Wednesday, September 9, 2015

Super Insulated Slab

Super Insulated Slab

Insulating a basement slab is incredibly uncommon. It is hardly ever done unless the code requires it. Walk out basements and radiant floors are about the only time you will see insulation under the slab - and you would be lucky to see more than R-10 (2" of extruded polystyrene (XPS) rigid foam).

An uninsulated slab will be sucking the heat out of the house all the time (since the ground temperature is semi-constant (between 40-50 degrees all year round).

Before I go into detail on how we did it, I will talk about the most common question... Why?
Passive House is as much of a comfort standard as it is an energy efficiency standard. All interior surfaces of a Passive House will be within 6 degrees of each other all year round - that includes the basement floor. The basement will be as comfortable as the bedrooms.

Rochester NY Soil Temperature  6 feet below grade can vary throughout the year from 57 degrees in the summer to 37 degrees in the winter. An uninsulated slab in the winter can be as cold as 40 degrees!

While it is true that hot air rises, heat doesn't necessarily rise. Heat (Energy) moves from high to low thanks to the 2nd Law of Thermodynamics. Your hot coffee mug will make the counter warm. Your house will lose it heat to the basement slab. While this may not be a large heat loss in a code built house, it will have a huge impact on a Passive House.

How much insulation is needed under the slab? There are a few variables.
  • The depth below finished grade (The soil temperature changes the closer to grade).   
  • Ratio of the area of the slab to conditioned floor area (a ranch slab will require more insulation than a 2 story house slab)
  • R-value of material you will be using
  • Available thicknesses of material you will be using.
In our case we will be using 8" of high density expanded polystyrene (R-38). We came to this thickness based on ability to do 2 layers of 4". 
  • 4" was a manageable thickness to work with, rigid enough to withstand minor "bridging" over the crushed stone. 
  • staggering seams allows for minor imperfections to be masked
  • the EPS foam comes in 36" x 36" x 96" cubes - so to maximize yield, 2 layers of 4" was less expensive than 1 layer of 8" due to waste created
Here are the details we were working with:

That is right, we are installing 8" of rigid foamboard under the basement support posts. We can do it since the compression strength of the foam is higher than the soil under it (If you soil can hold the weight, so can the foam)

We prepped the area where the basement posts were going to go
We installed the 8" of rigid foam under where the posts are going to go

We installed the 15 mil radon barrier (and taped the edges to the ICF - with a little help from a Pro Clima Tescon Primer)

A quick note on the radon barrier: we chose this barrier over a standard 6 mil poly for a couple reasons.

  1. High levels of radon have been found within a mile or so of the property, so it is more of a precaution
  2. The barrier comes cut to the custom size (150'x200' max) - so there are no seams
  3. 6 mil poly has a perm rating of  <0.1 perms, the radon barrier has been tested to 0.02 perms
  4. It also has a much higher tear and puncture resistance, so it will last in the field 
  5. It only added a few hundred dollars to the job.
Next we began the edge foamboard - by installing the edge 1st we eliminated the need to trim the 4" under slab foamboard to fit. 

Now we started laying down the sub slab rigid foamboard one layer at a time

We did leave the areas for the concrete piers free of insulation and built the forms out of pink XPS foamboard

With both layers of 4" rigid insulation installed and the wire mesh laid down the basement looked something like this

Now the basement slab is ready to be poured

Now we have a basement!

Next we will be water proofing, back filling and framing will be starting soon!  I'll keep you posted!


  1. It's really moving along! So happy to be able to keep up with progress and learn a little something along the way!

  2. Thanks for this great article! I was thinking that passive houses didn't have basements, usually, and am pleasantly surprised that that can, in fact, have them (and be functional, insulated and comfortable, too). Bummed I didn't get to come out for the open house but I haven't exhausted this site's information, either, so I'll keep learning til the next one. Thanks!

    1. Another a walk out basement possible in a passive house or would the large area of windows/doors be prohibitive? Thanks for the info!

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