EPS under slab requirements?

[Affinity Fab]

I had planned to insulate under my slab that I'm about to have poured with 2" EPS insulation. Of course now that I'm ready to pick it up the wholesaler I was planning to get it from is out of stock. I found that Menards carries it at a good price but it's spec'd at 10 psi minimum compressive strength. A lot of others I've looked at are rated at 25 psi, which they also carry & refer to it as "high density EPS" for 2.5x the price.
Searching I can't seem to find anything that states what the minimum requirements are under a 4" slab. Would 10 PSI be OK?

 

[ard]

I am pretty sure the low density eps is NOT suitable for under slab

Reading a bit about eps vs XPS, tge need for waterproofing and draining, and different densities, there are specific "types" of eps. It's not my area, so all I'd be doing is regurgitating Google content..... But be cautious ...

GL

 

[Dagny]

I know of lots of slabs with the regular foam under them with no issues.

 

[ard]

What is "regular foam"?

 

[Franz1.0©]

What is "regular foam"?

An assembly of microbubbles of air surrounded by a polymeric coating.

Why does everybody get wound up about subfloor foam and insulation and completely ignore frost fencing?

 

[Thumper68]

For the record the 10psi foam board will be just fine.

 

[Affinity Fab]

;6307979]An assembly of microbubbles of air surrounded by a polymeric coating.



Why does everybody get wound up about subfloor foam and insulation and completely ignore frost fencing?

Frost fencing? Are you referring to insulating around the perimeter of the slab or something I'm not familiar with?



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[Affinity Fab]

For the record the 10psi foam board will be just fine.

Any chance you you can direct me to a source that confirms that? I'm having trouble finding anything.


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[brownbagg]

well if the slab cracks, you will know why

 

[Affinity Fab]

well if the slab cracks, you will know why

Thanks, that's helpful.


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[Franz1.0©]

Frost begins on top of the ground, and initially travels horizontally. When it contacts your floor it begins sucking heat from the concrete. Frost only travels down AFTER expanding to the maximum surface area.
Therefore, more heat retention is available in a concrete slab by isolating the slab horizontally than is available by sitting a slab with ground contact on top of 6" of foam.

Secondary benefit is less foundation system is needed when the slab is isolated.

http://www.homeinnovation.com/~/med...de-to-Frost-Protected-Shallow-Foundations.pdf

 

[Affinity Fab]

Well I'm doing post construction & have had the walls spray foam insulated all the way down to the bottom of the grade board so the slab will be poured within that & insulated under it, therefore completely isolated.
The remaining issue I have is whether or not 10 psi foam is sufficient for under the slab?


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[matt_i]

Suppose you park a 1 ton truck, 6000 lbs of it, on the slab. Lets just say its on a 6" x 6" square contact pad on each of 4 wheels (assume not DRW for argument here). Thats 144sq inch of contact. Nearly 40psi.

That doesn't automatically make the foam fail but forces the concrete to bend and act like a washer more or less to distribute the concentrated load.

I don't think its a recipe for success. I also think the guys working the concrete are going to stomp the hell out of it causing more concrete to be needed incrementally, but enough to notice. Go buy one piece and walk all over it and see what happens.

Given the large investment in the foam the slab, and (I hope you are putting tubes in it for heat), I am going to say you have at least $7k on the line here that has a high potential for failure given poor subgrade. The ground, for reference has around 2000 psi bearing potential.

As far as heat travelling directionally, I would say it flows fastest to the greatest temperature differential, proportional to its area and insulation just like the equation

Q (dot sub A) = delta T/ R-value. The Qdot means its a flow, the sub A means per unit area. So if you have a 1000 sq ft isolated slab, 6" thick, it can have (25 x 0.5 x 2) + (40 x 0.5 x 2) = 85 square feet of "side area". Suppose 70 F inside, 30F outside and 50F ground temp, you lose 1000 sq ft x 20 degree delta vs. 85sq ft x 40 degree delta. R-values basically same for assumption.

Numbers don't add up very well to say that even with zero insulation anywhere you could lose more heat thru the edges. (the total heat flow above is around 5.8x as much into the ground as into the air, change numbers to suit your own conditions) If I was going to pick the biggest heat loss to attack with my insulation investment it would be the big flat area underneath.

IIRC owens-corning makes up to 40psi in Foamular, I had researched it as I thought I wanted a radiant floor that could support forklift traffic. Another app is airport runways in cold climates, they plow it very little to never, but that's because the slab is melting it off with the radiant heat. A jet does have a lot of tires but its also a hell of a load coming out of the sky off controlled stall.

 

[Franz1.0©]

Suppose you park a 1 ton truck, 6000 lbs of it, on the slab. Lets just say its on a 6" x 6" square contact pad on each of 4 wheels (assume not DRW for argument here). Thats 144sq inch of contact. Nearly 40psi.

That doesn't automatically make the foam fail but forces the concrete to bend and act like a washer more or less to distribute the concentrated load.

I don't think its a recipe for success. I also think the guys working the concrete are going to stomp the hell out of it causing more concrete to be needed incrementally, but enough to notice. Go buy one piece and walk all over it and see what happens.

Given the large investment in the foam the slab, and (I hope you are putting tubes in it for heat), I am going to say you have at least $7k on the line here that has a high potential for failure given poor subgrade. The ground, for reference has around 2000 psi bearing potential.

As far as heat travelling directionally, I would say it flows fastest to the greatest temperature differential, proportional to its area and insulation just like the equation

Q (dot sub A) = delta T/ R-value. The Qdot means its a flow, the sub A means per unit area. So if you have a 1000 sq ft isolated slab, 6" thick, it can have (25 x 0.5 x 2) + (40 x 0.5 x 2) = 85 square feet of "side area". Suppose 70 F inside, 30F outside and 50F ground temp, you lose 1000 sq ft x 20 degree delta vs. 85sq ft x 40 degree delta. R-values basically same for assumption.

Numbers don't add up very well to say that even with zero insulation anywhere you could lose more heat thru the edges. If I was going to pick the biggest heat loss to attack with my insulation investment it would be the big flat area underneath.

Your weight distribution math is totally screwed.

Your theory of concrete bending is from the planet Oblivion. Concrete has maximum stress handling in compression, so the slab will not bend more than .004 inches under the tire contact area.

Delta this, you need to learn a lot about frost. I suggest you begin study by finding some frozen ground and start digging a hole.

 

[wssix99]

The remaining issue I have is whether or not 10 psi foam is sufficient for under the slab?

That depends. How thick is your slab going to be.

Suppose you park a 1 ton truck, 6000 lbs of it, on the slab. Lets just say its on a 6" x 6" square contact pad on each of 4 wheels (assume not DRW for argument here). Thats 144sq inch of contact. Nearly 40psi.

Close, but your math is missing one dimension!

The pressures decreases as a square of the depth. So, the pressures that the slab sees are much less at the bottom/grade than at the top surface of the slab. (Your contact patch may be conservative, also - but that's fine.)

The stress at the top of the slab dissipates in the shape of a cone to the through the bottom of the slab. It's not a perfect cone due to irregularities in the material and calculating a cone requires Pi, which requires a slide rule - so you can liberally estimate a pyramid shape for the stress distribution. So:

In your example, that 6" X 6" contact patch at the top of a 4" slab would be spread out of approximately a 14" X 14" area at the base of the slab (where it rests on the foam). This is gives you 196 sq in of area and 7.6 psi of pressure, dividing the 6000 lb truck on 4 equal tires.

^ If you work out a more realistic contact patch for a 6000 lb truck, I expect you will see much smaller pressures at the foam.

^^ If the slab gets thicker, the stresses get spread out over a larger area and the foam will see even less pressure.

 

[matt_i]

Your weight distribution math is totally screwed.

Your theory of concrete bending is from the planet Oblivion. Concrete has maximum stress handling in compression, so the slab will not bend more than .004 inches under the tire contact area.

Delta this, you need to learn a lot about frost. I suggest you begin study by finding some frozen ground and start digging a hole.

Throw down some equations and how you arrived at your numbers and then we can judge your claims more seriously.

I'm not sure how frost enters into this, I don't think the guy is worried about his slab heaving, he never mentioned a frost-protected shallow foundation trying to entrap the latent heat in the ground. I took it as he is building a pole barn and intends to heat it and someone told him it would be good to insulate under the slab.

 

[Franz1.0©]

Well I'm doing post construction & have had the walls spray foam insulated all the way down to the bottom of the grade board so the slab will be poured within that & insulated under it, therefore completely isolated.
The remaining issue I have is whether or not 10 psi foam is sufficient for under the slab?


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Your warm foot factor will improve tremendously if you dig a 18" trench immediately beyond the poles and insert 2" foam to the bottom, and it will probably be a cost wash against spraying foam on the grade board.

As to floor load transmitted to underdeck foam, provide slab dimensions, for individual slabs within the building. I'll whip a pencil out and tell you what the actual load per square inch of foam/concrete contact is.

 

[pstnbly]

XPS 2.5 density, not EPS

 

[Franz1.0©]

Throw down some equations and how you arrived at your numbers and then we can judge your claims more seriously.

Just as soon as your PAYMENT arrives and clears the bank.

Your failure to complete 7th grade math is not a guaranteed path to me instructing you free of charge.

 

[joes169]

Given the large investment in the foam the slab, and (I hope you are putting tubes in it for heat), I am going to say you have at least $7k on the line here that has a high potential for failure given poor subgrade. The ground, for reference has around 2000 psi bearing potential.


IIRC owens-corning makes up to 40psi in Foamular, I had researched it as I thought I wanted a radiant floor that could support forklift traffic. Another app is airport runways in cold climates, they plow it very little to never, but that's because the slab is melting it off with the radiant heat. A jet does have a lot of tires but its also a hell of a load coming out of the sky off controlled stall.

2000 PSF, not psi. HUGE difference.

And DOW makes foam up to 100 psi as I recall. We used "Freezermate 60" probably 20 years ago under a freezer pad at the Milwaukee County Zoo. It's been around a long time.


If I were the OP, I would opt for using 1" XPS, 25 psi before using 2" EPS, if the cost is the determining factor. I last ordered some of it in December, and with delivery to the jobsite, it was right at $13.00 a sheet.

 

[bsg]

I had planned to insulate under my slab that I'm about to have poured with 2" EPS insulation. Of course now that I'm ready to pick it up the wholesaler I was planning to get it from is out of stock. I found that Menards carries it at a good price but it's spec'd at 10 psi minimum compressive strength. A lot of others I've looked at are rated at 25 psi, which they also carry & refer to it as "high density EPS" for 2.5x the price.
Searching I can't seem to find anything that states what the minimum requirements are under a 4" slab. Would 10 PSI be OK?

Try here;

http://williamsinsulationinc.com

This who I used, they were cheaper than anyone else and it was free delivery!
They offered a product that was comparable to XPS and was considerably cheaper.......IIRC it was around $22 for a 4' x 8' sheet.

Kevin

 

[matt_i]

Just as soon as your PAYMENT arrives and clears the bank.

Your failure to complete 7th grade math is not a guaranteed path to me instructing you free of charge.

I'm well aware the internet is full of tough guys wanting to talk a lot and never wanting to put down any math behind it, having frequented many technical forums. If you are posting on a forum which by nature dispenses free advice I don't get the deal with "wanting to get paid to dispense knowledge". But, like, whatever, man.

Not saying everything is correct as you see I am incorrect on some points above, I didn't mention the conical stress distribution and posted a ground pressure of 2ksi vs 2kpsf. OOPS! But at least we can have an honest technical discussion about it.

 

[Affinity Fab]

Throw down some equations and how you arrived at your numbers and then we can judge your claims more seriously.

I'm not sure how frost enters into this, I don't think the guy is worried about his slab heaving, he never mentioned a frost-protected shallow foundation trying to entrap the latent heat in the ground. I took it as he is building a pole barn and intends to heat it and someone told him it would be good to insulate under the slab.

That is more or less correct which brings me full circle back to my initial quandary of if it's even worth it? No I'm not running tubes for heat, I will be going with forced air. Initially I planned to but I've decided for my given use it's not worth the investment. I was thinking that once the slab warmed up the foam would keep the ground from sucking the heat out of it as quickly & help keep it more comfortable. Maybe this is false logic or at least false economy.

Your warm foot factor will improve tremendously if you dig a 18" trench immediately beyond the poles and insert 2" foam to the bottom, and it will probably be a cost wash against spraying foam on the grade board.

As to floor load transmitted to underdeck foam, provide slab dimensions, for individual slabs within the building. I'll whip a pencil out and tell you what the actual load per square inch of foam/concrete contact is.

I sprayed the grade board because I was spray foaming the entire building so it was simple just to go all the way to the bottom.
The slab will be 30'x56' 4" thick (might possibly opt for 5") with fibermesh & rebar.

Also a lot of reading I've been doing is saying that XPS used to the go-to but EPS has proven to be as good or better especially considering the cost difference. I'm not looking to go the cheap route by any means, I like to do things right & do them once. I planned to do this because I had a line on the insulation for a great price so I figured it couldn't hurt for the investment. Now that I'm almost ready to pour & they're out of the material I started looking at different options, which brought this up. If the 10PSI material is not suitable then I need to decide if it's worth doing at 2.5x the cost?

 

[matt_i]

I have a ~1000sf shop, no foam under the 'crete or anything. It takes about 2 therms of energy (1 therm ~ 1ccf nat gas ~ 100k BTU) from DTE to heat it up to 55F for 8-10 hours depending on exterior ambient (in typical Michigan 25F-30F). A therm is around $0.80 right now.

I just took Foamular 250 from Home D as an example, its $36.** for a 4'x8'x2" thick sheet. So in rough terms $1 per square foot. I didn't see your overall size of building mentioned, going to guess its between $500 and $2000 to put this down.

A lot depends on how often you are going to heat this up, are you going to 50F or 70F inside, etc. I personally don't lay on the slab a lot so this isn't a big factor to me. If I was going to heat the slab with liquid-transfer then it would be well worth it. I don't think I could save $1 in heat (if I sub-slab insulated) every time I went out to the shop and heated it for a full day (if so it would take 500-2000 days of winter-work to break even on the insulation).

What i would be concerned about is if the subgrade was prepped for the thickness of foam already, you are pretty much committed unless you put in washed limestone and compact it as a filler material.

 

[Affinity Fab]

So what is the general consensus on insulating under a slab that doesn't have hydronic heat, worth it or waste of money? This is my home hobby shop that I'll be using in the evenings & on weekends, therefore running the temp up & down frequently. I'll probably keep it above freezing at all times though.

Try here;

http://williamsinsulationinc.com

This who I used, they were cheaper than anyone else and it was free delivery!

Kevin

Thanks for the info, that's not too far even if I had to pick up.

 

[Affinity Fab]

I have a ~1000sf shop, no foam under the 'crete or anything. It takes about 2 therms of energy (1 therm ~ 1ccf nat gas ~ 100k BTU) from DTE to heat it up to 55F for 8-10 hours depending on exterior ambient (in typical Michigan 25F-30F). A therm is around $0.80 right now.

I just took Foamular 250 from Home D as an example, its $36.** for a 4'x8'x2" thick sheet. So in rough terms $1 per square foot. I didn't see your overall size of building mentioned, going to guess its between $500 and $2000 to put this down.

A lot depends on how often you are going to heat this up, are you going to 50F or 70F inside, etc. I personally don't lay on the slab a lot so this isn't a big factor to me. If I was going to heat the slab with liquid-transfer then it would be well worth it. I don't think I could save $1 in heat (if I sub-slab insulated) every time I went out to the shop and heated it for a full day (if so it would take 500-2000 days of winter-work to break even on the insulation).

What i would be concerned about is if the subgrade was prepped for the thickness of foam already, you are pretty much committed unless you put in washed limestone and compact it as a filler material.

Shop is 30'x56' with a 13' ceiling, so 1680 sq. ft., all walls spray foamed @ 2+" and blow-in going in the attic. I'm having the final grade prepped this week so I'm not completely committed one way or another.
Out of curiosity where in SE MI are you?

 

[matt_i]

Out of curiosity where in SE MI are you?

Im in the Ann Arbor area, out amongst the cornfields.

 

[Franz1.0©]

Based on steel reinforced concrete 4" thick, the slab with a 6000 pound truck sitting on it weighs 90,000± pounds. Spread that over the area of 1680 sf, and the slab exerts 53.7± of downforce per square foot or 3.7±pounds per square inch.
For comparison, a 120 pound woman in 3" spike heals exerts 28.235 pounds per square inch of downforce to the ground.

Slabs without hydronic heat do NOT act as thermal batterys, so insulation under the slab offers no value in terms of heat loss. Without a frost fence to block edge contact the slab will begin becoming a cold battery as frost forms outside and travels to the slab by contact. Frost will also travel under the slab by conduction if the slab is above grade.

When you look at PEX layout for a slab on ground you'll see double or triple the number of loops near the edge as you see in the center of the slab to compensate for loss to frost.

Were it my building, I would install a few PEX loops around the perimeter into the pour now. You aren't going to be able to install them after the pour. Double up the loop in door aprons, and add a pass just outside of the door to prevent freezedown of the door.

At a future time it's real easy to add a couple water tube solar panels and a draindown tank connected to the PEX. This allows you to begin charging the thermal battery early on before temperatures really drop, and minimize fuel cost through winter as well.

 

[Radix2]

This is the foam I used under my slab - they claim it is better than XPS and it is a Michigan company so perhaps you can find a good local deal. I used 2in over all and a second 2in layer for 4 feet around the perimeter. My foundation is made from ICF blocks giving about 4 in down the full 42in. So far I have been amazed at how well the radiant floor works.

http://atlaseps.com/products/thermalstar

 

[wssix99]

Based on steel reinforced concrete 4" thick, the slab with a 6000 pound truck sitting on it weighs 90,000± pounds. Spread that over the area of 1680 sf, and the slab exerts 53.7± of downforce per square foot or 3.7±pounds per square inch.

This isn't very helpful because the 6000 lb truck can't be spread over the floor like peanut butter.

You should also check that pencil as your psi number is off by a factor of 10.

Concrete is 150 pcf. The pad is 1/3 of a foot thick. There are 144 square inches in a square foot. Weight of slab = 150 / 3 / 144 = 0.35 psi of downforce from the slab.

Sorry for the OP - lots of bad numbers in this thread. Saturdays, alcohol, and first grade math obviously don't mix.

For comparison, a 120 pound woman in 3" spike heals exerts 28.235 pounds per square inch of downforce to the ground.

This is either fantasy or unsafe for a working garage.

 

[joes169]

So what is the general consensus on insulating under a slab that doesn't have hydronic heat, worth it or waste of money? This is my home hobby shop that I'll be using in the evenings & on weekends, therefore running the temp up & down frequently. I'll probably keep it above freezing at all times though.

I replaced about 60% of my floor years ago when I remodeled my shop. Put 2" underslab insulation and pex for infloor heat in the new part. I haven't had it on in years, waste of money IMO, but that's a whole 'nother thread. I did check both sides of my own floor last night, and can tell you that generally speaking, the floor over insulation is 1-2 degrees warmer than the non-insulated floor. The lowest temps are around the exterior walls, which can be as much as 10 degrees cooler than the middle of the floor.

Because of the loss around the exterior edge, I would consider just foaming the outside 2' or 4' horizontally. THis does require more work when it comes to grading obviously, as you need to grade and compact the outside perimeter first, and then grade the balance after the foam is set in place.

 

[bsg]

I built a new barn and attached it to the end of my old barn. The new barn is prepped for radiant floor heat, two inches of foam board and a vapor barrier under the six inch thick concrete, two inch foam board around the perimeter at two foot deep as well.

The old barn is 30' x 50' x 14' walls and the new building is 50' x 72' x 16' walls.
The older barn does not have any foam or vapor barrier down.
The other major differences, the old building has pliywood under the vinyl siding, the new building is just bare steel for now.

I can tell you there are two major differences between the two buildings, the floor temperature is warmer in the new building by a couple of degrees with no heat in either building or insulation yet.

The other major difference, when the dew point changes and the old building has moisture on everything from the floor to about 40" high, the new building has none.........if I was to build a new building I would not eliminate the vapor barrier even if I did not intend on heating it, keeping the moisture off of tools and equipment was well worth the extra cost IMHO!

Kevin

 

[brownbagg]

Your theory of concrete bending is from the planet Oblivion. Concrete has maximum stress handling in compression, so the slab will not bend more than .004 inches under the tire contact area.

Delta this, you need to learn a lot about frost. I suggest you begin study by finding some frozen ground and start digging a hole.

he is not wrong, concrete does have a compression strength but because the foam is not holding the load the concrete slab is now a simple beam with no enforcement under the center, so using the formula for a simple beam as in triple point loading, the slab has the strength of roughly 450-600 psi, before it will start cracking.

 

[wssix99]

So what is the general consensus on insulating under a slab that doesn't have hydronic heat, worth it or waste of money? This is my home hobby shop that I'll be using in the evenings & on weekends, therefore running the temp up & down frequently.

Consensus? Ha! (There is a mathematical right/wrong answer for you, but figuring out that payback includes a lot of variables and future-telling.)

Here is some perspective that you might find helpful in making a decision: The floor is like a giant window filled with concrete instead of glass. It has a poor R factor, just like glass - heat moves through it pretty quickly. Putting 2" of foam down will slow the heat transfer in a similar way that blocking up all your windows with 2" of foam would lower your house's heating bill a good bit.

The concrete floor has a high thermal mass - but that's a different thing.

I'll probably keep it above freezing at all times though.

This would be reason, alone, to assume insulating will have a near-term pay-back for you if you plan on staying with the house for a good bit of time.

Will investing on $1000 of insulation give you a payback if you sell in 3 years? No. Will that investment payback after 10 years if you keep the floor above freezing the entire time? Probably.

 

[wssix99]

Your theory of concrete bending is from the planet Oblivion. Concrete has maximum stress handling in compression, so the slab will not bend more than .004 inches under the tire contact area.

he is not wrong, concrete does have a compression strength but because the foam is not holding the load the concrete slab is now a simple beam with no enforcement under the center, so using the formula for a simple beam as in triple point loading, the slab has the strength of roughly 450-600 psi, before it will start cracking.

Simple pavements (like a floating slab in a house or garage) are "continuously supported slabs". Since they have very little capacity to hold tension, their resistance to bending is highly dependent on the ground underneath them. The slab will generally bend as much as the ground below allows it. Their free body diagram looks like a beam/slab with an infinite number of columns supporting the slab.

^ The inherent challenge here (assuming one wants bypass weekend sport watching and beer drinking by taking regular statics/dynamics equations and solving them with some calculus for the set of infinite supporting columns) is that the strength of those dirt "columns" are not typically well known in a residential scenario. So the slab thickness guidelines we are given are pretty conservative. (By the time one calculates pressures down through the slab and typical compacted bases, the pressures very very small.)

 

[matt_i]

The point about bending was more along the lines of everything has a Young's Modulus and deflects under loading. It seems like concrete (and glass) are rigid because they don't deflect much and then fracture. But the tensile and compression stresses are still there for any kind of loading.

The pyramid/conical distribution theory is actually very useful because otherwise every vehicle would be limited by the 2000 psf (got it right!) ground bearing capacity and then the cushion tire forklift would not be in existence, as those things sink *instantly* into the earth even when not carrying any load. Don't ask how I know this

The best analogy for the bending I can currently think of is a steel washer trying to compensate for an oversized hole. There isn't enough structure under the center of the washer to support anything, so stresses build in the cross-section to distribute the load outward until there is enough area to support the load. Just like concrete, a thin hardware store washer is going to deflect more than a 3/8" thick steel washer...a thick slab has more ability to support & transfer than the thin.

 

[WNYflyer]

Hope I am not being too technical but the insulation under the slab needs to meet the ATSM specification C578 for "Rigid, Cellular Polystyrene Insulation"

There a two categories of material under that spec,
Expanded Polystyrene, aka "EPS"
Extruded Polystyrene, aka "XPS"

and within both those categories are seven different classification types,

http://docserver.nrca.net/technical/9997.pdf

See the attached link above for the specified compressive strength of each of those materials. This shows you the 10,15,25,etc. psi materials

http://www.homeinnovation.com/~/med...de-to-Frost-Protected-Shallow-Foundations.pdf

See Table 2 on page 5 of the link above provided by Franz1.0c,

They don't come out and say it but it looks like they do not recommend C578 EPS Type I (10 psi) since it isn't even shown in the table. It also looks like they do not recommend EPS Type II (15 psi) either since the allowable bearing capacity column is left blank for that one.

So looking at table 2 it would seem at a minimum you should be using EPS type IX (25 psi material) or XPS Type IV (25 psi material),

So say you use 25 psi material, how much load should it really be subjected too? Well 25 psi x 144 sq in/sq ft = 3600 psf but that it is for short term loading. Should you have long term loading such as the dead weight of the slab, structure, pallet storage, etc, then you should have a factor of safety of 3.0 to cut down on the permanent long term deflection/compression of the insulation,

So, 3600/3.0 = 1200 psf (8.3 psi) which conveniently comes out to the same allowable bearing capacity shown in table 2!

Now don't know your loading on the slab so can't comment on whether on not the 4" slab is strong /stiff enough to limit the bearing stress on the insulation too 1150 psf (8.3 psi) but for normal garage loadings I doubt it is a problem.

After all that I wouldn't be putting anything in less than 25 psi material.
Also table 2 shows you the Max Effective R-values for different types based upon vertical or horizontal installation. This may help you determine thicknesses and type to be used to also satisfy any thermal requirements.

 

[lakeroadster]

This isn't very helpful because the 6000 lb truck can't be spread over the floor like peanut butter.

Interesting thread technically and comically.

From a layman's perspective... spending a lot of time, money and effort to compact the sub-grade in preparation for a slab makes sense. But then laying some squishy foam on top of it just seems wrong? I understand the reasoning, and the theory... but it seems wrong.

 

[GMCGarage]

Based on steel reinforced concrete 4" thick, the slab with a 6000 pound truck sitting on it weighs 90,000± pounds. Spread that over the area of 1680 sf, and the slab exerts 53.7± of downforce per square foot or 3.7±pounds per square inch.
For comparison, a 120 pound woman in 3" spike heals exerts 28.235 pounds per square inch of downforce to the ground.
/QUOTE]

90K/1680sf = 53.7psf = .37psi.

Isnt the 3" usually the height?

Where do I send the bill?

 

[ard]

Hope I am not being too technical but....

Very useful content, thanks.