The Pros and Cons of Under Slab Insulation (Is It Needed?)

Despite the many other options modern technology has made possible, under slab insulation is quickly becoming popular due to its sustainability and effectiveness.

However, like any other insulation method, it has its strengths and weaknesses. What are they?

The pros of under slab insulation are better energy efficiency and improved floor structural stability. It’s also environmentally friendly.

However, the insulation may cause moisture absorption, allow insect nesting, and compromise your home’s fire resistance.

In the rest of this post, I’ll explore the benefits and drawbacks of under-concrete insulation to help you decide whether your home needs it.

I’ll also go over some tips and tricks for buying a house with slab insulation.

Let’s get started!

The Pros of Under Slab Insulation

Here’s an in-depth look at the benefits of under slab insulation.

Lower Energy Bills

The cost of heating or cooling indoor spaces built on under slab insulation is a lot lower due to its effectiveness.

Some estimates say that slab insulation technology is an investment that pays for itself within the first decade through energy savings alone.

Insulating a slab prevents heat loss through conduction, which significantly offsets the energy costs of indoor climate control.

Along with using energy-efficient tax credits, this is one of the best ways to save money on energy bills.

Lower Environmental Impact

Homes with under-concrete insulation spend far less energy to heat or cool indoor spaces, which means they use less energy overall.

As such, slab insulation is a great way to reduce your carbon footprint effortlessly.

This system also reduces construction costs because it improves construction efficiency. That means the environmental impact is less from the construction phase and not only after you use the building.

This is your go-to insulation method if you want to incorporate eco-friendly construction methods into your projects.

Comprehensive Strength

Slab insulation makes the floor stronger.

Its comprehensive strength is convenient for building a multi-story home because it significantly boosts the foundation’s structural integrity.

The added structural strength from insulating under a slab is similar to reinforced concrete.

Better yet, under slab insulation is comparatively more efficient to install. That means you benefit from its structural strength for a lower energy (and monetary) cost.

Lower Construction Costs

Under slab insulation is the least expensive foundation system available today.

There are several reasons for its cost-effectiveness, but the main one has to be the fact that the insulation is built in.

For example, many houses built on a raised foundation need to insulate underneath using spray foam, which is costly.

Depending on the budget and the contractor’s preference, raised foundation homes might use high-density closed-cell spray foam or low-density open-cell spray foam.

A cheaper option to insulate under raised-foundation houses is installing insulation batts.

If you’ve installed under slab insulation, the foundation is already insulated, and there is no need to waste money on the insulation above.

Although very similar to ICF foundations, under-slab insulated foundations do not include a basement. Also, it takes a lot less time to dry slab-on-grade foundations.

Ultimately, these two factors reduce your construction costs by a lot.

Construction Efficiency

Another advantage of under slab insulation is construction efficiency.

For starters, it allows you to build a foundation and insulation layer simultaneously, saving time and other resources.

Since this type of foundation takes less time to dry than concrete foundations, it significantly improves the efficiency rating of the construction project.

Of course, this positively impacts how green and sustainable your project is and makes it much more affordable.

The Cons of Under Slab Insulation

Insulation systems aren’t flawless. So despite the many advantages of using under slab insulation, there are still a few drawbacks you may want to consider.

It Increases the Risk of Moisture Absorption

This is arguably the biggest drawback of under slab insulation.

Due to how the foundation is constructed, any moisture around it might be absorbed.

Moisture absorption can lead to slab cracking. A cracked slab is significantly weaker, and that weakness compromises the rest of the building’s structural integrity.

This may also decrease the value of a property, which means you might lose money if you intend to sell your house.

Note that fixing a crack in the foundation slab is doable. It happens through a process known as slab jacking, which involves raising the foundation and injecting fillers to restore the slab.

However, many prospective buyers prefer buying a house where the foundation has never been slab-jacked.

Also, heating systems and plumbing would need to be installed above ground to avoid moisture absorption. This might seem like an aesthetics issue, but it’s also a waste of space inside your home.

It Might Contribute to Insect Nesting

Pests cannot enter your home through under slab insulation systems.

However, some insects have been known to nest within the slab insulation, which makes it easier to infest the building through other openings (for example, through ground-floor windows).

In a way, slab insulation adds extra vulnerability.

Treat slab insulation with a good pesticide before installation to address this vulnerability.

Also, consider consulting a professional if your home’s location and natural surroundings make insects a serious problem.

Flammability

Some research has posed concerns about the flammability of slab insulation. However, we still need to learn more about this topic.

Slab insulation’s flammability is manageable if the house is up to code because building regulations boost fire resistance in other ways.

Nonetheless, this drawback is worth considering if you live in an area where wildfires are common.

If the EPS foam of under slab insulation is exposed to a fire, it will burn, and the fire would be difficult to put out.

No Basement for Hurricane Protection

Last but not least, using under slab insulation systems means you can’t have a basement.

This might reduce construction costs and increase efficiency, but basements are helpful for riding out hurricanes.

There may be better options than this if you live in a hurricane-prone area because you must consider hurricane protection measures when building.

On the other hand, the lack of a basement may be fine if you live in an area with little to no storms.

Is Under Slab Insulation Necessary?

For those living in sweltering climates, under slab insulation provides an efficient and affordable way to insulate and reinforce the home.

However, it isn’t necessary because other modern alternatives address its weaknesses.

I also don’t recommend under slab insulation for buildings in colder climates. There is a higher risk of slab cracking due to extreme temperature changes.

The Best Insulations for Under Concrete Slab

The commonly used insulations for under concrete slab include:

• Expanded polystyrene (EPS)
• Extruded polystyrene (XPS)
• Radiant insulation like Prodex

Let’s explore the basic qualities of each insulation type.

Extruded Polystyrene (XPS)

• An R-value of approximately 4.7 per inch
• The main colors are green, blue, or pink
• Either square or rectangular in shape
• Quite costly than expanded polystyrene

Expanded Polystyrene (EPS)

• Made up of 2% plastic and 98% trapped air
• An R-value of approximately 3.6 per inch
• Mainly white in color
• Releases moisture quickly

Prodex

• Is compression-resistant
• Prevents the entry of termites and other pests
• Is easy to install
• Is non-toxic
• It seals well around nails

Tips for Buying Houses With Slab Insulation

Here are a few things you need to keep in mind if you’re in the market for a house built on slab insulation.

• The slab should be intact. If there is already a crack in the slab, that significantly reduces the foundation’s lifespan and drops your property value. It’s best to hire a professional to inspect the foundation for cracks rather than judging on your own.
• Wood sidings attract insects. Even though pests won’t be able to eat through the insulation, they might still be able to get into your house if they’re nesting. If the house has wood sidings in contact with the ground, there is a greater chance you’ll have an insect problem.
• Ask about the type of insulation. Not all insulation is equal. You want to be sure the insulation used is not flammable and is treated with pesticide to avoid insect nesting. Don’t settle for just any insulation.
• Ask for a geological report. The soil under the slab should be hard. Soft soil will settle the house over time, and the only way to remedy that is to inject insulation in the slab to raise the building. Even with the insulation injection, settling would still be an ongoing problem. The house would depreciate much faster than any other real estate.
• Check if the building is up to code. Often, contractors and homeowners insulate under the slab without regard to building codes and regulations. As a result, buying such a house is a bad investment because you’ll need to spend a lot of money fixing it up to comply with government regulations and address safety concerns.

Final Thoughts

Now that you know the pros and cons of under slab insulation, you can see why this is a highly sought-after system.

Of course, no system is perfect, but modern technology has allowed us to overcome the negligible disadvantages of using under slab insulation.

If you live in an area with an extreme climate, under slab insulation is one the cheapest ways to reduce energy costs and maintain comfortable indoor temperatures.