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Community and Q&A

EPS deck for suspended concrete slab – good or not so good?

jberks | Posted in Energy Efficiency and Durability on

Hey y’all,

So I’m finishing up the foundation walls for the front basement extension of my reno. I am using 6″ core ICF to form the 3 walls which extends the basement forward and will eventually create a front porch. This is similar to how people do cold rooms under their porch, but I am doing this as part of the conditioned finished basement space.

For the concrete slab on top, I was planning on using an EPS concrete deck form. EPS deck is the product I’ve been in discussion with, but insul-deck is a more familiar brand that does the same thing.×156.jpg?crc=127511281

I figured it would be efficient with the build and with achieving good insulation for the envelope. The thickness of the EPS deck would be 10″ with a 3″ concrete slab on top. This thickness would just meet my code minimum of R31. The area of the top of the deck will be roughly 5’x14′.

I’d like to ask if anyone has any experience with these deck systems and to open discussion on alternatives.

After doing some thought on it, I now have some reservations. One being that EPS isn’t the most efficient in terms of R value per inch. I would just barely be meeting code. R31 isn’t bad but I’d like to do better for higher performance. Second I noticed the flutes that create the concrete joists are quite wide. They take up a lot of volume compared to normal joists, which take up a lot of potential insulation and performance as well.

I was wondering if I would be better off with a more “conventional” wood joist and plywood deck with concrete slab assembly, or a corrugated steel pan slab assembly. Either of these would allow me to 2lb spray foam underneath to really improve the R value & performance.

Does anyone have any experience with this?


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  1. GBA Editor
    Martin Holladay | | #1

    I imagine this is a good system, although it's probably expensive. If you want a higher R-value, you can always glue more EPS to the basement ceiling once the slab has been poured.

    If your porch gets any wind-driven rain or snow, remember that you'll need a waterproof layer above the concrete -- some type of peel-and-stick.

  2. jberks | | #2

    Hi Martin,

    Actually the cost isn't too bad for the EPS deck. They've quoted me about $2,000 CAD for the system. Considering the materials, labour & spray foam to do it another way, it's competitive. But just looking for advice on how other people did their assemblies.

    What I don't want to get into is like an 8" flat suspended concrete slab, which would take up a lot of overhead room and wouldn't leave much room below the deck to run mechanicals and get a good level of insulation (without dropping the ceiling height in the basement, which I don't want to do)

    In regards to waterproofing, I am planning to finish the concrete on the top side also this deck will not be covered overhead so it will be exposed to a lot of water. I'm planning to do a buffed epoxy on the top side for both aesthetics durability. I believe that will create a waterproof membrane from the top side. I'll also do a liquid based waterproof membrane on the underside of the slab (before the pour) as an extra insurance. Hopefully with this system, any small water leaks from the top would allow the slab to dry from the edges towards the outside.

  3. Expert Member


    I take it that all your choices include a concrete slab as the finished porch surface above for aesthetic reasons, as it complicates what you are doing considerably.

    if you want the slab exposed it needs a membrane below it, not to rely on an epoxy coating if you want the area underneath to be protected from leaks. Given that, why not frame the porch out of wood like the rest of the house? You can then pour a slab on top. My own preference, as I like to be able to get at potentially problematic assemblies like waterproofed roofs over conditioned spaces, would be to use pavers over the membrane. There are a number of proprietary systems that set the pavers on chairs to create a drainage plane below.

    Here is a small deck I did using them:

  4. jberks | | #4

    Hi Malcom,

    Awesome input, thank you.

    I get what you're saying with framing the porch deck with wood, and pouring a slab on top. I'd probably use I-joists so I can get my duct work through the webs and then spray foam the whole underside. I'd certainly get more insulation in there as the I joists have a lot more space between the webs compared to a concrete composite system or EPS deck.

    One issue I see with this is if I sandwich the 3/4 ply between the slab and closed cell spray foam, it will most likely rot. another issue is in the wood-to-concrete details, just how finnicky things might be like creating beam pockets in the ICF foundation walls to rest the joists at exactly the right height. etc.

    I'm going to give Canam a call tomorrow and see what they have to say. Their Hambro D500 Composite system apparently is just open web steel joists where you can form a 3" slab on top without an underside deck.

  5. Expert Member


    What concerns me is relying on the slab as a waterproof flat-roof. Because with Canam the slab is integral to the structure, it is harder to introduce a membrane into the roof than it is when the slab is just a finished deck surface. Ask Canam if they have a detail for what you are proposing. Maybe they have some work-around.

    Edit: With an all wood structure, couldn't the beams, and joists all sit on top of the ICF much as they do any concrete foundation?

  6. GBA Editor
    Martin Holladay | | #6

    Whatever you do, remember that a concrete slab is not roofing.

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