FHB Logo Facebook LinkedIn Email Pinterest Twitter Instagram Plus Icon Close Icon Navigation Search Icon Navigation Search Icon Arrow Down Icon Video Guide Icon Article Guide Icon Modal Close Icon Guide Search Icon Skip to content
Subscribe

In every issue you'll find...

  • Expert insights on techniques and principles
  • Unbiased tool reviews
  • Step-by-step details to master the job
  • Field-tested advice and know-how
Subscribe Now!
Subscribe
  • How-To
  • Design
  • Tools & Materials
  • Videos
  • Blogs
  • Forum
  • Magazine
  • Members
  • FHB House
  • Podcast
  • Join
  • Log In
Subscribe

Fine Homebuilding Project Guides

Insulation

Guide Home
Chapter
  • Energy Efficiency
  • Air-Sealing
  • Choosing Insulation
  • Installing Insulation
  • Water Management
Energy-Smart Details

Insulating a Slab-On-Grade Foundation

Depending on your location, you may need rigid-foam insulation under and around the edges of the slab.

By Martin Holladay Issue 245 - Aug/Sept 2014

While walls and roofs have long been a point of focus, foundations seem to be the final frontier when it comes to insulation. Energy-conscious builders are getting more and more refined with their insulation strategies and manufacturers are doing their part to be helpful, too. Sometimes, the codes are specific. Other times, they are unclear. When it comes to insulating foundations, you can look to the codes for requirements, but know that they may not represent best practices. Beefing up your R-values or insulation strategy, if the budget allows, is rarely a bad idea. And since most of the time, insulating a foundation is done with rigid foam, it’s good to know that you are using the right type of foam for the job.

When it comes to insulating slab on grade foundations, there are at least four factors to consider: the climate, the type of foundation, whether the slab includes hydronic tubing for radiant heat, and the severity of the local termite problem.

The most important factor is climate. In climate zones 1, 2, and 3, most builders don’t bother to install any insulation. While it could be argued that insulation might be useful in climate zone 3, it really isn’t needed in warmer climates, where an uninsulated slab helps lower air-conditioning bills compared to an insulated slab.

In climate zones 4 through 8, the most common locations for insulation are at the perimeter of the slab (installed vertically) and under the slab (installed horizontally). Less common is a third method: buried horizontal or sloped “wing” insulation installed around the exterior perimeter of the building to raise soil temperature.

Two types of rigid-foam insulation—expanded polystyrene (EPS) and extruded polystyrene (XPS)—are suitable for use in these positions. (Make sure EPS is rated for ground contact.)

During the winter, when the temperature of the outdoor air is colder than the soil temperature, the coldest part of a slab on grade is always the perimeter.

Stemwalls

In cold climates, most slab-on-grade foundations include perimeter stemwalls (or frost walls) that extend 3 ft. or 4 ft. below grade. The area between the stemwalls is usually filled with compacted gravel to support the slab. Because they are easier to insulate, slab foundations with stemwalls are preferable to monolithic slabs.

At a minimum, this type of foundation requires vertical perimeter insulation to separate the slab from the stemwall. Most builders install 2 in. of XPS or EPS insulation at this location. Thicker insulation is better, of course, as long as the builder can come up with a detail that allows finish flooring to be installed easily near the exterior walls—a task that can be difficult if the foam insulation is exposed at this location. Many builders bevel the top of the vertical insulation so that none of the rigid foam is visible after the concrete slab is placed.

In cold climates (climate zones 4 and higher), it’s important to install vertical insulation on the interior side of the stemwall all the way down to the footing.

Slab on grade

In most climates, it’s important to install a 4-ft.-wide band of horizontal insulation at the perimeter of a slab on grade and to insulate the interior of stemwalls with a minimum of R-10 insulation.

slab on grade

Bevel the foam

 

If the top of the vertical rigid foam installed at the slab perimeter is visible on the interior of the house, it can be hard to install flooring. The usual solution is to bevel the top of the foam so that concrete will then cover the bevel.Bevel the foam

 

Insulation requirements

In climate zones 4 and higher, it’s also a good idea to install some horizontal insulation under a slab on grade. If you want to save money, you can install a 4-ft.-wide band of horizontal R-10 (or higher) insulation at the perimeter of the slab in a picture-frame configuration. Builders with a bigger budget, especially those in cold climates, should consider installing continuous horizontal insulation under the entire slab. A continuous layer of horizontal insulation reduces heat loss in the winter, and it reduces condensation of humid air on the slab during the summer.

Slabs that include embedded hydronic tubing for radiant heat always need a continuous layer of horizontal insulation under the entire slab. If your slab is heated, it’s a good idea to increase the R-value of the vertical and the horizontal insulation to at least R-20.

Monolithic slabs

Monolithic, or thickened-edge, slabs can only be insulated on the exterior. In climate zones 4 and 5, building codes require a minimum of R-10 vertical insulation that extends down 2 ft. In climate zones 6, 7, and 8, it must extend down 4 ft. The main disadvantage of exterior foundation insulation is that the above-grade portion needs to be protected from physical damage. Materials options include a cementitious coating (ideally, one with chopped fiberglass), cement backerboard, pressure-treated plywood, metal flashing, or a proprietary plastic or peel-and-stick covering (Protecto Bond).

Monolithic slabs

 

What about termites?

Unless you live in a very cold climate, you probably have to consider termites. Most experts advise builders to treat the ground under slabs and near foundations with an insecticide that includes fipronil (for example, Termidor) and to install a termite shield, set in mastic, under sill plates. Requirements for protection vary widely from state to state, so consult your local building department to verify requirements before beginning construction.

The code is unclear

Insulation requirements for slab-on-grade floors can be found in section R402.2.9 of the 2012 IECC and section N1102.2.9 of the 2012 IRC. Both codes state, “Slab-edge insulation is not required in jurisdictions designated by the building official as having a very heavy termite infestation.” Otherwise, both codes require that “slab-on-grade floors with a floor surface less than 12 in. below grade” need vertical insulation at the slab perimeter. These codes do not require any insulation for slabs that are more than 12 in. below grade. The codes are silent about whether above-grade slabs need to be insulated.

This omission is curious, since most slab-on-grade homes have above-grade slabs. Most inspectors, however, require above-grade slabs to be insulated in the same way as belowgrade slabs. In climate zones 1, 2, and 3, slabs are not required to be insulated unless they include hydronic tubing, in which case they are required to have R-5 vertical insulation extending downward from the top of the slab to the footing.

In climate zones 4 and 5, the code requires certain slabs (below-grade slabs that are less than 12 in. below grade) to have R-10 vertical insulation at the slab perimeter, extending downward from the top of the slab to a depth of at least 2 ft. If the slab has hydronic tubing, the minimum R-value of this insulation increases to R-15. The same R-value requirements apply in climate zones 6, 7, and 8, but the insulation must extend to a depth of at least 4 ft.

There are no requirements for horizontal insulation under slabs, but the requirements for perimeter insulation can be met partially with horizontal insulation. This compliance option is explained this way: “Insulation located below grade shall be extended the distance provided in Table N1102.1.1 [namely, 2 ft. in climates zones 4 and 5, and 4 ft. in climate zones 6, 7, and 8] by any combination of vertical insulation, insulation extending under the slab, or insulation extending out from the building.”

Drawings: Steve Baczek, Architect

For more information:

  • What’s Better: Crawlspace or Slab-on-Grade?
  • Foundation in 15 Photos: Details on prepping and pouring the slab
  • Slab and finished floor questions
  • Insulate existing slab on grade vs. thermal mass

Sign up for eletters today and get the latest how-to from Fine Homebuilding, plus special offers.

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters
Previous: Crawlspaces That Work Next: Assembling a Concrete-Free Slab

Guide

Insulation

Chapter

Installing Insulation

View Comments

  1. GBatch | Jan 17, 2017 09:01pm | #1

    I am building a house in a cave near Moab, UT. The main floor has sandstone rock for grade. When my neighbors don't heat their homes, the interior temperature is around 65 degrees. I plan to do HE Pex in floor radiant water heat. My downstairs square footage is around 5,000 sq.ft. I am torn over whether to use Insul-Tarp under the slab. What do you think?

  2. PeterBerrill | Mar 24, 2021 01:07pm | #2

    >> In climate zones 4 and 5, the code requires certain slabs (below-grade slabs that are less than 12 in. below grade) to have R-10 vertical insulation at the slab perimeter, extending downward from the top of the slab to a depth of at least 2 ft. If the slab has hydronic tubing, the minimum R-value of this insulation increases to R-15. The same R-value requirements apply in climate zones 6, 7, and 8, but the insulation must extend to a depth of at least 4 ft.

    What happens if the footing below the slab, or the thickened-edge of a monolithic slab does not extend down 2ft or 4ft below grade? Or does that never happen (i.e. do the footings/monolithic slab edges always extend at least 4ft deep)?

Log in or create an account to post a comment.

Sign up Log in

Become a member and get full access to FineHomebuilding.com

Insulation

Insulation

Trusted, comprehensive guidance from the pros for a home that is healthy, comfortable, and energy efficient

View Project Guide

View All Project Guides »

Become a member and get unlimited site access, including the Insulation Project Guide.

Start Free Trial

Energy Efficiency
  • Understanding Energy and Houses
  • Building Energy-Efficient Homes
Air-Sealing
  • Air-Sealing Basics
  • Air-Sealing Tools and Materials
  • Foundations and Floors
  • Walls, Windows, and Doors
  • Attics and Roofs
Choosing Insulation
  • Insulation Basics
  • Material Choices
Installing Insulation
  • Insulating Attics and Roofs
  • Insulating Walls
  • Insulating Floors, Footings, and Slabs
  • Insulating Foundation Walls
Water Management
  • Water-Management Principles
  • Materials
  • Installation Methods

Fine Homebuilding

Follow

  • twitter
  • facebook
  • instagram
  • pinterest

Newsletter

Get home building tips, offers, and expert advice in your inbox

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

Membership & Magazine

  • Online Archive
  • Start Free Trial
  • Magazine Subscription
  • Magazine Renewal
  • Gift a Subscription
  • Customer Support
  • Privacy Preferences

Taunton Network

  • Green Building Advisor
  • Fine Woodworking
  • Fine Gardening
  • Threads
  • About
  • Contact
  • Advertise
  • Careers
  • Copyright
  • Terms of Use
  • Site Map
  • Do not sell or share my information
  • Privacy Policy
  • Accessibility
  • California Privacy Rights

© 2023 The Taunton Press, Inc. All rights reserved.

X
X
This is a dialog window which overlays the main content of the page. The modal window is a 'site map' of the most critical areas of the site. Pressing the Escape (ESC) button will close the modal and bring you back to where you were on the page.

Main Menu

  • How-To
  • Design
  • Tools & Materials
  • Video
  • Blogs
  • Forum
  • Reader Projects
  • Magazine
  • Members
  • FHB House

Podcasts

  • FHB Podcast
  • ProTalk

Webinars

  • Upcoming and On-Demand

Account

  • Log In
  • Join

Podcasts

  • FHB Podcast
  • ProTalk

Webinars

  • Upcoming and On-Demand

Popular Topics

  • Kitchens
  • Business
  • Bedrooms
  • Roofs
  • Architecture and Design
  • Green Building
  • Decks
  • Framing
  • Safety
  • Remodeling
  • Bathrooms
  • Windows
  • Tilework
  • Ceilings
  • HVAC

Magazine

  • Current Issue
  • Past Issues
  • Magazine Index
  • Subscribe
  • Online Archive
  • Author Guidelines

All Access

  • Member Home
  • Start Free Trial
  • Gift Membership

Shop the Store

  • Books
  • DVDs
  • Taunton Workshops

More

  • FHB Ambassadors
  • Reader Projects
  • Podcast
  • Customer Support

Account

  • Log In
  • Join

Newsletter

Get home building tips, offers, and expert advice in your inbox

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

Follow

  • twitter
  • facebook
  • instagram
  • pinterest

Join All Access

Become a member and get instant access to thousands of videos, how-tos, tool reviews, and design features.

Start Your Free Trial

Subscribe

FHB Magazine

Start your subscription today and save up to 70%

Subscribe

We hope you’ve enjoyed your free articles. To keep reading, become a member today.

Get complete site access to expert advice, how-to videos, Code Check, and more, plus the print magazine.

Start your FREE trial

Already a member? Log in