How To Insulate Under Basement Stairs

Insulating your home is not rocket science. However, the space under the stairs in your basement is usually forgotten and can result in a substantial amount of heat loss. We have consulted with the experts to guide us on how best to insulate it.

The methods for insulating the underside of the stairs include:

  • Foam board insulation
  • Spray foam insulation

To insulate that little space under your stairs will not only keep your home warm and comfortable but also help to lower the energy bill. Continue reading to get a complete picture of how to insulate under the basement stairs, including the adjacent space and the floor.

spray foam and foam board insulation side by side used in basement. How To Insulate Under Basement Stairs

Basement Stairs Insulation Techniques

We'll tackle the simplest method of insulating under basement stairs, foam board insulation, and the more difficult method, spray foam.

Insulating under basement stairs involves working on the adjacent wall and the floor underneath to ensure there is no heat loss. But first, let’s start with how to insulate the underside.

Foam Board Insulation

Foam board is an excellent material for insulation because it is resistant to moisture and is easy to work with. The basic steps to follow include:

Prep Tools And Materials Needed

  • Nail gun
  • Foam board
  • 1 by 3-inch wood strapping
  • Drywall tape and putty
  • Drywall
  • 10d nails

Step-By-Step Guide

  1. Start by strapping the wood to the joists on the wall under the staircase. Place the wood perpendicular to the joist every 16 inches.
  2. At 8 inch intervals, tightly nail the board to the strapping wood on the top, bottom, and the cavities on the sides of the staircase. You can safely use a 1-inch board in thickness, but if you live in extremely cold areas, 2-inch thickness will be fine.
  3. Finally, for extra security, add the drywall onto the strapping wood. Then tape the edges of the drywall and putty the seams for a smooth finish. If you wish, you can paint the adjacent wall to blend with the rest of the decor, or simply leave it as it is.

Foam board insulation can also be used on the wall adjacent to the stairs.

Construction worker fixing wooden stairs with polyurethane spray gun

Spray Foam

Spray foam is another good option for sealing under the stairs of your basement. However, this technique is a little more challenging to use, and you may require the help of an expert. The advantage of spray foam is that it seals any air leaks providing excellent insulation.

The foam sealant is a liquid chemical product that expands very slowly. This is unlike the other sprays that you know that expand fast in seconds. The slow rise gives it time to penetrate even the smallest crack or hole.

You can also use it on the wall adjacent to the basement stairs. But, remember it cannot be added to your wall if it has already been insulated using fiberglass.

Click here to see this spray foam sealant on Amazon.

How Do You Drywall Basement Stairs?

After you finish fixing the foam board insulation on the underside of your stairs, you may need to think about drywalling to improve the aesthetic look and improve the safety of your home, as it can prevent fires.

The building codes in your area will dictate the type of drywall you will need. But most require that you install not less than ½ inch thick of drywall under the stairs. In some areas, it is not necessary to have fire-rated drywall, but it must not be lower than the recommended ½ inch thickness. 

Some excellent tips for successfully installing drywall include:

  • Include extra framing if needed. For example, if you only have two stringers on either edge of your staircase, then you may add an extra one at the center so that your drywall can have adequate support. By doing this, you will avoid sagging and bulging.

  • For extra stability of your drywall, you can also add 2-inch by 4-inch dry walls perpendicularly to the stringers at intervals of between 16 to 24 inches.

Should Basement Insulation Go To The Floor?

Yes, you should extend the basement insulation to the floor for the following benefits:

Prevents Loss Of Heat

As the basement accounts for one-third of the heat loss in your home, proper insulation can reduce heat loss.

You will need to know the type of foundation or ground your home is built on when choosing the right insulation. For example, if your home is built on rock, your home will lose more heat than if it were built on clay.

Prevents Moisture

Even though concrete appears to be non-permeable, it is relatively porous. This is because moisture slowly seeps from the ground, leading to the growth of mold and mildew. Insulating will tackle this effectively.


The floor can be very cold as it tends to suck in the heat around it. But, insulation greatly diminishes this, creating a much warmer and comfortable basement to live in or store your belongings.

How To Choose The Correct Insulation For Your Floor

If your ceiling is high enough, or you have enough ceiling clearance, then you can add extra inches to your floor. This can be done by installing a polyethylene that is an excellent vapor barrier sheet, which when installed goes upwards at 3 inches. It is then followed by foam insulation to seal at the seams using tape.

Afterward, you should install ¾ inch of sleepers that have been pressure treated. They are mainly planks of wood where the sub-floor will be nailed.

The sleepers are then glued down with adhesive and nailed to the concrete under the insulation. Next, install ¾ to 5/8 inches of plywood as the sub-floor. Finish off by installing the flooring of your choice.

If your ceiling height is low, your best option is the insulated subfloor panel. They consist of rigid foam insulation sandwiched between a sub-floor material such as plywood or OSB and a vapor barrier.

Types Of Basement Insulation

The following are some of the basement insulation methods you can use:

  • Radiant Heat Insulation - When you install this on top of your sub-floor, there will be heat sent throughout your floor, keeping you very warm during winter.

  • Electric Heating Mats - This goes very well with vinyl plank and laminate flooring, carpet, or even tile. The heating mats are usually installed on these types of floors. These mats work by connecting them to the thermostat using wires.

Where Should I Put The Vapor Barrier In My Basement?

A vapor barrier is excellent insulation in your basement floor and also prevents moisture from seeping into your basement. It also prevents the build-up of mold and mildew. As seen above, you normally work on the first layer of insulation before other layers of flooring are added.

Polyurethane is the most common sheet used as a vapor barrier. It is stapled to the frame on your basement wall before applying drywall.

It is, however, important to note that it does not completely stop moisture from seeping through into your basement, but it will prevent water from entering your basement.

Is Insulating A Basement Worth It?

Insulating a basement is definitely worth it. After identifying the correct insulation depending on the type of ground your home is built on, you will benefit by keeping warm throughout winter. This is because it prevents the loss of heat.

In addition, the stubborn mildew and mold that can damage your wooden furniture will be put at bay.

How about saving on some money? You will conserve energy by preventing heat loss, which will lower your utility bill.

A room at newly constructed home is sprayed with liquid insulating foam. How To Insulate Under Basement Stairs

In Closing

Insulating under the basement stairs is one crucial place that should never be overlooked. A lot of heat is lost from this small space if it is not insulated. And this is done using foam board insulation that will keep the heat in.

That notwithstanding, do not forget to insulate the adjacent wall with spray foam. The floor is also an important place to insulate to keep the heat within the room. With various methods to insulate the floor, choose one depending on the ground the house has been built on.

Finally, by insulating under your basement stairs, you will save on your energy bills.

Check out these previous posts and learn more about framing your basement and drywalling the basement ceiling:

What Wood To Use For Framing Basement

Should You Drywall Basement Ceiling?

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