Subscribe or Renew Membership Subscribe Renew

Insulation under a garage slab

greggo's picture

Hello Everyone: I'm getting ready to pour my 3 bay garage slab and had a few questions.I plan on pouring 4000lb mix and 5"thick in two of the bays and 6"-or 7" in the 3rd bay.The third bay will have a 2 post 10,000lb car lift in it. The lift specs say 4" minimum sounds a little thin to me. Also I'm adding # 3 rebar 18"on center criss crossed with fiber mesh.Garage is approximently 26'x36 with a sand base. Questions are:I want to install 2"foam board to give me a thermal break from the ground does that sound ok even though I'm not going to use radiant in the floor?Should the rebar be pinned to the side walls?And should I replace the sand base with 3/4" stone. I live in CT.Any help will be appreciated.




Thanks Greg


Good questions (post #196161, reply #1 of 27)

I would never, ever place concrete over sand, even with a vapor barrier under it. I would remove the sand and place at least one 4-6" lift of washed 3/4" gravel down, more if possible. Compact and level it. That gives you a reasonably good capillary break so that ground moisture can't so easily migrate up thru your slab prep and into the slab itself. Then... lay down Stego Wrap or Tu-Tuf or similar over the gravel, tape the seams and penetrations per the manufacturer's details. Then, lay down 2" XPS foam rated for 25PSI, such as Foamular 250. Butt the sheets tight together, and use foam at the edges to isolate the slab thermally from the exterior concrete walls. Lay down your rebar grid on 1-1/2" dobies and pour the concrete. I always order a low-water mix with a plasticizer, less water in the batch is a lot better IMO. If you need bearing locations for the lift, you could cut out small areas of the foam where the lift will sit, but that's probably not necessary, and if you really want to know you will need an engineer to look at the loads and the slab performance.

Hi Dave:    I'm not to crazy (post #196161, reply #2 of 27)

Hi Dave:    I'm not to crazy about the sand neither but I wasnt sure if the foam board would take care of that.I have a BobCat 843 and a 10,000lb dump trailer so I can get in there and get the sand out .I have used Tu-TuF in my basement and it worked great.I wasnt sure if it was over kill for the garage.Is the Foamular 250@ 25psi enough for a diesel pick-up to be parked? Would you pin the rebar ends?I had two concrete guys tell me that they put flat wire mesh on top of the rebar?



Thanks for the fast reply!




The Foamular 250 (post #196161, reply #3 of 27)

is enough for an F-250 truck, but if you have an F-350 then you need to go with Foamular 350.

Just kidding. The 250 should be fine for a garage slab, assuming the slab is done correctly. You could talk to the Dow Corning engineers and get their take on it. I would absolutely get the sand out, especially now that I know you have a machine to do it easily. If you can get clean crushed rock without fines in it, that's your best bet. Around here I can get what's called drain rock, it's a crushed product that's graded at 7/8" and has a little bit of small stuff, but is mostly large and irregular shaped so it compacts well enough. I would not use driveway gravel.

 Dave:    LOL You almost had (post #196161, reply #4 of 27)

 Dave:    LOL You almost had me for a second with the F350.The truck is  a GMC 2500 HD(7000lbs).Not sure what driveway gravel is .That might be what we call 3/4" processed I think?  Is 3/4 "washed stone the same thing as drain rock?What is pinning the apron?Is that where the  concrete door opening is and rebar is put in a certain way?


Thanks Greg


Driveway gravel (post #196161, reply #5 of 27)

is crushed rock with a lot of fines (grit) in it. It will hold moisture just like sand and I wouldn't put it under a slab, except maybe an outdoor slab. You have to look at what gravel you can get locally, and what it's good for. Our "drain rock" is a crushed product and it compacts reasonably well, although not as well as rock with fines in it. In other places, drain rock is river gravel that's nicely rounded but won't compact worth a damn, so it would be hard to work with as slab base. 

I'm not familiar with "pinning the apron" but it sounds like rebar that connects the slab (interior? exterior apron slab?) to the concrete foundation. In most cases I would not do that.

Pinning (post #196161, reply #8 of 27)

Pinning the apron to the slab is just doweling some #4 rebar into the garage slab at each door. Generally done after the largeer slab is poured and the forms wrecked. (BTW I like to thicken the slab at the door openings). I have installed dowels through the door form boards and stubbed them intop the apron area but it is a real PITA to wreck forms if done that way.

The pins from the garage slab into the apron slab reduce the risk of the apron lab pulling away or moving up or down.

Perimeter insulation, as suggested, by some one will reduce your heat loss through the slab edge, where it is the greatest, but no insulation will stop it completely, only slow it down. Many insulated slabs are only insulated for the first 4 to 6 feet around the slab edge because the soil temp. varies very little after that distance. It takes a long time for soil temperature to below earth temp inside and under a concrete slab, paricularly if the sub base remains dry.

Heat the Garage? (post #196161, reply #6 of 27)

Are you planning to heat the garage now or eventually?

Then insulating under the slab is not the way to go.

The foundation walls should be insulated instead, either outside or inside, from the sill plate to the top of the footing or frost depth whichever comes first.

Otherwise you will lose energy through the edge of the slab and any masonry exposed above grade.


PS. Otherwise I agree with the use of washed 3/4" stone as a base.

You should also use a minimum 6 mil plastic vapor retarder over the stone and under the concrete.

Energy code here (post #196161, reply #7 of 27)

requires R-10 under the slab unless the building is unheated. You are correct in pointing out that the foundation walls should be insulated as well. Here that would be R-10 inside the walls to a depth of 24" or more.

Does Not Require Both (post #196161, reply #11 of 27)

Underslab insulation is not  required under most energy codes (like the IECC) and in fact is prohibited from being used instead of foundation insulation by such codes.

Of course, there may be some local variations of these national requirements.

Generally speaking, however, the only place underslab insulation is required is when the slab itself is heated.

Otherwise the majority of heat loss is through the upper portions of the building, not through the soil or slab, and adding insulation under the slab provides only added cost while offering little or no energy savings benefit at all.

Needs both (post #196161, reply #9 of 27)

insulating perimeter does nothing to prevent heat lose to ground, only to perimeter. Local conditions vary too. Soil with water moving through it carries more heat away. ave ground temp will vary also



Oh Well,

We did the best we could...

I will probably  throw a (post #196161, reply #10 of 27)

I will probably  throw a modine heater in there eventually.We dug out a lot of sand today(anyone need some fill lol)Getting approximately 16 ton of 3/4" washed stone tomorrow.Spread that around,compact .If I can get the tu tuf vapor barrior locally somewhere in CT I will use that,otherwise its 6 mill polly.2"Foamboard,rebar,cement to be poured Monday.I had a mishape with the Bobcat. I just hit a row(nicked) of 2x4 studs between two bays holding up three LVLs  26'x18"x1 /3/4"  I knocked out the bottom plate 1/2 way and bowed the wall out at the bottom.I thought I was doing good considering I only got about 5 hours run time on the machine.Lesson learned (slow the heck down)I'll jack up the LVL tomorrow and try to bang the wall back in.I will also put foamboard on the inside perimeter walls as far down as the remaining sand to the sill plate.


Should the rebar be drilled into the perimeter walls or is that over kill?


Thanks Guys!!




Make sure you are using (post #196161, reply #14 of 27)

XPS foam insulation.

I must disagree ... "Then (post #196161, reply #12 of 27)

I must disagree ...

"Then insulating under the slab is not the way to go."   That is purely opinion and not really science as you imply.

While admitedly from an energy point of view, it may not be considered the best, it's a choice and it isn't necessarily unreasonable. Assuming that when you insulate the slab, you also do something about the perimeter, then there is nothing wrong with it. Since he's heating w/ e.g. a Modine, it will temper the slab temp a bit.

There ain't NO free lunch. Not no how, not no where!

Feel free to disagree. But (post #196161, reply #15 of 27)

Feel free to disagree.

But most energy codes prohibit the practice of insulating under slab-on-grade construction when the area above is conditioned,  requiring instead that the foundation be insulated to frost depth or the top of the footing... whichever comes first.

The exception is when the slab itself is heated.

One will also gain little if anything by insulating the perimeter of the slab except if the slab edge is above grade and/or the slab is heated.

Otherwise insulating under the slab is not an approved  alternative, does not comply with energy code requirements, and adds nothing to the energy or functional efficiency of the concrete floor,  serving merely as little more than an expensive and unecessary "expansion joint".

I know of no energy code that (post #196161, reply #16 of 27)

I know of no energy code that would PROHIBIT insulating under a slab. Some energy codes REQUIRE it to be insulated as you imply when the slab is heated. Some codes require the foundation to be insulated as you indicate. Some allow you to insulate the slab edge and under the slab e.g. 2 ft as an option to insulating the foundation walls.

Insulating the foundation is much equivalent to insulating the slab perimeter. Slabs typically have to be or normally are above grade. Assuming you wouldn't want your foundation wall imposing on your finished space and the code requires e.g. 6-8" of clearance between the bottom wall plate and the finished grade, you effectively always have your on grade slab ... above grade. If it is below grade ... e.g. basement or 1/2 basement, then no insulation would normally be required.

Insulating under a slab is not an issue of 'approved' for nothing (i.e. code) prohibits it. It does comply with energy code requirements ... or at least the energy code doesn't likely say 'you can't do that'. Whether it will contribute neutrally or positively to the functional energy efficiency may be arguable, but most agree that enclosing the mass in the space could generally be considered a positive energy strategy.

Insulating an unheated slab in a conditioned space is not much different than insulating an unheated slab in a conditioned space. In both situatitions the insulation controls the heat loss to the ground ... which is simply greater with the in slab heating than with the above slab heating.

The slab and ground beneath are unlimited heat sinks that will draw energy out of your heated space. In radiant slabs, the slab temperature is much higher and it makes most sense to insulate because of that. But insulation should benefit either condition ... albeit maybe fairly small in the unheated slab condition ... but to imply it provides NO benefit and is in fact PROHIBITED by an energy code is IMO incorrect.

I'd invite you to post one solid thread of evidence that any energy code would specifically say that insulating under a slab is not allowed (notwithstanding any requirement to insulate the edge of the slab and/or the foundation wall).

There ain't NO free lunch. Not no how, not no where!

Wow. WWF AND rebar? Seems (post #196161, reply #13 of 27)

Wow. WWF AND rebar? Seems like overkill to me. But hey it'll be strong. As far as your 10KLB lifts ... if it says 4" and you are doing 5+ then you should be OK Concrete at 3,000 psi distributed across a metal plate say 8x8 gives you a lot of capacity when you think about it. Also, if in doubt, simply cut away the insulation around your mounting area and thicken the slab ... standard detail for column support in the center of a slab.

There ain't NO free lunch. Not no how, not no where!

Thanks for every one that (post #196161, reply #17 of 27)

Thanks for every one that replyied.Very much appreciated!! The Garage is looking good.



Clewless :what's WWF?





WWF (post #196161, reply #18 of 27)

World Wildlife Fund


the ever popular

World Wrestling Federation

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


welded wire fabric AKA welded (post #196161, reply #19 of 27)

welded wire fabric AKA welded wire mesh, too, I think (WWM). comes in various grid sizes and maybe various wire gage I'm guessing. Common is e.g. 6x6 grid, but I think 4x4 is also used. You can get it flat or in rolls. I prefer flat so I don't struggle w/ rolling it out and getting it flat. As always ... pay your money, take your choice, eh?

There ain't NO free lunch. Not no how, not no where!

Clewless: Thanks for the (post #196161, reply #20 of 27)

Clewless: Thanks for the reply.I used #3 rebar 18"on center,no wire mesh.4000lb mix.We cut out an area approx.where I thought the lift should go(hard to get that info) Made it extra large 5'x12'  to CMA 5" concrete and 7" for the pad.I think I goofed.I put the foam board on the interior outside walls below the slab was that the right way to do it?Looks like water could get under the slab and freeze if I wash the truck.



Thanks again for the replies




A long time ago............... (post #196161, reply #21 of 27)

when we poured service station flat work we were instructed to add a shim (3/8's") to the top of the expansion b/4 the pour (the expansion was set 3/8" below the screed line..  This allowed us to screed to that shimmed expansion.  Later we removed the shim and used Urethane caulk (self-leveling where applicable) to fill the gap.  This gave us the expansioin joint and sealed the opening.  Can't say it worked forever, but in theory I guess the specs were right at least for the interior.

You could so the same if you ran the foam only to the top of the concrete-a propane torch might "lower" the top of the foam a bit if you were careful.  If the foam runs up the wall above the slab-run a nice looking beat of Sonolast Urethane caulk (or one of the Tremco urethanes) available at a good concrete/masonry supply.  It'll bond to the concrete as well as the foam.  Sonolast for sure has the qt tube, maybe the tremco's also.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


Your description sounds like (post #196161, reply #23 of 27)

Your description sounds like a control joint, not an expansion joint ... there is a difference. Most residential work only requires a control joint.

There ain't NO free lunch. Not no how, not no where!

Well, whatever you say. (post #196161, reply #24 of 27)

But in this case-the gas station pours, we did it at all expansion joints

and just so happens, whenever called for-which made it on the control joints where expansion was used between pours.  Saw cuts where caulked also.

In other words, all joints in the pours.

So, you never have come across expansion joints doing residential work?  Maybe things are done differently in different areas.  Or perhaps you're talking about interior work only.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


IMO you have no issues w/ (post #196161, reply #22 of 27)

IMO you have no issues w/ possible freezing of water if I understand you right (which I may not). I advocate at a minimum using 1/2" celotex around the slab perimeter (the thickness of the slab). It provides a thermal break without eating into the floor area w/ e.g. 2" insulation (often a tough detail to deal with). If you set it a bit low and then clean out the concrete after the pour, you can caulk the edge w/ e.g. flowable urethane ... standard for concrete joints.

There ain't NO free lunch. Not no how, not no where!

Here's two shots of what I (post #196161, reply #25 of 27)

Here's two shots of what I did.



Thanks again to every one!




You are making me try to remember back 22 years (post #196161, reply #26 of 27)

I'm wondering if you shouldn't isolate the lift area that you have thickened by nixing the foam.

We poured an outlandish footing for under our several ton masonry heater and chimney inside the greatroom.  We tiled everything (HW Radiant).  No cracks anywhere, around interior block stem walls etc.

But, maybe, any thought on forming the lift area, placing expansion there-filling in.  And taking some bar around the perimeter of that lift area, rather than running bar through.

No concrete guy by any means.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


Just curious, thermal break (post #196161, reply #27 of 27)

Dave, are you saying you'd use a 2" foam thermal break (expansion joint) around the perimeter of the slab? Just curious if that's done now-a-days for energy reasons.