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Community and Q&A

Retrofitting a Concrete Slab

Matt2021 | Posted in General Questions on

EDITED

Hello!  We are turning an existing, screened, covered porch into a four-season room, or sunroom (which will become an integral part of our kitchen, and, so, needs to be full-proof comfortable).  Between the kitchen and the future sunroom, there is a 8.5″ step; the floor is slightly sloped towards the backyard (I will measure, but, end-to-end, the difference in height might be 1″-2″).  The current porch has a tiled floor over a concrete slab; everything is very solid.  The concrete slab is 4″ thick, reinforced with #10 wire.  Under the slab, there is a 1-inch styrofoam board and, under the styrofoam insulation, a 6 mil plastic vapor barrier.  Further below, there is a 4″ layer of clean stone, and then earth fill.  The porch has a footing, “minimum 36″ below grade.”

As the house has hardwood floors throughout, including the kitchen, the new room will have a matching hardwood floor.  Since the slab is only (minimally?) insulated thanks to the styrofoam board, and already has a vapor barrier underneath, do I need/is it recommended that I add any insulation or additional vapor barrier above the tiled floor/concrete slab, “sandwiched” between the existing floor/sban and the new OSB subfloor?  If we were to install the OSB subfloor directly on the concrete slab (that is, the tiled floor, after it has been leveled), we could take 1.5″ from the (now too high) step between the kitchen and the new room; a 7-inch step would be perfect.  I fear, however, that a floor placed, basically, directly on concrete, despite the existing under-slab one-inch insulation, might be cold.  (The house is in New Jersey.)  In the event that adding insulation above the existing floor is after all a good idea, 1) should add another vapor barrier (that would create two barriers, one above, one below the concrete slab)?  And, in case, which insulation should I go for?  Would adding a, say, half-inch insulation board above the existing floor make any sense?  (The concrete slab would then be sandwiched between the insulation layers.)

This helpful article by Martin Holladay talks about placing insulation over a BASEMENT concrete floor:

Installing Rigid Foam Above a Concrete Slab

I imagine that everything Martin says there could be applied to my situation.  However, I already have some insulation under that concrete slab, it turns out.  Also, I have height limitations: I cannot raise the floor the whole 8.5″ because the new room’s ceiling would become too low at one end; I cannot take away too many inches from the existing step between the kitchen and the future room because steps that are too low feel odd and are a trip hazard.

Any thoughts?  In my circumstances, am I at risk of having a cold, uncomfortable floor, if I do not try to add any above-slab insulation?  Yet, would it make even sense, or would it create condensation or other problems, to sandwich the concrete slab between the existing vapor barrier and a new barrier, or between the existing insulation and a new layer of insulation?

Thank you very much for any input on this!

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Replies

  1. Walter Ahlgrim | | #1

    The first question that comes to mind is if your new room started life as a patio without a footing set below the frost line? Generally patios just set on the surface and we don’t worry if the patio moves when the ground freezes but things get ugly when you start building on the patio.

    1. Matt2021 | | #2

      Thank you, Walter. Excellent question. We have all the plans and permits from 1985, when the porch was built.
      Indeed, your question made me discover details, in the documentation, that I had never noticed before -- and that importantly affect my original question (which I will edit):

      - It has a footing of "minimum 36 inches" below grade, as for the project presented to the township.
      - The concrete slab is 4" thick, with 6x6 #10 wire reinforcing.
      - Under the slab, there is a 1" styrofoam insulation board, and a 6 mil vapor barrier; then, 4" of "clean stone", followed by earth fill.

      So, it turns out that the concrete slab is already insulated, if unfortunately with only a one-inch board of styrofoam, and already has a vapor barrier. Perhaps, this settles the question: I should apply OSB right on to the tiled concrete slab, add no insulation and no additional vapor barrier. Am I correct?

  2. Andy S | | #3

    8.5" step? That's a good opportunity to raise the floor even with the rest of the house and provide insulation space below. You'll also need somewhere to run pipes and wires so that 8.5" space could become precious.

    1. Matt2021 | | #5

      Thank you, Andy. In fact, that was the original plan: to have one continuous floor between the kitchen and the new room. However, given that the porch's roof is "shed style," if we raise the floor, the ceiling will be only 7 feet high at its lowest point (which would be the length of the room, parallel to the house); the whole room would risk not looking quite right. That's why we decided to leave a step between the two rooms, though of 7 inches or so. We do not need to run any pipes or wires, by the way.

      The questions remain: whether that 1-inch of styrofoam insulation under the slab is enough for the floor not to feel cold, and whether, in case, it is advisable to add a thin foam board over the slab, under the underfloor of the new floor.

    2. Matt2021 | | #7

      Maybe I could aim for a five-inch step, but only if it is really important to have, say, at least two inches of foam insulation. The higher I can keep the ceiling, the better.

      1. Andy S | | #11

        8.5" is too big a step in my opinion. 7.5" should be the max, but either way a single step can be annoying if it isn't carefully integrated into the flow and style of the space.

  3. Jon R | | #4

    To the extent you have room, add foam above the slab.

    What flooring will be over the OSB? Don't put OSB in a moisture trap.

    1. Matt2021 | | #6

      Thanks, Jon.

      Is it OK to add a foam board over a slab that already has, under the slab, foam and a vapor barrier? Do I also add another vapor barrier? And is a, say, half-inch foam board going to do anything?

      The floor will be oak hardwood floor. By the way, from what you say,I should go for plywood rather than OSB, is that right?

    2. Matt2021 | | #8

      Maybe I could aim for a five-inch step, but only if it is really important to have, say, at least two inches of foam insulation. The higher I can keep the ceiling, the better.

  4. Walter Ahlgrim | | #9

    If this addition is really going to be integral part of your kitchen I say spend the money needed to keep the floors flat and the ceiling level so it feels like part of the kitchen and not like a cheesy addition.

    Yes it will cost more to replace the roof and build up the floor. Think about how many times you will be going up and down that step dozen of time a day for 20 or more years?

    Don’t get obsessed with below grade insulation it just does not make that much difference given that the ground under the slab is 50° or so.

    Walta

    1. Matt2021 | | #13

      Thank you very much! You definitely present a very refreshing perspective. I will definitely look into the cost of redoing the roof, so as to raise by 10" on one side (I know it will still have enough pitch), and go back to the original plan of raising the floor.

      On the insulation, am I right in reading what you are saying as implying: sure, good that there is that one inch of styrofoam board under the concrete slab, but it would have not been a great deal if it weren't there; rather, if you raise the floor, get the insulation above the slab. I imagine, at that point, that I would also add a vapor barrier. There are no problems in having two vapor barriers, one under the slab, one above the slab, right?

  5. Andy S | | #10

    One more point to add...unless you were there on the day that slab was poured you don't know for sure that the 1" foam is even really there or that it's continuous. Residential building is notorious for being a little freestyle on details like this that aren't code required or standard practice. I'm not saying evil intent was present, the original homeowner will often be the one to strike details like this for the sake of budget.

    1. Matt2021 | | #14

      Good point, Andy. In this case, I think the previous owner, with whom I am still in contact, did get there: he's very meticulous, drew all the drawings, and left me all the receipts for the job. Still, as you say, who knows? Either way, it does not make that much of a difference, right?

  6. Expert Member
    Malcolm Taylor | | #12

    Matt,

    Check your building code to make sure it allows a single step within a dwelling unit.

    1. Matt2021 | | #15

      Thanks Malcolm. That's interesting. I did not even think that having a step - that is, a single step - within the house, between rooms, may not be allowed. The uncertainty says a lot about the nuisance a step is inside a house. I really need to think this through very carefully!

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