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Finding a natural solution

Building the NewenHouse Kit Home: Choosing and Installing Slab Insulation

Sonya NewenhouseSonya Newenhouse, Ph.D. is an eco-entrepreneur who enjoys providing practical and creative solutions to help individuals and organizations live and manage green. Her firm, Madison Environmental Group, provides LEED green building and sustainability consulting services. She is also founder and president of Community Car, a car sharing organization in Madison Wisconsin. Currently she is developing NewenHouse, a business that will provide super-insulated sustainable kit homes. 

While I was at the Passive House conference, the utilities were brought into the site. I was disappointed to miss this excitement. The NewenHouse is being built next to our current home in Viroqua, a small town of 4,500 in southwest Wisconsin. Our house is in town, so it’s affordable to bring utilities into the building site which is a 44-foot-wide lot where Cecil and I held our wedding reception in August.

Upon returning from the conference, Carly Coulson and I had some productive phone meetings and confirmed that we needed to reduce the size of the slab (for an interior room dimension of 22-by-22 feet). It was poured in the nick of time on Friday, November 19, just before the weather turned wet and cold. Prior to pouring the slab, the weather was mostly sunny and above 35 degrees which was perfect for installing the insulation under it. We opted for a 7-inch monolithic slab so that it would be easier for the concrete crew to install the required 12 inches of insulation under the slab. Our first plan was designed with a 4-inch floating slab with 12-inch thickened edges. This configuration is more labor intensive and a bit more complicated for installing insulation under the slab and edges. The thicker slab also gives us more thermal mass to store passive solar heat. The slab prep work and pouring the slab reminded me of a large sewing project, where prep work of careful measuring, cutting material, and taping of the seams takes time while the last stage of pouring of the slab (similar to machine sewing a carefully pinned pattern) went quickly and—voila!—it was done. 

Sonya Newenhouse with pipes 
Sonya stands on new insulation. Photo Courtesy Sonya Newenhouse. 

Henry Hoefer’s concrete crew did an excellent job.  I worked beside them taping the 4-by-8 foot sheets of Expanded Polystyrene (EPS) insulation together. We special ordered a high density EPS foam to be cut at 4 inches thick rather than a typical 2-inch thickness. This way we only needed to place and tape three sheets of insulation on top of each other instead of six sheets.  EPS foam uses much fewer ozone depleting chemicals in its manufacturing process than Extruded Polystyrene (XPS) foam, which is why we chose it. (Check out this article to read about the comparison of EPS to XPS foam.) 

At the Passive House conference I also learned about the importance of air sealing. Air tightness is one of the three Passive House performance measures, thus it is critical to do the best job you possibly can as you will not have the chance to be certified if the blower door test (an energy performance test conducted by an energy specialist) does not pass. The first blower door test is done prior to insulating the walls, so you have an opportunity to fix the air leaks. The final blower door test needs to prove that the performance is equal to or better than 0.6 air-changes-per-hour. (Check out this article for more information on the Passive House energy model and performance measures.)

Sonya demonstrates how she and her team insulated under the slab with 12 inches of EPS foam, taping all of the seams. Video By Naomi Lipke. 

It was fun creating an airtight seal under the slab working alongside Henry’s crew. Getting a straight tape line is not as easy at it may seem. We alternated the direction of the insulation between each of the three layers of EPS, and I took the opportunity to add additional insulation to all of the plumbing and exhaust vent pipes. Flexible pipe insulation is not readily available in a 3-inch diameter, so I taped a few together to do the job.

The walls of the house will be insulated with dense-pack cellulose. We are choosing dense-pack cellulose because it is made with recycled newspaper; it provides the ability to build a breathable wall system; it’s affordable; it should be easy to separate and recycle if and when the house will be deconstructed; and it has a lower carbon footprint than foam. If the cellulose settles, you can add more cellulose to the walls. Because the dense pack cellulose has a lower R-value than foam, you need a thicker wall system. That’s why we designed a 16-inch thick wall to give us an R-value of approximately 57.

There is so much to learn about insulation, moisture issues and the long term effectiveness of the choices we make. All new homes and remodels are in essence part of a building science experiment. Today we can only try hard to make good decisions based on information we have available, move forward and continue learning and monitoring our work.