Plastic above or below insulation for slab?

   / Plastic above or below insulation for slab? #1  

JFoy

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Dec 23, 2015
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Clemmons/Lexington, NC
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Kubota BX25D
I am having a metal building erected that will be turned into an apartment. We are climate zone 3 so insulation under the slab is not required but I still want to put 1in foam board down under the slab. The foam board I have bought has foil on one side.

The question is whether or not the plastic sheeting goes above or below the foam board. Arguments both ways.

Sheeting above: Lower chance of underside moisture coming into contact with the slab.

Sheeting below: Insulation stays drier.

Any thoughts?

Thanks,
JFoy
 
   / Plastic above or below insulation for slab? #2  

RNeumann

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Aug 29, 2016
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North Idaho
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Mahindra 1538
Vapor barrier goes below the rigid insulation.

How are you insinuating the rim?
 
   / Plastic above or below insulation for slab? #4  

teejk

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My contractor did under. Just curious...if the barrier is under, where would the moisture come from?
 
   / Plastic above or below insulation for slab?
  • Thread Starter
#6  
OP
J

JFoy

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Dec 23, 2015
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Location
Clemmons/Lexington, NC
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Kubota BX25D
My contractor did under. Just curious...if the barrier is under, where would the moisture come from?

Rising water table perhaps. That is an issue on this plot - the water table is close to the surface at some point. Though, water tables following the land baffle me.

Since the slab will have footers on all sides, I'm going to install drain pipe from the gravel field to outside the building. Hopefully, all these steps will be enough and I won't have to trench a french drain as well.
 
   / Plastic above or below insulation for slab? #7  

tcreeley

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Hudson, Maine
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2003 NH TC30
Climate zone 3 - in Maine that would be the coldest part of the state - temps like -40F and frost depths of 5', etc. . Have they changed the climate zones, or what am I missing?
 
   / Plastic above or below insulation for slab?
  • Thread Starter
#8  
OP
J

JFoy

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Joined
Dec 23, 2015
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234
Location
Clemmons/Lexington, NC
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Kubota BX25D
Climate zone 3 - in Maine that would be the coldest part of the state - temps like -40F and frost depths of 5', etc. . Have they changed the climate zones, or what am I missing?

Odd. The "NC Builder's Code" calls this area "Climate Zone 3". I guess it varies by state. I know there is a "hardiness" zone for planting stuff.
 
   / Plastic above or below insulation for slab? #9  

jfh28

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Poconos, PA
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Power Trac 2425
I think you need to differentiate between a moisture barrier and a waterproofing barrier.....a 6 mil poly sheet is a moisture barrier and its purpose is to resist the moisture which is inherent in soil from passing thru the concrete and diffusing into the air.......anyone who has ever been in a musty smelling basement has experienced a good example of this.......to work best a moisture barrier must be placed as close to the concrete as possible so in your case this would be on top of the insulation and the slab would be poured right on top of the moisture barrier.......don't use 4 mil poly it rips to easily during the concrete pour and a ripped moisture barrier is worthless.......also the less seams in it the better......a moisture barrier will not stop water......so if a high water table is your problem then you need a different system which will stop the hydro-static pressure of water from infiltrating your foundation and that will be dependent on your individual situation....this would be a waterproofing system........the insulation you use should be rated for ground contact and so will not be effected by being on the ground beneath the moisture barrier......there should also be a 6" drainage layer below the insulation.....we use 3/4" stone here but your area may be different........would be best to drive around your area and see what contractors are doing or talk to your local building inspector as moisture and waterproofing methods vary immensely around the country based on specific geographic factors.........Jack
 
 
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