Slab insulation dilemma

[ilikedirt]

Long story short, I drew out my concrete drawing to detail 2" XPS over the entire floor, with the vapor barrier underneath the foam but above the 57 crushed stone.

Come home to find the contractor only insulated 2' from the footings and ran 2' down into the footer. Not the middle of the floor. Plus, they laid the vapor barrier down on top of the XPS.

Will I notice the lack of insulation in the floor? (building will be climate controlled)

Should the vapor barrier be above or below the insualtion? I thought it was supposed to be underneath....

 

[Pntyrmvr]

In the old days they used to leave the center of heated slabs uninsulated believing the heat would sink in to a certain point and then equalize thereby saving money on insulation.

Some argue that insulating foam boards are also vapor barriers. They are not specifically designed as under slab moisture barriers.

Sounds like you need to talk to the contractor about what specification or design he priced versus what you expected.

Drawings or specs before contract price was agreed on?


“Talk is cheap. Whiskey costs money.”

 

[TractorJeff]

AH!
I notice he is in Virginia.
Not an Expert but in my mind, 2 feet down on the footer and all but the center maybe OK?
If It was heated through the floor, then maybe I would push back for full under Slab insulation.

 

[mcbane]

Apparently there have been at least a few failures involving vapor barrier beneath the slab insulation. I haven’t seen exhaustive testing of that issue but would wonder why anyone would wish to tempt the fates by putting the vapor barrier under the insulation. Failure mode is purported to be “iceberg” where the foam floats into the slab. No way to control that during a pour.


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[Randy in Maine]

Have him fix it now.

 

[GeddyT]

I don't remember why, but I remember reading at the time that vapor barrier goes between insulation and slab, so that's how I did it.

International building code calls for R10 perimeter insulation to the footing if the building is climate controlled and full coverage R10 under-slab as well if you are heating with the slab itself (in-floor radiant). Sounds like you have somewhere in between.

So it's either overkill--probably based off stricter local code or insufficient if slab heating. Either way, I believe the vapor barrier is fine. At least I hope it is, as we're in the same boat...

 

[ford33]

I would think the crushed stone would eventually puncture the vapor barrier making the vapor barrier ineffective. The contractor likely helped you there. What mil thickness is your barrier?

I do not understand why the middle of your floor does not have a barrier. If you ever decide to paint or cover the floor you may have problems with coatings adhering to the concrete surface. You do have a contract and drawings?

 

[purediesel]

First thing you ought to do is pull the contract out and go over it with the contractor. If the contract says what you say, they should re-do everything to what you said in the contract. I would leave the 2' vertical insulation as this will help fight frost migration and fight frost heave. Then you ought to cover the entire floor with the 2"xps. If you used the proper class A synthetic vapor barrier you shouldn't have a problem with it laid over compacted stone. I used a 10mil Viper brand Class A barrier and its incredibly strong stuff.

 

[GeddyT]

I recommend you read the comments and maybe follow the links below this Green Building Advisor question. The consensus seems to be that vapor barrier goes above the under-slab insulation, and reasons are given.

As to the lack of full coverage under-slab insulation, what is the code requirement in your area? Again, IRC calls for perimeter insulation to the footings if the building is heated and full under-slab insulation if heating with the slab itself. What will be the heat source in this garage?

I agree that you should get what you contract and pay for. That being said, if this slab is already poured, I wouldn't have the contractor break it up and re-pour it. I'd negotiate a partial rebate for the missing insulation, time to install it, the frustration of not getting what you asked for, and the slightly higher heating bills moving forward. And it would be slightly. Hot air wants to rise, and as long as your foundation is insulated to the footings and the footings are below the frost line--as you say they are, then you should lose very little heat to beneath the center of the slab.

That being said, if this building is being heated by the slab itself, I'd make them rip it out, do it right, and re-pour. Anything short would be a code violation anyway.

 

[ilikedirt]

Thanks guys, an update to this...it was a miscommunication and they had no problem taking care of it. We ended up using the poly on top of the insulation. They poured the slab on Wednesday.

 

[walrus]

I think I put my vapor barrier under insulation , nothing floated during pour so I guess its ok. I did have #4 rebar 12 inches on center over the top of the insulation so maybe that kept it down?

 

[logcabin4life]

wall...perfect slab....perfect roof....I am gonna follow them...

 

[logcabin4life]