How to Insulate Interior Basement Walls

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Why Insulate your basement walls?

There are many benefits to insulating your basement walls and they all will help to save you money in the long run. First of all, insulating will help keep your basement warm, even in the really cold months of the year, which will allow your heating system to operate more efficiently. Secondly, it will reduce your exposure to moisture. Not only could moisture ruin your carpet and drywall and rot your framing, it can create mold which can put your health at risk.

What type of insulation?

For insulating against a concrete wall you will want to buy 2″ thick foam board, which has a R value of 10. R value represents the ability that the material has to resist heat flow, the higher the R value, the less heat loss you will have in your basement (which will save you lots of money on heating fuel). There are two types of rigid insulating material, XPS board and EPS foam board. They come in 2×8 and 4×8 sheets, which cost about $22 and $35 respectively.

I recommend getting the XPS board over the EPS board. XPS is more moisture resistant and will hold a better R Value when facing lower temperatures when water is present. For more information on that you can check out Owens Corning #sciencedoesntlie.

How to install insulation

Before you get started, make sure your concrete walls do not have any leaks and you have already applied concrete sealer. Be sure to take care of any waterproofing issues before installing insulation. For more on that, check out my blog on waterproofing.

Safety Equipment:

  • Gloves
  • Safety Glasses (optional)
  • Dust Mask (optional)

Tools:

  • Utility Knife
  • Combination Square
  • Tape Measure
  • Caulk Gun for Foam Board Adhesive
  • Saw Horses
  • Drill with 5/16” Masonry Drill Bit

Materials:

  • 4′ x 8′ x 2” R10 XPS Board – Owens Corning
  • Foam Board Adhesive – Loctite
  • Spray Insulation – Great Stuff
  • Gorilla Tape
  • 3 ½” Masonry Fasteners 

Measure and Cut Insulation

Measure the height of your wall and cut the foam board to that measurement. If you have a height of 88” and you know the foam board is 96” long, then you can set your combination square to 8” and place your utility knife at the edge of the square and score the height of the insulation board. Go back and make a deeper cut into the board and then you can snap off the the scrap insulation.

Quick Tip: Save your scraps for placing in between the floor joists, which we will get to later.

Securing Insulation

Load the foam board adhesive into the caulk gun and generously apply to the back of the cut insulation. Place it on the concrete wall and press all along the sides and the middle of the board for 10 or 15 seconds.

Once it feels relatively secure you can then take your drill and the 5/16” masonry bit and drill a couple of holes in the middle of the board. I would aim for the middle of the board at least a couple feet apart. Don’t go to deep, probably an inch into the concrete is far enough.

Next, take your masonry fasteners and tap them into your drilled holes. Take a piece of tape and cover the plastic fasteners. Once you have the insulation boards tightly secured next to each other, take the gorilla tape and apply it where the boards join.

Insulate the Floor Joist

Where the floor joist rests on the top of the concrete wall is a little square section that can allow a surprising amount of airflow, if not insulated correctly. You can take your scrap pieces of XPS board and cut them to fit in between the floor joists. Securing with just the adhesive should work fine. Use the spray insulation to fill around the perimeter gaps to reduce airflow.

Final Note

You should feel a difference in the temperature in the basement within a few days. With the insulation installed correctly, the temperature should never get too hot or too cool. If you find later that there are some areas that feel cooler than you can usually feel the airflow coming from a crack that didn’t get filled. Just use the spray insulation and fill in any areas that might be leaking air into your basement.

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