If your home underfloor needs insulation, it’s a good idea to know all the options available to you. This will help you choose the best insulation material to go with, in line with peculiarities unique to your home and your budget.

Underfloor insulation options include fiberglass batt, foam board, spray foam, and vapor barrier. Spray foam insulation is typically the costliest form of underfloor insulation, with the price per square foot reaching $2.50. Fiberglass and foam board insulation costs can reach $1 per square foot.

Determining what works best for you, including the costs, is as essential as correct installation. This article will provide all the details you need.

Consider Your Local Building Codes

You are responsible for your home being up to local building codes. For instance, if you add insulation to your concrete floor, this will necessarily raise the floor level.

Where ceiling height is regulated, this could put you at risk of violating those regulations. Get your questions answered before you get started.

Your best bet will be to get this information from a contractor, and it’s probably an even better idea to hire that contractor to do the insulation for you unless you are a seasoned DIY-er. Incorrect installation can eventually wreak real damage on your home at worst; at best, you won’t see any benefits from the insulation.

Underfloor Insulation R-Value

A word before we get into the options: You will also want to consider the R-value of the insulation you choose.

“R” stands for “resistance,” so R-value is the measure of how resistant a piece of insulation is to conductive heat flow. The higher the R-value, the better the insulation is at, well, insulating.

The United States Department of Energy provides a map of zones to help determine the minimum R-value recommended for your climate. The site also provides a corresponding chart to help determine the best insulation choices for your home.

Top Underfloor Insulation Options and Costs

Everyone knows there’s insulation in the walls and the attic, right? People rarely think of the floor, and if you’ve got a crawl space or some similar construction, the chances are good that you’re losing a lot of energy to that space beneath your house.

This is where the need for underfloor insulation comes in.

Here are the top four main insulation options:

  • Fiberglass batt
  • Foamboard
  • Spray foam
  • Vapor barrier

There are other kinds of insulation, but for our purposes here, dealing with underfloor concerns, these are our most common choices.

Each has its advantages, and very often, the specifics of your project may determine which you use, so you may not even have to decide on your own. A person can dream, right?

1. Fiberglass Batt

This is the insulation we see rolled out between joists in the attic floor and between the studs of exterior walls.

It is almost always fiberglass, which is excellent for insulating, but the filaments can be murder on the installers.

The batts are also available in cellulose, natural fibers, and plastic, but fiberglass is by far the most common type.

Try this Owens Corning’s R-13 Faced Insulation Roll, available through Amazon. Perfectly sized for 2×4 walls, one roll can cover around 40 square feet.

The R-value of batt insulation usually averages around 3.25 per inch of batting. As with any R-valued insulation, using multiple layers increases the value. Depending on many variables, expect to pay between $.30 and $1.00 per square foot of batt insulation.

2. Foam Board

Often having a polyurethane core, foam board is usually sold in 4’ x 8’ sheets and offers a lightweight, easily shapeable (via trimming) insulation.

This type of insulation has an added perk in that the exterior often acts as a moisture barrier. Building codes almost always require foam board to be covered by sheetrock, so this may not be best for crawl space applications.

The main draw to foam board is that it has a higher R-value in relation to its thickness than batting. Its R-value is usually just under 4 per inch of insulation.

It usually comes in inch-thick sheets and runs between $.30 and $.95 per square foot, but that price range does not include labor for installation, which can add almost $5 per square foot to the cost.

3. Spray Foam

Perhaps best used for hard-to-reach or oddly shaped areas, spray foam gets sprayed into the space, then expands to fill all the nooks and crannies.

It is available either in open- or closed-cell varieties. Closed-cell foam is denser, has a higher R-value, and is more expensive.

Spray foam, in general, costs more than other types of insulation per square foot and the R-value ranges from a little under 4 to about 6.5 per inch of thickness.

Consumers can buy spray foam in a 12-oz can, such as Loctite’s Tite Foam. It is flexible yet durable with strong bonding abilities to everything from wood to metal to PVC.

But this is typically for gaps here and there. For a whole floor, you’d need a much larger source of foam, and this may require a contractor’s assistance.

Spray foam can run as much as $2.50 per square foot. Still, as with other costs mentioned in this article, these are averages and will be affected positively and negatively by many, many variables specific to your project.

Do not base your project’s budget on these figures alone.

4. Vapor Barrier

The vapor barrier isn’t so much a kind of insulation as it is a necessary part of the insulation process, especially when you’re dealing with underfloor insulation.

It will stop water vapor from entering your walls, ceilings, and floors. Moisture, as always, can cause mold and mildew, but it can also lower the R-value of the insulation in the walls.

Vapor barriers do not have R-values.

Read also: The Best Flooring for Insulation and Energy Efficiency

Tips to Keep in Mind About Insulating Specific Floors

Different underfloors present unique challenges when it comes to insulation.

Crawl Spaces

Insulating this type of floor is probably the easiest of all, provided you can access the crawl space (there are homes whose crawl spaces inexplicably do not have access hatches).

More than likely, when you get under the house and lie on your back and look up, you’re going to be looking at the bottom of your home’s floor. If there is not a vapor barrier installed, you’ll probably want to install one before proceeding.

In the case of a crawl space, fiberglass batt insulation is probably your easiest and best bet for the project. You’ll be able to staple the batting in place and instantly add to your home’s ability to retain heat.

This will be logistically more difficult than rolling it out on your attic floor was, but an extra pair of hands can help hold things in place while you staple or otherwise fasten the batting in place.

Another option here is using foam board instead of the batts. It easily cuts to fit with a utility knife, though there exists a higher possibility of gaps that will negate some of the benefits of insulating the floor.

No matter which you choose, resist the temptation to close up all the ventilation ducts. Crawl spaces must have some ventilation. Without it, moisture will build up down there, causing mold, mildew, and rot that can cause catastrophic damage to your property.

Floating Timber Floors

Often a Victorian house feature, the floating timber floor will look like the floor above a crawl space.

The problem, if there is one, can come in that without a crawl space, you have no ease of access. Your choices then get more daunting than getting under the house with a hired neighborhood kid whose job it is to hold the batt in place.

You may face the task of pulling up your floor to lay the batt down. This may be prohibitively expensive, or it may be too big a task for you to take on by yourself, with or without the hired neighborhood kid.

Spray foam is another option here. Since many insulation companies offer injection foam as an option for your pre-existing walls, so you don’t have to rip up the drywall, the same principle works here.

You may be able to inject the foam into small holes in the floor and fill the voids between the floor joists.

Concrete Floors

There are two approaches here, one for before the concrete is poured and one after. A vapor barrier, followed by foam board, can be laid down before the concrete goes in, and this will sufficiently insulate the floor that the concrete will either become or support.

If the concrete has already been poured, you will need to decide if ceiling height is a concern.

If it isn’t– meaning if you have plenty of headroom to deal with– you might consider building a subfloor, insulating as you would a floating timber or crawl space floor, then putting flooring down on top of that now-insulated subfloor.

If you don’t have the clearance to add a couple of inches to the height of the floor, you’ll want to use a modified version of foam board specifically for this type of job.

They combine a vapor barrier, insulation, and a subfloor-like backing that will face up and be ready for you to lay flooring on top of it.

Either way, insulating your concrete floor is as vital to your home’s efficiency as doing the same to your crawl space floor.


Insulating your floor is an often overlooked task that, when left undone, can cost you money each and every month. Taking the time and budgeting for the project will increase your home’s efficiency and save you money in the long run.

Properly installed vapor barriers can stave off decay and damage to your home caused by moisture. Correctly installed insulation will make your home more comfortable and leave you with one less thing to worry about when it comes to your homeownership odyssey.

You may also like: Are Squeaky Floors a Structural Problem?

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