Slab-on-grade perimeter insulation, or slab-edge insulation, is one of the most commonly abused construction details. When installed properly, this insulation offers significant improvements in energy efficiency and, perhaps more importantly, occupant comfort. Unfortunately, getting the details of this insulation right can be complicated. Architects and engineers struggle to find easier methods, but far too often the details they come up with are completely ineffective.
How not to install slab-edge insulation
The installation process for the details above is easy, which is probably why they are used so often. They don’t require protection of the insulation or exotic concrete forms. The problem is that they are utterly useless—you can easily see where wintertime heat loss will occur in the drawings. These three methods provide no measureable insulating value. When energy raters and inspectors see applications like this, they should rate it the same as the slab having no insulation at all.
What the building code says
Section N1102.2.10 in the 2018 International Residential Code (IRC) and section C402.2.5 of the 2018 International Building Code (IBC) contain specific requirements governing the installation of slab-edge insulation. Here is what it says:
N1102.2.10 (R402.2.10) Slab-on-grade floors.
Slab-on-grade floors with a floor surface less than 12 inches (305 mm) below grade shall be insulated in accordance with Table N1102.1.2. The Insulation shall extend downward from the top of the slab on the outside or inside of the foundation wall. Insulation located below grade shall be extended the distance provided in Table N1102.1.2 by any combination of vertical insulation, insulation extending under the slab or insulation extending out from the building. Insulation extending away from the building shall be protected by pavement or by not less than 10 inches (254 mm) of soil. The top edge of the insulation between the exterior wall and the edge of the interior slab shall be permitted to be cut at a 45-degree (0.79 rad) angle away from the exterior wall. Slab-edge insulation is not required in jurisdictions designated by the building official as having a very heavy termite infestation.
This language has remained unchanged since the 2006 codes.
The image below is a cross-section of an uninsulated slab edge with a 2×6 wall with R-19 cavity insulation and the bottom plate installed flush with the edge of the slab (more on this to come). The colors represent different temperatures, as you can see in the legend above. These images were produced using software from Lawrence Berkeley National Labs called Therm. It calculates steady-state heat transfer and the resulting temperature profile of an assembly using finite element analysis.
At a glance, this demonstrates the importance of proper slab-edge insulation. It is often mistakenly assumed that material as solid and thick as concrete must provide excellent protection against the elements; not where heat flow is concerned. It takes an entire foot (12 inches) of solid concrete to achieve only R-1. Soil is only slightly better. In contrast, solid wood is about R-1 per inch.
There is a path of only six inches from the inside, bottom of the wall through the concrete. This yields an R-value of only 0.5. Contrast this with the R-19 insulation in the 2×6 wall. Such a concentrated area of heat loss is what creates the major comfort issue for owners of slab-on-grade buildings.
It’s not hard to see why the energy code considers this problem area of the envelope such a high priority. Yet simply meeting the IRC requirements may not be the most cost-effective path to an efficient slab.
In Climate Zone 4 (CZ-4), the IRC prescribes R-10 slab-edge insulation to a depth of 2 feet (measured from the top of the slab down). The IRC also allows you to chamfer the top edge of the foam up to 45° to provide drainage, although this decreases the effectiveness somewhat. The temperature just inside the wall is now 54°F, a bit more comfortable.
A simpler alternative
If the builder uses the Performance or ERI path to IRC compliance rather than the prescribed code, is there a simpler alternative that might offer acceptable results? The next example shows insulation extended just 12 inches down from the top of the slab.
If you use 2x10s to form the slab, the forms can be elevated and the insulation can simply be placed on the form prior to placing the concrete. No additional insulation is placed in the trench footing. Two-inch foam is quite strong and will hold back the weight of the concrete without adding taller forms.
The image above also shows the 2×6 wall cantilevered to flush the face of the sheathing with the face of the foam thereby avoiding the chamfer. One can see that just twelve inches of foam provides a result that is superior to the code prescribed method with chamfered foam. Now the inside corner is 57°F, the best performance yet. The software that many energy raters use confirms this result. If one compares the prescribed two feet of foam (placed like the image above) with just one foot in CZ-4, the results are within 96%. Ask your rater to try it on your plans.
Another detail that is often overlooked is that the entire building envelope must be insulated. This includes the separation between the house and any attached garage (whether the garage is insulated or not). This necessitates using a separate pour for the garage with slab-edge insulation placed in between. If both the interior slab and the garage slab are to bear on the footing, the intervening foam will hit the top of the footing. This is no problem when using just one foot of foam. The garage floor can also be placed a couple of inches lower.
Energy cost impact
So what is the effect of these various methods on energy efficiency? REM/Design software was used to simulate annual energy use for a 1,835 square-foot slab-on-grade rancher with an attached two-car garage in Kansas City (CZ-4). The only impact is to heating costs. The results are tabulated below. Four feet of foam has been included to demonstrate the reduced impact with depth. The upcoming 2021 IRC may prescribe 4 feet in CZ-4.With the results from both REM/Design and Therm in apparent agreement, these are significant numbers that justify the additional cost of adding slab-edge insulation as long as it can be accomplished cost-effectively. But again, the most profound improvement is comfort. So 1 foot of slab-edge would save $137/year. But more importantly, this savings represents 40% of total heating costs which yields a substantial increase in comfort.
Exterior insulation protection
It should be clear that one cannot simply expose foam insulation to the rigors it is likely to encounter like ultra-violet radiation from the sun and the assault from motorized weed trimmers. Here’s what the IRC has to say about protecting slab-edge insulation:
N1101.11.1 Protection of exposed foundation insulation.
Insulation applied to the exterior of basement walls, crawlspace walls and the perimeter of slab-on-grade floors shall have a rigid, opaque and weather-resistant protective covering to prevent the degradation of the insulation’s thermal performance. The protective covering shall cover the exposed exterior insulation and extend not less than 6 inches (153 mm) below grade.
Protection is commonly provided using sheet metal flashing, though it is difficult to apply and hard to make look good with many visible dents and wrinkles in the metal. Constant contact with moist soil also greatly shortens the lifetime of even galvanized sheet metal. Stainless steel could be used, but the cost would most likely be prohibitive. Some apply a stucco coating to the foam, but its durability would be questionable in the presence of weed trimmers. Another technique is to use fiber-reinforced cement board with a stucco coating and metal flashing under the wall.
A better alternative is Ethylene Propylene Diene Monomer (EPDM) rubber. EPDM is a synthetic rubber that is commonly used for commercial roofs. In its sheet form it most closely resembles a thick tire inner-tube. It is low cost, highly weather-resistant, and long lasting. The typical thickness used is 45 mil.
Many contractors already use EPDM to flash the point of contact between walls and porches and patios. DIY stores sell it in 15 mil thickness for use as pond liners. It is quite paintable and impervious to weed trimmers. The sheets can be laid out on the new slab and cut into strips. The strips are adhered to the foam and the top of the slab using spray, contact cement. Joints can be welded with adhesive specifically designed for the purpose. Folding the material over the top of the slab blocks the path of termites through the foam.
The bottom line
The importance of slab-edge insulation at first seems counterintuitive, but the images from Therm offer vivid confirmation of the transfer of heat under extreme conditions. It is important to understand that the impact of slab-edge insulation decreases with depth and insulating the upper portion that is exposed to outside air is the most critical. Proper insulation protection is a must and special attention must be given to avoid the concealed intrusion of termites.
There are many techniques for slab-edge insulation that have not been covered here. Each has its own unique challenges. The goal of this article is to simply encourage the proper insulation of this important part of the envelope by offering a cost-effective, simple, and long lasting alternative that provides good performance in a variety of locations. This includes between conditioned space and attached garages, porches, and patios.
-Neal Ezell is a third-generation residential contractor with a bachelor’s degree in Engineering Physics. At Ezell-Morgan Construction in Lawrence, Kansas, Neal and his brother Brian build cost-effective homes that consistently rate in the low 50s on the HERS scale. Illustrations and photos courtesy of the author.