How To Insulate A Cement Or Concrete Floor

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Concrete or cement floors are notoriously cold. Either to fix this issue or as part of a room remodel, you might be wondering how to insulate a cement or concrete floor. In this post, we combine industry professional knowledge and up-to-date research to thoroughly answer your question.

To insulate an existing cement or concrete floor there are two main options. You can install premade tongue-and-groove insulation panels, or you can add foam board on top of the floor. To add foam board, follow these directions:

  1. Repair and clean flooring
  2. Install vapor barrier
  3. Add foam board insulation
  4. Tape seams in foam board
  5. Add sleepers or plywood
  6. Add flooring or plywood
  7. Add thresholds as needed

Keep reading the rest of this post for details on how to fully accomplish this insulation job on your own. Here, we'll focus on installing foam board panels but also touch on the premade insulation in a later section. To conclude, we'll answer a few questions related to this post.

Extruded polystyrene foam boards laid on balcony concrete floor ready to start the application of thermal insulation, How To Insulate A Cement Or Concrete Floor

How To Insulate A Cement Or Concrete Floor

While adding rigid foam panels is a popular way to insulate concrete and cement floors, it does come with a major downside. Namely, this method adds several inches to half a foot of height to your floor.

In some cases, the added height may make the room unusable for your desired purpose. In these situations, using the thinner premade tongue-and-groove insulating panels is a good option. The directions for the specific application products are provided on their packaging.

Best insulation over a concrete or cement floor

Here, we'll cover how to add rigid foam board to concrete or cement flooring. We'll detail this method because it is considered the best insulation technique for this situation. Read the following subsections for a step-by-step breakdown of this process.

1. Repair and Clean Flooring

After installing the insulation, it will be very difficult and costly to reach the floor to make any repairs. Further, if the concrete or cement is out of level, sagging, or cracked, these imperfections will persist through the insulation up to the new flooring.

For these reasons, it is imperative to clean and repair your floor before taking any other steps. Use a self-leveling compound to even out any areas as needed. For cracks, it may be necessary to use a concrete patching material.

Click here for a crack and self-leveling repair product from Amazon.

2. Install Vapor Barrier

Polyethylene protection vapour barrier

Once all your repairs have fully cured, it is time to lay down a vapor barrier. According to the US Department of Energy, this layer is important because the concrete or cement is actually relatively porous and will allow moisture through.

Without a vapor barrier, that moisture would damage the insulation and wood elements you are about to install.

Click here for 6-mil plastic sheeting from Amazon.

Generally, 6-mil plastic sheeting is best for this job. All you need to do is lay this out so it covers the entire floor. Overlap all seams by 6-8 inches and run the sheeting up the wall far enough to account for your future insulation/flooring assembly.

Click here for construction seaming tape from Amazon.

Once down, tape all seams and edges with seam sealing construction tape. This tape step is very important because it facilitates a continuous vapor barrier layer. For best results, be sure to press the tape firmly into place.

3. Add Foam Board Insulation

Extruded polystyrene foam boards

Now, simply set the foam board on the ground. You can add one or two layers, but the more layers, the higher the final R-value. A higher R-value means lower utility bills and greater comfort. If you do add more than one layer, change directions for the second layer to increase total strength.

Click here for foam board insulation from Amazon.

The goal is to cover 100% of the floor. This will require cutting boards around obstructions and probably at the edges of the room. Cut the boards with a drywall saw, hack saw, and/or utility knife. To give yourself a little room for foam expansion and contraction, leave about 1/4 inch on all edges.

If there are areas where the foam board is being difficult and will not lay flat against the concrete, consider using masonry nails and washers to hold the foam board snugly down.

4. Tape Seams in Foam Board

Now that the foam board is down, use tape to seal all seams and edges. Once again, it is important to use heavy-duty construction tape. Duct tape and other household tapes will not last long enough or hold strong enough for this application.

5. Add Sleepers or Plywood

Now, it is time to add the next layer of material. For this, you can either add sleepers or plywood on top of the foam board. We'll cover each technique in the following two subsections.

Sleepers

Sleepers, in this case, would be about 3/4-inch pieces of lumber that will function to hold the subfloor and flooring up. Given the proximity to concrete, be sure to use pressure-treated wood for the sleepers.

You can attach the sleepers with a combination of foam board adhesive and masonry nails. Usually, flooring sleepers or joists are installed at least every 16 inches or even closer.

Click here for foam board adhesive from Amazon.

Importantly, the distance needs to be consistent between each sleeper and needs to be a multiple of 8 feet. This is so that standard 8-foot pieces of plywood will end and begin on the support structure of the sleeper joists.

Plywood

Sheets of stacked plywood in lumberyard

For the plywood option, be sure to use exterior-grade material. Generally, it is best to add two layers of 1/2-inch plywood. Alternate the directions of the plywood sheet from one layer to the next. This alternation provides a much more solid final subfloor.

For the first layer, you can simply lay it on top of the insulation. If you are having trouble getting it to lay flat, foam board adhesive and/or masonry nails may be appropriate. For the second layer, fasten it to the first layer with standard 7/8-inch screws.

6. Add Flooring or Subfloor

Now that you have added either sleepers or plywood, you can add a subfloor or flooring over the sleepers or plywood. Alternatively, you can also finish the plywood with paint or stain depending on how fine of a floor you want.

Man nailing down plywood sub-floor

Because of all the prep work, you can now add the flooring or subflooring of this layer as you normally would over wood framing. Be sure to cut the plastic vapor barrier that runs up the walls down to below either baseboard trim or the flooring level.

If you do want to add subfloor over your sleepers, it is common practice to see 3/4-inch sheets. Lay these perpendicular to the direction of the sleepers for a more solid easy installation job.

7. Add Thresholds as Needed

Now that you have added all this material, the old floor is much more insulated but also much taller. Thus, you will need to add thresholds at all doors to make the transition from the old flooring smoother.

Click here for a 2-inch rise threshold from Amazon.

Generally, you can find premade thresholds of most common height but you may need to frame or make something yourself.

An Alternative Method

Above, we discussed adding the sleepers on top of the panels. Generally, this provides plenty of support for the flooring. However, if you want to attach your sleepers more solidly, add them directly to the concrete floor on top of the vapor barrier.

In this case, the sleepers still need to be pressure treated but should be tall enough to stick up past however much depth of foam board you plan on adding.

The downside of this method is that it requires you to cut the foam board to fit between the sleepers. Also, it requires much more taping to create a solid barrier.

What insulation goes under a concrete floor?

Generally, rigid foam insulation is also the best choice for insulation under a concrete floor. Be sure to do your research on how to install this material, as there are a few tricks to the trade that will help with longevity and product quality.

Extruded polystyrene foam boards laid on floor

How thick should concrete floor insulation be?

There is no rule on how thick your concrete insulation should be. However, the thicker the insulation is, the better the R-value is. Thus, consider your budget, local climate, and room height needs when choosing how thick to make your concrete floor insulation.

Should you put Styrofoam under concrete?

No. Instead of styrofoam, use rigid foam insulation under your concrete. Styrofoam is not a standard product that builders put under concrete.

In Closing

In this post, we answered the question of how to insulate a concrete or cement floor. Here, we mainly discussed ways to accomplish this goal as a remodel. To conclude, we answered several questions related to the topic of this post. Good luck!

To learn more about insulating floors, check out these articles:

How Thick Should Concrete Floor Insulation Be?

How To Insulate The Kitchen Floor

How To Insulate Mobile Home Underbelly And Floor

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