I'm reading that the U.S. Department of Energy has developed a list of recommended insulation levels for different climate zones. They provide an R-Value for the floor, in zone 4, of R-Value 25-30. That seems pretty high for a slab foundation? That is like 5 or 6 inches of rigid insulation. What am I missing because most builders do not insulate the slabs at all, I figured an R-10 foundation would be pretty good. This is my source: http://www.greenbuildingadvisor.com/green-basics/insulation-overview

The R value for a floor in that article is for a floor over unheated space NOT for a slab.  If you read the paragraph under the table in the referenced article, it says Betsy Pettit and Joseph Lstiburek recommend R10 for basement slab insulation.

Thank you NH Homeowner. I am new to this stuff and some things come across confusing. I appreciate the feedback.

Please also keep in mind James that the R10 recommendation is for unheated basement slab. For heated slab-on-grade, it is typically recommended that the MINIMUM under-slab insulation R-value be:

Rm = 0.125 x (Ti - To)

where:

Rm = Minimum R-value
Ti = Indoor Design Temp (F)
To = Outdoor Deign Temp (F)

So a 70F Ti and 0F To would call for R8.75 MINIMUM. I would consider multiplying this typically recommended MINIMUM R-value by 1.5 - 2.5 to determine the actual installed insulation R-value for an energy efficient home (i.e., R13.125 - 21.875).

Thank you sailaway. That makes a lot of sense. If I may be ask another question, the basic layers of a basement slab is thus: dirt, gravel, EPS, & then slab? And if so, EPS is tough enough to withstand the weight of the slab, furniture & be exposed to the gravel, dirt for the life of the house?


If that's correct, then I've studied well on this site, if not, please guide me!!


Thanks!

Yes James, you have apparently studied well and earned an A+!

You should also have some vapor barrier/retarder under the slab and it often debated whether this should be placed below or above the EPS. We tend to favor placing vapor barrier (10-15 mils meeting ASTM 1745) on top of the EPS as this forces the concrete to absorb all the water during the concrete curing process. Placing it on the bottom can sometimes result in water collecting in between the vapor barrier and the EPS that can take a long time to vaporize away and this can cause problems depending on how you will finish the concrete surface. Dana and others likely has other good guidance to consider on this subject as well.

Interesting. Thanks sailaway. It's a shame most home builders don't insulated more than they do. The home I'm currently in was advertised as a "energy-efficient house" but it's basically bare bones and terribly insulated. I am still grasping the whole vapor barrier/retardant stuff but I got a good gist of what your saying. Thanks for the help.

Glad I could help. Yes, advertising is often very deceptive…Caveat Emptor!

The accepted "best practice" for insulating below a radiant heated basement slab or slab-on-ground here in Minneapolis is 2" of 15 psi XPS. We will use more at the perimeter --2x is plenty--for slab-on-grade residential designs, but over-insulating the "field" of any slab in the lower 48 is one of the least cost effective ways to "save fuel" or dollars. The return on investment is poor and the payback is all intents and purposes, forever.

This makes sense, since the deep ground temperature in Minneapolis is 47°F since the temperature differential driving heat flow is calculated by the difference of slab vs ground. In the case of my own garage that difference is 6° F during design conditions of -13°F. Doubling down on under-slab insulation out-of-hand, is neither good science nor good advice.

Caveat Lector!

Right, states that don’t have license requirements only have “best practice”. Unfortunately, the companies that “practice” in these states are often far from competent.