Hi all.. I'm getting closer to building my little house in WI (zone 6) with R40 walls, R60 ceiling, on a slab. How much sub-slab insulation should I use? Two inches are specified now under the slab and on the inside of the 4' deep frost walls. Am I going to have problems with damp (from condensation) or cold floors? should I go to 3"? Any thoughts appreciated! -m

Use a minimum of 4" EPS. 6" would not be out of the question. It is critical that the floor not touch the foundation so use 2" foam for a thermal break.

should I go to 3"

If I had to do it again, I'd use a minimum of 4".

2" of what?

Will the slab be radiated?

Sub-slab insulation should be specified per design. If you have adequate insulation to the frost line using a standard foundation then 2" of XPS is the standard in most of the mid-west. The ground temperature in in the high 40's so more insulation under the slab is very low on my list. I would argue the perimeter insulation makes sense, especially to the frost line.

So there seem to be varying opinions......??? Any experiences with too little insulation? Thank-you -m

IRC 2012 code min for sub-slab insulation in zone 6 is R10 down to 4', with another R5 edge for heated slabs.  MN code is based on IRC 2006, but with many modifications to the IRC about foundation insulation- I'd have to look it up.

There's a long term rationale for R10 full-coverage under slabs in zone 6 even without heating the slab (see Table 2, p10.) With a radiant slab adding another R5  wouldn't be outrageous. If you layered 3" of Type-II EPS under an inch of XPS (better staple retention if you're stapling the tubing to the foam) you'd be at about R17.5 for about the same cost as R15 in XPS (3"). 

In 50 years after most of the blowing agent has escaped the XPS will have the same R/inch as EPS, and the stackup will settle in at about R17, whereas 3" of XPS would end up somewhere beteen R12.5-R13 over the long haul. If you can find a source for reclaimed roofing foam you can go north of R25 for less cost than 3" of XPS. (Stick to EPS & XPS when using reclaimed goods, and NOT polyiso, which can slowly take on water over time.)

XPS is blown with a hefty amount of HFC134a (automotive AC refrigerant) which has a huge global warming potential (about 1400x CO2), whereas EPS is blown with pentane (about 7x CO2), so whenever virgin-stock XPS is be swapped out for EPS or reclaimed goods it's a far greener approach.  Low global warming blowing agents for XPS are on the market (notably HFO1234yf at about 4x CO2, under a couple of different trade names), but SFAIK none of the US vendors of XPS are using it.  Unlike HFC134a the low GWP blowing agents are somewhat flammable, but contained inside the mostly closed-cell structure of XPS and under 4" of concrete there's no hazard to using it in building applications. (It's among the refrigerants approved for use in Europe, where R134a has been banned in air conditioning.)

Thanks for the link. It sounds like 2" would do the trick. I've also read (from you and others) that I should try to use EPS. In that case I'd try to get 3" under the slab and on the perimeter...

As for the poly, it sounds like it should go on top of the foam. By the way, I'm not using in-floor heating, but rather a mini-split.

Thanks.
-m

R-10 XPS is the standard for radiant floors (and heated driveways here in Minneapolis). Mostly due to availability.

You have to be careful of reclaimed XPS as it is unlikely to sport any information pertaining to its density. We like to use 25 psi for most of our work, though I would not be quite as particular in a basement for instance.

I rather like the idea of XPS over EPS, but you have to buy the short staple to make it work.

Dana has you covered and your plan sounds good to me. If your were doing HR heating, there is an equation that defines the minimum slab insulation that should be used and folks than consider economics to decide how more above minimum they want to pursue.

if you're feeling adventurous, you can have 300 mm eps like we have in Sweden :)