insulation under poured concrete for radiant.

mayna123

I have heard two opinions on insulation for a poured concrete radiant system in a garage. First, the use of foam board will cause the floor to settle and break the floor. The second is the mat does nothing to help with insulating. Any thoughts to share with me?

kcopp

If the floor under the insulation is not properly prepared and leveled then sure you could have settling....r10 board.
You also need to insulate around the perimeter. r5

When you say mat...please clarify. Did you mean Bubble foil packing material?

Jamie Hall

If you use good foam under there -- blue board -- and your slab is designed properly reinforced and the subgrade and finish grade underneath the whole thing is fully compacted and levelled -- you shouldn't have a problem. If any of those things are missing, you will have slab cracking, whether you have insulation under there or not.

Gordy

250 pink 2" foam. Is 25psi. The 150 pink foam board is 15psi. Use the 250.

FranklinD

I just had a new 24x28 garage/shop slab poured and used the 2" 25psi foam board, no worries. As was stated, it's all about the material prep before the foam and the pour. We spent days compacting and leveling prior to putting the foam down, and ended up using 1/2" rebar on 24" centers (minimum code here is 36" centers). I'll be using 3" foam board around the perimeter, up to the sill plates (we poured a 6"x6" curb as part of the slab as well to make sure the sill plates stay high n dry). I don't have any concerns about settling or cracking...even the local inspector was very impressed by the setup when he came to sign off for the pour.

It cost a bit more, for sure, but concrete is something you only want to do once.

mayna123

kcoop....The mat I was referring to is Insul-Tarp. Thanks for your input.

Canucker

Insultarp? Don't take this the wrong way but it isn't worth the amount you paid for the electrons to display it online, in this application. There is no cheap way to do this properly, but you're on the right track with your questions.

Jamie Hall

The claimed R value for Insul-Tarp is 5.9, uncompressed, which would probably drop to around 4, compressed under the slab. It does seem to be a decent vapour barrier, provided it is installed properly, but that could be said about a lot of other materials as well. 2 inches of rigid poly would give you an R of about 10. Then add in the R values for the concrete -- but don't count anything underneath the vapour/moisture barrier.

Harvey Ramer

How much does insulation really compress under a slab? And bigger question, by the time you calculate the bridging properties of the concrete, based on reinforment and thickness, what is the minimum compressive strength insulation that will do? According to some research I've read, it's quite a bit lower than most people realize.

Jamie Hall

Harvey Ramer said:

How much does insulation really compress under a slab? And bigger question, by the time you calculate the bridging properties of the concrete, based on reinforment and thickness, what is the minimum compressive strength insulation that will do? According to some research I've read, it's quite a bit lower than most people realize.

Hard to say exactly, @Harvey Ramer -- but look at it this way. The rated compressive strength of most foam board is in the 15 to 25 psi range. A typical concrete floor might be six inches. That's going to be 1 psi. Lets then suppose you put a superduty pickup in there. The tire pressure is about 80 psi -- at least on mine. By the time that is transmitted through the concrete it might be as much as 10 psi. Maybe. Should be a good bit less, as the concrete should distribute the total load over the total footprint of the truck - which would be no more than 2 psi.

So. Bottom line -- no problem. Unless, as I said earlier, the subgrade under the insulation isn't compacted properly, in which case the insulation doesn't matter a hoot; the floor will crack. The presence of insulation won't affect it, one way or another.

ScottSecor

Jamie, I think your math may be off on PSI calculations for the pickup truck. I'm only guessing here but let's say the loaded pickup diesel pickup truck weighs 8000 pounds. 8000 lbs divided by four tires is 2000 pounds per tire. Each tire has about 8 square inches of surface that actually touches the ground. 2000 divided by 8 equals 250 pounds per square inch. I know this is not very scientific, but I just wanted to point our the actual pressure on the slab. Actual tire footprint may be way off, just guessing here.

For my concrete radiant slab in the recent addition (dining room/living room) to our home I used 2" R10 250 pink board underneath. Been almost exactly twelve months and no cracks or issues. Only complaint is if you tap on slab surface it does sound hollow when compared to a slab without foam underneath.

Zman

For vehicle traffic, in my mind, the minimum compression strength should be 25psi. I like to think of the load path being a 1 to 1 ratio through the concrete, this accounts for poorly placed reinforcement (welded wire) and other variations that happen in the real world.
The contact area of a heavy duty truck would likely be 4"x8", distributed through a marginally reinforce 4" slab, that would apply the pressure over a 12"x16" area of insulation. That would make the 2500 lb point loads required by code apply approx 13psi to the insulation.You then need to add the weight of the concrete (approx 1 psi) and a safety factor.

As for the r-value of Insultarp, I have used the product and been a victim of it's under performance. Our local building department no longer allows it because the manufacture cannot produce an ICC-ES report validating it's true R-value.

That's all I have to say about that.....

Jamie Hall

On tire loads... For almost all vehicles and almost all tires (there are a few exceptions for the tires) the pressure on the pavement is only fractionally greater than the air pressure in the tire; the area of the contact patch thus varies with the load on the tire and the air pressure. This is particularly true of radial construction tires, where the sidewall rigidity is almost zero. Bias ply tires are a little more rigid -- but not much.

The primary exceptions are certain types of construction equipment, using more or less solid foam fill rather than air.

Things get a little flaky when the tire air pressure is too low and one is on a deformable material, such as mud or sand, where the belt rigidity and lack of elasticity in the side walls comes into play.

Gordy

This is what Harvey is trying to convey. Go to the website for formulas.

Jamie Hall

@Gordy -- I was trying to avoid all that... shades of the past (there was a time when I taught woebegone students to do that sort of thing!). But since it's there -- a couple of things. First, that applies to flexible pavement (or no pavement at all). Second, note that the width of the contact patch is a factor (so is the length) -- which is what I was trying to get it in a shorthand way, since it is related to tire design and tire pressure. Rigid pavement, such as concrete, adds some very real complications to the computations, since rigid pavement isn't really quite rigid, so you have to account for the deflection of the pavement under the load; the principal being that the deflection is resisted by bending in the pavement and by the transmitted load to the base materials and the ability of the base material to resist deflecting or compressing under the load.

Which goes back to what I said earlier -- the integrity of your slab is dependent on the design of the slab -- compressive strength of the concrete, placement and design of rebar, etc. -- and the compaction of the base material.

If there is no base material -- the slab can be considered as supported on only two or four edges, or on columns (say as it might be a parking garage above grade, or a bridge), then the tire loads can be usefully considered as point loads (sort of as @ScottSecor was thinking) and the whole design process is quite different (and, paradoxically, in some ways a good bit simpler, since you don't have to worry about base reactions).

Gordy

Yes Jamie it pertains to flexible pavement however it does pertain to rigid pavements in as you say a bit more complexity. The real deal is that the load is not the foot print of the tire straight down.

Just keeping it simple. The above I posted is oriented to road construction. We can get into all kinds of loading. Especially when there is bumps in the pavement that cantipult loads, and then concentrate them in a much greater force at the point of rentry.

The thrust of everything is well prepared subbase. It's everything. Unless you want to drive piling, and use extensive reinforcing steel.

Harvey Ramer

There is a lot of point load deflection in a reinforced concrete slab. Slabs are typically relieved in not greater than 20' squares. Along which they do crack, unless the concrete is slow cured, in which case they can be larger. Would I be safe to say that the point load gets deflected over the entire 20' square. Obviously with a lessening gradient towards the perimeter?

Gordy

Yes Harvey you would. Rigid PCC pavement can disperse the load over a greater area than asphalt pavements. That's why asphalt needs to be thicker to give the same load distribution.

Around here rigid road pavements are sawed transversely at 15' no greater than 20'. This is also tied in to expansion/contraction from temperature. Not just loading. Reinforcing Baskets are at each saw cut. Unless its continual reinforced pavement