Building SIPsmart: Considering Concrete and Solar Hot Water

Reader Contribution by Michael Morley
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Michael Morley will post regular updates about his progress building a unique green home with structural insulated panels (SIPs). For an introduction to this project, read Building Smart with SIPS.

There are several images that accompany this post. The photograph is of the completed building pad marked out for the trench footings. This was one of a series of 150 pictures produced by a Bushnell Trail Cam that can take time lapse pictures on a programmed interval. I set this camera up to take two pictures every eight minutes if there was motion to trigger the camera at the site. This worked great and during construction we will have a record of the installation of all materials. These stills will be assembled into a video, which we hope to share with you readers. The other image is a Sketch-up rendering of what the completed ARCX building will look like

The building site at 13th and New Jersey Street has more of a slope across the lot then it appears. We had geotechnical engineer, Dean Grob, monitor our work as we brought in engineer to fill to raise the site to a level pad. Mack Grant brought in his equipment and spread eight truckloads of crushed, recycled concrete as our base material. The topsoil and vegetation that Mack removed was then spread around the perimeter to contain the fill and provide a rough grade for the finished building. I spent several hours laying out and marking the building perimeter for the trench footings that will be going in soon. We’ll be using a 16-inch-wide by 36-inch-deep footing with 2 inches of rigid insulation installed before the pour around the exterior of the footings.

It looks like we have a dry week coming up, and we hope to have Burlinggame Concrete Company trench and pour footings this week. After that I will go in with a helper and build the wood forms for the concrete slab.

While we’re waiting for the snow to melt and the concrete to be poured, Rebecca, Jeremi and I have been trying to resolve several issues. The first issue is how to accomplish the colored concrete that we want to use as the finished floor. One option is to have the color integrated into the batch for the slab and pour and finish in the typical manner. This involves an additional charge of about $1,000 and then we would need to protect the floor during construction. The other option, which Tenants to Homeowners has used on various projects, is to pour and finish the concrete in the typical manner and then etch, stain and finish the floor as part of the finishes in the project area. The problem they have had in the past is that the finish, usually an epoxy, is relatively fragile and can be damaged with sharp objects and general wear and tear. What Rebecca and Jeremi have decided to use is a moss green color integrated into the concrete. This will be a durable, attractive finish, and I have no problem with this approach.

The other issue we are dealing with is the heating, cooling, ventilation and domestic hot water for the building. Originally, I proposed using an 80 gallon hot water tank with a copper coil heat exchanger in the tank. This would be combined with two 4-by-10-foot flat plate solar collectors on the roof. The tank was to have a backup heat source with an electric heat element in the lower part of the tank. We would then take the heated water and pump it through a loop of PEX tubing embedded in the concrete floor. I liked the idea because of the green elements of the solar hot water and the ability of the building to retain heat. I asked various people in the heating and cooling business for their opinions.

Paul Oehlert of Oehlert Heating and Air had recently installed a similar system at his home south of Lawrence. His experience was that in the winter season, with a much reduced solar angle and many, many cloudy days, the system did not provide much solar gain. When the system operated on its electric backup for much of the heating season, it became a very expensive option. In the summer months, there was way too much solar capacity from the panels and no need for the radiant heating capability. The basic system designed for the building was for an air source heat pump with 1 1/2 tons of capacity and a 16 SEER rating. The system would provide all of the heating and cooling needs of the building for under a dollar a day. I am reluctant to try any other system that would ultimately cost more to provide basic heating and cooling needs. Rebecca and Jeremi agreed with this assessment, and we settled on the high-efficiency air source heat pump as our primary heating and cooling.

That left water heating to be determined. I felt reasonably comfortable with the solar setup as described earlier for domestic hot water only with some reservation about the electric backup costs. Jeremi’s research came up with a new, but highly promising approach. It is called a heat pump water heater. A standard 50 gallon insulated water tank has a small air source heat pump sitting on top of the water heater. It draws heat from the ambient 70 degree air and converts that to heat applied to the storage water tank. Although not much history is available for this type of system, it claims a 50 percent reduction in cost to heat water. From what I can read, this system has a lot of promise, and I agree that this is the way we will go.