insulation under slab on grade

Looking for insight regarding insulation of my ongoing garage project. The architect who drafted our garage is a 'green' builder. She has been great so far but I am starting to wonder about a few things.

The garage is 43'6 x 25' with 15' high ceilings. Probably more of a barn/fortress than garage but nomenclature aside, it's big. The foundation is 3'6" down with waterproofing on the outside. I believe the walls will be insulated with Roxul and Zip wall sheathing on the outside, then Hardie Board siding. She convinced us not to use spray foam on the walls because we have kids and the apparent harmful off-gassing can last for over a month.

The roof is going to be standing seam metal with a vapor barrier and Zip sheathing. The drawings specify 3" spray foam insulation but I'm thinking that's not right based on the off-gassing thing so basically I'm not sure what will be on the ceiling.

Garage heat is undecided but it will likely be a Modine type deal. Also figuring for reversible ceiling fans to keep the heat down during colder months, perhaps even thermostatically controlled.

Not sure on doors but they will be good quality, no flimsy plastic junk. Couple windows on the one long side that faces out, the opposite wall is shared with a mudroom area.

So under the slab, (which will be 4" thick, originally spec'd for 6") the architect is calling for 6-8" of EPS foam board for an R value of 30-40. Again, this is below the slab so the insulation is above the gravel and below a vapor barrier.

The boards come 2" thick and at almost 1,100 sq. ft, it needs to cover a LOT of area 3-4 times over to achieve the specified R value.

Unless I am missing something, 6-8" of rigid foam board insulation for R30-40 seems completely excessive.

Can anyone give a me a good reason for having so much insulation under the slab?

I know I can decide to have whatever I want there, just making sure I am not missing something significant that would warrant such a large amount of material. Our contractor thinks it's unnecessary. So do I.

Thanks for any input.

- Greg

Nothing to add but wow.


No location is listed, but are you on some planet where heat falls instead of rises?

Yikes! That does seem excessive. 2" rigid has been the std. for garages for a long time. I have seen the 6-8" on commercial sites lately though.

You may want to consider to running some pex tubing above the insulation before they pour the concrete. There is nothing more comfortable for heating than radiant in the floor. Almost no temperature differential from the floor to ceiling. In that case, I wouldn't talk you out of more insulation.

Anyway, there's a lot of information out there for you to ponder.

Regards,

Dan

First off, if she speced out EPS(Expanded Polystyrene, do not use it. It has very minimal compressive strength when compared to Extruded Polystyrene (XPS) Secondly, yes, that is very excessive. Heat rises. The ground temp wont change to much under the building. 2" XPS foam around me is usually the norm. If it were mine, I would do the 2" XPS and 6" fiber reinforced concrete.

Here is a link for info on different types of rigid foam insulation.
https://www.insulfoam.com/insulation-comparisons/

chaosracing said:

First off, if she speced out EPS(Expanded Polystyrene, do not use it. It has very minimal compressive strength when compared to Extruded Polystyrene (XPS) Secondly, yes, that is very excessive. Heat rises. The ground temp wont change to much under the building. 2" XPS foam around me is usually the norm. If it were mine, I would do the 2" XPS and 6" fiber reinforced concrete.

Here is a link for info on different types of rigid foam insulation.
https://www.insulfoam.com/insulation-comparisons/

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What he said. That is overkill on a constant ground temp of 55. Money is much better spent elsewhere.

Your slab is a huge heat/cold sink so keeping the heat in your slab is important but r 30 to r 40 is cra cra.

I used 3" rigid 60 psi because I had a bunch leftover from a job.

I put it vertically at the edge of slab as well as underneath.

Lost of the loss comes from the edges.

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I would ask the architect if you can see the heat loss calcs that support the use of that much insulation below the slab. The calcs should have assumptions for slab temperature and soil temperature, since heat loss is proportional to the difference in those temperatures. Make sure those temperature assumptions are realistic. Not every location has 50+ degree soil (34 degrees at 3 ft depth at my location). And not everyone is running hydronics, where the slab temperature is a lot higher than the air temperature. If you arent running hydronics you will have a cooler slab and will lose a lot less heat for any amount of slab insulation.

Also, as was mentioned by machsnell, slab edge is where half or more of slab heat loss happens. Typically you insulate the outside of the footing, for the full height of the footing, and spend less money insulating under the slab.

Thanks for the replies guys, much appreciated. For those that asked, I'm in CT, about an hour north of NYC.

This morning me, the builder and architect discussed the insulation since the material is ordered and we are pouring concrete for our addition crawlspace tomorrow. What happened is that the initial bid drawings from several months prior did not contain enough insulation info. The subs all bid thinking that insulation would be standard 2" but the builders numbers did not match the later approved construction drawings which showed the 6-8" R30-40 spec. Apparently, the builder missed that little tidbit. He said if we ran over the allowance, he would eat the extra cost since it was his mistake. I agreed with that!

The other misleading piece to this was that since we are building an addition as well as the barn/garage, the living space has different requirements. There is a new crawl space under the addition which will require at least 4" of insulation and a 4" rat slab. The foundation walls will be getting 2" vertical insulation glued on.

Since the garage is not technically living space (though I will probably be 'living' in there when it's ready) the insulation requirements are apparently not as stringent, like some previous posts confirmed.

Regardless, we all agreed that 4" of insulation in the garage would be more than enough, and furthermore, if I only wanted 2", that would be fine, it's my call. They do seem to think the the slab should remain at 6". Not sure why but there is time to decide that since it actually won't be poured for at least a month, maybe more. The builder has that cost figured in already so either way, I'm not getting into change orders there and would save money if we stay with 4".

To that point - the excavator spoke with the manufacturer regarding the compression of the boards and whatever it is EPS or XPS, it's correct for the application. Also, the 2" thick board provide R10 insulation value.

Thanks again for the replies, was very helpful. Gotta keep your eyes open with this stuff and hope your builder does not just follow the drawings blindly. Thankfully he didn't but if the question didn't come up, he may have. Could have been an expensive lesson, these foam boards are like $35-40 a pop!

chaosracing.......EPS can have compressive ratings qual to XPS. Depends on the mfg. process. Did you read your link?

rburke65 said:

chaosracing.......EPS can have compressive ratings qual to XPS. Depends on the mfg. process. Did you read your link?

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Yes I did, but that is a one manufacturer specific product from that link (Carlisle Insulfoam) General everyday EPS that is readily available at big box stores and material suppliers is low density. Till you would pay for the Insulfoam product, it would still be cheaper and easier to get XPS foam. You would also have to go to the Type IX just to match XPS R value and you would have to go to Type XIV to meet minimum XPS compressive strength.

mcbane said:

Also, as was mentioned by machsnell, slab edge is where half or more of slab heat loss happens. Typically you insulate the outside of the footing, for the full height of the footing, and spend less money insulating under the slab.

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+3

rmchrgr said:

Regardless, we all agreed that 4" of insulation in the garage would be more than enough...

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I think this is an excellent place to land. You will not regret it. Energy prices are not going to go down in the future and we continually increase our minimum insulation values. Yes, heat does rise, BUT heat moves to cold, regardless of the direction. It's the laws of thermodynamics. You're just a tad bit ahead of the curve. But then again, this is coming from a guy who just build a house with 10" thick walls @ ~R34.

rburke65 said:

chaosracing.......EPS can have compressive ratings qual to XPS. Depends on the mfg. process. Did you read your link?

Click to expand...

I know a builder nearby who used 12" of EPS under the slab of a home. Geofoam was the brand I think. I felt a sample, I couldn't believe how dense it was. Pretty incredible stuff. Used commercially a lot.

It's a big slab and w 15 foot ceilings I would imagine some big "things" to go in it.

I know you dont want co's but go for at least a 5 inch slab. 4 is pretty thin for what it sounds like might be in there.

I believe the foam also helps the concrete move and resist cracking by reduced friction under slab.

What psi foam are you getting?

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