Building My Net Zero Energy Home: Key Early Decisions

http://www.motherearthnews.com/green-homes/building-my-net-zero-energy-home-key-early-decisions.aspx

When I decided to rebuild my home at The Evergreen Institute in east-central Missouri, which as many readers know, burned in a fire in January 2011, I had to make some decisions before I could begin work with my architect, James Plagmann of HumaNature Architecture. 

The first decision was whether I would retain the existing foundation or start anew. In my case, the choice was made by the fire and the building department. As it turns out, the south wall of the foundation had been so badly damaged in the fire that it had to go. That was the building department’s recommendation.

To create a net zero energy home, I knew I had to pour on the insulation. So, I decided to build with insulated concrete forms to achieve an R value of around 30. I also decided to place 5 inches of extruded polystyrene under the slab to achieve an R-value of around 30. At this writing, we’ve already poured the foundation walls.

insulated concrete form 

I next turned my attention to the design of the exterior walls. To achieve net zero energy, I decided on R-50 in the walls. That limited my options considerably. 

The first option was straw bale. While it is one of my favorite building materials, straw bale seemed impractical. An 18-inch wide straw bale has an R-value of only around R-32. To achieve R-50, I’d have to build a second wall out of two by fours and install additional insulation. I’d need another 6 inches of wall depth. A 24-inch wall seemed absurd, so straw bale was eliminated from the mix. Besides, my county prohibits straw bale construction.

I then began to compare three other options: insulated concrete forms or ICFs, structural insulated panels or SIPs, and a super-insulated double 2 x 4 wall. 

As just noted, I used ICFs for the basement, so it would have been an excellent option. Problem was, to achieve R-50 I still would have had to pile on a ton of insulation. Besides, I didn’t want to use that much concrete, which has a pretty high embodied energy.

Structural insulated panels seemed like a great option, too, but as in the case of straw bales and ICFs, I’d very like still need to add more insulation to achieve the R value I was after. Furthermore, I received a bid from ICS, a Colorado-based manufacturer of an excellent ICF. The bid came in at over $22,000. I’d need to rent a small crane to install the ICFs for a day or two, and hire a couple workers to help me install them. Furthermore, I’d have to ship the ICFs by truck around 800 miles, burning a considerable amount of fossil fuel.

Another serious issue I had with SIPs was bridging loss around all the windows and doors and at the top plate and bottom plate. We would need to install solid dimensional lumber in all of these locations and wood as an R-value of only 1 per inch. In this home, I wanted to eliminate or at least greatly reduce bridging losses to achieve net zero energy.

My third option was to build a double two by four wall. I had remodeled my classroom using this technique and was pretty impressed with it. What made this option more desirable to me was that I can buy locally produced 2 x 4s, and create a wall with no bridging loss around doors and windows or at the top plate and bottom plate. I could also use advanced framing techniques to reduce lumber use and costs.  Furthermore, I could use cellulose (recycled newsprint) to insulate the exterior walls.

So, this seemed like one of the greenest options and most effective options.

To create an airtight wall, I opted to blow in two inches of liquid foam insulation with an R-value of 6.8 per inch. With 12 additional inches of cellulose, I could easily achieve an R-value of 52.  By my estimates, the wall construction and insulation would cost me about $12,000 without labor. Labor could add another $3000 to the cost, so it turned out far cheaper than SIPs.

In my next blog, I’ll discuss my choice of roof framing…


Contributing editor Dan Chiras is a renewable energy and green homes expert who has spent a lifetime learning life’s lessons, which he shares in his popular blog, Dan Chiras on Loving Life. He’s the founder and director of The Evergreen Institute and president of Sustainable Systems Design. Contact him by visiting his website or finding him on .