Insulating a radiant heat floor

[B&H]

I own an auto repair shop in Central New York. 4 or 5 years ago, I had a contractor remove the crumbling existing concrete floor and pour a new floor. We also added a radiant floor heating system. When we were discussing the insulation for the slab, the contractor did want to use foam board for fear of failure (compression). He had seen on a few of his previous jobs where the foam collapsed under the concrete near the transition from the apron to inside of the building. He wanted to use the foil blanket style. When I researched it at that time, seems like the consensus was that the foil blanket was not too effective, so I really wanted to use foam board. The contractor talked me out of it, and I accepted his way more so for warranty issues.

Fast forward to now, and we are starting an addition onto the shop and I am now at the same crossroad. I have a different contractor, and he wants to use foam board under the new slab, and now I am not so sure. Our energy bills went way down when we installed radiant floor heat in the original part of the building, even with foil blanket under the slab. We have no wall insulation or perimeter insulation, and the used oil we generate throughout the year is sold and pays for the natural gas that we use during the winter.

I realize that most of the heat loss from radiant floor heating is at the perimeter. My concerns with foam board under the slab are cost and the fact that we will be obviously installing lifts on the concrete. Not sure I trust the foam board. One of the workers on the job stated that they just installed a radiant floor heating system at a concession stand at a college. I think he said that they used 100 psi board instead of the standard 25 psi, and that it was very expensive. I wouldn't be concerned at all in a concession stand with foam board, but I have some major concerns in an auto repair shop with my name on it.

We briefly discussed the foam board vs foil blanket months ago with the owner of the contracting company when we were going over the build. He claims that they have never had a problem with foam and had never heard of foam collapsing. He has never used the blanket.

Looking for opinions on what product to use and why. I don't want to be an *** and demand something from the contractor like I know what I'm talking about. However, cost savings of material and time and possibly a safer set-up using the blanket is really weighing on me.

 

[bazzateer]

If the concrete floor is thick enough and properly re-inforced I don't see how it could collapse enough to compress the foam board?

 

[bgarrett]

If the concrete floor is thick enough and properly re-inforced I don't see how it could collapse enough to compress the foam board?

Its not that the concrete would collapse, its the weight.

I have wondered about the weight of the concrete crushing the foam board but did not know about 100psi foam.

What kind of insulation can be used in the Southern States where there are termites?

 

[RTcat]

Around here everyone uses the 2" or 2 1/2" thick high density rigid foam board from Menards. They are pink in color.

From the ground up:

compacted base
plastic
foam board
pex
rebar

Concrete workers standing on plywood during the install. Maiden lift after the installers completed the install. This was two years ago and no cracks or issues.

Enjoy your addition.

 

[B&H]

RTcat,

How thick did is your pad and did you pour thicker under the lift columns? If you poured thicker under the columns, how was the foam board handled?

 

[Rookie2]

I used dow grey board back in 96, it was denser for drive in freezers. foam board has a shrinkage rate that is in the spec sheets ,i think i remember somewhere around .02% over 20 yrs. i made sure that the concrete was floating on the foamboard (no piers or footer) also the heat will expand the concrete a very small percentage. 5" 4000# concrete weighs around 0.44# per sq inch. = 63.5 # sq.ft. approx.

 

[wssix99]

When we were discussing the insulation for the slab, the contractor did want to use foam board for fear of failure (compression). He had seen on a few of his previous jobs where the foam collapsed under the concrete near the transition from the apron to inside of the building.

He either doesn't know what he's doing (with regard to foam selection) or what happened on the other jobs that failed.

The foam under the floor should be at least a 60 PSI board. (You can't get this from Home Depot or Menards.) For added piece of mine, you could go up to the 100 PSI.

If you take the weight of a car or the weight on a lift and divide it by the contact patch or base plate surface area, you'll get the downward compressive force. For example, a 4000 lb car with tires that have a 20 sq. in. contact patch, puts 4000/4/20= 40 PSI on the ground. (The contact patch in this example is really small and I just used it so the numbers would be easy.)

In reality, the force on the top of the slab radiates down in a cone shape to the base, where the foam is, so the forces it sees is even more distributed.

As you get up in to 60 or 100 PSI, its possible that the dirt under the base can't even support this load without compacting. This may have been the problem on the contractor's other jobs and he might just be blaming it on the foam.


Personally, I'd look for another contractor who has the experience and confidence to do this correctly.

 

[theoldwizard1]

A well compacted base is very important !

If it were me, I would go with 2 layers of 2" high density foam. Seems should be overlapped.

Fiber added to the concrete mix will help.

 

[B&H]

wssix99, the FIRST contractor on a renovation 4 or 5 years ago didn't want to use the foam board do to collapsing issues and we went with the blanket. My thoughts were similar - the base wasn't prepped correctly. The current contractor wants to use 100 psi board, and it's me that's questioning whether there will be issues or not.

How perfect with regards to the flatness does the base have to be to use the foam board? What if my resident moles chew through the foam board??

If we pour deeper under the lifts, how is the foam installed?

Yes, we will be using fiber in the 4000# mix.

 

[Kevin C]

http://www.owenscorning.com/worldwide/admin/tempupload/pdf.077_HighDensity_E_1.pdf

DOW brochure with info on how to calculate loading. EDIT: Page 8 has a few examples already worked out using 5" thick 4000 PSI concrete. For Foamular 400 @3" thick 7200 lbs placed on a 5" diameter contact area results in an acceptable unit loading of the concrete and the foam.

Three factors that matter:
1: Concrete thickness ( thicker spreads the load better).
2: Foam rating.
3: Thickness of foam (thicker foam increase the loading of the concrete) .

Not covered: Effect of Rebar.

The guide should give you a good idea of what is possible. I don't see any reason as to why a 6" slab w foam would have a problem if a lift was attached to it.

 

[Highbeam]

B&H, just don't put foam under the lift supports. It's insulation and you don't need it that much as you move away from the edge of the slab. You want a thicker slab under the lift anyway so omit the foam here. No harm done, I wouldn't blame you a bit.

Do realize that 25 psi foam will be stronger than the earth beneath it. If the foam collapses, then so would the earth. In reality, the concrete only weighs 0.5 psi and the load from a truck tire is ditributed since concrete is a "rigid" pavement.

Any blanket insulation is dumb. Only good as a vapor barrier if it is poly faced. The insulative properties of a material come from trapped air. The blanket will be crushed under the concrete and offer no insulative value.

 

[wedge40]

Thanks Highbeam, some common sense put into this thread.

From my research, MOST people use 2" of XPS (extruded polystyrene) under the slab and around the perimeter. There are many post on this site about the process of placing the foam and PEX. I laid out two 4'x4' areas that I avoided with the pex and made it a little thicker for a 2 post lift in the future. Most people use 1/2" PEX placed on 1 foot centers.

Wedge

 

[RTcat]

RTcat,

How thick did is your pad and did you pour thicker under the lift columns? If you poured thicker under the columns, how was the foam board handled?

6" for the whole pad. Nothing special for the lift columns. I just marked an area on foamboard where the columns would be bolted and made sure my pex lines stayed out of that marked area. I conferred with the lift salesman and the installers and both said with 6" of concrete on foam that no footer was needed for the lift.

 

[akpolaris]

Put some stem walls of extra concrete where you wan0at the lift posts to hold their weight. Lay some heat loops at the perimeter since that si the greatest heat loss areas. make sure the base is compacted well and all should be good.

 

[rburke65]

wssix99.....do you have any credible reference for the requirement of "at least 60 psi foam board"? I would really be interested in your source. Thanks.

 

[Kevin C]

I don't really feel like writing a long involved post on this. There are a few assumptions being made that are incorrect. If you want to be safe, my recommendation is to use the guide from DOW.

A concrete slab must be capable of distributing loads over an area of
sufficient size so that pressure on underlying layers does not exceed
allowable limits. When FOAMULAR®
Extruded Polystyrene Insulation
is used below the slab, allowable stress limits are defined based upon
a percentage of Foamular®
insulation’s minimum compressive strength.
(Please refer to the Recommended Stress Limits table on Page 4.)

Use the following charts and formulas to determine the stress present
on the concrete slab and insulation layers. To determine the stress that
Foamular®
insulation will experience, you will need to know the deflection
of the concrete slab (see Concrete Slab Design Formulas on page 7) as
well as the foundation modulus.
Foundation modulus is a measure of how much a substrate deflects under
a given load, expressed as inches deflection per inch of thickness or “pci”.
The foundation modulus for various thicknesses of Foamular®
insulation
can be found in the table below:

IMHO common sense is good... But with this large an investment, your may want to back it up with a bit of number crunching. They give examples and all the info you need to calculate your setup. Since your dealing with a lift supporting vehicles I don't think this is overkill.

 

[Rookie2]

Wait a minute ! If I jack up my F150 with a floor jack , I would blow a hole in my 5" concrete . Hasn't happened !

 

[wssix99]

Its not that the concrete would collapse, its the weight.

I have wondered about the weight of the concrete crushing the foam board but did not know about 100psi foam.

What kind of insulation can be used in the Southern States where there are termites?

Concrete weighs approximately 150 lbs. per cubic foot. So, the weight of a 6" slab is 150 pcf / 2 (1/2 foot thick slab) /144 (sq in per sq ft) = 1/2 psi for the weight of the concrete in the slab. (It's virtually nothing.)

RTcat,

How thick did is your pad and did you pour thicker under the lift columns? If you poured thicker under the columns, how was the foam board handled?

You want a consistent slab thickness, just like the lift instructions indicate. If the floor slab were steel, this would be a good idea - however concrete is a totally different animal. A non-uniform thickness will create odd shrinkage stresses and will put cracks exactly where you don't want them in relation to your lift.

wssix99, the FIRST contractor on a renovation 4 or 5 years ago didn't want to use the foam board do to collapsing issues and we went with the blanket. My thoughts were similar - the base wasn't prepped correctly. The current contractor wants to use 100 psi board, and it's me that's questioning whether there will be issues or not.

IMO - 100 psi is overkill, but I haven't priced out the difference between 60 and 100. In my case, I won't even come close to 60psi stresses on my floor - in fact I could probably get away with 40, but I'm close to that in my calcs.

I've seen 100 marketed as "airport grade" and I'd say that's about right. I expect 100 would only be needed for a building floor if there is heavy industrial lift truck traffic, etc.

How perfect with regards to the flatness does the base have to be to use the foam board? What if my resident moles chew through the foam board??

You can put the foam above the stone base, so getting it flat shouldn't be an issue. The foam should conform to any irregularities underneath.

If we pour deeper under the lifts, how is the foam installed?

As above, you want a consistent depth. If you want stronger, go thicker across the entire floor. Your biggest challenge will be your contraction joints. You should plan these well in advance so they don't interfere with your lift and so you can plan your radiant floor. Everywhere the radiant tubing crosses an expansion or contraction joint, you will want to have a protective sheath around the tube, per the manufacturer's specs.

wssix99.....do you have any credible reference for the requirement of "at least 60 psi foam board"? I would really be interested in your source. Thanks.

I should have qualified my comment - sorry. For a lift, 60 is a good idea. For a floor w/o a lift, the regular 40 psi foam is just fine.

For the lift I'm installing, I took the maximum load for my lift, 10,000 lbs. and divided by my estimated area of the pad 144 sq. in. That took me to 35 psi and, so I wanted to have a little higher factor of safety - so I'm going with the 60 psi. I calculated some estimated moments for an unbalanced load and saw they were minimal compared to the downward force on the posts for a balanced load.

 

[Highbeam]

Your biggest challenge will be your contraction joints.

Everywhere the radiant tubing crosses an expansion or contraction joint, you will want to have a protective sheath around the tube, per the manufacturer's specs.

Nobody does this in a typical slab with sawcut expansion joints. Are you talking about full expansion joints with joint material or sawcuts?


For the lift I'm installing, I took the maximum load for my lift, 10,000 lbs. and divided by my estimated area of the pad 144 sq. in. That took me to 35 psi and, so I wanted to have a little higher factor of safety - so I'm going with the 60 psi. I calculated some estimated moments for an unbalanced load and saw they were minimal compared to the downward force on the posts for a balanced load.

So your mistake is that you assume that only a 144 square inch area is supporting the lift. That would only be true if you were not using a slab and were bolting it to the foam. With a rigid slab, the bearing area is significantly larger. Large enough to allow the use of standard 25 or even 15 psi foam.

It is ridiculous to use 60 or 100 psi foam. Don't you realize that the ground beneath the foam also has a bearing capacity?

COmments in blue.

 

[wssix99]

^^^ It's all in the instructions. Every PEX system I've seen calls for protection for sawcut contraction joints and full expansion joints. If "nobody" wants to follow the manufacturer's instructions, that's their decision.

No mistakes. Like I said, my calculation has an added factor of safety. If the load is off center, the load will be concentrated on less than the full plate. The forces are spread through the concrete in a cone, but I have yet to find a calculation for that effect except for the edges and corners of the slab. If the ground beneath has a weaker bearing capacity, stronger foam is indeed a waste.

One could probably get by with a 40 psi foam or weaker. In my situation, with a lift manufacturer not willing to sign up for a spec., the cost of confirming the soil bearing capacity being high, the cost of an insured professional engineer's time even more expensive, I'm going with a conservative calculation. The added cost is a small insurance policy for my situation.

 

[Kevin C]

So your mistake is that you assume that only a 144 square inch area is supporting the lift. That would only be true if you were not using a slab and were bolting it to the foam. With a rigid slab, the bearing area is significantly larger. Large enough to allow the use of standard 25 or even 15 psi foam.

It is ridiculous to use 60 or 100 psi foam. Don't you realize that the ground beneath the foam also has a bearing capacity

I think you may want to consider that the modulus of elasticity for the foam and the concrete are dramatically different, and the way your calculating loading assumes they are the same.

Similarly, the modulus of elasticity of compacted gravel is a lot higher than the foam.

What happens? As the concrete is loaded, it's stresses go up very quickly. The foam, tends to give a bit as its loaded and does not see nearly as much loading.

This increases the localized loading of the concrete. All materials deflect as they are loaded, even the concrete floor.

The foam failing is not the issue, localized loading of the slab is. The foam with the higher PSI rating is also stiffer (increased modulus of elasticity).

The increase in foam stiffness provides better support for the concrete, reducing deflection and therefore strain (the foam board starts to behave more like soil, but still has a long way to go.

The main reason to specify stronger foam is that as its strength goes up, it becomes more stiffer. That's also why DOW recommends calculating using a percentage of the foams rated strength (not its full advertised strength).

 

[B&H]

Thanks for all of the advice! You've relieved my worries about using foam board under the pad, and we spoke to the GC today and he is going to check on the XPS rating (either 60 or 100). He said that he won't use the 25 psi stuff under a concrete pad. We've decided to pour the entire pad at a depth of 5" using 4000 psi fiber reinforced concrete, so that it will be uniform. That will be more than sufficient for both the Hunter and Rotary lifts that we will be installing, and we will keep the Pex away from the anchors.

The block walls will be waterproofed and 2" of foam boards will be insulating the block on the inside. The footers have been poured and the masons finished up the block today. Decided on Rotary Lifts for the 2-posts (I have been using Challengers but decided to try Rotary. Challenger Lifts was sold to Snap On just yesterday). We are also adding a Hunter scissors lift and their Hawkeye Elite alignment system. I never thought I'd get into alignments, but it would be nice to not have to rely on someone else to work on your customer's cars.