Should You Put Rigid Insulation Under Concrete Slab?

Tired of paying astronomically high heating bills? Don’t want to have the same struggle in your new home? Putting rigid insulation under the concrete slab is a great way to ensure that your new space will be energy-efficient for as long as it stands! It’s required for many new structures to meet code, and it can also protect the concrete from water damage.

To ensure that the foundation of your home isn’t a major source of heat loss, you should install rigid insulation before pouring concrete. Doing this protects the concrete from moisture as well as keeps heat inside the home and is required by many modern building codes.

By the time you’ve finished reading, you’ll know the benefits of rigid insulation, different material choices, and the basics of installing rigid installation. Let’s get started!

Should You Put Rigid Insulation Under Concrete Slab?

Rigid insulation underneath concrete is an important step in construction. Although many current codes recognize its value and require it, it isn’t required by all building codes. Concrete may seem like an impervious material, but it’s actually quite vulnerable to moisture and is terrible at trapping heat.

Rigid insulation is the perfect way to keep your heating bills lower while simultaneously protecting your home’s concrete slab. It’s very popular in green building circles because it drastically cuts energy consumption. It also does a great job of keeping water away from the slab, preventing cracks and structural damage!

As much as 10% of the heat in a home – and therefore 10% of your energy bill – escapes from the floor. A 2-inch layer of rigid foam insulation is forty times more efficient at trapping heat than concrete alone. Although rigid foam can be expensive at the outset, the savings over the next few decades will far outweigh it.

The average home in America spends about $1,200 on heating each year. If you’re cutting your heating cost by 10%, that amounts to $120 a year – thousands of dollars saved over the life of the home. That makes it nice for resale value, too, especially considering the rising importance of environmentally-friendly lifestyles.

If you live in a dry or hot climate, such as the American Southwest, rigid foam insulation might not be necessary. Since you don’t experience seasonal freezing, it’s unlikely that your heating bill will change much. If you experience all four seasons – especially harsh winters – using rigid foam insulation under the concrete slab is a must!

Rigid foam is installed on top of the gravel and covered with a vapor barrier, reinforced concrete is poured directly on top. Why not do it the other way around with concrete on the very bottom? Heat-wise, you may not notice a difference, but your foundation will crack and crumble over time due to water damage.

What Kind of Insulation Goes Under Concrete Slab?

Compressive strength – the amount of weight the foam panels can support – is an important determining factor in under-grade insulation. The insulation used under a slab needs to meet strength requirements, or else it can slowly collapse under the weight, destroying the foundation of the home. Compressive strength is measured in PSI and refers to how many pounds one square inch of the insulation can support.

Since the weight of the concrete slab is spread equally across the whole foundation, the insulation doesn’t have to be as strong as you’d think. ASTM standards dictate a minimum of 10 PSI for under slab rigid insulation, and 15 is the most common.

Of course, you are free to choose rigid insulation with higher compressive strength, but your costs may go up unnecessarily. It may be a good idea, though, if you live in an area with loose/wet soil or lots of clay. Rigid insulation rated at 25 PSI will be stronger than the earth beneath it!

R-value is one of the most important specifications of under-grade insulation. R-value describes a material’s ability to resist heat transfer. The higher the R-value, the better it insulates. A 4-inch concrete slab has an R-value of 0.4, which is very low; heat escapes through concrete very easily.

XPS and EPS rigid insulation have an R-value of 4.7 and 3.6, respectively. But just because XPS has a higher R-value doesn’t make it the clear-cut best choice for rigid insulation.

EPS vs XPS Rigid Insulation

There are two common types of rigid insulation: EPS (expanded polystyrene) and XPS (extruded polystyrene). They’re both made of a synthetic material called polystyrene, which is similar to other oil-based materials like polyester and polypropylene. Although similar, they are different. It all boils down to how they perform. Insulation needs to perform well against both temperature and moisture to be effective.

R-value is very important when choosing insulation, and XPS has an R-value of 4.7 compared to EPS’ 3.6. It might seem like that puts XPS over the top, and for a while it did. XPS was the insulation of choice when laying foundations for decades, while EPS was used more in roofs and walls. Over time, though, EPS became more popular in use under concrete slabs.

Seeing that both EPS and XPS foam insulation was popular, Stork Twin City Testing decided to put them to the test. They found that, after being in the ground for 15 years in St. Paul, Minnesota, EPS and XPS performed drastically differently. The XPS had lost half its R-value, while the EPS retained 94% of its R-value.

On top of that, EPS absorbed far less water (still within the range of the ASTM standards) than XPS (18% over the water-absorption limit). That difference in moisture, in a very wet climate like Minnesota, effectively destroyed the XPS foam’s ability to do its job. Structurally, it was still sound, but it did not do a good job of insulating; EPS did.

So, which is better? If you live in a dry climate, it’s likely that you’ll never have a drastic reduction in r-value if you choose XPS rigid insulation. For those who live in very wet climates, it’s best to go with EPS rigid insulation for longevity and moisture resistance!

Rigid Insulation and Building Codes

Depending on where you live, rigid insulation under a concrete slab might be required! The IECC, a regulatory code that ensures new construction is environmentally conscious, requires a minimum of R-10 insulation in Climate Zones 4-8. You can check if you live in one of these zones on the IECC zone map.

If the IECC requires rigid insulation under concrete slab, you’ll also need to get an inspection before the concrete is poured. If you live in climate zones 1-3, rigid insulation is optional but can still be a good idea. No matter what, it’s always advisable to check local building codes for requirements about rigid insulation under slab-on-grade and basements!

Do You Need a Vapor Barrier Under a Concrete Slab?

A vapor barrier is an essential part of a good slab foundation. Since water from the soil soaks into the rigid insulation over time, it’s necessary to put a barrier between the foam and the concrete. In the past, 6-mi (6/1000”) vapor barriers were quite common, but the American Concrete Institute now states that a vapor barrier should never be less than 10-mi (10/1000”) thick.

Polyethylene material isn’t totally waterproof, so it isn’t just the thickness of the vapor barrier that matters. It needs to be dense enough to stop even microscopic water droplets. It is required that a vapor barrier have a permeance (moisture permeance rate) of 0.3 perms – meaning that only 0.3 grains of water vapor can pass through the barrier.

If you’re searching for a vapor barrier that will truly stop water in its tracks, look for vapor barriers that conform to ACI specifications. If a product is labeled as a “vapor retard”, it likely does not fully meet the ACI requirements, and shouldn’t be used in under-slab construction.

How to Insulate Concrete Slab Before Pouring

It’s not as simple as digging a hole and filling it in – although that’s exactly what I thought as a kid! Before the concrete is poured, these steps must be taken to ensure construction quality, code compliance, and structural integrity:

1. Dig to the appropriate depth. This could range from a few inches to several feet depending on the type of construction
2. Smooth and tamp the excavated ground. Rake out rough spots, fill in low spots with dirt. Then, tamp the ground until it is smooth and firm. This packs the soil down, providing more strength.
3. Spread a 4-inch layer of crushed stone/gravel over the dirt. Ensure that it is raked smooth and level, with no jagged edges sticking up.
4. Lay down rigid foam panels across the entire surface of the foundation. Butt the boards together lightly. Make sure all the boards stay flat to ensure maximum efficiency. You’ll also need to lay a 4-inch tall section of foam around the perimeter of the foundation- enough to be level with the concrete after it is poured.
5. Lay down the vapor barrier according to the manufacturer’s instructions. Make sure there is a 6-inch overlap on the edges of each roll.
6. Make sure everything is level and continuous, and you’re ready to pour!

How Much Does Rigid Insulation Cost?

In general, this is what you can expect to pay for rigid insulation, both per square foot and for common foundation areas. Prices can change depending on where you purchase, but this should give you a rough idea:

Type of Insulation (2-inch boards)	Price Per Square Foot	For 900 sq foot slab	For 1500 sq. foot slab	For 2200 sq. foot slab
EPS	$.77	$693	$1,155	$1,694
XPS	$1.01	$909	$1,515	$2,222

Note: these prices are what’s common at home improvement stores. You can likely purchase EPS and XPS cheaper through a different construction supplier; these numbers are simply to provide illustration. They also don’t include the cost of labor, gravel, or vapor barriers.

Can You Use Spray Foam Under Concrete Slab?

Using SPF (sprayable polyurethane foam) to insulate a foundation is an alternative to rigid foam panels that is comparable in cost. However, you must make sure that 2-pound closed-cell SPF is being used; any other type of sprayable foam won’t be dense enough to support the slab.

SPF is more expensive than rigid foam, but the labor is much cheaper. So, using sprayable foam under a concrete slab isn’t a bad idea! The R-value and compressive strength of SPF are roughly the same as EPS/XPS rigid insulation, so it will still meet all building codes.

Best Under Slab Insulation

The best way to purchase under-slab insulation is directly from a supplier, prices at home improvement stores are likely to be inflated. Below, you’ll find two of the best, most highly-recommended under-slab insulations. One of them is EPS, and the other is XPS:

1. Insulfoam R-Tech Panels

R-tech EPS rigid panels are one of the most popular insulations for walls, roofs, and below-grade uses. It’s available in a variety of sizes and thicknesses. It meets all code requirements and is affordably priced. If you drive past any local residential construction sites, odds are pretty high that you’ll see R-Tech being used somewhere. 2-Inch R-tech panels have a compressive strength of 13 PSI and an R-10 rating, making them suitable for every building code.

2. Owens-Corning Foamular 250

This is, far and away, the most popular XPS rigid insulation on the market. A 2-inch panel of Foamular 250 has 25 PSI and R-11 ratings, making it equally heat-efficient and twice as strong as R-Tech. It’s a perfect way to get a remodel or new construction up to code and keep it toasty for years to come! It was even used to insulate One World Trade Center in New York City!

Conclusion

Rigid foam under a concrete slab is a complex but important topic to dig into, especially when building codes come into play. Using foam insulation under a slab drastically lowers your energy bill, complies with modern codes, and protects the life of the slab. So, did you learn what you needed for your project? Still have questions or suggestions for other readers? Let us know in the comments section below!