Under slab insulation

  • Active since 1995, Hearth.com is THE place on the internet for free information and advice about wood stoves, pellet stoves and other energy saving equipment.

    We strive to provide opinions, articles, discussions and history related to Hearth Products and in a more general sense, energy issues.

    We promote the EFFICIENT, RESPONSIBLE, CLEAN and SAFE use of all fuels, whether renewable or fossil.
Status
Not open for further replies.
Buzz Saw

Buzz Saw

Minister of Fire
Jan 18, 2014
523
Attica, Ohio
How deep should I put foam board on the perimeter of my new building? From the top of the concrete to the ground is about 30"ish. Should I put foam board all the way down before I start dumping stone or just the last 5 inches where concrete will be?

Thanks

uploadfromtaptalk1441069425379.jpg uploadfromtaptalk1441069966160.jpg
 
If you are going to use foam, use it. Run it all the way down and bury a foot in the ground. That will help with frost.
 
When we did our house, the foam board was placed on the level gravel, then concrete poured over the foam. It's really no difference for insulation IMHO as all you are doing is isolating the ground temperature from your building temps. As Isaac says, bury the foam at the walls, otherwise you'll get frost creeping in the edges. I assume you are fully insulating the building? If not, no need for any foam.

You might want to consider running a plastic pipe loop through the concrete and have solar collectors on the roof heating the circuit in the floor. I would sure use all that roof space for solar of one kind or another.
 
when I did mine the foam sat on the footing 3 feet deep, not sure on a pole type structure. I would agree with above of at least a foot deep

just reread your post, I would go down the full 30" depth. also nothing under slab unless you are doing radiant floor heat.
 
I have a pole barn like yours and I insulated with foam and installed the radiant heat tubes. No heat in the tubes yet. I also insulated the walls and ceiling.

It drives me crazy to see a building built like yours where they failed to place and compact the fill before building a building. Now you need to go in there and try to do it without damaging the building and you can't possibly compact the edges of the slab. Oh well, let's look ahead.

The folks that know about these things say that most heat is lost from the edge of the slab. The ground under the slab is always 50 degrees after all. They say to run the foam vertical from top of slab to the frost line. Place all your fill, inside and outside of the building, place all your gravel to grade, then 6 mil poly, and then 2" foam either under the whole slab (what I did) or for the first 5 to 10 feet around the perimeter. Then staple the tubes to the foam, and then steel mesh on top of tubes, then pour concrete.

I would insulate the slab like this regardless of whether or not you plan to use in floor heat. The tubes in the slab only add a small cost to the project so put them in to make the slab "radiant ready".

Oh and I'm a big fan of EPS (25psi) for under the slab but for vertical I used XPS since it just holds up better to abrasion during the backfill.

IMAG0234.jpg IMAG0518.jpg IMAG0523.jpg shoptube.jpg truckfoam.jpg
 
Last edited by a moderator:
DougA said:
When we did our house, the foam board was placed on the level gravel, then concrete poured over the foam. It's really no difference for insulation IMHO as all you are doing is isolating the ground temperature from your building temps. As Isaac says, bury the foam at the walls, otherwise you'll get frost creeping in the edges. I assume you are fully insulating the building? If not, no need for any foam.

You might want to consider running a plastic pipe loop through the concrete and have solar collectors on the roof heating the circuit in the floor. I would sure use all that roof space for solar of one kind or another.
Click to expand...
Yes, the whole building will be eventually be insulated.

I have plans for solar after the building is completed. The dream is to have solar heating my thermal storage, ideally a Garn. Lots of work till I can set in a Garn or any boiler for that matter.
 
ironpony said:
when I did mine the foam sat on the footing 3 feet deep, not sure on a pole type structure. I would agree with above of at least a foot deep

just reread your post, I would go down the full 30" depth. also nothing under slab unless you are doing radiant floor heat.
Click to expand...
So 30" to my current ground level and call that good?

After the outside is graged into place only about 4" of the top treated 2x8 will be visible.

If I can get dig some dirt out with my FEL I'll take out as much as I can.
 
Highbeam said:
I have a pole barn like yours and I insulated with foam and installed the radiant heat tubes. No heat in the tubes yet. I also insulated the walls and ceiling.

It drives me crazy to see a building built like yours where they failed to place and compact the fill before building a building. Now you need to go in there and try to do it without damaging the building and you can't possibly compact the edges of the slab. Oh well, let's look ahead.

The folks that know about these things say that most heat is lost from the edge of the slab. The ground under the slab is always 50 degrees after all. They say to run the foam vertical from top of slab to the frost line. Place all your fill, inside and outside of the building, place all your gravel to grade, then 6 mil poly, and then 2" foam either under the whole slab (what I did) or for the first 5 to 10 feet around the perimeter. Then staple the tubes to the foam, and then steel mesh on top of tubes, then pour concrete.

I would insulate the slab like this regardless of whether or not you plan to use in floor heat. The tubes in the slab only add a small cost to the project so put them in to make the slab "radiant ready".

Oh and I'm a big fan of EPS (25psi) for under the slab but for vertical I used XPS since it just holds up better to abrasion during the backfill.

View attachment 161117 View attachment 161118 View attachment 161119 View attachment 161120 View attachment 161121
Click to expand...
Not building the pad up was my call. Reason being I live close to several quarries and I can get 304's trucked in for a comparable price(or cheaper) as dirt. As far as compacting a vibtrating double roller does a pretty good job on all but the outer 12" or so. The last 12" I can water down, hopefully to get enough settling.

Knowing what I know now I would look closer at pouring a foundation and building a pole barn on top of the foundation. Also if redoing a pole barn I would have opened up a trench and out foam into a 4" wide trench right next to the poles... Oh well moving forward.

What are the differences in foam board? I take it not all foam board is created equal?
 
There is EPS and XPS foam board. Both are polystyrene, both are available in 25 psi compressive strength, and both are made for underslab installations. The EPS is expanded (bunch of little balls melted together) and the XPS is extruded (more like a cast liquid material). Cost for XPS is way way higher but the only benefit is better abrasion resistance during construction and I find the XPS easier to shape with a saw as it "chunks" less. Both are R-5 per inch. You don't need 25 psi, 15 psi is usually stronger than the ground under it, but 25 psi is better.

It is not too late to install vertical foam from top of slab to finished grade frost line. Do it. Your biggest challenge with using stone as backfill is making sure that surface water doesn't pool under your building. Grade the dirt to drain then fill with stone. Hopefully this 304 stone you speak of (stone names are a regional thing) is crushed rock with fines so it will lock in and compact.

Oh and you can still pour your slab with a thickened edge to act as a foundation for the interior walls. I framed out the pole barn bays and insulated. Shot nails through a treated sill plate into the top of slab.
 
Highbeam said:
There is EPS and XPS foam board. Both are polystyrene, both are available in 25 psi compressive strength, and both are made for underslab installations. The EPS is expanded (bunch of little balls melted together) and the XPS is extruded (more like a cast liquid material). Cost for XPS is way way higher but the only benefit is better abrasion resistance during construction and I find the XPS easier to shape with a saw as it "chunks" less. Both are R-5 per inch. You don't need 25 psi, 15 psi is usually stronger than the ground under it, but 25 psi is better.

It is not too late to install vertical foam from top of slab to finished grade frost line. Do it. Your biggest challenge with using stone as backfill is making sure that surface water doesn't pool under your building. Grade the dirt to drain then fill with stone. Hopefully this 304 stone you speak of (stone names are a regional thing) is crushed rock with fines so it will lock in and compact.

Oh and you can still pour your slab with a thickened edge to act as a foundation for the interior walls. I framed out the pole barn bays and insulated. Shot nails through a treated sill plate into the top of slab.
Click to expand...

So ESP is the type of foam used for packing material? XPS is the typical foam board I see in construction, typically blue or pink.

I'm not to concerned with water pool under the building because there is a 4" perforated tile that runs under the building. In theory that drain like a footer tile.

304 stone is crushed limestone with powder. The stone is about the size of a flattened golfball, and that is on the big end for size. This stuff locks in tight after being compacted.

After I get the mess cleaned up left by the builders (are all builders slobs or just Mennonite's?) I plan on digging out some more dirt. I'll get out as much as I can with my little tractor and I'll have to call it good enough unless I can borrow a mini excavator to really get some work done.

When you shot nails into the concrete weren't you concerned about hitting your pex lines?
 
Buzz Saw said:
When you shot nails into the concrete weren't you concerned about hitting your pex lines?
Click to expand...
The lines are under the slab, which should be 4"+. I hit one when I drilled to put in a lag bolt once. What are the odds?
 
How should I insulated were my garage doors will be? If I have concrete that comes out a foot in front of the door do I insulate on the edge of were the concrete will be or not insulate at all and just let the heat loss be what it is in that area?
 
DougA said:
The lines are under the slab, which should be 4"+. I hit one when I drilled to put in a lag bolt once. What are the odds?
Click to expand...
I thought the lines were supposed to be closer to the surface. Will it heat differently at different depths?

Was the PEX the repairable? If so how can you fix it? Fill the hole with hydraulic concrete?
 
Buzz Saw said:
If so how can you fix it?
Click to expand...
It was pvc not pex that I used 3 decades ago. I put loops of pipe in the concrete in our south facing window wall with the intention that someday I might hook it up to solar collectors and pre-heat hot water. Someday has not arrived, been too busy. :rolleyes::rolleyes:
Soooo, hole was never fixed. The hook-ups got tiled over a few years ago so no one will ever know. The only reason I know I hit the pipe is that the drill bit came back up with bits of black plastic on it. Sh!t happens.
 
Buzz Saw said:
So ESP is the type of foam used for packing material? XPS is the typical foam board I see in construction, typically blue or pink.

I'm not to concerned with water pool under the building because there is a 4" perforated tile that runs under the building. In theory that drain like a footer tile.

304 stone is crushed limestone with powder. The stone is about the size of a flattened golfball, and that is on the big end for size. This stuff locks in tight after being compacted.

After I get the mess cleaned up left by the builders (are all builders slobs or just Mennonite's?) I plan on digging out some more dirt. I'll get out as much as I can with my little tractor and I'll have to call it good enough unless I can borrow a mini excavator to really get some work done.

When you shot nails into the concrete weren't you concerned about hitting your pex lines?
Click to expand...

The pex is laid out with high precision. First loop and was 6" inside of the poles and the nails shot into boards between the poles. Also, as mentioned, the pex is on the bottom of 5" of concrete.
 
Buzz Saw said:
How should I insulated were my garage doors will be? If I have concrete that comes out a foot in front of the door do I insulate on the edge of were the concrete will be or not insulate at all and just let the heat loss be what it is in that area?
Click to expand...

Good question. There will be heat loss where the heated slab runs under the door. This will prevent the door seal from freezing shut. Also, the cat loves setting there.
 
Highbeam said:
Good question. There will be heat loss where the heated slab runs under the door. This will prevent the door seal from freezing shut. Also, the cat loves setting there.
Click to expand...


I would cut a slot with a concrete saw and create a thermal break. 1/2 inch wide and foam fill it. or if you still havent poured it, form it to the inside of door, then foam or expansion board which lines up under door and pour apron up to that. even a piece of PT would create a thermal break.
 
ironpony said:
I would cut a slot with a concrete saw and create a thermal break. 1/2 inch wide and foam fill it. or if you still havent poured it, form it to the inside of door, then foam or expansion board which lines up under door and pour apron up to that. even a piece of PT would create a thermal break.
Click to expand...

Just remember how structurally important this spot is. Right in the wheelpath and there is usually some sort of ramp into the slab. People thicken this area for a reason.
 
Put in a big wood stove and a little cold seeping in won't matter.
 
Status
Not open for further replies.

Similar threads

I
Foam board insulation
Replies
15
Views
864
DIY and General non-hearth advice
EbS-P
EbS-P
EatenByLimestone
Laying tile
Replies
12
Views
423
DIY and General non-hearth advice
EbS-P
EbS-P
K
Hearthpad
Replies
1
Views
510
The Hearth Room - Wood Stoves and Fireplaces
begreen
begreen
I
foam board insulation
Replies
6
Views
1K
DIY and General non-hearth advice
bogieb
bogieb
francesstoves
Underlayment for tile hearth beneath Woodstock Cottage Franklin soapstone gas stove
Replies
0
Views
457
It's a Gas!
francesstoves
francesstoves
Top Bottom
Back