Protecting under slab insulation from fuel leaks

[StefanNH]

I'm pouring a 40x60 frost protected monolithic slab, which includes 3 inches of EPS foam under the concrete to prevent the subgrade from freezing and heaving. My original plan was to first install the vapor barrier, then the foam, then staple to PEX directly to the foam board.

But after thinking about it some, I am worried than if there is fuel leak or solvent leak on the slab or right next to it, it could compromise my foundation by eating away the EPS insulation and cause the slab to crack and fail. So to mitigate this risk my new plan is to put a heavy duty 15 mil vapor barrier on top of the foam board, and tying the PEX right to the rebar. The only obvious draw back from this is accidently cutting it with the saw afterwards.

Obviously I'm not going to go out of the way and pour fuel all over my shop but as we all know accidents happen and I'd rather not have to worry about it.

Anyone one else think about this or am I overthinking it?

 

[manwithtools]

Anyone one else think about this or am I overthinking it?

Yes, you are overthinking it!

Do you know how much fuel it would take to eat away the foam appreciably? It would have to be gallons that got spilled and not cleaned up quickly. I would be more worried about the fire risk that might create at that point.

 

[mike93lx]

If you have a fuel leak getting into your subgradez you have a bigger problem than it's impact on foam

 

[StefanNH]

Yes, you are overthinking it!

Do you know how much fuel it would take to eat away the foam appreciably?

Were I work now the floor tends to get greasy and covered in solvents, I'm sure to dissolve the entirety of the 800cu ft of foam I need it would take many barrels of solvent, but I still think a small leak around one of the load supporting columns could cause some significant degradation.

 

[manwithtools]

Nope, not worth worrying about. Think how many shops with insulated floors would have experienced this failure already if it was a concern. Epoxy the floors and eliminate the worry entirely.

 

[pcmeiners]

Good question.....


"Yes, gasoline travels through concrete. Concrete is a highly porous material, and spilled gas quickly infiltrates these microscopic capillaries rather than just evaporating off the top. Over time, these hydrocarbons can seep entirely through a typical slab and contaminate the soil and groundwater beneath"

"EPS (Expanded Polystyrene) is highly moisture-resistant but not entirely waterproof. Because of its closed-cell bead structure, it absorbs minimal water. However, water vapor can diffuse through the material, and bulk moisture can get trapped between the fused beads"

" Molecular Permeation: Gasoline is a volatile mixture of petroleum hydrocarbons. The molecules are small enough to squeeze through the dense but permeable molecular chains of standard plastic sheeting"


Best answer....

"Polyethylene (LDPE/HDPE) has good chemical resistance to gasoline. However, most standard plastic sheeting is not 100% vapor-proof. For true fuel and vapor resistance, you need specific barrier films or coated sheets"

Polyethylene (LDPE/HDPE) on top of the insulation, hoping you have fairly dry substrate under the foam or another vapor barrier below the insulation.

99% of slab owners only have a single vapor barrier using standard poly sheeting.

 

[StefanNH]

99% of slab owners only have a single vapor barrier using standard poly sheeting

I wouldn't want to make a "vapor barrier sandwich" because the foam needs to breath one way or another, whether its through the ground or the slab. Still begs the question, are people tying PEX to their rebar?

 

[pcmeiners]

Foam does not need to breathe nor could it under a slab, the moisture it absorbs will be pretty much static though out it's life. Pex, question no idea

 

[jblnut]

You are over thinking it. Go dump some gasoline or whatever solvent you'd like onto the poly sheeting you plan to use to protect your foam. I bet you'll be surprised to see it'll most likely eat through it.

 

[jack stand]

I tied my pex to the 6x6 mesh that's tied to the rebar on a 2' grid. All above the foam and poly vb.

 

[finn]

There are literally millions of square feet pf foam under slabs, nearly all of which have experienced petroleum based chemical leaks at one time or another. Even if 55 gallons of gas spilled on your slab, either your shop would burn down or the gas would evaporate before it damaged the foam, especially if you’d pouring a thicker than 4” slab to accomodate the tubes (my thirty year old slab is predominantly 4” thick, and according to one guy who worked for the logger that previously owned the shop, was frequently covered with hydraulic oil spilled while rebuilding forestry slashers).

In the event the spill did soak through and permeate the foam, the concrete would have enough tensile and compressive strength to bridge the damage, especially if your using rebar and have an overly thick slab, as is seemingly standard practice here.

In a word, you’re overthinking this.

Ask around to see how factory or commercial shop floors that have radiant heat are constructed in your locale. They’re going to have far more frequent and serious spills than you ever will.

A good epoxy or the like floor covering would be a far better solution.

And keep those liquids in a separate, fire resistant shed.

 

[Leaflessshadetree]

I wanted the PEX in the center of the slab and tied it to the wire mesh.

 

[Superbowl]

If you are worried, put a epoxy sealer that will stand up to gas and oil ON TOP of the slab.

That is way better than trapping gasoline between a sheet of plastic and the bottom of the slab.

 

[StefanNH]

Thanks for all the responses, with all the money on the line I tend to go off the deep end trying sure things are done right! In other words I'm going to stick to my original plan.

 

[finn]

Thanks for all the responses, with all the money on the line I tend to go off the deep end trying sure things are done right! In other words I'm going to stick to my original plan.

So, if your mind was made up, why did you ask?

 

[StefanNH]

So, if your mind was made up, why did you ask?

Must have been a miscommunication, you guys convinced me to NOT to worry about it and place my vapor barrier underneath the foam.

 

[finn]

Must have been a miscommunication, you guys convinced me to NOT to worry about it and place my vapor barrier underneath the foam.

I misunderstood, so I apologize

Hope your build turns out well. It’ll be an adventure for sure.

 

[wssix99]

I think you should treat yourself and have a professional company come in and give you a nice epoxy floor on top of that concrete. Cleanup of everything will become super-easy.