Questions, answers, comments & suggestions about radiant heating system design or installation mistakes.

This article series 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.

In the photo above, 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. Tthe owners ultimately had to abandon the entire radiant heated floor system.

FAQs on Radiant Heat Floor Mistakes to Avoid

These questions & answers on problems with radiant-heated floor slabs were posted originally at RADIANT HEAT MISTAKES - be sure to see the discussion there.

The correct solution is to insulate to the siding and protect the insulation.

"Insulate the slab perimeter, making sure that the insulation design does not rely on foam placed against the slab perimeter and extending above grade up to siding where it will invite termites or carpenter ants into the structure".

This is a little confusing. If the insulation does not extend above grade, it would leave a gap for cold transfer.

Also, do termites and carpenter ants eat foam insulation. If so, which do they like better, polystyrene, or Polyiso?

Does this mean ICF's are a scam?.

The correct solution is to insulate to the siding and protect the insulation. - On 2018-05-20 by Kevin -

Reply by (mod) - what insulation or construction details will permit us to insulate the perimeter of a concrete floor slab

Kevin, thank you for asking an important question: what insulation or construction details will permit us to insulate the perimeter of a concrete floor slab or a monolithic foundation and slab while at the same time avoiding a termite or carpenter ant problem .

Here's the image from Minnesota.

[Click to enlarge any image]

To be clear, termites or carpenter ants don't "eat" foam insulation, but they find it easy to tunnel through it or between the insulation and the foundation wall, to achieve an easy path to wood construction materials that are nearby.

In areas of termite risk in particular, (as it's termites that will come up into the building through the soil) I'd like to see use of termite shields

See TERMITE SHIELDS vs TERMITICIDE

The most-severe heat loss through a monolithic slab perimeter or a stem wall or foundation perimeter is where the foundation is in contact with soil. Above grade, in contact with air the heat loss rate is much less.

So some alternative designs stop the perimeter insulation just slightly above grade and below the building siding.

Another design, shown in my photograph is one that that you'll see in my TERMITE SHIELD article above. This approach, embedding the foam insulation within the monolithic slab perimeter, is used in northern Minnesota such as at the Minneapolis home where I photographed this detail.

The risk of insect attack with or without foam providing a protected path into the structure is always greater when there is water pooling around the foundation or when there are leaks into the wall or floor cavities of the building.

Choosing among different formulations of foam insulation (as you asked in your posting) does no,t in my opinion, make a material difference in the risk of termite or carpenter ant damage.

Some writers such as Holladay suggest that the solution is to fully insulate the slab perimeter inside or outside (I'll attach an excerpt to illustrate), combined with pre-treatment of the building perimeter with a termiticide.

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. - Holladay, Martin, "Insulating a slab on grade - What’s the best way to insulate a slab-on-grade foundation?" Fine Homebuilding (No. 145, 2014) http://www.finehomebuilding.com/2014/07/09/insulating-a-slab-on-grade

I am nervous about termiticide pre-treatment as with the response of regulation to environmental concerns with use of some termiticides and pesticides, I'm not sure of the long term durability of that approach.

Below are sketches from Holladay's article

Your comment that "The correct solution is to insulate to the siding and protect the insulation." doesn't address the issue of heat loss into surrounding soil. But a termite shield can protect against termites entering the wood frame of the wall sills and studs.

 

Why was your cabin floor slab so thick?

You say the tubing was seven to eighteen inches deep in the concrete. Why would you have such a thick floor in a cabin? - On 2018-05-07 by Don -

Reply by (mod) - Why would you have such a thick radiant heat slab floor in a cabin?

The floor shown at RADIANT HEAT MISTAKES was poured as part of a monolithic slab. The tubing was placed at the bottom of the pour.

The slab thickness was just about seven inches - entirely normal.

The 18-inch thickness in my article and that doubtless prompted your question, was the depth of the integral footing that was poured as part of the monolithic slab.

Unfortunately in some areas of the actual floor the excavator and contractor actually dropped the floor area of the slab and the radiant heat tubing as well as the wire mesh down to that deeper level.

We used 800 gallons of propane to heat our radiant slab floor in eight weeks!

We had a system installed this past summer. The contractor insisted that they had installed hundreds of these systems and that they are far more cost efficient than forced air heat.

They didn't install any styrofoam under the slab but dis use a thicker moisture barrier that looked like bubble wrap. The system was heating just fine until we got a cold snap, the heater isn't keeping up and the building dropped 20 degrees in 1 day and it appears that the Navien unit was running the entire time.

It's a 3,456 square foot building with 2" of foam insulation in the walls and a 12' dropped ceiling with R30 insulation above the ceiling.

We started using the system on October 25th and it heated the building up quickly (2 days from 50 degree temperature up to 68 inside), We had a propane tank with a bad gauge that wasn't telling us how much we were using.

We went through 400 gallons of propane in 6 weeks, then another 200 gallons in less than 2 weeks. Do you think we are blowing through propane because of the lack of foam insulation or do you think that there are other issues involved? We will go broke heating the building at this rate. It is an auction house where we only have a sale once every 5 weeks. Thanks for any advice. On 2018-01-01 by Wylie -

Reply by (mod) - They didn't install any styrofoam under the slab - heating costs $340. week!

Wylie

Where is this home located? In a warm climate you might survive with no real insulation under the slab but otherwise I'm doubtful that the radiant heat can work effectively.

A contractor who says he has installed hundreds of something may be speaking accurately, or not, but it's also possible that he has never read the instructions on the box and has installed hundreds of his somethings in correctly.

You might check first with your propane company about what they consider typical fuel usage for your type of home - size, insulation, windows doors and local climate and the number of degree days.

With propane costs in New York (for example) averaging about $3.38 / gallon, you would have been spending about $340./week to heat the home with the un-insulated floor slab you describe.

What's the best radiant heat floor design for central Missouri

Slab on Grade, 2,400 sq.ft. post framed residence, Central Missouri. Clients want radiant floor heat.

Can we pour two separate lifts of concrete to achieve this. Prepared clay building pad, proof rolled.

2x8 skirt boards attached to post frame for exterior forms covered vertically on interior with 1.5" thick Foamular 2500 board. Install 4" lift of one inch clean limestone, 6 mil poly moisture barrier, 6x6-W2.9xW2.9 steel reinforcement.

Pour lower lift of 4,000 psi concrete mix, 3" thick with bull float finish, saw cut control joints for 150 sq.ft. increments. Install 1.5" foamular 2500 foam board on top of lower lift. Install 1/2" pex tubing, secured in place. Pour upper lift of 3,500 psi concrete mix, 2" thick with added nylon fiber reinforcement, finished for application of concrete stain.

Does this sound like a proper method to achieve desired end result? On 2017-11-22 by EZZE

Reply by (mod) - Can we pour two separate lifts of concrete?

Ezze,

The most reliable answer to your question would come from a concrete engineer familiar with radiant slab heating. I don't want to pretend that I'm a P.E.

That said, I have a question. I'm not sure why you would Place insulation in intermediate layers in a slab rather than putting all of it underneath the bottom lift.
Are you thinking that you are going to isolate cracking in the bottom slab from the top to reduce the chance of cracks coming up through the top pour?

And are you building the top layer of slab with no control joints?
I'm guessing that the clients want a finished concrete surface and don't want to see control joints. I'd be worried about a cracking complaint down the road.

Put down plastic under radiant heat tubing on Stone ?

Do I need to put plastic down on stone subgrade before foam and tubing goes in. My job is installing a floor heat system in a barn - On 2017-11-04 by Gary

Reply by (mod) -

Watch out: Plastic isn't the first question here. If you are putting radiant heat tubing on Stone which is in turn on dirt than your heating system probably will not work as more heat will be flowing down through Stone and soil than up through floor.

I would want solid foam insulation between my tubing and stone. That's more critical than the plastic barrier about which you asked.

Could the original freezing and subsequent problems have caused a problem with the heating system?

We are considering buying a strawbale house, off grid, with radiant floor solar heat in a cement slab. The house froze several years ago, and three of the rooms had two feet from the floors covered with black mold.

Supposedly now the mold was remediated and the work was warranted. The tank that held the hot water that had been heated in the solar panel froze which caused the flooding. The problem now is that the radiant floor heating system doesn't work.

Could the original freezing and subsequent problems have caused a problem with the heating system? On 2017-10-31 by Pamela -
Thank you.

Reply by (mod) -

Yes.

Need advice for radiant heat under a wood floor in Northern Minnesota

I am building a house that will be slab on grade in northern MN. We want in floor heat and were thinking in the slab and the slab is of course over rigid insulation.

My question tho is because we want hardwood flooring would it be better and more efficient to put the pex tubing on top of the slab and fir up the floor with say 1x1s on 16in center, then place 3/4 subflooring to finally put the hardwood on? - On 2017-09-25 by mike

Reply by (mod) -

Mike, especially in northern MN you will see better heat transfer into the occupied space when the tubing is closer to the warm side - or the floor in your description.

But I would beware of causing shrinkage cracks in the finished floor. Before going to that design check with your radiant heat system manufacturer about their recommended operating temperature range and their view of having the tubing right below the wood floor.

Also be darn sure your contractor, unlike mine, provides adequate thickness of foam insulation and drainage below the slab and that the insulation is installed meticulously, that it's complete, and that the perimeter is well insulated.

See FLOOR, WOOD RADIANT HEAT for some useful details.

Effect of missing insulation under radiant heated apartment floor: hot ceilings below

I’ve hired a company to install a radiant floor heating system in my apartment a few years back, but did not ask for the insulation to protect the heat from going downwards.

Now my neighbours (living below me) claim that I’m heating their apartment to extreme temperatures.

The floor is made of concrete and it was cut into just enough to put the tubes in it and the floor on top is the laminate, so I don’t think we lose much heat going up. They reported that their ceiling heated to 80F when my thermostat was at 70F.

Could you advise if this is possible at all that I could be heating their apartment to 70F and maybe refer to some articles? - On 2017-05-09 by Yev -

Reply by (mod) - radiant heat floor is heating the apartment below

Yev I suppose it's possible; I'd like to see objective data - actual measurements of the neighbor's ceiling temperature and changes in that temperature when you turn your floor heating system on.

If there is no time correlation between your heat being on and the neighbor's ceiling reaching a high temperature, then their ceiling may be warm from their own heating system having heated their space.

Do I need a water softener for my radiant heat system?

adding water softener with 14 year old Radiant heating system good or bad idea ? - On 2017-03-06 by WILLIAM BERRY -

Reply by (mod) - Not normally

William

I don't think the softener will make much difference to how your radiant heating system is working, though it will reduce mineral scaling and clogging in other parts of the building water supply system, particularly the hot water heater and its piping.

That's because in most hydronic (hot water) heating system, regardless of their form as radiant heat tubing or radiators or baseboards, the same physical volume of water remains closed in the system, circulating around heated in response to heat.

Because new water is not constantly being added to a hydronic system, the total quantity of dissolved minerals in the water is limited and tends not to be a serious problem. (Steam heat is a different matter entirely).

Can over-heated water damage radiant heat tubing?

Will over heated water (180 degrees ) cause damage to concrete slab poly piping - On 2017-01-23 by Al -

Reply by (mod) - yes over heated water (180 degrees ) will cause damage to concrete slab poly piping

Yes, polyethyelen tubing has temperature limits to which it can be exposed without damage

See

RADIANT HEAT TEMPERATURES

and also

temperature-related leaks discussed

at RADIANT SLAB TUBING LEAKS

My radiant heat floor Heating bills are astronomical! Will slab perimeter insulation help?

It's been about 25 years ago that my builder step-dad (now deceased) built his retirement home. A 30' x 76' slab on grade inside a 42" frost footed stem wall (western WI) with a staggered stud (double wall) single story super structure with about 7" of insulation. He placed pex tube over fairly clean 3/4" gravel and under 4" of concrete.

The slab & foundation have no insulation anywhere. The unheated double garage juts into the floor plan for an integral (monolithic) edge 13' x 24'.

He had a medical condition where he needed ambient temperature of at least 78°F to be comfortable and had to add a 2nd 35k BTU propane fired source to the liquid manifold.

Heating bills are astronomical! Would placing 2" rigid insulation along perimiter stem walls down to the footings and cutting a few inches out from the garage perimerer gain any recoverable savings? - On 2016-12-06 by mouselb -

Reply by (mod) - Yes, heat loss through the slab and foundation perimeter are significant.

I think so, mouse. Other studies of insulation placement in frame construction show that heat loss through the slab and foundation perimeter are significant.

...

Continue reading at RADIANT HEAT MISTAKES or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see RADIANT HEAT MISTAKES FAQs-2 - more-recent Q&A about what goes wrong in radiant heated floor slab installations

Or see these

Radiant Heating System Articles

• FLOOR TILE INSTALL on CONCRETE
• RADIANT HEAT - home
• SLAB INSULATION, RADIANT / PASSIVE SOLAR
• SOIL R-VALUES
• THERMAL MASS in BUILDINGS
• TRUSS UPLIFT, ROOF

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