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Old 09-30-16, 02:58 AM   #1
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Default Foam insulation under concrete slab

Ok, be kind, I'm new at this-

where do I go, as a DIYer, to get foam insulation to put under slab- on- grade for a shop?
What grade of foam do I need for what's effectively going to be used as a garage floor?

The backstory is, we're having a barn built to use as storage and work space, and it
seems shortsighted to NOT put in the tubing for hydronic heating. No immediate heating
plans, (as in, this is a 'storage' building until the permit's signed off, and probably for a while after)
but we have room for a geothermal field and also lots of maples that are going to eventually
be removed...

Thanks!

t

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Old 09-30-16, 11:28 AM   #2
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Quote:
Originally Posted by TobyB View Post
Ok, be kind, I'm new at this-

where do I go, as a DIYer, to get foam insulation to put under slab- on- grade for a shop?
What grade of foam do I need for what's effectively going to be used as a garage floor?

The backstory is, we're having a barn built to use as storage and work space, and it
seems shortsighted to NOT put in the tubing for hydronic heating. No immediate heating
plans, (as in, this is a 'storage' building until the permit's signed off, and probably for a while after)
but we have room for a geothermal field and also lots of maples that are going to eventually
be removed...

Thanks!

t
Welcome to the forums, Toby!

It depends on how thick of a slab you will be pouring and what kind of traffic the building will see. If it's a thin (4 inch or less) slab and will be seeing foot and/or light duty vehicle traffic, just about anything (10 to 30 psi) will hold up. For a thicker slab, or commercial vehicle traffic (heavy/super duty truck, tractors, etc.), a higher compressive strength product (over 30 psi) is advised. Naturally, the higher the strength spec, the higher the cost per square foot, so don't over-spec the insulation or you will pay dearly for added strength you may never need.

The only grade you are going to find at the big box stores is the standard-strength stuff. The higher strength products are not considered a DIY product, so they will have to be sourced through a commercial building supply house. The least expensive path to this end is usually having an order drop-shipped to your site or to the nearest distributor. Being close to Seattle, you should have no problem finding a dealer. That being said, if you have a trailer large enough and the time to go get the material, you could save a decent amount on freight charges, especially if the dealer is only a few miles away.
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Old 09-30-16, 12:26 PM   #3
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Thanks, Jeff-

that's what I've been finding- anything over 15 psi is hard to get ahold of. Add to that
our mild climate, and no- one locally carries much in the way of higher strength insulation.
Odd, given our supposed 'green' label and relatively high energy costs,
but we're pretty mediocre in attitudes towards insulation.

I'm planning on a 5" slab, and it will probably see pretty limited loading-
smaller cars, mostly. Do you think the 15psi's ok, or should I start shopping for
something stronger?

t
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Old 09-30-16, 01:21 PM   #4
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Welcome to the site TobyB.

I hope you don't mind, but I moved your thread out on to its own. It'll get more attention this way.
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Old 10-05-16, 11:51 AM   #5
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Quote:
Originally Posted by TobyB View Post
Thanks, Jeff-

that's what I've been finding- anything over 15 psi is hard to get ahold of. Add to that
our mild climate, and no- one locally carries much in the way of higher strength insulation.
Odd, given our supposed 'green' label and relatively high energy costs,
but we're pretty mediocre in attitudes towards insulation.

I'm planning on a 5" slab, and it will probably see pretty limited loading-
smaller cars, mostly. Do you think the 15psi's ok, or should I start shopping for
something stronger?

t
I've looked over the problem of high-strength rigid foam, also.

I goggled: "high load insulating foam", and found several choices, with tech papers to guide your decision.

There are other types of foam panels that are stronger than what is usually sold in the Big Box stores, but may be more accessible than the real High Load stuff.

And that would be rigid foam insulating panels that are specifically made for roofing applications. I have seen some with fibers combined in with the insulating board, to increase compressive strength.

It might do it.

If you read the application specs for some of the extreme High Load stuff, you will find that it is certified for use as airplane landing insulation, to go beneath the surface material. You will not need anything like this.

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Old 10-06-16, 02:32 PM   #6
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You need to make certain that the insulation will not take up water, if the foam soaks up water it loses much of its insulating value. Pretty much the only two types that are monocellular are the blue and the pink.
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Old 10-07-16, 02:54 PM   #7
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Thanks, all-
yes, EPS (expanded polystyrene) and XPS (extruded polystyrene)
are the two that are permitted around here. But by abandoning
the internet and picking up the phone, I discovered that the local
inspectors/installers etc expect to see XPS, even though it's slightly
worse at resisting water. EPS looks too much like styofoam packing, I guess.

So what I ended up with was Dow type IV XPS with a 25 psi compressive strength.
The local insulation wholesaler for Dow (Apec, in Kent, WA) recommended it for what I'm doing
and beyond, and were happy to sell me 60 sheets and fork it into the pickup and onto
my trailer, and even gave a hand lashing it all down. It was quite a bit cheaper than the lighter
(15 psi) stuff at the big boxes, too.

Now I'm off to compact and re- level the gravel one more time, and then start checking
my work to see if it's all level(ish).

Then vapor barrier, insulation, remesh, tubing, 2500 zip- ties- and concrete.
I should be done by lunch, right?

t
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Old 10-07-16, 03:04 PM   #8
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With the foam, you do not need to be quite so fussy with compaction, a good firm base is good.
I use either sand and wet it down well, or epscrete 1 cement, 2 sand, and 5 or 8 eps grindings by volume.

Also in your slab mix use superplasticizer to cut water use, fiber to help stop cracking. 5" is very thick, you may be good with a thinner slab, do a bit of research.
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Old 10-07-16, 11:16 PM   #9
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Epscrete- THAT looks like fun!
Hadn't heard of it before.

Yeah, I need the extra inch for anchors and such- with the tubing, 4" is just too close
for comfort. The guy who's pouring the slab says he'll do the tricks to give it the
best chance of working, and he's not too worried- especially because if it's on the thin
side of 5", it's still thick enough.

I have a pretty level, pretty smooth base of coarse sand (I'm using fine next time!!!)
and it's time for foam.

Tomorrow.

t
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Old 10-07-16, 11:40 PM   #10
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EPScrete is cool stuff, insulating concrete, I built a little machine to grind up styrofoam, and use epscrete extensively, last place I used it was behind the chimney on a fireplace, wanted to keep the chimney heat in the house, put a rock & mortar wall at the sides and front of the chimney, the rocks get nice and warm while the outer wall stays cold.
Have offered in the past to describe the techniques if there is any interest.

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