Well, that’s a question-and-a–half!
At least we know we’ve decided to use SIPs.
I was going to say, “Let’s start with the EPS vs Urethane question first,” and then I realized that it really doesn’t work that way at all. The first thing to do is put these questions aside. Start with the design of the building. The principle argument here is that one can’t make fully intelligent decisions about component parts of a project out of context and therefore one starts with the forest, not the trees.
The design process is not all intuitive; there is a methodology and a process that enables the experienced designer to work efficiently. Generally, we begin with the development of a program. For a residence this may be a short simple list of requirements including the number of bedrooms, whether the day spaces are arranged along an “open plan” or not, additional space requirements such as a garage, the direction of the major view(s), relationship of the spaces to each other (en suite bathrooms?), site features and requirements…you get the idea. We then create a rough “bubble” diagram that satisfies the program. The final hard-line proposal takes off from there. Now we finally have a design.
Features of the design are then examined to see what bearing they may or may not have on the choice of panel type and thickness. Typical feature impacts on panel selection work out like this:
1. Number of stories.
Generally if a building has two or more stories the surface area of the walls is significantly greater than that of the roof. When we look at heat loss calculations we see that it may pay off to use thicker SIPs for these walls as there may be a significant savings in heating and cooling operational costs that justify a 2” or even 4” increase in the amount of wall insulation.
We take our building design and quickly enter it into our little spreadsheet program (available for free download, see note at end of article) and check this out.
2. Plan efficiency and window trim costs.
On a house of 1400 square feet (just a rule of thumb, not to be taken too literally!) or larger, it is hard to show or justify a significant impact on the plan in changing the exterior walls from 6” to 4” (all such designations herein are nominal) or the other way around. When the plan gets so tight that you find you are taking the gypsum wallboard thickness into account, then it is a good bet that the exterior wall thickness does count. One is trying to hold the plan exterior to a 4 foot module, or be sure that stairwells and corridors maintain no less than 36” clear. Two inches may make or break some tight plans. If, in this case, we find we need the thinner exterior walls, but still find we need a high R-value for the walls, than we should look at urethane core SIPs. Most urethane core SIP manufacturers are charging about the same per R-value as EPS core guys. In other words, a 4” urethane core SIP costs roughly the same as a 6” EPS core SIP and their R-values are also approximately equal.
We may also try to hold down our interior trim costs by not having to buy or install extension jambs on our windows and exterior doors. If we are using drywall returns anyway, it may not figure into the equation.
3. Roof construction and spans.
When we check out the roof designs and their structural supports we usually find that structural span capability controls thickness selection rather than thermal requirements. In a “true” SIP structure, with virtually no additional ridge beams, purlins, or bearing partitions the larger spans in excess of 12 feet usually call for panels 8” or thicker. This tends to rule out urethane core SIPs which are usually not available over 6” thick. In some designs it may be possible to insert purlins to cut the spans, their cost must be evaluated compared to the savings realized by the thinner panels now allowable.
4. Additional services and program requirements.
Some urethane core SIP manufacturers offer additional services in the form of electrical conduit and boxes foamed into place in the panels. This may become a necessity if one can’t find an electrician who doesn’t balk at the way the EPS core guys handle electrical system installation. However, many EPS core SIP manufacturers offer the ability to install additional structural elements within the SIP such as extra blocking or whole flitch plates where they may be required. There is differentiation between the various SIP manufacturers in terms of other services and minor design features such as different structural spline configurations, accessory services like beveled wall top plates, different pre-cutting and installation services, delivery services – boom trucks or large trucks, design and other support services.
5. Panel size.
Some companies offer 8 foot wide SIPs, some only offer SIPs in the convenient 4 foot width. Our local installer prefers to use the jumbos and eliminate as many panel joints as possible. Some jobs don’t have the room for crane access and the whole job has to be assembled by hand with all the SIPs carried up into place. The 4 foot wide SIPs have the edge here.
All of these factors bear on SIP selection, both core type and thickness, but you can see that both the design and site conditions must be known before proper selection can be made. The more we work with SIPs and investigate the various offerings from different manufacturers, the more we realize that best value is delivered to our clients by checking out all the different manufacturer’s options against the particular design, the budget, and the site determinants.
When we really get sophisticated and are working in an “inspired” fashion, the process become interactive and synergistic, with early decisions about SIPs informing the design and visa versa. This more dialectic approach ultimately yields the best design. It is not the simple, clear linear process outlined above, in fact-it can be quite “messy” with a lot of back-and-forth – but it yields a more tightly integrated solution.
Now that we’ve made the process more complex I hope you’re not discouraged from entering fully into the fracas. A better design with an intelligent selection of SIPs is now possible. Who knows, you may wind up with 4” urethane walls and 10” EPS roofs!