Originally written: Jun 2003
Most of the questions that get asked about SIPs are set in the context of how are SIPs different (or the same) as conventional stick construction in a very strict comparison fashion - almost a panel-for-2x4 direct substitution. Many SIP-experienced people attempt to deal with this as it is delivered, knowing that the author of the question expects the answer to fit into his or her paradigm of simple substitution. An example of this would be like asking if cherry finish flooring might be a preferred substitution for oak strip finish flooring. I can sense the uneasiness in the responses of those who are knowledgeable about SIPs. Just as they know it is usually best to respond to a question in a way that that is expected, the truth is that SIPs are different enough in their entire aspect from stick framing so that the example is more like asking, "Is tile an optional substitution for an oak strip finish floor?"
Let's stay with this tile-oak thing for a little bit. .....
Let's stay with this tile-oak thing for a little bit. .....
Oak strip finish flooring usually presumes a substructure of plywood subflooring over a wood joist system, over a crawl space, full basement, or floor below. If we switch to tile, it is true that we may achieve "satisfactory" or expected results by applying the tile in an approved (Tile Council of America specifications) manner over the same structural system, but the best substrate for a tile finish would be a concrete slab. If this is recognized, and the floor structure is to change, this opens the door to all kinds of other changes that go back up a decision tree so that major design elements and program benchmarks are revisited. The tile main floor may now become a finish to a slab-on-grade and the passive solar performance may so improve from this revision that the HVAC system should be radically downsized. As whatever activity in the former basement has lost its subterranean location, the function will have to be provided for somewhere else and the floor plans and site plan will change. So we see that the floor finish is integrated with the structure that supports it and even has ties to the thermal performance and the floor and site plans! So, not just a simple substitution at all. This is typical of the cyclical or helical design process where, in a thoroughly integrated design, apparently small pieces of the design are changed which results in a re-examination of the whole scheme.
It is true that I have been discussing how this works with an integrated design; just what is this so-called "integrated design?" The finished product, in this case a house which is the successful outcome of an integrating process, has the major subsystems tied to one another in such a fashion that they all work together efficiently and economically, just the way Mother Nature does it. Hopefully the result is also aesthetically pleasing, just as it always is when Mother does it. In the example above, the tile finish and the slab are "analogous" materials that work extremely well together both structurally and thermally. Where the thickened edge of the slab may also serve as the "foundation," then an additional saving is picked up with this system as well. Of course, much of the initial construction cost saving will be lost when the above ground portion of the building gets enlarged, but then again, maybe it doesn't. Perhaps the garage "attic" is designed to accommodate the function that was originally supposed to go in the cellar.
Whew! This is getting complicated! Well, yes it is, but it is worth the effort because the end product promises so much more. Any additional front-end cost incurred is usually more than offset by significantly lower operating and maintenance costs. Additionally, this kind of building will also build and retain higher value in the market place.
So, how does this play out with SIPs?
SIP structures are fundamentally different from stick structures in two immensely important ways, and these qualities should be fully exploited by the astute designer.
Firstly, SIP structures are "thin shell" structures that work by distributing large point loads throughout their entire surface so that the stress at any one specific location is very low, usually much lower than that which the SIP materials are capable of safely handling. Stick structures collect and distribute loads along the component members, each one of which must be sized or engineered together with its connections to its adjacent members. Conventional stick framing is mostly like post and beam construction, where the beams are analyzed primarily for their resistance to bending, and posts primarily for their resistance to axial compression. SIP walls act more like giant box beams and are capable of spanning very large openings or serving as great cantilevers. Most conventional openings will require no headers in SIP construction. Taking these features all together, a designer should be able to come up with forms and designs that are pretty exciting, or if staying with a traditional stylistic vocabulary, do so at greater savings by eliminating many of the usual large framing members that are a necessity with stick construction.
Secondly, SIPs are their own vapor/air barrier. Stick construction, which has recently been forced to meet modern thermal performance benchmarks, is showing itself to be fraught with inherent problems. The stud bay cavities are excellent places for condensation to occur and all those joints and seams are a devil to properly seal. The time and material that have to be "remedially' applied to a stick structure in order to upgrade its thermal performance to required standards is considerable. They are also difficult, if not impossible, to inspect for compliance in this regard. Here again, the designer may exploit this advantage of SIPs by having conditioned spaces over garages or "the outside" without fear of "drafty" floors. Additionally, buildings with high performance envelopes lend themselves to very different HVAC system strategies that are more economical to install as well as operate.
These are only two ways that SIPs may have a large positive impact on building design, but the most important point remains that SIPs should be seen as a subsystem component -- a very important one, the structure! -- to an integrated design where each component is interdependent on the others.
It is true that I have been discussing how this works with an integrated design; just what is this so-called "integrated design?" The finished product, in this case a house which is the successful outcome of an integrating process, has the major subsystems tied to one another in such a fashion that they all work together efficiently and economically, just the way Mother Nature does it. Hopefully the result is also aesthetically pleasing, just as it always is when Mother does it. In the example above, the tile finish and the slab are "analogous" materials that work extremely well together both structurally and thermally. Where the thickened edge of the slab may also serve as the "foundation," then an additional saving is picked up with this system as well. Of course, much of the initial construction cost saving will be lost when the above ground portion of the building gets enlarged, but then again, maybe it doesn't. Perhaps the garage "attic" is designed to accommodate the function that was originally supposed to go in the cellar.
Whew! This is getting complicated! Well, yes it is, but it is worth the effort because the end product promises so much more. Any additional front-end cost incurred is usually more than offset by significantly lower operating and maintenance costs. Additionally, this kind of building will also build and retain higher value in the market place.
So, how does this play out with SIPs?
SIP structures are fundamentally different from stick structures in two immensely important ways, and these qualities should be fully exploited by the astute designer.
Firstly, SIP structures are "thin shell" structures that work by distributing large point loads throughout their entire surface so that the stress at any one specific location is very low, usually much lower than that which the SIP materials are capable of safely handling. Stick structures collect and distribute loads along the component members, each one of which must be sized or engineered together with its connections to its adjacent members. Conventional stick framing is mostly like post and beam construction, where the beams are analyzed primarily for their resistance to bending, and posts primarily for their resistance to axial compression. SIP walls act more like giant box beams and are capable of spanning very large openings or serving as great cantilevers. Most conventional openings will require no headers in SIP construction. Taking these features all together, a designer should be able to come up with forms and designs that are pretty exciting, or if staying with a traditional stylistic vocabulary, do so at greater savings by eliminating many of the usual large framing members that are a necessity with stick construction.
Secondly, SIPs are their own vapor/air barrier. Stick construction, which has recently been forced to meet modern thermal performance benchmarks, is showing itself to be fraught with inherent problems. The stud bay cavities are excellent places for condensation to occur and all those joints and seams are a devil to properly seal. The time and material that have to be "remedially' applied to a stick structure in order to upgrade its thermal performance to required standards is considerable. They are also difficult, if not impossible, to inspect for compliance in this regard. Here again, the designer may exploit this advantage of SIPs by having conditioned spaces over garages or "the outside" without fear of "drafty" floors. Additionally, buildings with high performance envelopes lend themselves to very different HVAC system strategies that are more economical to install as well as operate.
These are only two ways that SIPs may have a large positive impact on building design, but the most important point remains that SIPs should be seen as a subsystem component -- a very important one, the structure! -- to an integrated design where each component is interdependent on the others.
Originally written: Jun 2003
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