By: Todd Bryson, Pacific Insulation

Many homeowners don’t think about insulating their crawl space or don’t know how to do it. A crawl space is the hollow area under the floors of some houses, between the ground and the first floor. It’s generally 1-3 feet high and is usually accessed as the name implies—by crawling. Crawl spaces elevate houses from the ground and provide an area to contain important home elements such as air conditioning and heating units, duct work, plumbing, electrical wiring, and insulation.

So, how do you insulate a crawl space?

Installing insulation in a crawl space is similar to other places you would insulate in your home, though the space is generally much smaller and less accessible. It’s important to properly prepare before installing insulation in your crawl space and consider insulation material that will not deteriorate due to moisture across time. Homeowners should also consider ventilation and whether it’s best to insulate the under floors or walls for the greatest efficiency.

Though most homeowners with crawl spaces don’t give them much thought, if left unsealed or uninsulated, this area will let outdoor air into the space, which creates an uncontrolled home environment. This can affect the air quality and temperature of the house. Therefore, properly insulating your crawl space is one of the primary ways to improve energy efficiency and indoor air quality, as well as reducing energy bills.

Insulation for Crawl Space

Many homes with crawl space foundations reveal basic signs of poor crawl space insulation. When it’s cold, the floor areas directly above the crawl space may be cold and even uncomfortable to walk on with bare feet or in just socks. In addition, another sign of improper crawl space insulation is musty odors coming up from the space under the house.

Using the wrong type of insulation for your crawl space can actually increase the risk that mold and mildew will grow in this dark, damp area. 

Therefore, it’s essential to make the right choice when it comes to the best insulation material to install in a crawl space.

Traditionally, crawl spaces were insulated with the installation of fiberglass batt insulation material between the exposed joists of the crawl space. Fiberglass is an inexpensive material compared to other types of insulation, and it’s very available as well.

However, fiberglass has a tendency to absorb moisture, support mold, harbor mice, and even sag or fall out of place. These negative qualities are indications that fiberglass is a viable option for crawl space insulation, but not necessarily the best choice.

A better choice when it comes to insulation material for crawl spaces is rigid foam insulation board. 

Instead of installing foam insulation between floor joists in a crawl space, the foam board can be installed against the crawl space foundation walls. Unlike fiberglass, rigid foam insulation does not absorb water, support mold, or allow air to pass through. This is especially true if the joints between the boards are sealed with tape.

Insulation Preparation for Crawl Space

It’s important to prepare before installing insulation in a crawl space since the area is limited and tight. 

In addition, crawl spaces can be very dirty and even feel a bit claustrophobic. Therefore, it’s essential to adequately prepare this space for insulation to get it right the first time.

It’s important to protect the rest of the home when insulating a crawl space. Here are some ways to prepare for insulation so that your house is not damaged due to the installation:

  • Remove any items stored under your house
  • Remove any lingering debris from under the house
  • Even out the dirt in the crawl space
  • Fix any standing water problems under the house
  • Cover and secure the area under the house with a later of plastic sheeting, running all the way to the foundation walls

It’s vital when installing insulation into any crawl space that you measure and cut each piece of material so that it fits correctly. There may be variations in the length of insulation needed due to slope, dirt, or pipes present. Measuring accurately will prevent any insulation gaps or problems during and after installation.

If possible, it’s best to properly ventilate an unventilated crawl space. This is essential to keeping your home dry, fighting mold, and protecting the structural integrity of the house as well as maintaining the health of those who live in it. Without ventilation, moisture can move from the ground up through cracks in the floor inside the home, which can breed mold, fungi, pests, and even rodents.

Insulating an Unventilated Crawl Space

To best insulate an unventilated crawl space, a homeowner should focus on installing insulation in the walls of the crawl space, not the subfloor of the home. This requires less insulation and can eliminate the need to separately insulate ducts and pipes.

In addition, an unventilated crawl space will heat or cool with the house, in which case faced insulation is recommended. Faced insulation is a blanket-type insulation material, typically made of fiberglass. Faced insulation has a vapor barrier, generally made of kraft paper, that blocks moisture from moving through one space to another.

Insulating a Ventilated Crawl Space

A ventilated crawl space is conducive for insulation because it aids in the elimination of moisture, which can cause eventual structural damage to the home. In the case of a ventilated area, homeowners should focus on installing insulation under the subfloor of the home. Fiberglass insulation can be easily installed under the subfloor, between the floor joists.

With a ventilated crawl space, unfaced insulation is the best choice. Unfaced insulation does not have the vapor barrier that faced insulation does. It’s important to secure insulation and cover it with a vapor barrier in order to prevent the harmful effects of moisture and mold.

Insulating Under Floors

If a crawl space is ventilated, then insulation can be easily installed under the subfloor between the floor joists.

The best choice when it comes to installing insulation between the floor joists in a crawl space is fiberglass batts or rolls.

This insulation material is economical, and the easiest for do it yourself (DIY) installation.
It’s recommended to install the insulation with the paper vapor barrier facing up towards the heated living space. You should make sure that the insulation fits tightly between the joists.

The insulation can be held in place from below with the use of insulation support wires or by using chicken wire to staple to the bottom of the joists. Stay or tension rods should be avoided, as they can come loose or compress the fiberglass and lessen the insulation value.
Holes in the subfloor for electrical wiring and plumbing should be sealed with spray foam insulation. Plumbing pipes and HVAC ducts should be insulated as well to prevent heat loss and freezing.


Insulating Crawl Space Walls

If a crawl space is not ventilated, it’s best to insulate the walls of the space rather than the subfloor of the room above. This is especially true for cold climates in which the most efficient technique is to insulate crawl space walls and close them off from the elements by sealing all air leaks. In this way, plumbing pipes and HVAC ducts are protected from freezing temperatures, which helps to conserve energy.

The best method for installing insulation in crawl space walls is to use rigid insulation material. This can be challenging and time-consuming, as well as more expensive, but it offers a permanent solution. In addition, if there are vents in the foundation, they must be removed with the holes sealed. Closing the vents is not sufficient.
Homeowners can glue rigid foam insulation board to the inside of foundation walls with a

waterproof construction adhesive. Seams can be sealed with waterproof tape. Crawl space floors incur moisture, whether it’s earth, gravel, or concrete. Therefore, a vapor barrier is necessary to prevent ground moisture.

If the crawl space is sealed from cold and moisture, it can be linked to the house’s HVAC system with vents to circulate warm air under the home’s floors. In warmer weather, vents can be closed when the air conditioner is running.

Benefits of Insulating a Crawl Space

Most homeowners think of insulating larger spaces in their homes such as attics and basements. Often, the benefits of insulating a crawl space aren’t considered. However, no matter what type of crawl space you have, there are benefits to insulating it so that it is a dry, usable space that adds value to your home. Some of these benefits are:

  • Lower utility bills
  • Safer home structure
  • Better indoor air quality
  • Indoor comfort
  • Less risk of moisture damage and pest problems

Uninsulated or under-insulated homes will end up costing homeowners more for trying to maintain comfortable temperatures through heating and cooling or resulting in heavy repairs due to potential water damage.

Insulating and sealing your crawl space will not only improve the energy efficiency and comfort of your home, but it will also prevent issues due to mold, mildew, and pests which can lead to health problems for home occupants.