The finished concrete floor after the application of stain and grouting of the score lines.
As mentioned in article 1, thermal mass walls and floors help to heat and cool the home. The floor of this house is exposed concrete that has been acid-stained to a stone-like patina. Since the use of concrete should be minimized because of environmental considerations (it is energy-intensive to manufacture), careful thought was given to the pros and cons of various types of flooring. Some of the choices considered were: a concrete floor with excellent thermal mass properties that would also be a good conductor of heat from the embedded hydronic heating system (a backup heating system is required by code); an earthen floor that has very low-embodied energy; a brick-on-sand floor with the radiant heating in a sand base below the bricks; flagstone with a concrete or sand base; or a wood or carpeted floor.Wood flooring and carpet were out of the question due to the lack of thermal mass. Carpets wear out and thus create a disposal problem and long-term costs. Brick-on-sand would compromise the transmission of heat from the heating tubes to the dry sand and then to the bricks. Brick-on-concrete would work but would still involve the use of some concrete in addition to the brick. Flagstone had the same problem as the brick-on-sand and, in my opinion, is a bit more challenging to seal in an environmentally sound and aesthetic manner. An earthen floor would be a great alternative. However, due to the small footprint of the home and with 80% of the floor area being high traffic/high wear, this could have become a greater maintenance problem. An earthen floor, with the addition of aggregate, can perform quite well as a thermal mass. Many individuals, however, are not receptive to earth as an alternative flooring.
The concrete was poured in sections, scored, and hand finished to the desired texture.
In conclusion, concrete was chosen because it is commonly used as a hidden base under a wide variety of finished floors. It also provides a great thermal mass, is visually pleasing, is easily maintained, and will not wear out, thus eliminating replacement surface materials for the life of the building. The downside is that it is higher in embodied energy. Looking at the overall energy efficiency of the building, I selected this choice as it followed our rule that, if concrete was used, it had to provide thermal mass, good aesthetics, extreme durability, and structural integrity. You may come to a different conclusion based on your needs. By adding 10-25% fly ash to the mix (a waste product from coal-fired electrical generation plants), the concrete becomes more environmentally sound. The important point is to look at all of the options, with their pros and cons, and then to make a decision based on your best judgement. I definitely would have used earthen floors in low-traffic areas if the home were to have multiple bedrooms and closets.
There are more environmentally sound alternatives to the acid staining of concrete. In hindsight, I would have used an iron-based wash (the iron component that is used as an agricultural additive to soil) or would have added integral color to the concrete mix (although I rarely consider this choice the most aesthetic). That said, it is hard to beat the depth of color and beauty of an acid-stained concrete floor.
In the next column, I will talk more about the construction process.
Update: Part 9 is now available.
[This has been a guest post by Ted Owens, a green designer and filmmaker. More details on green building design and construction can be found on his website and in the Building with Awareness DVD and Guidebook. -Ed.]