Our builder put 3" of XPS below our basement slab in Raleigh, NC.

I agree that the contractor is a total idiot and probably either terribly inexperienced or such an old dog that he can't even remember how to learn new tricks. You need to clarify exactly what his position is on this build. Is he the General Contractor or a concrete sub or does he have some other position? What his position is and whether or not you have entered into a contract with him will depend on how much trouble you're in.

Concrete does not "dry", it hardens by a chemical process called "hydration" that does not require exposure to air or any absorbent material. The longer it takes to hydrate and the less water it gives up, the harder the concrete will be and the less it will crack. This is the purpose of the "curing" process applied immediately after the concrete sets up.

If water is allowed to seep into the substrate, it will later try to rise up through the slab and that will destroy the adhesion of adhesive applied resilient flooring.

The place for the code required vapor retarder is directly below the slab, or a layer of rigid foam boards that keep the floor from being the temperature of the ground (essentially close to the temperature of well water) throughout the year. This insulation should be dictated by code.

Your contractor sounds like the concrete guys who poured my basement slab.

I don't think that he was referring to long time curing of concrete and getting a dry floor. Instead I assume he was referring to the actual concrete pour day.

The ambient temperatures when my floor was poured was 50F, overcast skies.
I did all the prep work which included 4" insulation and a 15 mil vapor barrier. This was required by the engineer.

The concrete contractor wanted to poke holes into the vapor barrier so that some water can drain from the uncured, fresh concrete.

I told him no.

The concrete was poured at 8am in the morning. The bleed water didn't evaporate for about 18 hours (the contractor was not happy) so the finishing was delayed until late at night.

If there would have been no vapor barrier, the soil would have soaked up and drained water, which would have accelerated finishing the concrete.

lzerac: "new energy code states r-15 below slab"

The Ontario Building Code governs in Ontario. The table below from the Code shows the minimum thermal resistance required of building assemblies

Please note the bottom line. (To convert RSI to R divide RSI by .1761.)

lzerac: "with an uninsulated slab in ON, you would constantly be living on top of an ice cube."

Four feet or more down, the soil is 55 degrees F. more or less year round.

It's true that underslab insulation will save some heating costs. But I've been building in southern Ontario for more than 20 years and have never installed underslab insulation except for a home that used radiant heat in the basement.

As for vapour barriers over underslab insulation, thisdiscussion from the Journal of Light Construction points out a divergence of opinion between informed construction experts. For instance, Dr. Joe Lstiburek specifically says no vapour barrier should be used between underslab foam and poured floors.

In other words, the issue is not so cut and dried and your builder is not an uninformed idiot.

The P. Eng I rely on, who has 40 years in construction with many large commercial projects under his belt, has specifically directed me not use poly under concrete slabs.

However, if the OP is very energy conscious, by all means it is his choice to call for underslab insulation.

If water is allowed to seep into the substrate, it will later try to rise up through the slab and that will destroy the adhesion of adhesive applied resilient flooring.

In any case, the thousands of pounds of water in a concrete slab will dissipate upwards. That's why adhesive resilient flooring should never be applied directly to the concrete, but to a subfloor system that provides for escape of moisture such as XPS, Dri-Core or HDPE dimpled membranes such as Platon.

Excessive bleed water can be addressed to some degree with well-graded aggregate and lower water content but contractors often don't like the greater effort required to place it.

I agree a subfloor system is best when the owner is willing to include it in the budget and that seems to be increasingly the case. As with all other construction elements, a slab should be designed in conjunction with all of the other project features, systems and conditions. It's never simple.

Yes, your contractor is an idiot or a liar, perhaps both. With the Superior Walls foundation I have, the continuous gravel bed under the wall units and the entire floor affords a drainage plane for any water which the french drains do not carry away. My basement is daylight at the back and the gravel bed extends out 6' beyond the foundation on that side. The yard is graded to move surface water away from the house. No matter what type basement walls are installed, the secret to a dry basement is to first keep the water from the house and the second is to provide a path for the water to run off below the level of the basement floor. Completely sealing out the water is impossible.

Yes, your contractor is an idiot or a liar, perhaps both

Just curious as to any relationship this conclusion has to the details of your Superior Walls Foundation?

I think the previous posts were concerned with moisture migration in the form of water vapor rather than hydrostatic pressure under the slab from inadequate site or subsurface drainage. These issues vary considerably with local climate and soil conditions. In any case, a vapor retarder membrane would not preclude a crushed stone drainage break nor would it be considered waterproofing.

To the OP:

Be sure that the plan includes frost protection for the area around the basement walkout. That would be an appropriate thickness of XPS or EPS adhered below grade to the interior or exterior of the foundation,perferably down to the footings. I have seen this detail overlooked by new home inspectors.

Worthy
Forgive my assumptions I do make from time to time. I was going off of 2009 international energy code, assuming it would be similar or used and not the Ontario code. However I do not have a copy of r2000 in front of me to confirm that, but typically it is higher then IEC. However it appears you do not use the r2000.
Also, in this situation, it is a walkout basement. Around here, those are considered and treated to a slab on grade, so your soil temps are not rated at 4-5' down. The slab (at least closest to the walkout section) will remain colder then a typical basement slab. However even if the slab was at ground temp, that is still cold (IMO) and would suck the heat from the air.
Also because something has been done for 20 years, does not make it ideal. Check any heat loss program, and you will see significant losses through uninsulated slabs. I am quit familiar with Dr. Joe and is semi comedic way of presenting the science behind energy conservation, and I mean that in a very respectable way as I read many of his publications (his air sealing around windows always sticks in my head...as he refers to the sill flashing adhesive wraps as the "black thingies"). However as you know, increasing XPS foams decreases perm ratings, thus thick foam below slab would greatly reduce water migration into the slab from below, especially over a bed of clean gravel. In these situations of thick XPS, which he typically recommends (sometimes higher density EPS foams) then moisture barriers can become less needed as the foam is acting has a very low perm vapor retarder.
For commercial jobs, we always specify a moisture barrier below slab on 6" gravel min. Typically 2" perimeter XPS, about 2' into the building. However we also never use surface treatments to elevate fully adhered products (such as vct and carpet tiles). Instead we use water sealing products and treatment of the slabs as well as close measurement of water content of the slab prior to installation of adhered products.

So the fact of the matter is...and I stand correct...is rigid required? no, unless you intend to hit energy code (which at least in the states is not enforced for all projects). However why you wouldn't is a different story. And the only reason I can think of not to is to reduce upfront costs, which ironically will ultimately cost you far more in the long run. I guess we can agree a below slab vapor barrier will vary in requirement from place to place, one person's opinion to the next. Around here with all designers and contractors, it is definitely the norm without any considerations not too.

Just to chime in.

Back to Raleigh where one of the posters had insulated under the slab. This seems unusual. While it may help slightly in the winter, it probably hurts an equal amount in the summer. Either way - not much of an issue but why would you go through the expense. Raleigh is roughly an equal cost heating and a/c (depending on design of house and how you heat) climate. The foam under the slab reduces the natural cooling of the basement by the earth. My walkout basement with 150 sq ft of windows was never above 84 or below 50 before I finished it - and that was before air sealing. I think with air sealing (prior to HVAC), it would have been between 55 and 80 year round.

As far as moisture, I just know that I have been in many moldy smelling basements in the area. My builder's never smell and he doesn't use foam under the slab. I think the key here is ground sloping and siting rather than under floor moisture migration but this issue has to be incredibly local.

I have to imagine in Ontario, some underslab insulation is in order. But, if the space is part time heated, it may not be a big deal. If you have NG and just use the space 10 hrs a week, then we are talking pennies in the grand scheme of things. Sure, it might help a tiny bit with duct losses but again pennies.

"Four feet or more down, the soil is 55 degrees F. more or less year round."

The problem occurs with walkout basements.

The slab is NOT "four feet or more down" along the walkout wall, but a surface slab.

The frost depth will extend under that edge of the slab, thus the requirement to go down to the frost depth for the footers on the walkout wall.

Blindly following rules without understanding the how or why is often a recipe for disaster.

brickeyee said: The slab is NOT "four feet or more down" along the walkout wall, but a surface slab.

That's why you're required to treat the area around the walkout as if it were at grade and then insulate down or out accordingly as I noted to the OP above.

What's always puzzled me are the many retrofitted basement walkouts I've seen around downtown TO that completely ignore that principle and seemingly exhibit no ill effects. All I can conclude is that poor insulation practices allow enough heat to escape to warm up the surrounding soil and prevent frost heave.

Frost Heaved Foundation Source: My Foundation Repairs.com

"All I can conclude is that poor insulation practices allow enough heat to escape to warm up the surrounding soil and prevent frost heave. "

Likely, but a rather expensive heat load.

Is this house condemned or do people actually live in this death trap?

I just received a cal for a walk ut that has a severly cracked basement slab.

The owner claims the builder placed insulation under thelast 8-10 feet of the slab on the walkout wall.

There is a massive crack about 8-10 feet from the walkout wall on the back of the house and some evidence of excessive settling in the basement walls and both floors above (floor to ceiling drywall cracks that are (no real surprise) aligned with a few feet of the basement slab crack.

I told him the first thing to do is find out if the builder's company is 'suit proof' and if the corporation has any recoverable assets.

No sense in trying to get money out of an empty shell.

Worthy:
The contractor did recommend foam on the exterior portion of the walkout foundation.
Another question about waterproofing. He said that because of my soil conditions I did not need waterproofing membrane on the foundation, just the normal waterproofing coating. Does this sound right?

What your contractor is probably suggesting is a thin asphalt based "dampproofing" coating instead of "waterproofing" like cold spray-on rubberized asphalt. In my experience, dampproofing is virtually worthless and waterproofing is cheap insurance. They look similar but the latter is flexible enough to bridge imperfections in the concrete.

The waterproofing membrane should be protected with an insulation board that is porous or has grooves that promotes drainage to a continuous drain below the level of the bottom of the footings.

Anyone have an opinion of using Insul-Tarp instead of rigid foam to save costs?

Besides heating/cooling and water penetration, one other major consideration is condensation. A cold basement slab at 55 degrees will cause warm humid air to condense. Not a good thing if you have carpet, furniture, etc over the concrete.

If your house is well air conditioned to reduce inside air temperature and humidity, it may not be as much of an issue. (Just speculating.) But our house is not air conditioned, and during the summer our newly finished basement gets quite musty. Really the slab was insulated underneath!

Side note, I'm not sure where Lstiburek specifically says that "no vapour barrier should be used between underslab foam and poured floors". In at least one publication, he suggests that vapor resistance of foam insulations may reduce the need for a vapor barrier, but then goes on to discuss the other functions that a poly sheet can provide. His recommendation is that insulation "should be placed under the slab, and the slab placed on poly sheet."

See page 35: http://www.buildingscience.com/documents/reports/rr-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones