Which Foam Board Insulation Is Best For Basement Walls?

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Are you planning to add insulation to your basement, and you’re wondering which foam board insulation is best for basement walls? Wonder no more, for we researched this question and have the answer for you.

The best foam board insulation to use for basements is the polyisocyanurate foam board. Specifically, you'll want to select a polyisocyanurate foam board with a coated glass face (CGF).

Unfortunately, not all basements can be insulated. There are conditions that a basement should meet so that it can be insulated. We will talk about all of that in the succeeding sections. Read on!

Construction worker installing styrofoam insulation sheets on house facade wall for thermal protection Which Foam Board Insulation Is Best For Basement

Which foam board insulation is best for basement walls?

Construction worker installing styrofoam insulation sheets on house facade wall for thermal protection.

Polyisocyanurate foam board starts in liquid form. Then it is sprayed on a substrate where it dries into its rigid board form. Because of this, all polyisocyanurate foam boards are faced. A common material used for facing polyisocyanurate foam board is aluminum foil.

However, when picking an insulation material for basement insulation, the material must be semi-permeable. This is necessary to allow moisture from the basement walls to dry internally.

Without a semi-permeable layer, the moisture will get trapped on the basement wall layer, and it can cause mold growth. Thus, the right polyisocyanurate foam board for basement insulation should have a semi-permeable facing instead of the usual aluminum.

Polyisocyanurate foam board with coated glass facer (CGF) is the best type to use on basement walls with its semi-permeable facer. A polyisocyanurate foam board with an aluminum foil facer can be used for basement ceilings instead.

What alternative foam boards can be used for basement wall insulation?

If your basement does not need a very high R-value, you can consider using other foam boards to insulate your basement walls.

Expanded polystyrene foam board (EPS)

At 4.6 R-value per inch, expanded polystyrene foam board has the advantage of having the highest R-value per dollar because of its low price. Despite its price, it meets or exceeds all building and energy codes.

EPS foam board doesn’t retain water, and it can be bought in faced or unfaced versions. Faced versions that are vapor retardant can be used as vapor barriers.

Extruded polystyrene foam board (XPS)

XPS foam boards are easy to recognize because they are normally colored bright blue, pink, or green. XPS takes the middle ground between an EPS and a polyisocyanurate foam board in cost and R-value. XPS foam boards are naturally semi-permeable, which makes them a common insulation material for basement walls.

It uses recyclable materials in manufacturing and is available in faced and unfaced versions. Its special properties make it a vapor retarder but not a vapor barrier.

Its disadvantage is that it can retain moisture for a long time.

How to tell if a basement can be insulated?

Basements are often left looking like this after initial construction, a cost saving move which allows for future expansion.

Not all basements are created equal; not all basement walls can be insulated. We listed the requirements that should be met before and during insulation in the sections below.

The good news is that most of these requirements can easily be achieved. The bad news, however, is that one of them can be impossible to accomplish in a finished house.

Filled cores

Concrete masonry walls that are below grade -these are walls or part of the walls that are below outside ground level- should be completely filled. If unfilled masonry is insulated, this can force moisture to accumulate within the hollow parts of the wall.

Moisture inside the basement walls has very little chance to evaporate anywhere else, and this can degrade the wall from within.

Maintain drying capacity on the interior side

The installation of insulation in basement walls should ensure that below-grade walls will retain their capacity to dry on the interior side. Below-grade walls cannot dry on their exterior side because it is below ground level, leaving their interior side as the only side where they can dry.

We provided installation steps below that can maintain the ability of a basement wall to dry on the interior side.

The basement wall system must be sealed from moisture-rich basement air

Once the insulation is installed, the air from the basement must no longer be able to reach the walls below grade. We have included this step in the basement wall insulation steps below.

How to install basement wall insulation?

Basement or crawl space with upper floor insulation and wooden support beams.

Now that we’re done talking about the requirements for installing insulation in the basement, and you’ve checked whether your basement can have insulation installed or not, we should proceed to the steps to properly install basement wall insulation.

Materials needed

Here are the materials that you would need to install foam board insulation on your basement walls:

  • Foam board adhesive
  • Square ruler
  • Marker
  • Measuring tape
  • Saw
  • Builder’s tape or duct tape
  • Polyurethane caulk
  • Gypsum board
  • Polyisocyanurate foam board with CGF
  • Canned spray foam
  • Cardboard cutter

The Dap polyurethane construction adhesive and sealant in 10.1 oz packaging is available on Amazon. Check it out through this link.

Installing basement wall insulation

Here are the steps to install basement wall insulation:

1. Prepare the foam board for installation

Cutting styrofoam block

  1. Measure the height of your basement wall up to the tip of the rim joist.
  2. If there are pipes along the basement wall, measure up to an inch from the bottom of the pipes. Make another measurement an inch from the top of the pipes to the top of your basement wall at the tip of the rim joist.
  3. Trim the foam board using the saw to match the height of the basement wall.
  4. Measure the thickness of your foam board.

2. Install foam board on the basement wall

  1. Apply a layer of polyurethane caulk on the basement floor, where you will install the polyisocyanurate foam board. Make sure to spread the caulk evenly to the surface where the edge of the polyisocyanurate foam board will connect the floor. This will create an air seal between the bottom of the polyisocyanurate foam board and the basement floor plate.
  2. Apply foam board adhesive on the side that does not have the facing.
  3. Attach the foam board on the basement wall with just enough force to let the adhesive stick to the wall. Do not apply too much pressure.
  4. Wait until the foam board adhesive is strong enough to take over, then let go.
  5. Repeat the same step until you have covered one side of the wall with polyisocyanurate foam board.
  6. Seal the seams between the foam boards with builder’s tape.
  7. When starting with the next wall, apply polyurethane caulk on the floor, same as before, and on the part of the installed foam board that will connect with the foam board that you will install.
  8. Repeat steps 1 to 7 until you have covered all the walls with polyisocyanurate foam board.
  9. Apply polyurethane caulk on top of the foam board where it makes contact with the rim joist. This is to seal this side of the insulation.
  10. Double-check the foam board for any gap or hole and seal them with the builder’s tape.

The Duck Tape Max Strength all-weather performance duct tape in silver color is available on Amazon. Check it out through this link.

3. Install the gypsum wall covering

  1. Trim the gypsum board based on the measurement of the wall minus half an inch.
  2. Apply an even layer of adhesive on the gypsum board.
  3. Attach the gypsum board to the polyisocyanurate foam board. Make sure that the gypsum board is half an inch away from the floor. This small space will serve as an exhaust for any moisture that will get to the basement wall. This will allow moisture on the basement wall to dry on the interior side of the wall. Attach screws every 12 to 16 inches, letting it sink a little without breaking the paper facing.
  4. Repeat the previous steps until you have covered all the walls with the gypsum board. Gypsum board is fire resistant, and this helps counter foam boards that are vulnerable to fire.

4. Seal gaps

  1. If you had to cut two pieces of foam board to make way for pipes, seal the gap between the insulation by spraying canned spray foam starting from the areas behind the pipe.
  2. Let the spray foam settle and dry overnight.
  3. Trim any excess foam with a cardboard cutter. Check for any gaps and seal them with the canned spray foam once more.

Conclusion

It is best to use polyisocyanurate foam board with CGF to insulate basement walls.

If you enjoyed this article, why not check these two other articles too:

How To Hide The Furnace In Your Basement [Inc. Unfinished One]

How Can I Make My Basement Warmer In The Summer?

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