When Cheryl and Eric Cosway decided to build their dream home in the hill country south of Austin they teamed up with Bill Moore, Austin’s original green builder. Their passion for building green, combined with Bill’s technical knowledge, led to a truly sustainable home – built with sensitivity to the health of it occupants, use of energy, cost factors, footprint on the land, and to its impact on the environment as a whole.
This is a blog series on the green elements of this “5-Star Energy Rated” house. Thanks to Cheryl Cosway and Bill Moore for sharing their thoughts.
Roselind: I understand that you used a SIPS roof ? What is this? Why was it selected?
Cheryl: Yes, we decided to use a SIPS roof, which stands for Structural Insulated Panels. These panels are usually made from expanded polystyrene sandwiched between two sheets of OSB. Bill Moore recommended that we consider the SIPS roof because it is extremely well insulated and sealed. There is also very little waste since the panels are pre-cut by the fabricator. The SIPS roof appeared to be our “greenest” option.
Bill: Every custom job has its own set of design variables. The site location dictates the view, the direction of cooling winds, and how the passing of the sun relates to the house design. In this case the view was to the north and the breezes came from the south.
So, the roof was designed with a shallow pitch that mostly faced south. The south facing roof design prepared the home for future photo voltaic panel placement. It also shades the interior from the summer sun by putting the ends of the house on the east/west axis.
When viewing the plan, I noticed how the simple roof design and the vaulted ceilings were well suited for a SIP (Structural-Insulate-Panel) system for the roof. This system was developed to reduce the energy load of the house, and shorten construction time.
I prefer to only use a metal standing seam roof on these panels because they are a structural unit. If the roof leaks, the structural strength can be compromised, and replacing a panel is extraordinarily difficult. But the standing seam metal roof was already a part of the design parameters, so that was not an issue.
Also this site is on a bluff with the potential for severe wind storms. This interlocking roof panel system is designed for high wind speeds. After Cheryl and I discussed the benefits of this greener approach, she agreed to go this route.
Roselind: So, the long, simple roof design made it a good candidate for a SIP system. Let’s talk about the panels themselves. Cheryl said that SIPS are made from rigid foam insulation sandwiched between two sheets of OSB. OSB is oriented strand board – made from wood strips laid in cross orientation, similar to plywood. The foam insulation core makes it very efficient in keeping a stable temperature inside the house.
What about the cost? How do you compare between SIPS and conventional roofs?
Bill: As is usually the case with comparisons – it is a difficult question. The SIPs panels are expensive and were ordered months in advance. We needed a forklift to unload the delivery truck, and we needed a small crane for a couple of days to put them up. But, it only took a few days to install a very large roof system. And, all the high value foam insulation was part of the system, with no waste and much less labor. All total, it may have cost a few thousand more total, but the energy savings should make up the difference in a few years with a return of continued savings after that.
Roselind: I can see that it is not possible to make an apples-to-apples comparison. In a standard roof system, you would buy lumber for roof rafters or trusses, plus a layer of plywood over the rafters, plus the insulation on the floor of the attic. In the SIP system the structural and insulation components are combined.
Although SIP’s provide excellent insulation and stability, they may not work well for complicated roofs. In order to realize the maximum benefit of the SIP system, you would build a simple pitched roof, with a vaulted interior ceiling, covered by standing seam metal. The panels are pre-made, so they can be put up faster, but there is some cost for machinery to lift them up, since they are heavy. Also, I imagine there is a learning curve to installing the panels, so it would be important to use a seasoned contractor.
