(C) Daniel FriedmanConcrete Slab Insulation for (Solar) Radiant Heat

  • SLAB INSULATION, PASSIVE SOLAR - CONTENTS: What insulation thickness is needed under a passive-solar heated slab floor? Where should floor slab insulation be placed to avoid heat loss? Where does the vapor barrier go under an insulated, heated floor slab? Heated floors for car wash bays. Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices
  • POST a QUESTION or READ FAQs about how to best insulate under a concrete slab where radiant or solar radiant heat will be installed

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

This article explains the proper insulation amount and proper vapor barrier location below a passive solar heated concrete slab floor.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Sub Slab Insulation Amount, Location, & Vapor Barrier Placement for Heated Slabs

Dow Super Tuff-R foam board insulation (C) Daniel Friedman - at Home Depot stores

Accompanying text is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Our page top photograph shows a horrible radiant floor slab insulation scheme installed by a Minnesota contractor - this radiant floor system never worked because the tubing was placed too deep in the concrete slab and the insulation system was incomplete.

The question-and-answer article about proper insulation amount and proper vapor barrier location below a passive solar heated concrete slab floor, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.

Questions about a heated slab: What thickness of insulation should I use below a 6-inch slab that has SolaRoll™ (passive solar floor heating system) auxiliary heating installed in the concrete? Where is the vapor barrier placed? 


I am in the process of designing a car wash that will use passive solar heating in the self-service bays. In the article "Choices in Underground INsulation," Solar Age, 3/83, you discussed below-grade insulation and waterproofing and recommended the use of extruded polystyrene.

What insulation thickness should I use under a 6-inch concrete slab that has SolaRoll™ auxiliary heating installed in the concrete?

What is the best arrangement and material to use as a vapor barrier under the floor slab?

Insulating a radiant slab (C) Daniel Friedman

I have received contradictory advice from solar dealers regarding the placement of the vapor barrier - above and below the sub-slab insulation have both been recommended. - J.W., Gettysburg PA

Our photo (above) shows a well-designed insulated radiant-heated concrete slab with an insulated foundation perimeter being installed in Two Harbors, Minnesota in 2007.


Heated floor slab insulation detail (C) Daniel Friedman

In a heated floor slab, heat losses into the ground and outward through the edges of the slab and foundation wall are quite significant. (See RADIANT HEAT FLOOR MISTAKES). Designers generally insulate both directly under the slab and around the perimeter as well. In cold climates it is advisable to use a minimum of 2 inches of extruded polystyrene under the slab.

Many increase the under-slab insulating foam board thickness to 3 or 4 inches at the outer 2 to 4 feet of the slab.

On the perimeter foundation wall, 2 inches of solid foam insulation are commonly used, again increasing the thickness toward the ground's surface. Our photo (left) shows insulation placed at the perimeter of a heated garage floor slab in Minneapolis, MN in 2007.

In any case, careful attention should be paid to insulating the slab edge, where heat loss is greatest. Details that thermally isolate the slab edge from the foundation wall and outdoors appear to work the best.

For a vapor barrier below a concrete floor slab, most choose 6- to 10-mil polyethylene. because extruded polystyrene insulating board will absorb little moisture, the vapor barrier can be placed above or below the slab insulation. Some builders prefer to place the vapor barrier below the insulation or even below the gravel bed because they find it easier to work on preparing the slab itself without destroying the vapor barrier membrane.

If you want to pour the concrete directly onto the plastic vapor barrier, you might protect it first with a layer of heavy felt building paper. If migration or moisture into the building is not a concern, as we suspect not in a car wash bay, you could eliminate the vapor barrier altogether.

Readers constructing an insulated slab with radiant floor heating, whether by passive solar or any other means, should see FLOOR, CONCRETE SLAB CHOICES and should not fail to read RADIANT HEAT FLOOR MISTAKES.

We discuss whether basement slab insulation is or is not recommended for ordinary basement floor slabs (non-solar, non-radiant heat) at INSULATION LOCATION for BASEMENT FLOORS.

Readers should also see FLOOR, CONCRETE SLAB CHOICES for a discussion of choice of finish floor materials to preserve use of thermal mass of a concrete floor slab.

Readers concerned about termite damage associated with foam, fiberglass, or other building insulation materials should also see TERMITE SHIELDS vs TERMITICIDE, and Insects & Foam Insulation. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

"Insulating a Slab" - links to the original article in PDF form immediately below are preceded by an expanded/updated online version of this article

Reader Question: Insulation Plan for Solar Earth Box or "earthbox" heat storage design

(Oct 2, 2014) jim said:
should a subslab solar storage earthbox be insulated on the top also



In a passive energy solar earth box or "earthbox" design, using the model described by Summit Solar Systems, a hydronic solar design, installation, and consulting company, at

it's apparent that we are constructing a heat storage system whose heat input is from sun shining onto the surface of the earth box. So you would not want to insulate the top of the box, as doing so would prevent it from gaining solar energy.

In a different design, if for example you were pumping hot water from solar panels through a solar storage medium, then as the heat source is the heated water, not direct sunlight, in that case all sides of the earth box or "earthbox" would be insulated.

Please also see our article series on passive solar designs, beginning at


Or see our complete solar energy article index at



Continue reading at INSULATION LOCATION - WHERE TO PUT IT or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Suggested citation for this web page

SLAB INSULATION, PASSIVE SOLAR at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia

Or see


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Questions & answers or comments about about how to best insulate under a concrete slab where radiant or solar radiant heat will be installed.

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

HTML Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher's Google+ Page by Daniel Friedman