How to Heat a Crawl Space | Step-by-Step Insulation Procedure

If you own a home with a crawl space (as most of us do), you know how frosty it can get during the cold months of winter.
When this happens, most homeowners usually rush to heat it with bulbs and other space heaters. Like most HVAC experts, I do not recommend this.

Placing a light bulb or crawlspace heater in an unattended crawl space could be a fire hazard, endangering your home.

Read along to learn how to heat a crawl space while keeping your home and family safe. 

Effective Ways of How to Heat a Crawl Space

The most effective way to winter-proof your crawl space is by insulating it to keep the cold outside air out.

Now, installing space heating devices in this part of your home may sound like a quick fix. But as we started by saying, it could put your home in danger of fire as the crawl space is generally unattended.

More importantly, a lot of the heat from the crawl space is likely to escape through its poorly insulated walls and vents. 

Uninsulated crawl space walls are a common problem in many homes. Usually, they have block or stone walls that are very porous and allow chilly outside air to come in. 

Other than the walls, these underground spaces tend to receive the least care during building construction.

As such, there are often gaps between the ground and the walls, openings around air conditioning lines and rim joists, wall cracks, and more. 

Together with drafty windows and doors, all these provide a lot of room for frosty outdoor air to enter the room, creating an unconditioned space right underneath your feet.

As a result, the floors are often too cold, significantly increasing your heating energy expenditure. 

Also, if you install or modify your existing space heater’s ductwork to make it heat the crawl, you run the risk of condensation.

The warm HVAC ducts inside a chilly underground space create the problem of condensation by adding extra moisture into the crawl.

Condensate drain lines can also leak the liquid down there, increasing the risk of molds and rot. 

How to Heat a Crawl Space and Insulating it

Closing vents is part of making a crawl space energy efficient and your home more comfortable. You also need to seal the rim joists and insulate the walls with appropriate material. 

What you’ll need for this task will depend on whether the crawl has a concrete or dirt floor.

A dirt floor tends to experience waterlogging and is a significant outlet for energy loss, so installing an insulation system on it is crucial.

For a concrete floor, you only need to insulate the walls. The walls require a vapor barrier, whether the floor is dirt or concrete.

The barrier is an effective way of preventing cold air from entering your home from outside through the walls.

What you will need

Here is a quick peek at the list of items you will need for the insulation task. We have selected these for the best results, but notice that the brands and types of insulation material can differ from one technician or project to another.

  • Spray foam (homemade spray foam insulation kit)
  • Foam boards (SilverGlow foam insulation at least 2 inches thick)
  • Drainage matting (preferably dimpled polyethylene sheet)
  • Spikes for hooking the matting to the floor
  • Hammer
  • TerraBlock insulation boards (to go over the drainage matting)
  • CleanSpace crawl space encapsulation membrane
  • Mastic tape

Insulating the crawl space – step by step 

The objective here is to completely envelope the crawl space, isolating it from the ground and the outside environment. 

To do that, we start by closing any vents and then insulating the entryways. After that, we insulate the perimeter joists, exposed ductwork, water pipes, and the perimeter foundation walls from the inside of the crawl space. 

For a crawl with a dirt floor, we install a polyurethane vapor barrier covering the entire floor and sealing it off from the inside of this subfloor space. 

Step 1: Seal off any vents

Many traditional-type homes have vents in their crawl spaces. While these are ideal for removing excess moisture during hot months of the year, they can disastrous your energy expenditure in extreme weather conditions. 

So the first thing to do is close these vents and seal them off with thick SilverGlow foam insulation. Manufacturers generally assign an R-value to the different insulation thicknesses.

For better insulation, you will want to go for a higher R-value.

Step 2: Plug any holes in the walls. 

Before installing perimeter insulation boards on the walls, it is important to seal off any holes bringing cold air in from outside. Begin by checking anywhere with piping or wiring running up into the house above. 

Often, you will find a significant amount of space left between these pipes or cables and the walls or joists. You must fill any such holes with mold-resistant insulation. 

We recommend using an expanding spray foam insulation which is a quick and very effective solution. However, the spray foam insulation does not support molds and mildew and will not allow these fungi to grow beneath it.

Step 3: Insulate piping and ducts.

The crawl space is typically a hotbed of ducts and wiring. You will find plumbing pipes and air ducts running between the joists and forming a complex network inside the crawl space in most homes.

Take a careful look at each of these pipes and ducts; study them closely for any damp spots that could indicate leaking areas. 

You will need to plug any such holes to stop the leaks before installing your insulation. In addition, any holes in the ducts connected to your home’s ventilation system create lots of room for energy loss. Again, these are hurting your wallet.

You can use mastic tape or another duct tape you can get on Amazon to seal the holes.

While at it, ensure you inspect all connection points into the floors or walls for openings into the outside environment. If there are any open seams, seal them properly with spray foam to increase efficiency.

Once this is done, install insulation over the exposed ductwork to prevent any heat loss and possible condensation during cold weather. 

You should find various insulation duct wraps for your home on Amazon or your local stress. The type and size you buy will depend on the kind of ductwork within your crawl space. 

In any case, the insulation should be made of fiberglass material. Usually, the fiberglass will be enclosed in a protective foil for mold resistance and to prevent condensation.

You may want to talk to your HVAC technician beforehand since different residential areas have different codes to follow. The contractor should be able to offer advice and appropriate guidance on what is allowed in your area. 

Step 4: Insulate the crawl space access door.

Every entry point to this space needs to be as airtight as possible, and the door is key among them. 

Doors accessible from the outside are particularly a major concern because it allows for chilly air to enter the crawl space directly from outside. You want to make this the most airtight it can be. 

Whether you have a make-shift or custom-made door for this part of your house, ensure you cover it with a foam insulation board inside and seal the outside edges with weather strips.  

Crawl space with a concrete floor 

At this point, the steps vary slightly for crawl space with concrete floors and one with a dirt floor. 

A dirt floor will require insulating the floor itself, while that step will not be necessary for a concrete floor. In the latter case, the general rule is to install insulation material on the underside of the sub-floor. 

Step 5: Insulating the sub-floor 

Measure the length and breadth of each cavity left by the sub-floor rim joists. After that, cut your insulation material in the form of long strips that sit snugly and tightly in between the joists, filling the gaps with the paper side pressed against the sub-floor. 

The idea is to close even the smallest gaps, allowing cold air through to the floor. We recommend using some form of support wire to secure the insulation setup in place.   

Step 6: Insulating the foundation walls

Now that you are done insulating the top, it is time to keep the cold outside air out by lining the walls with SilverGlow rigid foam boards.

Ensure the boards are long and large enough to envelop the entire perimeter of the foundation walls all around.

Also, they should be long enough to run from the stringer joists to the bottom, with about one to two feet extra sitting on the floor. 

To secure the insulation blanket in place, you will want to nail it down on the wooden sill plate with the help of furring strips.

To ensure the whole setup is completely airtight, seal up any joints and cracks with duct tape and foam spray.

The line between the insulation blanket and the sill plate will particularly require generous amounts of foam spray to seal it up. 

Step 7: Insulating the crawl space floor

This step is for a dirt floor. 

Dirt crawl space floors can often get pretty wet. To keep this wetness and the frosty temperatures associated with it sealed off underneath, cover the entire floor with a drainage mat. A dimpled polyethylene sheet will do a marvelous job at this. 

On top of the mat lay TerraBlock insulation boards to prevent heat loss through the ground. Then, follow this with CleanSpace encapsulation membrane lining the entire floor. 

This multi-layered plastic liner is one of a kind. It is UV treated and completely resistant to puncture. The CleanSpace sheet also has an antimicrobial layer built-in to prevent mold growth. 

As a final touch, fasten the clean space sheet to the walls and floor to get the whole place air sealed.

Final Thoughts

According to the Energy Star program, your crawl space is one of the primary opportunities to save energy – particularly if you live in a cold (or hot) climate where an inefficient crawl space can lead to quick energy losses.

While there are many variants to the methods of insulating your crawl space, what we have here is a superior thermal protection approach that curbs energy losses by preventing all access of the cold outside air into your home through the crawl space.

We hope that now you know how to heat a crawl space as well as insulate the space to prevent cold outside air from coming inside your home. Let us know your thoughts in the comment section below.

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Ronald Watford, the owner of Quality Home Air Care, is a qualified HVACR technician and manages the team of expert writers on this site. He believes that educating homeowners about HVAC systems is one of the most impactful aspects of his job.