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Monolithic slab

JLazaro317 | Posted in Construction Techniques on March 1, 2008 08:27am

I’ve got a 16’x18′ sunroom coming up. I’ve never done a monolithic pour before and was wondering some opinions. I will not be doing this myself but my concrete guy will.  He said no problem. I bid the job figuring footers, block walls, and slab pour. Frost depth is 30″.  Neither pour would met concrete minimums and going mono would avoid bringing in block sub, block, mortar, sand, etc. My biggest concern is insulating so there is not a cold slab. Doing it my way, I would have insulted on the inside of the block with foam for a thermal break. With a mono pour, it will use a little more concrete but job will be done in one whack with 1 pour. Seems more efficient to me. We usually don’t build on slabs and only do slabs on basement floors, garage floors, or commercial jobs. I have no problem with doing it the way I originally bid, but is mono better?

Any opinions or recommendations?

John

J.R. Lazaro Builders, Inc.

Indianapolis, In.

 

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  1. davidmeiland | Mar 02, 2008 02:06am | #1

    I'm not sure how you can insulate the perimeter of a mono slab, aside from installing foam board against the outside of it. What about the 6" or more that's above grade, does that get insulated or not? I do footings/walls first and then pour a slab within, with a thermal break. 

    1. JLazaro317 | Mar 02, 2008 05:14am | #4

      That's how I do large slabs. This one is so small it almost seems like a waste. My concern is that without the thermal break, I won't like the performance.John

      J.R. Lazaro Builders, Inc.

      Indianapolis, In.

       

  2. Marson | Mar 02, 2008 02:40am | #2

    We pour frost protected slabs with 18" footings (thickened edges, really) in one pour. We line everything including the bottom of the footings with 2" of 25 psi foam, though I don't know if I would be comfortable with the foam under the footing if it is an addition to an existing house on a conventional foundation. The foam comes up to the base of the wall plate and gets covered with a trowel on coating like "tuff II" or covered with coil stock. Wall plates are hung 2" so that the siding can extend a bit over the foam.

  3. Riversong | Mar 02, 2008 03:33am | #3

    If the sunroom will be heated space, all you need for a shallow frost-protected foundation in Indianpolis is a perimeter 12" below grade with R-5 vertical insulation.

    http://www.toolbase.org/Design-Construction-Guides/Foundations/Design-Guide-Frost-Protected-Shallow-Foundation

    If you want to go beyond the minimum for frost-protection, I would use R-5 under the slab and R-10 at the edges.  I use surface-bonding cement to stucco the exterior foundation insulation on a base of 1/2" hardware cloth stapled to the overhanging sills.

     
    Riversong HouseWright
    Design *  * Build *  * Renovate *  * Consult
    Solar & Super-Insulated Healthy Homes
    1. JLazaro317 | Mar 02, 2008 05:19am | #5

      I thought about a FPSF but I'm kinda old fashioned. And I didn't want the perimeter foam exposed. Your idea of coating the foam with stucco would work, but I may have to go the old way to put my mind at ease. This sunroom will finish with cement siding and I don't want the base insulation to stick past the siding. I could hide that with a water table, but the rest of the house has neither. I may just have to match the old because it'll have at least 6" exposed.John

      J.R. Lazaro Builders, Inc.

      Indianapolis, In.

       

    2. User avater
      Matt | Mar 02, 2008 04:10pm | #6

      That's a very interesting document...  We don't see FPSF here because our frost line is negligible so we are only going 12" deep with our footings.  Mono-slabs are very popular though...

      RE:

      >> If you want to go beyond the minimum for frost-protection, I would use R-5 under the slab and R-10 at the edges.  I use surface-bonding cement to stucco the exterior foundation insulation on a base of 1/2" hardware cloth stapled to the overhanging sills. <<

      I have some Qs... 

      1) Fastening of vertical insulation: If you install the vertical perimeter insulation inside the form boards prior to concrete being placed is there sufficient adhesion between the concrete and the foam to hold the foam in place?  When you pull the form boards does the insulation get somewhat torn up?  I assume if the insulation is nailed to the form boards this can cause a problem...?  Or maybe you install the insulation after forms are stripped?

      2) Why use hardware cloth rather than say metal lathe?  Or maybe either one is fine?  Maybe lathe is not as readily available where you live?

      3) Would you use 2x4 or 2x6 walls with this method you describe?

       

      1. Riversong | Mar 03, 2008 02:43am | #8

        I have some Qs... 

        1) Fastening of vertical insulation: If you install the vertical perimeter insulation inside the form boards prior to concrete being placed is there sufficient adhesion between the concrete and the foam to hold the foam in place?  When you pull the form boards does the insulation get somewhat torn up? 

        I nail the foam board to the outer form with small finish nails that pull through when the foorms are stripped.  And I sink galvanized drywall screws into the inside of the foam, protruding out an inch to bond it to the concrete.

        2) Why use hardware cloth rather than say metal lathe?  Or maybe either one is fine?  Maybe lathe is not as readily available where you live?

        Metal lathe would work but would require a thicker stucco coat.  With hardware cloth an 1/8" coat of block bond does the job and creates an almost indestructible protective barrier.

        3) Would you use 2x4 or 2x6 walls with this method you describe?

        I use a 2x12 sill and double walls 12" thick.  But, if you wanted to overhang 2" of foam, you'd need a 2x6 sill, plate and wall to have enough bearing on the slab.

        Another option would be to set the sill & wall at the slab edge and attach foam board on the outside of the frame the same thickness as the slab edge insulation to keep the siding in plane and break the thermal bridging through the studs and band joists. 

        Riversong HouseWright

        Design *  * Build *  * Renovate *  * ConsultSolar & Super-Insulated Healthy Homes

        1. User avater
          Matt | Mar 03, 2008 04:07am | #9

          I guess the 12" think eall would be what I've heard you refer to as superinsulated.  Sounds like some awesome construction....

          OTOH, athough I build some slab houses, I still prefer a wood floor....

  4. User avater
    shelternerd | Mar 02, 2008 06:32pm | #7

    PolySteel Insulated concrete forms can be used as a slab form for radiant slabs with good results. Just pour your footing below frost level as usual and come up to floor level with the ICFs brace the tops with a continuous 2x6 around the exterior to run your screed board on and cut down the interior to allow the concrete to run down the interior to the footing. Drill rebar pins into the footing per ICF specs and tie the vertical rebar into the slab rebar and the horizontal rebar that runs around the top of the ICFs.

    We run 3/8" rebar 30" each way rather than using fiber or 6x6 mesh on the advice of our engineer easy to tie the tubing to and easier to set up on chairs to assure proper placement in the pour.

    ------------------

    "You cannot work hard enough to make up for a sloppy estimate."

    1. JLazaro317 | Mar 03, 2008 04:19am | #10

      I was contemplating that as an option. I haven't done any icf yet but have always been interested. Last time we priced it to be a 15% upcharge from conventional construction. But for this job, I really like your suggestion. Only problem is that the foam will still protrude beyond exterior of wall. Do you also do a stucco coat as Riversong suggested?John

      J.R. Lazaro Builders, Inc.

      Indianapolis, In.

       

      1. User avater
        shelternerd | Mar 03, 2008 04:43am | #11

        Yes we do stucco coat it and have been using the wire lath and surface bonding cement similar to what Robert uses except that we're using expanded wire lath rather than hardware cloth (or rat wire, seems like it would be just as good his way though) We are looking at the BASF system where a screen glues on directly to the foam and the top coat goes on over that. It costs about $1.25 prer sf plus labor. It looks to be a whole lot faster and smoother than mechanically anchored lath. We'll be giving it a shot on our current project but not for a few months because I want to get the landscaping wrapped up to avoid damaging the top coat. If you do want to go with a mechanically anchored lath take a look at http://www.SpiderLath.com I saw it at the Builders Show and was very impressed.Sorry about the giant photos, I'm feeling under the weather with bad hay fever (spring in North Carolina, tee shirt weather and I'm miserable, not fair!)------------------

        "You cannot work hard enough to make up for a sloppy estimate."

        File format File format
  5. Piffin | Mar 03, 2008 06:53am | #12

    Google frost protected shallow foundats - FPSF

     

     

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