slope away from the house.

3. Insulate the Rim Joist. This is the wood 2”x10” or 2”x12” board that sits on the foundation. It might have air vents, a dryer vent, pipes or windows in it. Where there isn’t a window, cover the area with cut pieces of rigid foam and use cans of spray foam around the openings.

This is the perfect place, in our opinion, for closed cell spray foam insulation. It seals those air leaks and gaps better than any other type of insulation.

Pro Tip: Crawl space contractor Armando Cobo suggests, “If you have ducts or pipes in a well-sealed and unvented crawl space, I usually prefer to insulate the perimeter wall with rigid foam and spray foam around the rim joist; that helps keep pipes and ducts warm.” It also, just as importantly, stops air leaks as we’ve been emphasizing.

Now, on to the walls.

4. Insulate the Foundation Walls

If you use rigid foam boards, tape all seams and locations where the boards meet the rim joist insulation. Inspect the tape at all seams at least every year. Twice per year, is better, and secure any location the tape has come loose.

If you use spray foam insulation, apply the proper thickness for the R-value you want. Apply the coats evenly, and make sure it adheres well.

Pro Tip: This is why we recommend pro installation for spray foam. It must be properly applied, or the higher cost of the material can easily be wasted.

5. Consider an Air/Vapor Barrier. Moisture migrates up through concrete into your basement Seems hard to believe, but the porous nature of concrete makes this possible.

Choose a heavy-gauge barrier, especially if you store stuff in your crawl space or want to once it gets properly insulated. Clean the floor, so debris beneath it won’t poke through the barrier. Then overlap the barrier seams by a few inches, and heavily tape the seams and where the barrier meets the foundation.

6. Take it to the Next Level. This technique is generally called “crawl space encapsulation.” The goal is to stop all air in and out and the moisture that can come in with it.

Pro Tip: Find a contractor that knows what they are doing and has a lot of experience. One online contractor has this view of encapsulation, “The barriers that are placed throughout your crawl space will block some of the outside air from coming in and making your floors cold, or affecting your energy costs.” Wrong!

The goal of encapsulation is to block “all” or at least “almost all – all but the tiniest amount” of outside air. “Will block some of the outside air…” isn’t good enough, and that is not encapsulation.

Now, while we don’t recommend it from a cost-effective standpoint, some homeowners go to the extreme of also insulating the crawl space ceiling where the floor joists are to create a full encapsulation. In very cold climates, it might be worth considering.

According to the Green Building Advisor, if you are going to do it, then, “insulate under the floor joists with a continuous layer of 2-inch-thick polyisocyanurate.” We agree.

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