Plastic above or below insulation for slab?

[JFoy]

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

 

[RNeumann]

Vapor barrier goes below the rigid insulation.

How are you insinuating the rim?

 

[JFoy]

How are you insinuating the rim?

Outside of the footers - vertically. Won't have anything under the base of the footer itself.

 

[teejk]

My contractor did under. Just curious...if the barrier is under, where would the moisture come from?

 

[dmccarty]

Under the insulation.

Later,
Dan

 

[JFoy]

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.

 

[tcreeley]

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?

 

[JFoy]

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.

 

[jfh28]

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

 

[RNeumann]

Slab Edge Insulation | Building Science Corporation

This site has hundreds of articles on all types of building construction principles.

 

[Tomtint]

When I built our current home, I used 14mill Polly under the slab , on top of the gravel. I did not use any rigid insulation as we are not in a very cold climate and the Polly was primarily to eliminate and moisture from the ground as well as radon mitigation. .. Our basement is bone dry and even after 8 years there is absolutely no "basement" smell. We actually have a giant room finished that is nicer than most living rooms.

 

[mikester]

From bottom up:
clear pea gravel
poly
rigid insulation
concrete

Lately I've been seeing guys using 2" closed cell spray foam on the gravel base to the walls. I think I'd do that next time

 

[majorwager]

The higher the zone #, the colder the winter climate. Would use an extruded insulation product like DOW or Formular by O.C. (not expanded type) Also would use High Density, min 60 psi. Otherwise you will break the sheets when pouring concrete even if you use a pump. Important that the subgrade, stone/gravel is uniform and level. Built a Large Head House for the USDA, years ago, they required the insulation over crushed limestone base, the insulation, then 12 inches of course concrete sand. Somewhat of a heat sink but they did NOT require radiant piping imbedded in the sand. Guess they thought heat would not permeate the slab. Did result in a true floating slab and it never did display a crack. Vapor barrier under slab, not insulation. Pouring concrete over sand directly, would allow moisture from concrete to be absorbed by sand, The bottom of the slab would cure faster than the top, the slab would likely suffer a strength failure, by interrupting the chemical reaction of the Portland cement component. (separation)

Note, Head house is the staging/equipment monitoring, and operations area, Structure for large attached green houses. All masonry construction. The green houses they later attached were 150 ft long in three directions. Tax $ at work.

EDIT: Pea gravel is unstable, will swim all over while working on top, does not compact. Never seen closed cell spray below grade? wonder the density, would it crack and separate from weight of concrete buggy, or old school wheel borrow? Long term performance of closed cell spray below grade? Generally agree VB should be under insulation, the proper insulation.

 

[dmccarty]

Slab Edge Insulation | Building Science Corporation

This site has hundreds of articles on all types of building construction principles.

We used one of the building books from that link to specify many details on our house, including the slab, which has 4 inches of insulation and a plastic barrier.

Later,
Dan

 

[JFoy]

Much appreciate all the advice.