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Perimeter Slab/Footing Insulation Question

Raywatt Member Posts: 6
I have a 26'x16' slab inside 60'x26' pole building that I instaled 1/2" Wirsbo PEX tubing installed at time of 4" slab pour. The outside edges of tubing sit at aprox 25'x 15'. There is 2" foam insulation with vapor barrier under slab and around perimeter of slab. I want to build a 10 ton recording studio (double wall/4 layers 5/8" sheetrock) on slab and have been advised to create monolithic footings with rebar at least 10" deep and 12" wide under slab to support this amount of weight. My plan is to dig back under from the outside on three outside 3 walls. The one inside wall I will cut trench with concrete saw and dig back under slab. I will core drill 6" holes every 4-5 feet around perimeter of room between outside of slab and the inset PEX tubing (inset about 10-12" from edge of slab) that will allow cement to be line pumped through 6" holes into dug footings under the slab. I will build perimeter forms.

My question. Can I leave existing vapor barrier and foam insulation and pour footings directly beneath foam insulation. Or do I need to remove the existing foam under slab to the backside of footing trench and insulate footing trenches with 2" foam and new perimeter foam. Either way the weight is sitting on the foam. But to remove existing foam and piece in footing foam and perimeter foam would make this unpleasent job even more strenuous.

I've read a lot on net about need to insulate slabs for radiant heat especially perimeter. In first scenario above would much radiant heat be lost by moving from slab downwards through the 6" holes through 10-12" of cement in footings and dissipate into the bare ground of footing trenchs?


  • Gordy
    Gordy Member Posts: 9,537

    What you are trying to do is not "monolithic". All though your footing pour in itself will be monolithic. What should have been done is the footings poured with the slab which would truly be a "monolithic footed slab". Monolithic is one continuous pour with no cold joints.

     The re-bar for the footing should be poured into the slab also. What you will have if doing what you posted is a reinforced footing sitting under a non reinforced slab with no re-bar tieing the two together. In other words the slab is free floating from the footing. Even if the slab is reinforced in itself the two components are separate.

    Pumping concrete into holes core drilled 4-5 feet apart will not give the best results at making sure the concrete is getting up to the bottom of the slab 100% around its perimeter. What usually happens is it gets over vibrated to try to move the concrete, and bring it up to the slab bottom. This causes the aggregate to sink leaving cream in contact with the bottom of the slab.

     I also have to ask what kind of subgrade is 10" down? Digging into undisturbed native soil to pour a footing is one thing. But if fill was brought on site to level up the subgrade for the pole barn, and was not properly compacted this could lead to failure after all your efforts.

     Basically your walls will weigh 238 pounds per linear foot if that is your 10 ton calculations. I would much rather see a detail as drawn. Drilling L bars into the perimeter extending into the footing with 2 or 3 horizontals . Pouring the concrete would be easier, and a much better sucess at full contact reinforced slab to footing detail.
  • Raywatt
    Raywatt Member Posts: 6
    More Questions

    Thanks Gordy for your response

    Here is a quick drawing of materials.

    I included crude drawing of a suggested method to tie into slab with rebar, but suggestion was without foam and person recommended underpinning slab 8”. I would think heat from slab would transfer down 6” holes to footing then out to heat sucking cold earth if no insulation under footing. If there is insulation around footing it seems that would isolate radiant heated cement from cold earth. The 2" foam is obviously bearing weight of the slab, but could foam bear weight of structure and footings? Are there weight bearing calculations for 2" foam?

    Is your 238 lbs per lineal foot figured with 6" width and lineal length (weight on 2x6 sill plate)? There will be two walls each with 2 layers of 5/8 sheetrock. One 2x6 wall and one 2x4 wall with 1" space between for a 11 1/4" double wall width with sheet rock.

    I re-measured and L Bars could be inserted 6-7” into side of slab without hitting PEX tubing. How far spaced, what size rebar/holes and how deep would they have to be? I would have to cut (where the cut needs to be is a bottom drip sill on metal building) and remove over half of width of 2x6, the perimeter foam and under slab foam back 18". Could I drill through perimeter 2x6 and perimeter foam leaving them in place? Also leave under slab foam in place? My concern if removal of foam is heat loss. I could line footing trench with foam, but with your suggested method of drilling in L bars the foam perimeter insulation would be aprox 6-10” outside the building to the outside of footing. Add 18" dug out under slab to get footing wide enough to support walls and I'm looking at 24-28" wide footing. Wouldn’t the footing outside of building be a heat loss potential? Also would I need to tie a vapor barrier under footing to existing vapor barrier under slab? From the one picture I could find on web, the insulation (although not foam) was under the footing with what appears to be a foam perimeter piece (turquoise piece) not labeled. Here it is: 


    The earth is fill that is a combo of gravel and dirt and was compacted by driving machinery and a heavy truck over fill combo. I know that is not a correct method of compaction, but from my test digging it is very compacted. To dig out under slab a large bar is needed to loosen soil/gravel mix and digging is very difficult to due to compaction. For the first method above the concrete would be a pump mix with pea gravel. I guess that can be just as over vibrated as regular cement.

    I know I’m trying to go about this bassackwards due to not knowing about the need for footings in this heavy structure when slab was poured. So what I’d like to know is, can footings be made under existing slab and possibly foam someway to work to hold weight and retain radiant heat?

    When a normal monolithic slab/footing is poured for radiant heat, what is the right way to insulate where slab meets footing and under the footing with 2" foam insulation?

    I would be grateful if you could be as specific as possible.

    I appreciate your help and thank you for your patience.
  • Gordy
    Gordy Member Posts: 9,537
    edited May 2011


     I assumed that the 10 tons was a aggregate wieght of the materials going into the walls. I also assumed that the walls would be the entire perimeter of the slab including the exterior walls. So 20,000 lbs/ Perimeter is the 238 lbs per linear foot.

    Your walls are essentially 1' thick so if your weight is right its 238 lbs a squre foot. If you used 250 psi foam you should be good. If your 10 ton weight is a guess then my calcs go out the window. I can figure it if you give me some time here.

     What centers will your studs be?  How tall will the walls be?  And what type of roof will it be?

     5/8" Drywall is 2.6 pounds a square foot. so if your wall is 8' tall that is 83.2 pounds a squre foot in drywall alone. A 2x6 is 2 pounds per foot so thats another 37 pounds for a running total of 120.2 pounds if studs are 16" oc. need details on ceiling or roof.
  • Raywatt
    Raywatt Member Posts: 6
    Room Dimensions and Weights


    First off this is going to be a recording studio built with sound reduction construction to the best of my ability and affordability.These are free standing rooms, one inside of the other. each independent of one an other and not attached in any way to pole building to reduce flanking, The only part of room touching anything is 2x6 treated sill plate sitting on slab for outside room 8'11"'Hx14'10"Wx26'6"L  and 2x4 treated sill plate sitting on slab for inside room. Inside main mixing/multi purpose room is 8'5 1/4"H x 13'10W x 16'L and in back third of inside room will be Drum room,Vocal booth and Bath in area 8'5 1/4H x 14'4"W x 10'L . All dimensions are inside dimensions of rooms.The inside main mixing room will be built with dimensions close to mathematical ratios figured out by sound professionals to eliminate standing waves and reflections as to have a acoustically tuned room for mixing. The best way to achieve sound reduction is mass and isolation, equating 2 layers of 5/8" sheetrock on the outside of outside room with R-19 insulation and nothing on inside of same wall but bare exposed insulation. Then 1"(or more) dead air space. Then inside 2x4 room will have R-11 exposed insulation facing 2x6 wall exposed R-19 insulation with 2 layers of sheetrock on inside walls and ceilings.There are two 7"x4 1/2"x16' beams per room that hold the ceiling joists at 16" centers on posts made with six 2x6's or 2x4's(inside room) bolted together. This creates loading on slab at bottoms of posts from ceiling and beam weight. The weight of four beams I've figured at 150lbs each. Beams are located at aprox 1/3 and 2/3 of room's length. Outside room ceiling will be two layers of sheetrock with layer of 5/8 plywood on top screwed to top of room.Inside room ceiling is two layers sheetrock on inside. I added all the wood and sheetrock and came up with 19,355lbs no screws,caulk,doors or windows.

    I figured weight out by using 73 lbs per 4x8 5/8 sheetrock (140 sheets)

    2x6's 2lbs per foot (16" centers).

    2x4's 1.28lbs per foot  (16"centers)

    Beams aprox 150 lbs each.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Recording studios.

    Years ago a friend of mine built a sound recording studio inside an existing building (The old Pierce-Arrow factory in Buffalo where the cars were made and sold).


    This building was all reinforced concrete with steam heat. He set up interior walls made of cinder block, and filled the hollow space in the cinder blocks with dry sand. He did nothing to the concrete floors. The space between the inner and outer walls was just air, if I remember correctly, but it might have been fiberglass insulation. I do not recall what he did with the ceilings. There was no problem with sound leaking into the rooms where the performers were. And enough heat leaked into the studios from the outer space so no heat was specifically needed for the studios themselves. This was on the second floor of the administration building. Apparently their showroom was on the second floor. I thought that was funny.

    But I cannot imagine that any two studios would be the same as far as heatning and cooling needs were concerned.
  • Gordy
    Gordy Member Posts: 9,537
    Ceiling Loads.

     Ray sounds from your discription of your beams, and post layouts that your ceiling loads will be well distributed on the walls, and slab. That is why I asked. I was wondering if the ceiling joists were only going to bear on the 26' walls.

     Another question is the rest of the pole barn going to be conditioned space? Or is this a pole barn with a corner studio that is conditioned?  Thinking frost heaving slab.

    Was the slab reinforced with heavy gauge wire mesh? What bag mix was used?

     Personally I would rather see the foam go from bottom of slab down inside face of footer wall, and the outside face of footer wall up to slab perimeter. You could use the foam as your forms. 

    Sorry for all the questions, but I hate to see you go through more than needed. Or maybe not enough in areas.
  • Gordy
    Gordy Member Posts: 9,537
    My thoughts

     If it were me Ray I think I would do this.

    Since you are already going to core drill 6" holes in the slab around perimeter. I would go every 4'. That would hit every 4 th stud on the perimeter.  Auger 4" hole 42" deep in each core hole. Bell the top of the hole large enough so new concrete bears under slab were hole was cored. Fill the hole with concrete, and float 3 #4 bars 40' long in the hole. Done. If you can layout your support posts at your 1/3 points for ceiling beams to miss tubing do the same there.

    Seems that would be easier than trying to trench under the slab the whole perimeter. for a footer wall. If 4' seems to far go every other stud 32" oc with holes.
  • Raywatt
    Raywatt Member Posts: 6
    edited May 2011
    Confession and More Questions


    Due to the embarrassment of already starting this project with pre conceived ideas that were bassackward, I now will meekly admit I've already built the outside room and am trying to salvage this project without tearing down room.You can see what I've done at:


    I have since learned from my mistakes by reading the book Home Recording Studio Build It Like The Pros and some good help from the author Ron Gervais and terse answers from the forum recording.org.

    I overlooked answering your previous questions about ceiling loads,conditioned space and slab. The ceilings joists 16" oc are supported on the 16' walls and the 16' beams with beam posts in the 26' walls.With the 16' x 14 1/2" x 4 1/2" beams I'm planing on cutting a 14' 6" x 7"x 4 1/2" chunk out of beams for the beams for inner room.The studio area 26' x 16' is the only conditioned space(if by conditioned you mean heating and cooling)in the 26' x 60' pole building. The 4" slab is a 6 sack mix with fiber mesh and heavier type flat(not rolled) mesh sitting on 2" dobies(chairs) with PEX tubing attached to mesh.

    I like your suggestion to do belled pillars 42" under slab. But wouldn't these pillars be a direct conduit for heat loss directly to the ground? Core drilling would have to be through 2 x 6 sill plates which would most likely defeat the whole purpose of this idea as 2 x 6 sill plates would be hacked through where cement pillars are. Possibly could pour cement to top of 2 x 6 sill plate then bolt down a 14 1/2" 2 x 6 over top of cement in cored holes.

    I've read a lot of horror stories on the web of unheatable or massive heat loss in uninsulated, improperly insulated, improperly installed or no perimeter insulation on radiant heat installations in slabs.

    Your previous suggestion of drilling and re-baring was good also. That is why I previously asked if a properly installed monolithic slab with footing has 2" foam under footing. From your suggestion to have "the foam go from bottom of slab down inside face of footer wall, and the outside face of footer wall up to slab perimeter. You could use the foam as your forms" Would this include foam along the bottom of footer walls or just the sides? 

    Seems any uninsulated cement in footer wall could cause a significant heat loss to ground and air. In your first plan the top of footing (6-8") outside building would sit bare at earth grade level creating a source of heat loss to air.. To put 2" foam on top of footer wall and cover with dirt 3-4" would cover the metal siding with 5-6" of dirt which I don't think would not be good for metal. I could put custom cut foam barrier on corrugated metal and remove drip sill to do back fill, but what a pain.

    If a properly installed footing does have 2" foam under entire footing supporting weight above, it would seem I could pour footings directly under my existing 2" foam under slab and support room weight fine, but it would not be tied in as you mentioned in your first reply "In other words the slab is free floating from the footing. Even if the slab is reinforced in itself the two components are separate" What makes this a bad thing? 

    From what I'm gathering, it sounds as if cement to cement contact with re bar is necessary to gain proper reinforcement and strength for support. With a ton of work I could achieve an completely insulated footing. Although belled pillar idea is less work, I'm not seeing a way to insulate pillars from earth (if that is critical). Thanks for coming up with two good ideas, but please elaborate on insulation of radiant heat slabs and (especially) footings.

  • Gordy
    Gordy Member Posts: 9,537
    edited May 2011
    Insulated footings

     Here's the thing. Is the pole barn in which the studio is located conditioned space?

    If it is not, or if it is but maybe not always your slab is susceptible to movement if footings are not below the frost line. Regardless your 2 exterior perimeters are going to be to the weather.

    This means that the slab will lift if ground freezes, and settle when it thaws. Unless there is a footing that is below the frost-line, and the slab is anchored monolithic, or through reinforcement. Simply pouring a slab on a footing with no connection will only prevent settling not heaving.

     I do not know your climate so I don't know the winters in your location. Maybe you live in the south?

    I will get to the insulation.

  • Raywatt
    Raywatt Member Posts: 6
    Not Conditioned Space North of Seattle

    Not conditioned space. No heat in building. 16' x 26' studio will hopefully have radiant heat when completed. Studio is only area in need of footing and heat.

    Thanks for describing the principles behind floating/detached slabs/footings and attached monolithic. Heaving does not sound good, though I'm in the Pacific NW a hour north of Seattle and it's fairly mild year round we do get some extensive sub freezing cold spells from time to time. I found this on net: The UFC 3-310-04 available on line at the Whole Building Design Guide lists Seattle at 9" for frost penetration. 

    Still waiting patiently on insulation under footing question.
  • Gordy
    Gordy Member Posts: 9,537
    I should have

    Asked about your location in the beginning.  42" is extreme then if the frost is 9".

    One thing I see though if you undercut the slab to do footers you have weight on it all ready. I think if your slab has wire mesh, and fiber mesh I would skip the footings with the climate you are in. I see a lot of garage floors poured 4". Think of the weight of a vehicle on 4 tires verses an evenly distributed wall on the perimeter.
  • Raywatt
    Raywatt Member Posts: 6
    edited May 2011
    Thanks for recommendation

    of no footings. Originally I was not going to do footings as I went through same thought process as I had a 8000lb truck with camper parked on slab with no problems. Others like the pole building builder, a home inspector friend and the cement company that provided cement have said it will most likely be just fine to load slab with 10 tons.

    The advise I got from pro studio builder put the fear in me that my concrete slab at 200lb per sq in would be compressing down on the marshmallow like soil with 200lb per sq ft bearing capacity and soil could push outwards like soft earth does when you drive a car over it. Also the fact I'm building on perimeter of the slab. But I'm really starting to lean the way of no footings. Not because I don't want to do it right or avoid the work, but I feel I've exhausted all "doable" methods while still keeping my radiant heat intact and having complete uncompromised insulation for the slab.

    With the recommended soil bearing capacity of 2000lb per sq ft I figured I have 13,229lb bearing capacity on 82 lineal feet of 2x6 and 98 lineal  feet of 2x4 sill plate. Technically not enough for the distribution of the 20,000lb load evenly. Any thoughts on this?

    What do you mean 42" is extreme if the frost is 9"?

    Would still like to know how to insulate under and around a monolithic slab with radiant heat tubing especially under the footings.
  • Gordy
    Gordy Member Posts: 9,537
    edited May 2011
    Studio designer

     Is all wet with the analogy of driving a car over soft earth verses driving same car over reinforced concrete. The concrete will bridge.  I was thinking more on the lines of ground freezing being the issue.

     Concrete will crack. If you don't want it to leave it in the truck. Its controlling those cracks through properly placed joint tooling/ saw cutting, and reinforcing to keep the cracks where you want them, and hold it all together. If you used wire mesh chaired up, and fiber mesh. I certainly would lose no sleep over it.

     If you are on jello type soil, and deep bass for extended periods will chime out of the studio through high wattage equipment.......Liquefaction could be a concern.

    About the insulation UNDER the footing. Let it go. I never have seen a detail for insulation under a footing for a radiant heated slab. You insulated the slab so you are in good shape. The penalty for heat loss conducted through a footer is far less than the penalty for pouring a footing on foam verses undisturbed earth.
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