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

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.

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

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.

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.

-AC

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.

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

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.

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

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.

Glad to hear you found what you were looking for easily. The supply houses can also offer you products and expertise the big box stores don't even care about. If you run short on supplies, most of the "little guys" can run you a short order right to the job site with very little persuasion or hassle.

Make sure and take lots of pics, uploading them as you execute this project. Many DIY projecteers considering building something similar visit the site. More real-time, in-the-flesh success stories add to the "git 'er done" nature of this site. Also, seeing what you accomplish as the project is in progress can help members spot potential mistakes or help you improve certain aspects before you finish. Especially with a poured slab, once it's in there, it's too late to save something easily. Many members have completed better projects than they started (myself included) using the advice provided by fellow members.

Yup, this is certainly one of those- it started as 'yeah, I want a pad in the barn'
and is currently at 'yeah, NOW I want a pad that I don't use high quality/cost energy to heat'.
A/C, I blame the hacked heatpump thread for this, of course-
I had one contractor scratch his head and say "this sounds like a science experiment"
and then get really confused when I said "Exactly! That's the fun part!"

It just seemed silly not to spend the $500 on tubing... well, and the insulation...
and the extra construction... and time... to get the 'radiator' built.
After that, I can add heat when we get there.

Nibs, I'd be interested to read an epscrete thread.

t

it seems that the thickness in inches of EPS or XPS under a slab should be = to your climate zone, ie Zone 4 = 4"

Never use Polyiso (the yellow stuff) under a slab as many have.

I'm using 6" of type 2 EPS in climate zone 5 (only a few miles from a county that is zone 6)

Things I found for the floor seem to stop at R10, and that at about zone 4.

The insulation I'm using says not to go over 4", or R20 with it.

So there will be pictures when my hands heal, but I got the grade pretty level, the
foam's down, and the remesh is almost done. Of the 6 or so ways to do it, I think I picked
the hardest. Getting rolled remesh to sit flat is not easy. And dang, that stuff likes
to roll back up and take a bit of foam- and flesh, if it can get it- when it gets loose.
The 12' door is the worst- as I bend it down, wire- by- wire, it wants to lift 5' behind the
door. I stopped tonight because I ran out of staples (yes, I'm stapling it to the treated
skirting boards. It's a pole barn. I gave up) but with more staples, I'm going to stretch
it sideways. And then jump on it.

At least it's stopped raining.

t

Update from the dead-
5 loops under 1800 square feet of concrete, all hold pressure, and ironically, it's been too cold
to do the sealing and finish work to seal up the floor and get a wood stove going!
But the barn's structurally done. The concrete finisher scored the surface, so I'm going
to embed temperature sensors into the scores. I didn't put them into the
slab itself, and then regretted it. But I want to seal the surface where he scored
it, so I've got some tiny sensors, some tiny wire, and a lot of flexible concrete
'crack sealer' and I'll see how it goes. One active and one passive sensor
per zone, roughly in the middle of each. Just for fun.

One thing I learned: when dealing with permits and inspectors,
I didn't think about the ramifications of putting part of a heating system
into a building that was permitted as 'unheated'- I had a heating inspector
who looked at it, made a note, then said "I was never here- this is an unheated building"
winked, and drove off. I can imagine one with less of a sense of humor would
have made my life a lot more difficult...

t

In my new house build I am using Tekmar 519 thermostats with slab sensors.
A good method is to put sensor into 1/2" pex (end plugged) down into slab & the top end goes right into thermostat wall box. Sensors can be easily pulled & replaced.

Yeah, I had read about doing it that way, and had meant to,
but in the panic over inspections, it didn't get done...
...This is going to be a total science project anyway, so this
is just one more... variable? HacK?

Thanks,

t

Be prepared for extra time and effort with laying your concrete- you layer of insulation will act as a water/ vapour barrier, it takes longer to tighten up and kick.

A foot (300mm) of EPS is standard under slab here or alternatively 150mm-200mm of polyurethane foam under screeds. This is in a temperate climate.
Just to put things into perspective.

Holy Cats! Yeah, our climate's analogous to yours- same south sea warming, precipitation, etc. 5c winters, 25c summers.
A foot of EPS is an R- value of something over 40.
Whereas our codes seem to stop at R10 for underslab, nationwide. And it gets a LOT colder in the midwest...

Interesting. Thanks!

t

Hi Toby, no problem.
150mm of PUR is just about minimum here to meet code.
You'll find that you'll probably end up in the same place in terms of minimum R value as us eventually, just takes years for things to change.
Also another point. If you ever intend to run the underfloor heating from a heat pump you want the tightest possible pipe spacing throughout - generally 100mm.
This improves the thermal transfer to the slab allowing you to run lower flow temperatures, which in turn increases the COP of your HP.

Let's all recall that we are in vastly different climates with code (or rational engineering) being different in Ireland vs Oklahoma where I live.

In general, the further north toward the respective pole (north or south) you go, means that a thicker underconcrete foam layer is needed and has a quick return on investment.

In Oklahoma, our deep soil temp, which become the underslap temp, is about 60 F, whereas Ireland is likely some 20 degrees less.

Even here in Oklahoma, I would put in some subslab insulation, especially if I do the labor (which is minimal. But maybe only 1 inch - ~ 25 mm. But guess, what - code here does not even address subslab insulation.

I tip my hat to the Irish, with their colder soil temperatures, for insisting (code) on at least 150 mm subslab insulation. Not only does this save money, but the slab floor is just SO much more comfortable!


Steve

I'm building a new house in climate zone 5 dry.

Average air temperature here is 41*F (5*C) so that is also the 10' deep soil average temperature.

I'm using 6" of EPS type 2 under my radiant slab and running down the inside of my foundation stem walls.

180' 1/2" pex-A loops 7” OC using constant circulation with Grundfos Alpha pumps on low, with Erie BB3000 injection Outdoor Reset controller.

Thermostats are Tekmar PWM type, that learn, with slab sensors.

You also need spray for foam insulation,its mechanical systems that heat and cool buildings are continuously operating; reducing extreme temperature variations saves on the overuse of mechanical systems and leads to lower energy bills