Construction Design-Closed Cell Spray Foam Below a Slab

This post originally appeared on GreenBuildingAdvisor – Green Building Advisor is the one-stop source for builders, remodelers, architects & homeowners looking for expert advice on green products, strategies & proven construction details.

I was introduced to using closed cell spray foam (CCSF) below a slab a few years ago by New England contractor Wade Paquin of WKP Construction. He was insulating the below grade slabs of his new homes by spraying a couple inches over a bed of stone, then pouring the concrete. I have now had the opportunity to try this insulation method over a couple projects, I feel there are many benefits to this below grade insulating technique.  This method isn’t for everyone, some people have strong feeling both pro and con about using any foam insulation products.  I fall in the middle, I feel there are times when it is the best solution, but if I have another option, I will definitely consider it.

Photos by Wade Paquin

Common in my market.

Frost protected slab on grade construction has been common in my market for several years. I collaborated with my concrete contractor to design an insulation system for a frost protected shallow foundation which we used on several different projects.  I have not had any call-backs on any of these projects, so I’m assuming everything has been working as designed. The drawback is the amount of time it takes to do a good job installing the insulation and then detailing the polyethylene sheeting used for air, vapor and radon control, often taking multiple days, especially on larger projects.  The photo below shows a smaller guest house I built in 2011, the project took two days to install and seal the sub-slab insulation. There are two inches of XPS foam under the slab and footing, two inches covering the slab edge, which was cast in place during the concrete pour, and then four feet of two-inch XPS used as a wing for the perimeter foundation insulation. (See the IRC Table R403.3(1) for local requirements).

A problem I had identified.

During blower door testing, I have identified on several different slab on grade homes, air leaking up through penetrations in the slab. I believe the air is coming down the radon pipe on the roof, entering the radon rock bed and then finding a path along a pipe, cast in place tub box or other floor penetration.  If air is able to enter through these areas, so can radon. Sealing these penetrations often is not well executed and easily damaged during the concrete pour. Finding an alternative to taping poly to pipes was something on my mind.

Typical sub-slab poly installation in my market.

The Code Minimum Build.

My first time using CCSF below grade was on a build I was involved with in 2020 we named the Code Minimum House. A Code-Minimum Case Study – GreenBuildingAdvisor  We used a manufactured form called the Mono Slab EZ Form for the slab edge and wing requirements of a frost protected slab on grade foundation.  But instead of placing rigid foam under the slab, we instead had a spray foam contractor come in and apply 1.5 inches of closed cell spray foam.  This CCSF is a two pound density with an insulation value of R-6.9 per inch, meeting our sub-slab insulation requirement of R-10.

The main heating system for the home is radiant hot water, ½ inch tubing was installed prior to the concrete slab pour. The spray foam insulation installation on this 1500 square foot home and 750 square foot attached garage took four hours, saving roughly a couple days of labor costs.

The Barndominium Project.

Our current project using sub-slab CCSF is a barndominium or post and frame build. This is a frost protected foundation using all closed cell spray foam, both on the under-slab interior and as the slab edge/foundation wing insulation. There is approximately 8,000 square feet of heated storage and an additional 1500 square feet of living quarters (two levels of 750 square feet) in this building. The homeowner had a few requirements in this build, first is it’s generational, meaning he is planning on this being his family’s property for generations, and second, he wanted the building to be as efficient as possible. The post and frame structure has its challenges, but we designed the slab insulation to lose as little heat as possible. We choose to install 3 inches of closed cell spray foam below the slab, mostly because the heat source for this structure is hot water geo-thermal. This operates at a lower water temperature than a gas fired or electric boiler, which required the in-floor tubing to be at a tighter spacing and at an increased size (5/8”) over what would typically be used. We wanted as much heat as possible to move up into the structure. The insulation contractor was on site for a little over two days, the first day was spent on the 8000 square feet of the storage slab area and less than half day spraying the living quarters. A final day was needed to address the slab edge, insulation under several concrete aprons and the horizontal insulation wing. My estimate is that it would have taken over a week to install and seal rigid foam for this project. This project is being completed late in our building season, cold weather was coming, speed of completion was a concern.

Observations and suggestions

I would recommend installing the CCSF over stone. The foam can be installed in one lift and the product works its way into the cracks and crevasses of the stone, locking it in. Often the stone is already in place for the radon system. That being said, CCSF can be installed over sand, but it requires an additional step.  A thin layer or flash coat of foam must be placed first to keep the sand from being pulled up into the product as it’s being applied. This thin flash coating locks the sand in place, the additional step slightly slows the process. If you are using a mat style radon system, these are typically placed on sand, no rock would be present.

Expect to use a little more concrete. The crews we used were checking heights with a laser periodically as they sprayed, they did a good job keeping the surface of the CCSF fairly flat, but it is an expanding product, some high and low spots will be normal.

You also want to pour the concrete as quickly as possible. Drying a puddled bed of CCSF after a heavy rain takes some time, and if concrete is coming, it may require a shop vac.


There are a few advantages using CCSF over a rigid insulation. Two have been mentioned, speed and the ability to air seal. Another is the higher listed R-value of the product. R-3.5 to R-5 for standard rigid insulation, whether that’s EPS, XPS or rigid mineral wool to roughly an R-7 per inch of the CCSF. The spray foam also becomes both the vapor and radon and other sub-soil gas barriers.


Cost is higher using CCSF over other rigid foams. It becomes a little more competitive if a heavy sheet of poly is added to the rigid sheet insulation for vapor and soil gas sealing. An unknown for me is if the CCSF is going to absorb moisture like I’ve seen some other rigid foams do. I’m hoping to perform a back yard wing-nut test to see if CCSF will absorb moisture when buried.

Final thoughts

Many of the spray foam manufacturers have modified their products to have better blowing agents and lower global warming effects. That said, spray foam isn’t right for everyone. The insulation contractor that worked on the Barndominium project indicated he can still get the “bad for the environment” foams, and he said some contractors prefer using those products in cold weather, but they need to be special ordered.  Chat with your insulation contractor and foam manufacturer to determine which products would be best for sub-slab installations.

I have had several discussions with other contractors about using CCSF, not only below the slab, but also it’s use in general.  Most are concerned with the products global warming potential.  Here’s a link to a very good article on the Green Building Advisor’s website about the climate impacts of using different insulations.

Choosing Low-Carbon Insulation

Wade Paquin suggests having the concrete control joints cut within 48 hours of the pour.  This was completed on the “Barndominium” project, but we did not have control joints cut in the “Code Minimum” project.  We did have one long crack that formed in that slab of that project,  no separation or lifting, just a long crack.

The installation of closed cell spray foam has been approved by at least the manufacturer we used on both projects.  Here is a link to their approval letter.

Be sure to have a conversation with a structural engineer and your local building official during the planning stages. Soil testing on a frost protected shallow foundation may be a good idea as well.

So far, I have been impressed with using closed cell spray foam as an option for sub-slab insulation. Both times I’ve used it have worked out well for all involved. Stay tuned for future content on the Barndominium project.

One last photo, installing closed cell spray foam on the ground or floor will be hazardous to your footwear!

Leave a Reply

Your email address will not be published. Required fields are marked *