under slab insulation

hydronicsmike

...of course one should install a strip of foam liner around the perimeter of the poor, between the slab and the walls. Right?

Mike

Dan_2

under slab insulation

I am installing how water heat in a 6 inch slab of concrete in a large garage 30 x 36 feet. We were planning to put 2 inches off blue board insulation underneath. I was told recently by a contractor that this is no longer the way to insulate under cement slabs. The concrete is to heavy it crushes the insulation and the slab sinks. Is this true? What is the best insulation for under 6 inch concrete slab? Thanks in advance.

hydronicsmike

It would be...

...the first that I would see.

I'd insulate below it.

Mike

Ted_5

They do make higher denc. foam

that works well under heavy use slabs. If it is not in heavy use the gray DOW is for below grade and that preforms the best in my oppinion.

Ted

delmas

I know there are a lot of misconceptions out there about under slab insulation. You should use a density that is rated for under the slab. Owens Corning Foamular 250 is one of them with a 25psi rating. Dow also has some that is made for under the slab.
You definitely should insulate to get a top performing system.
Delmas

kevin coppinger_4

Gee, if the...

area under the slab is prepared properly (graded and compacted flat) there should be no issue. am not sold yet on the bubble-foil stuff. I have heard that the insul tarp that HR uses is pretty good.
What thicknees/R-value were you going to go with?Were you going to staple it down or go with the tie down method? kpc

hot_rod

What's the load on the slab?

As the others mentioned foam is available in ranges from 15 psi all the way to 100 psi. You could park a 747 on a properly insulated slab.

Of course the slab will only be as solid as the ground below it. If this is a high load slab, best to have a compaction test done. Otherwise the foam will get the blame for cracking and shifting

hot rod

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marc

CUT TO THE CHASE

if the subgrade is done properly (as mentioned) above and the foam is rated for under cement (psi) why would it not work. the problem we have is with the finish. slab cracks all to hell. this usually is not due to settling but improper installation of concrete. you need vapor barrier-2 in. foam-then 2 in. of sand. this gives water in cement somewhere to go besides up. ever see finisher trying to get water off of top to finish. this is because all the water must go up, meanwhile slab is curing. also, almost every concrete installer adds water to mix to make it easier to work with. i have seen 6000 ft slabs turn out like children finished them. the sad thing is everyone says this would not happen if there was no foam/tubing. have you ever seen a slab that has no cracks (i have not) the key is proper mix , and timely finish. if you add water to mix it has to go somewhere. it would be hard to finish with water on top.

hydronicsmike

Cracked Slabs...

...I've seen were mostly caused by excessive supply water temperatures. I still remember this firehall about 4 years ago in Olds, AB. They ran 180°F straight through poly-b through the slab. Did this slab crack? It sure did.
Saved the firehalls live in dogs life. Not too long after the install, the dog started "hauling" when it ran across the slab and it would never lay down. They thought something ought to be wrong with the dog. My infrared gun prooved them wrong. Surface temperatures of close to 140°F.

We installed a heat exchager to separate the poly-b from the cast iron boiler and installed propper mixing.

Mike

hot_rod

You nailed it, Marc

generally it is the fault of the placement of the "mud" Every gallon of water added to a yard of concrete reduces the psi rating by 200 psi. The mix generally leaves the plant "correct" Although batch plants make mistakes also, even the computer controled mixes. I recently saw a drive that was ordered as a 6 bag mix, but the computer missed the proper portland mix and it actuallt tested as a 2 bag. It crumbled, and the concrete yard admitted the mistake and picked up the redo tab.

You can request the plant send out someone to test the mud at the time of pour for piece of mind. They can slump test before your eyes and take samples for other testing. Most of my commercial jobs insist on this. It protects all parties. Generally the redi mix rep will not allow excessive water to be added at time of pour.

There are numerous admixes and products to spray on that will slow the cure and help prevent cracking. liquid monomolecular spray on products, dry shake, foggers, even plastic sheets will help, although they can make for an ugly look! Especially important in hot, low humidity, breezy days.

The slower the cure the better the finish. Runs counter to the concrete finishers goal, sometimes, as they get paid by the yard around here. Two pours a day pays better!

Some of the finishers I work with lighten or delete the fly ash (it slows the setup in hot weather)percentage on large pours in hot weather. Costs the plant more to add portland in leiu of fly ash "filler" however.

It can be addresses if the contractor and mix plant plan and work together. Generally excess water will undo you, regardless of what the finishers want to speed their job.

hot rod

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hot_rod

Yes and no, Mike

Concrete weighs about 4000 lbs per yard. Takes a lot of BTU's and flow to quickly heat up a slab. Generally most residential systems do not have the boiler horsepower, and pump/ tubing capacity to shock the slab. Especially if it is started in cold weather with the building load imposed.

The slab is generally in charge, easily overpowering the boiler, and typically low return water to the heat sourse are the biggest concerns with slab start ups.

External restraint cracking is another one to watch for if the slab is poured within concrete walls without foam or expansion strips around the perimeter. That slab will try to move,somewhere!

Not saying that it never happens, of course, and temperature high limit control is always important!

hot rod

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hydronicsmike

Fair enough.

Thanks Hot Rod. All makes sense. Have seen those cracked slabs though, caused by excessive surface temperatures.

So what do you deem a bigger problem when it comes to insulating the slab. Uneven ground to begin with, or insulation that is to 'soft'? Or is it both maybe?

Thanks again.

Mike

hot_rod

Mike, by far

improper prep work by either the excavator or concrete contractors. Like any trade there is a right and wrong way to do it!

Often excavators fill the excavation with the soil that they dug out. often a mix of top soil, loam, clay, etc. rarely do this fill with just backhoe compaction meet the loads! definitally not for bearing slabs and footers. Compaction is really an often overlooked part of good concrete prep work.

Even the basic 25 psi foam will handle most residential and light commercial applications. Really the contractor installing the foam should be responsible for the load rating and the building requirements. the info to make this decision is readily available from the foam suppliers and building designers or owners. Just need to ask

Back to the other topic regarding slabs and boilers. Most often I see boilers connected to slabs without adequate return temperature protection. A slab started in winter sitting at 30 degrees will keep a boiler in condensing mode for days, maybe even a week. of course condensing equipment is able to handle this. Not so with the more common cast or copper tube boilers.

As you well know the only ABSOLUTE way to protect the heat sourse is to monitor the return temperature at the boiler and have controls that can react. Yes, tekmar is a leader in that respect, with this feature, wisely, built into virtually every one of your controls.

Good for you, good for the contractor, and real good for the consumer to protect their investment. Keep spreading the word.

hot rod

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hot_rod

Mike, by far

improper prep work by either the excavator or concrete contractors. Like any trade there is a right and wrong way to do it!

Often excavators fill the excavation with the soil that they dug out. often a mix of top soil, loam, clay, etc. rarely do this fill with just backhoe compaction meet the loads! definitally not for bearing slabs and footers. Compaction is really an often overlooked part of good concrete prep work.

Even the basic 25 psi foam will handle most residential and light commercial applications. Really the contractor installing the foam should be responsible for the load rating and the building requirements. the info to make this decision is readily available from the foam suppliers and building designers or owners. Just need to ask

Back to the other topic regarding slabs and boilers. Most often I see boilers connected to slabs without adequate return temperature protection. A slab started in winter sitting at 30 degrees will keep a boiler in condensing mode for days, maybe even a week. of course condensing equipment is able to handle this. Not so with the more common cast or copper tube boilers.

As you well know the only ABSOLUTE way to protect the heat sourse is to monitor the return temperature at the boiler and have controls that can react. Yes, tekmar is a leader in that respect, with this feature, wisely, built into virtually every one of your controls.

Good for you, good for the contractor, and real good for the consumer to protect their investment. Keep spreading the word.

hot rod

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hydronicsmike

Thanks Hot Rod

I truly appreciate you taking the time to explain it. Makes sense to me.

So to fully answer this gentlemans question, I would assume you agree that one should use Insulation underneath radiant heating! But make sure that the ground was properly prepared before the insulation goes down and that you should consider either mixing or a condensing heat source for the fluid temperature control.

thanks for the remarks as well.

Mike

hot_rod

Yes, but

but that band of foam between the foudation wall and the finished floor pour is a trickey detail.

Looks bad to have an inch or more of foam strip showing if the walls are not to be furred to hide it.

I have tried the 45 degree angle cut to make that disappear, but it leaves a very thin taper edge. A for sure crack and snap spot. I don't have a great answer for that joint.

Probably expansion strip with butyl caulk works, and seals, best. Works best at garage slab to concrete drive seams. Pricey! if you are talking several hundred feet.

hot rod

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