eps or epx foam under slab

[curtis]

As i continue to research and plan for my garage i have come to the point where i am trying to figure out how to insulate the slab. I have read that the epx foam board at 27 bucks a sheet for 2 inch is the way to go. But have also read that the 12 dollar sheets of eps foam will work just as good. The epx is a r-10 and the eps is a r-7.5 in 2 inch think sheets. Has anyone done this before and can tell me witch way to go? Im thinking i will need around 48 sheets so the $15 price difference will add up fast.

 

[heaterman]

Curtis we did a 9,000 sq ft church this past fall/winter. The only place we used 2" foam was around the edge of the slab down vertically about 20". The entire floor was done with 1". It heats like a dream.
By far the vast majority of heat loss from a slab is on the edge rather than the interior of the slab itself. Just think about this for a minute... The edge is going to see ground temperatures next to it that may be well below freezing while the ground underneath the interior of the slab only drops to maybe 50-60*. When you have temperature differentials that low in the slab to ground interface you don't need all that much insulation to stop the heat flow.

A local business asked me to give them a price on doing a 10,000 sq ft equipment repair garage for their logging operation. I priced it out like above and lost the job because someone came in and told them all they needed was bubble foil under the slab and nothing on the edge because there was so little area exposed. Long story short, after burning up a couple of the huge Centrals inside of 6 years they are now building their own boiler that will take logs in 8' lengths and there is a 4 foot wide strip of green grass all the way around the building even in the dead of winter.

Take care in insulating the edge and use some 2" there. The interior of the slab is not as critical.

 

[curtis]

Are you using the eps foam or the epx?

 

[BoilerMan]

XPS, expanded polystyrene, or blue board as most of us in the trade call it. The white "beadboard" does not have the same compression strength and water will penetrate it over time. Use the blueboard or whatever color it is in your area. As Heaterman pointed out once it's done it's done all you can change it the heat source, unless you like big jackhammers.

In my northern climate it's 2" everywhere in any residential slabs, which are relitivly small square footage, therefore not wide enough to justify the transition to thinner foam in the middle. Large slabs (over 40' wide) I'd consider thinner once your 8 or 10 feet in. And leave NO exposed concrete to the elements, the heatloss is HUGE! I use double 2" on the vertical perimiter of monolithic slab pours (my house included).

TS

 

[Fred61]

I personally always use a vapor barrier beneath the insulation.

 

[BoilerMan]

I personally always use a vapor barrier beneath the insulation.

Same here at least 8mil poly or better.

TS

 

[curtis]

Taylor, how do you insulate a monolithic slab? I thought of doing that type of slab instead of a footing with block but wasn't sure how to go about insulating it.

 

[Fred61]

My best friend is having a large house built on a lake in New Hampshire and after some words back and forth with his contractor he had to raise his voice in order to get him to lay vapor barrier. Contractor kept telling him it wasn't needed and they never install it.

 

[BoilerMan]

I insulate the whole thing, I drew a crude picture of how I've done it and not had any issues doing it this way. The thickened edge is 12-16" thick and goes into the slab 12" plenty of steel in all of this with the rebar bent into hooks tied to the thick edge lateral rebar as well. And as Fred said, I use poly under the whole thing, I like to tape the poly and overlap it at least a foot, does it do anything? Don't know but it's cheap insurance and a damp slab is no fun!

TS

 

[BoilerMan]

Oh and for what it's worth, the slab the chair is on which I'm trying from is done this way and has been here for 4 winters and I havn't had any sheetrock crack yet! The sub-grade is the critical factor as well as quality concrete. Dig out any organic matter and fill with coarse gravel. I use regular 4,000 pound concrete with fiber in it as well as 1/2" rebar no wider that 24" O.C. Over-kill in this stage is money well spent in my opinion, a building is only as good as the "rock which it is built on".

TS

 

[curtis]

Is there a reason for that one horizontal piece to extend out past the wall that far?

 

[ewdudley]

Is there a reason for that one horizontal piece to extend out past the wall that far?

In order for frost to get under the slab it has to travel down to the level of the horizontal piece, then the frost has to travel horizontally the length of the horizontal piece.

When I proposed this type of slab to our building inspector in NY he dug into the code and found specs. In NY it's down 1' and out 2' for a heated space, and out further for an unheated space. In NY I had to leave out a foot of insulation under the perimeter beam to allow some heat flow out to meet the frost headed in.

And FWIW, in my experience for high and dry Corning pink is OK, but for beneath grade it's gotta be Dow blue.

 

[jebatty]

I think it is a big mistake not to put 2" under the entire slab, especially in a cold climate like N.MI. Assuming ground temp is around 45-50F before being cooled further by the cold of winter, would you not insulate your walls and ceiling in a climate with outside air at this same temp? The floor has the same square footage as the ceiling, same area of endless heat loss to 45-50 "climate." You likely will insulate your ceiling much more than the R factor of 2" foam. So leave the floor uninsulated? Makes no sense.

With my 32 x 48 pole barn shop, I used 2" foam vertical down 4' around the outside perimeter, 2" foam between the concrete and the inside perimeter of the shop, and 2" foam under the entire slab. I set the floor temp to constant 61F as read by a sensor in the middle of the concrete about 3' into the slab from the perimeter; with a mixing valve supply the floor with 100F water; and the shop keeps an air temp between about 53F (when it's well below 0F outside), and about 60F (outside temp +20F and higher).

Do it the first time. You never will have a 2nd chance to add insulation under concrete.

 

[maple1]

On the vapour barrier - are you saying put it on the ground then the insulation on top of it? That's what I was reading - but doesn't vapour barrier always go on the warm side of insulation? Which would mean insulation down then vapour barrier.

 

[jebatty]

Vapour barrier, then foam, then concrete. The vapour barrier keeps water from penetrating the foam and therefore reduces heat transfer to the ground via water. It also would be good to tape your foam seams.

 

[Fred61]

Don't forget that the soil prep comes first. Vapor barrier and insulation base must be high and dry which means good drainage and very well draining material under the slab and around the perimeter. That's another place that you can't re-work later. Doing this incorrectly could rob you for a lifetime and you may not even be aware of it.

I think I have kept the local crushed stone supplier in business over the years. It's worth the extra effort and the extra bucks. Crushed stone is about 90% compacted as dumped so compacting is a small job.

I once had this wild idea of using epdm pond liner instead of polyethelene because it's less porous but for some reason during that phase of construction it's rush--rush--rush and there's no time to screw around.

 

[peakbagger]

Dick Hill and Tom in Maine (a hearth member) did a TV show several years back on frost heaves and slab insulation. I dont think the video is available anymore but Dicks paper is

(broken link removed to http://www.hotandcold.tv/professor%20hill.html)

Tom stocks and sells lots of surplus foam to contractors in Maine for this type of insulation. It may not look pretty but the price is right (unfortunately I dont think it would be worth shipping to the OPs location.

 

[Bret Chase]

I insulate the whole thing, I drew a crude picture of how I've done it and not had any issues doing it this way. The thickened edge is 12-16" thick and goes into the slab 12" plenty of steel in all of this with the rebar bent into hooks tied to the thick edge lateral rebar as well. And as Fred said, I use poly under the whole thing, I like to tape the poly and overlap it at least a foot, does it do anything? Don't know but it's cheap insurance and a damp slab is no fun!

TS

View attachment 98606

how did you get that past codes?

 

[BoilerMan]

What do you mean? That was code then. Actually above code standards, I'm not one to just follow rules, I like to make up better ones LOL

Now that we have the MUBC well I guess we'd have to have a 6' frost wall. I can't stand wasting time and material on things that have NO bearing on anything.

TS

 

[Bret Chase]

I don't know what code your municpality was using, but a 4' frost wall has been code at this end of the state for decades.

 

[curtis]

Im not sure on houses but I know for a garage you can pour a monolithic slab here if it isnt attatched to a building but if it is you have to go down 42'' with a footing.

 

[heaterman]

Using the church I spoke of as an example..........when you do the math on it you'll find there is less than 5000 btu difference in heat transfer between 1" and 2" foam used under the slab. That's on a 9,000 sq ft building.
R-value of the product is of significant importance but realize that when talking about underslab insulation you are dealing with a strictly conductive heat loss and R-value as a measurement does not express that factor very well.

 

[711mhw]

Im thinking i will need around 48 sheets so the $15 price difference will add up fast.

Bite the bullit!
I've used these folks several times. http://www.insulationdepot.com/ They may have a warehouse near you. They will negotioate, but like to sell trailer loads. I have allways picked up at their warehouse with a large trailer but not a full 48' van. I did a turned down slab like Taylors (excelent) drawing and used a product that they (insul. depot) had that was I think removed from a commercial flat, rubber roof, & that it is used for it's insulation and instead of the old stone "ballast" that they used to use. It was 2'x4' t&g panels that had 2" of Dow "blue" with a little 3/8 to 1/2 in concrete surface on it that look's like a fine exposed aggregate finish. I used it under the slab, and vertically along the perimeter. It really worked well, it did 2 things for me. 1. it is ridgid enough to be the formwork. 2. The concrete finish looks nice above grade and protects the foam.

 

[Bret Chase]

Im not sure on houses but I know for a garage you can pour a monolithic slab here if it isnt attatched to a building but if it is you have to go down 42'' with a footing.

In Maine, or at least southern Maine.... municipalities using a variety of IBC and IRC codes since I entered the industry in '89 requires a 3' 10" frost wall with a footing for any building that has human occupation as a use. The only monolithic slabs I have poured have been for external slabs... or a fuel containment structure...

 

[easternbob]

I used EPS under my slabs and it is fine. You have to make sure to get the higher density EPS for under the slab though. Under the slab is 1.5 lb foam, the foam I used on the inside of the basement walls is 1.0 lb.
This is the company I got my foam from. (broken link removed to http://www.thermalfoams.com/default.asp) They were very helpful.