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Radiant heating system design or installation mistakes that must be avoided. This article explains how to avoid some fatal mistakes when installing radiant heat in a concrete floor slab by describing an incompetent radiant heat floor installation along with an explanation of why things went wrong and how to avoid these errors.
The workers in the photograph at page top, where our concrete slab was being poured, were not guilty of a thing. But the contractor who prepared the forms and under-slab insulation placed radiant heat floor tubing too deep in the slab and he omitted proper under-slab insulation. The result: the owners ultimately had to abandon the entire radiant heated floor system.
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Original Research Identified Heat Loss Rates Through a Concrete Slab on Grade with Various Insulation Schemes
The nonsensical view that one can heat up the soil below an building slab on grade and that the soil would magically stay warm forever was put to the test experts when the author was five years old and the contractor was not even a gleam in his daddy's eye.
During February and March 1948, using a specially built, instrumented structure, Harlan Bareither and other experts and students at the University of Illinois Department of Mechanical Engineering conducted careful tests of various slab on grade floor and insulation designs to map heat loss, temperature, and moisture permeation characteristics of nine types of concrete slab subfloor constructions laid on the ground. 
Previously, the US National Bureau of Standards had already indicated that the heat loss of a concrete slab (floor) on grade (on the ground) is proportional to the perimeter of the building. Bareither et als. referred to that work, but because the original testing was in warmer conditions (outside temperature had not been below 35°F. for more than three successive days), they recognized the need to test slab-on-grade floors in still colder conditions - in a climate where the ground is frozen during much of the heating season.
The 1948 heat loss research was important in part because it recognized that the rate of heat transfer from the heated building to the outside (earth and surrounding air) would be greater in proportion to the temperature difference between the heated space and the surrounding soils.
This research on floor slab heat loss rates confirmed that
Nightmare-Construction's Insulation Scheme & Radiant Tubing Location Details
One exception to the general order of priorities of where to insulate in buildings concerns homes built with slab-on-grade construction, particularly homes which have used radiant heat in the floor slab.
The contractor (Nightmare-Works Construction) for a small cabin in the North combined being opinionated and a bully with dismal ignorance of how to construct a properly insulated radiant floor slab. (See Slab Log Cabin Siding for cabin photos and other comments).
Not only did the owners have to battle with the bully to put insulation under the entire slab (he thought that Mother Earth would be warming the home from "ground heat" (which is below 40 °F in winter there).
Owners also lost a battle to have the contractor install proper insulation around the slab perimeter with a frost wall before the floor was poured (he insisted on a floating slab with no inside-perimeter insulation plan).
Worst of all, the contractor also pushed the radiant heat tubing so deep into the concrete (ranging from 7" deep to more than 18" deep) that the entire radiant heat system was not useable at all. Running the radiant heat pushed heat faster into the ground than it did up into the building, even with foam insulation under the slab. We had to abandon the (expensive) radiant floor system and install alternate heating.
The Results of Combined Incomplete Floor Slab Insulation near the Perimeter and "Too-Deep" Radiant Tubing Placement were an Abandoned Heating System
Details about this radiant floor slab heat failure and and research on its cause are provided below. Also see SLAB INSULATION, PASSIVE SOLAR for a discussion of proper insulation below a heated floor slab.
Critical Design Details for a Radiant Heated Concrete Floor
Don't permit your contractor to make the (many) mistakes this one did. Insist that radiant heating in a poured concrete slab have these attributes:
Radiant Heat Floor Slab Design Specification Details
After reviewing photographs taken during installation of the radiant heat floor slab described above, here's what we wrote to the owner and to the contractor:
The bully contractor, who originally estimated the monthly heating cost for this small and otherwise well-insulated building, had said the owners would face winter heating bills of about $30./month based on his prior experience. Stunning heating bills arrived, exceeding $400./month or more than ten times the estimated amount. That's when we began digging into the installation details of this project. The floor slab and radiant heat tubing had been placed by the contractor while we were unable to attend the jobsite.
When the heating bills were excessive and when the heat, running 24-hours a day for weeks, was unable to raise the interior temperatures above 60 deg .F., the contractor offered to "correct" the problem by installing larger capacity circulator pumps.
The "option" of adding larger pumps for this radiant heat floor was not a proper solution for several reasons:
The most economical fall back is to install electric baseboard heating or possibly hydronic heating using the existing electric boiler which was installed to pump heated water through the radiant tubing in the concrete floor.
Meanwhile we shut down this unfortunate radiant slab heat system, installed a few portable electric heaters, and given the tight, well-insulated construction, we found we can keep the little cabin comfortable for a fraction of the cost of heating the earth underneath our floor with the contractor's heating installation.
Here are the details of the errors visible in photographs taken during installation of the radiant floor:
Discussion of the Above Radiant Slab Heat Performance Case Study
James Darling, General Manager of Preferred Heating LLC, in Eagle River, WI commented on this article that the contractor's promise of heating the building for $20. a month was an unreasonable promise not to be relied on - one that could make the article above misleading. We agreed that the description of the failure of this installation needed some clarification, and added the following information that should be considered:
Actual Heating Costs for the Building Described Above
Keep in mind that this was a small new structure (624 sq.ft.) whose construction details, methods, materials were unusually well documented as a project. So the insulation, air tightness, materials, heating details were known.
The building was super insulated, tiny, airtight, with double-glazing throughout, leading to an expected low heating cost. If the owner's actual heating bills for the structure had been even five times what was promised for this building that was occupied only part-time, the owners would have been happy. Heating bills weren't the arm-waving promise of $20 per month, they were not $200. per month. They were more.
In fact, the utility cost to heat this tiny cabin resulted in bills that more than doubled the corresponding costs of the nearby 1960's vintage two story large old, comparatively poorly-insulated house on the same property, exposed to the same conditions. And the exploding heating costs were observed when heating the building well before the coldest part of the heating season.
Heating Capability Limitations of an Improperly-Installed Radiant Floor Slab
The effects of putting the tubing deep into the slab created a problem of heat transfer losses to the ground, not just a matter of longer response time to warm the building. Even if money had been no object, the system simply could not heat the building to an acceptable temperature.
The problem with very deep radiant-heat tubing, combined with incomplete insulation, is that even with just 12 to 18" of concrete above the tubing, heat flowed enough into the ground below the building that even with the thermostat set to maximum, and running heat continuously for a week solid, in moderately cold weather (in the 40's in Northern Minnesota where in winter it can drop to 20 deg F below zero) we never ever could get the indoor temperature above 59 to 60 °F. And this was in a new, small, airtight one-story well-insulated building.
even if we had continuous solid foam insulation under the slab, say R-10 for simplicity, if we have enough inches of concrete above, even though the "R" of concrete is much lower than the insulation, it's the total heat resistance by the total inches that comes into play.
If we have enough thickness of concrete above the tubing (Where 1" to 2" tubing depth is the best design and 6" is considered a lot, in this building we are looking at 18" or more at least in many areas, maybe 24"). With radiant tubing at those depths, the concrete begins to offer not just a lag time in heating (Mr. Darling's point) but also an actual resistance to heat transfer until we begin losing at least some heat into the ground.
The contractor and others tried to improve the system's performance by changing the boiler settings from those set by the manufacturer on its integrated circuit control board, upping the circulator size and capacity, checking flow rate through the system, checking the thermostat controls.
What Caused the Failure of This Radiant Floor Heating System?
Our photo (left) shows where we found the radiant heat floor tubing when we later broke open a section of the floor slab. Radiant tubing was at the bottom of the slab, in this area more than seven inches down in the concrete, and set atop the foam sub-slab insulation.
Our photo above on this page shows that tubing was in some sections more than 18" deep, and adjacent to a large area where sub-slab insulation was simply omitted by the contractor.
We also measured floor temperatures in different areas of the building, mapping clearly where the radiant heat tubing dropped to the bottom of the footing-portion of the monolithic-slab footings! That deep run, probably combined with the incomplete insulation at the level drop between slab bottom and the integrated footings, were almost certainly the prime cause of the failure of this system to heat the building.
As our reference document(s) below show by calculation and model, ultimately, the heat flow into the ground for tubing really too deep in the slab can be significant, even if there is insulation below all or part of the slab. In the structure described here, not only was some tubing 12 to 18" or even more below the slab top, the insulation below the slab was incomplete, inviting ready heat flow into surrounding soils.
Despite varying opinion by some radiant floor installers, consumers, and installers as well should be wary of ignoring the advice of the radiant heating design experts and heat transfer engineers about tubing depth in radiant floor slabs shown just below.
Worse than too-deep radiant floor heating tubing, in this case, because the contractor put NO insulation at the area of soil where he stepped the slab down to the depth of the monolithic integrated footings, we have heat transfer from some of the tubing through concrete right into the cold soil, not just through concrete up into the room through the ceramic tile floor.
In this egregious error, even worse than putting radiant heat tubing too deep in the slab, insulation was simply omitted where the floating-slab monolithic footings were poured. The R-value of concrete is roughly .08/inch (US DOE). The builder located sections of the radiant tubing so that there was about 6" or less of concrete (in the 12" footing section") between the tubing and the cold soil, giving us a heat transmission path (tubing to soil) of R 0.24 or less. This is a likely area of heat loss at all four sides of the building: where the slab dropped down to form footings. (See INSULATION R-Values & Properties)
As an aside the ceramic tile on the finished floor slab was set in mastic - leaving some air spaces and mastic that is a poor conductor compared with tile set in concrete (optimal) - but we doubt that's nearly as important in the system failure in this case.
References for proper radiant heat concrete slab design
Our photo (left) illustrates a successful radiant heat system installation in Minneapolis, MN - a climate simliar to that where we had trouble with the Two Harbors system above.
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Questions & answers on radiant floor heating problems
Question: 2006 IECC: effectiveness of foundation perimeter insulation and insulation recommendations for radiant-heated floor slab designs
I would like to know what the persons that wrote and researched this article thinks about what Montana has on research. On their web page MONTANA SLAB EDGE INSULATION ANALYSIS FOR 2006 IECC ADOPTION. There seem to be so many theories on this.
One thing we have found that if the soil conditions are quite damp, there definitely needs to have some type of insulation under the slab.
Another theory I have read is that the heat as it goes down, which it will, some is that it radiates horizontally, which makes insulating the edge quite well. - Wendell Schubloom
Reply: thorough under-slab and perimeter insulation and proper tubing depth are critical for radiant heat floor slab designs
Wendell, there is not actually any contradiction between the Montana (DOE) research you cite above and radiant heat floor slab insulation requirements. The study you cite does not focus on radiant slab heating designs but or a more narrow question about the benefits of foundation/floor slab perimeter insulation. The DOE photo (below left) shows a typical Montana construction practice that gives a thermal break between a concrete floor slab (not yet poured) and the exterior foundation wall.
I've read quite a lot of supporting research on slab and slab perimeter insulation for radiant heat flooring, and I have some direct experience with installing radiant heat and more with inspecting radiant heat flooring problems.
Quoting from the conclusions of the Montana DOE-sponsored study you cite,  [photo at left showing interior foundtation insulation before the slab is poured, U.S. DOE, op cit.]
This study, using eQUEST, Version 3.0 simulation modeling, compared full versus partial slab perimeter insulation schemes and found that there was useful energy cost savings even with partial insulation. The study data includes comparison with fully-insulated slabs too, but most important for our discussion, it does not address radiant-in-floor-slab heating designs that, without full insulation, can find an easier heat flow into the ground than into the building - not what we want to see nor pay for in heating bills. Quoting:
The risk in misinterpreting the Montana study conclusions above would be to apply them generally to radiant heat floor designs and that to improperly infer that complete under-radiant-heat-floor-slab insulation is not needed in cold climates. That study makes a general conclusion for all Montana buildings and by no means does the conclusion adequately address radiant in-slab heating system designs. The fallacious concept held by the contractor in our horror story was that "once you heat up the earth below your building it will start "giving back" heat to the building and you'll be just fine. His theory was nonsense, as both expert advice and actual field experience proved.
The earth in a cold climate like Montana or Minnesota, is for practical and design purposes, an infinite heat sink. A radiant floor slab heating system will, if improperly designed, keep pumping heat into the ground as long as the heat is turned on. Forever. We saw this in astronomical heating bills and a cold building interior in the Minnesota home discussed above. Heat always flows, and continues to flow from a warmer material into a cooler material.
As the principal author of this material I relied largely on the concrete industry and the radiant flooring industry's radiant floor slab design specifications and advice  as they, above all, have a huge vested interest in their installations being successful. There is no doubt that in virtually every radiant-heat-floor-slab design we need continuous insulation under the slab and at slab perimeter, though the appropriate insulation amount might vary depending on the local climate. The folks who seem to disagree have been people like the bully contractor who himself admitted he had never read instructions, attended a class, nor asked for expert advice. As is often the case with small contractors in remote areas and without expertise, he was "winging it". Don't try mentioning "thermodymics" or "heat flow theory" to a bully.
But in the horrible installation we describe in these articles, the contractor not only provided incomplete and no perimeter slab insulation, he also buried the tubing so deep in the concrete that heat moved much more down into the cold earth than upwards into the occupied space. There was so much heat loss that we could not get the room temperature up even in cold but not bitter cold weather, and even though the same contractor had done a great job insulating the upper portions of the structure's roof and walls. (He was a framer/carpenter, and should not have attempted radiant slab installation nor tile work.) That's why we had to abandon the whole radiant floor installation.
If the floor slab had been very well insulated, the installation still would not have performed well because of the excessive tubing depth in the slab ( over 12" down in some sections ).
We are in the steel bldg business so we have alot of infloor heat done. with the experienced heating people we use, have had no problems. But the question I have is- in North and South Dakota there is a Cat dealer by the name of ButlerCat. they have built huge shops and I found out this spring what they do for floor hear. They place the foam down and put the pex directly to this and then place 4 to 6" of sand on top before pouring the floor. I ask why and was told if the have any floor problems they can remove any thing need to. They done this on I think four bldg's Waht are your thought's
Wendell it's a fair question, and I welcome the disccussion. But I suspect this may be a case of intelligent people who think things up on their own, make up an explanation that sounds reasonable, but may not know the whole story.
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