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Under Slab Insulation choice

newbuilder | Posted in Construction Techniques on September 4, 2005 11:42am

I’m getting a reasonable price on this slab on grade I’m having put in this week, so I said I’d have the under-slab insulation on site for them before the pour.

Tonight at HD there were three different possibilities and the guy on the floor .. young guy who I have no idea if he really knows … said that any of them would do.  They are all 2 inches thick … here is the info I jotted down on them.

1 – Western “R-Tech Insulfoam”: R-8.33 @ 2″ … 4×8 sheets .. (semi-flexible.. blue and white) — $18.50 / 32 sf

2 – Owens Corning “Foamula R 250″ extruded polystyrene insulation.  R-10 @ 2” … 2×8 sheets .. (rigid .. pink!) — $29.00 / 32 sf

3 – [offered as a “new material”]:  Dow “Tuff-R Commercial” Polyisocyanurate insulation … R-13 @ 2″ … 4×8 sheets …(very rigid .. yellow between to ‘foil’ faces) — $22.25 / 32 sf

MY QUESTION IS … is one of these THE one that really is the BEST choice for laying under a newly poured slab?  Alternatively .. is one of these clearly NOT to be used for underlayment?  I looked at everything that was there in terms od information and only ONE … the “R-Tech Insulfoam” even ‘mentioned’ laying under a slab on grade.  But the builders had all mentioned “2×8 sheets” .. which only the Owens Corning pink stuff came in.    Surely some of you folks here must know which is ok(?)

thanks!

Terry

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Replies

  1. MikeSmith | Sep 04, 2005 12:44pm | #1

    4>  PerformGuard EPS.....  2"  R7....treated with borates...2x8 sheets...call for a quote from your nearest franchise... about half  per BF of what you were quoting above

    google on R-Control

    Mike Smith Rhode Island : Design / Build / Repair / Restore
    1. bigman | Sep 05, 2005 02:19am | #8

      The R-control is also available in 4'x8' & 4'x16' !!! I cant find my quote,  but the type II was .17 per board foot, I will be placing an order shortly for my basement slab.

  2. User avater
    Matt | Sep 04, 2005 03:56pm | #2

    To add to what Mike said, the borates act as an insecticide.  The significants of this is dependant on how much of a problem wood destroying insects (primarily termites) are in your area.  Also, I assume you are going to have the substrate treated by a soil poisoning company, so this may effect the need for the borates. 

    Re the 2x8 sheets - any of the stuff is easily cut with a sharp utility knife, and some comes with a perforation down the center to help with this.

    JLC had an article on rigid foam a few years back and went through the characteristics, advantages and disadvantages of the 3 basic types of rigid foam - polyiso, expanded (EPS), and extruded (XPS) polystyrene.

    If you don't have access to JLC just do a web search for "XPS EPS polyiso" or similar.

    I'm building 2 slab on grade houses right now and I think I used the R-tech product.   It was a few months ago.  My pest control sub treated the substrate (stone) with a few hundred gallons of the latest chemical they think is safe - something called Premise - so I'm not particularly worried about termites.

  3. CJD | Sep 04, 2005 04:47pm | #3

    The "best" would be dependant on your load bearing requirements. I used 4" thick borate treated 2lb/Ft³ density EPS which has a 25 PSI compressive strength (3,600 Lbs/Ft²). I don’t remember the load bearing or density of the borate treated EPS at Home Depot but it was too low for requirement.

    Engineering technical support at Premier/Insulfoam is very good: http://www.pbspanel.com/

  4. joeh | Sep 04, 2005 06:33pm | #4

    Terry, R-Control EPS is the stuff.

    It's about half the price of pink or blue board.

    Type II is sufficient for under slab use at 15 PSI.

    They have manufacturing around the country & will sell direct. EPS is available in 36" thick blocks that they slice to whatever thickness you want. Free delivery if you order enough, I think about $1600 was the price when I got some early last year.

    That's a lot of foam to store, but you can pick up smaller amounts at their facility if you're close.

    Joe H

     

    1. newbuilder | Sep 05, 2005 12:59am | #6

      Ohhhh...  what a shame that the R-control eps people have no outlets near Seattle!  My god, they even have outlets on the big island of Hawaii!!  I'd say I need it here a little more than THEY do!  So ... though the product looks GREAT ... I don't see a nearby source.  On Tuesday I'll call and see if there is some place out here where I could pick some up.   Need enough to cover about 490 sq. ft. 

      Btw -- people here speak of a wall to wall ground cover placed under the 2" of insulation ... but not one of the guys that has come by to talk, look and give me a bid has EVER mentioned 'insect control' chemicals.  I don't think that's in the cards here in the PNW. 

      Another by the way:  are the 2" insulation sheets really simply 'laid' on the ground and poured over?   No taping of seems or fastening together so that they don't separate under weight? 

      And ... guys here have told me to ONLY go with #4 rebar network under slab .. whereas my guy is talking a 'wire mesh'.  Should I *insist* on the rebar? ... or rebar over the mesh? 

      First time having ANYTHING to do with a slab .. or ANY foundation work for that matter.

      thanks!

      Terry

       

      1. WayneL5 | Sep 05, 2005 01:49am | #7

        Don't use the polyisocyanurate, it's not going to hold up underground.

        Pretty much the insulation is just layed right on the ground, with no taping of seams.  The "ground" should be compacted crushed stone ideally, though sand is ok.  Having ground that won't settle or heave is more important to the solidity of the slab than the reinforcement or care in finishing.

        The metal reinforcement goes in the slab, not under it.  In my opinion the wire mesh (called "WWF -- welded wire fabric) is more appropriate for a slab for a home or garage on grade.

        I've never heard of insect control chemicals under a slab, but I lived in an area that is too cold for termites.

      2. User avater
        AdamGreisz | Sep 05, 2005 09:00am | #9

        I too build in the Great NW. Around here we do not typically spray pesticides or use the borate treated foam. The 2' x 8' pink foam @ 2" thick is the local standard.

        Reinforcement for the slab can be welded wire, #3 bar or #4 bar. On center requirements depend on soil conditions. If your subcontractor uses mesh make sure it is pulled into the center of the slab during the pour. It is very important to have good drainage under the slab. Sand works very well as a base. I typically install 6-8" of sand over a 12" layer of compacted gravel (these are high end jobs). Local building inspector will want to see a vapor barrier (typically 6 mil visqueen). Many concrete finishers do not like the VB visqueen due to the fact that it slows rate of curing (only allows moisture to cure out of the top) and means they will be finishing the hard troweled surface into the evening.

         

         Wood is Good

        Adam Greisz<!----><!----><!---->

        <!----><!----> 

        1. newbuilder | Sep 05, 2005 12:12pm | #10

          Adam,

          If I'd been aware of you earlier I would have called you!

          thanks again, everyone -- great info that I can really use!

           

          Terry

      3. User avater
        Matt | Sep 05, 2005 12:13pm | #11

        This is the way we do slab on grade here in nc - stone -> insecticide -> insulation -> poly -> WWF and then steel (rebar) if you think the soil under the stone is not real stable and uniform.  Some kind of chairs are required if you want to use the rebar.  I read they now have plastic ones but haven't seen them myself.  We have under slab inspections during which they check for the stone, insecticide, insulation and poly.

        No way in h&!! would I let anyone pour a slab to be enclosed without putting down poly first.

        1. JTC1 | Sep 05, 2005 07:52pm | #13

          Same drill here in Delaware.  Lots of hungry sub ter termites.  Rebar in slab is rare - need the little chairs if you do use it.

          Jim

          1. newbuilder | Sep 05, 2005 11:39pm | #14

            From what I've heard I believe the build-up from grade is: 

            ~~~ compacted soil ... some kind of gravel or sand ... vapor/moisture barrier ... 2" poly ... wire mesh ... 4" slab ~~~

            I've heard SO much about how that mesh just slips to the bottom and does NOT really sit 'within' the slab that I was asking for rebar on chairs .. but I'm thinking he's not used to doing that and I don't wanna force the issue .. especially since many here have mentioned wire-mesh as OK.  wish the mesh were up on 'chairs' the way bar is set.

            thanks for the great responses .. and Stainless .. thanks for the slab-painting tutorial .. I'll print it up and make us of it later!

            Terry

             

             

          2. User avater
            Matt | Sep 06, 2005 02:28am | #15

            When they pour the slab they pull wire up so that it doesn't lay on the bottom.  I confess I really don't know how well it works, but it is a very common practice.  That is what that hook is for on the concrete worker's come-a-long (a garden rake shaped tool that has a blade with a slight cup rather than tines).

          3. JTC1 | Sep 07, 2005 02:53am | #16

            I understand what you are saying, and this might happen if the wire mesh were really planar ( perfectly flat ).

            In practice, the mesh is not flat but somewhat wavy to begin with.  The guys who are pouring the slab will be tromping around  / wheeling concrete / moving chutes, etc. all of which serves to squash the mesh toward the bottom of the slab. 

            A knowledgeable crew will be hooking the mesh with their placers (rakes) and pulling it upward as the last step during the screeing process, just before the first pass with the bull float.   

            Most of the mesh reinforced concrete which I have torn out reveals the mesh in the middle third or half of the slab.

            Jim

            Never underestimate the value of a sharp pencil or good light. 

            Edited 9/6/2005 7:55 pm ET by JTC1

  5. Piffin | Sep 04, 2005 07:50pm | #5

    Definitely NOT the polyiso. It is notwaterproof. It will get saturated and lose any of it's effectiveness.

     

     

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  6. STAINLESS | Sep 05, 2005 03:56pm | #12

    Hi New! Here is yet 1 more vote for under-slab poly & XPS insulation!

    You may read my earlier response to a question from KB a few days ago which is quite similar to your own. Since then I've thought to add a suggestion regarding eventual painting of your floor (should you decide to), ;  I've had my best results with a 2-part EPOXY which I put on as soon as possible. (My floors get so grungy after even a few months that my paint prep. work time goes up immensely if I wait a year or so).

    I like to use 2 coats with the 1st coat thinned 50% for good penetration. Since I'm supposing you WILL be using a poly sheet of some type under your slab, water vapour transmission through your concrete will not be causing you the problem of paint lifting after a number of years. I've been told it is critical ,however, to be absolutely sure your concrete is well cured and dry, or the entrapped mix water will produce a vapour pressure which can lift any paint. (A 2' square of poly, duct-taped around it's perimiter to a clean, flat spot on your floor should show no condensation on it's underside after a week of being down.) Let it cure another few weeks if you still show moisture, before testing again.

    I've had excellent results with a floor now more than 18 yrs old. After damp curing the concrete for 2 months,I etched it with Muriatic acid ( AKA Hydrochloric) , rinsed it with clean water several times before allowing it to dry for a month or so. Once the poly test showed DRY, I vacuumed it and painted it with 2 coats of 2-part epoxy applied by roller. (1st coat thinned 50 %)

    This is a 5-1/2" thick, rebar-reinforced  slab, poured over 2" of DOW CORNING HI 60 XPS covered with a 6 mil. poly slip sheet. My base is 2' of clean sand damp-compacted in 6" lifts in an unheated building. Although winter temps. down to -40 C are quite routine where I live, this paint has withstood everything I've thrown (or spilled!) on it. Gasoline, brake fluid, diesel fuel, naptha, Xylol & Tolulene thinners, grease, engine oil, dog urine(!), methanol, beer, water. It has no gloss at all left but has not lifted anywhere, not even flaked!

    If you ever think you may need to paint your slab, I can heartily give thumbs up to this method. If you don't figure to ever paint your floor, just ignore completely the foregoing dissertation. This message will not self-destruct in 10 seconds, however.

    May the force be with you. LOL

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