A client comes to see you. He may be a developer who wants three or four models for his 60-unit subdivision, a manufacturer with a tricky commercial project that needs some engineering, layout and detailing work, a representative from a church building committee, or a young couple with hopes for their dream house. In all cases, if you say, “Yes, I’d be glad to help out and do this design project for you - and how nice to hear you want to build with SIPs, I was just going to suggest we use them in this case,” you are accepting responsibility for proper specification of SIPs.
What does accepting this responsibility mean? It means that one should have a thorough understanding of the properties of the material so that only appropriate applications are utilized. By “properties” we mean the physical characteristics, the structural/mechanical aspects as well as perm ratings, resistance to ultraviolet light, ozone exposure, construction loading and handling requirements, manufacturing tolerances, and familiarity with accessory components - fasteners, splines, foams, caulks and assembly sequences. This is a lot to integrate, but it’s no more than what we deal with for all the other construction materials used all the time. (All the issues around cast-in-place reinforced concrete make me dizzy just thinking about them!) But deal with them we must or we are sure to suffer from either short term problems that might be “just” cosmetic, or long term problems that might cause structural failure. Essentially, we must be aware of the physics of the entire situation. We must be knowledgeable of code requirements, but also mindful of contextual issues that may not be anticipated by the code and arise from user patterns (normal or deviant, but foreseeable), or extreme conditions that arise from micro-climatic abnormalities (carpeting gets moldy and sick occupants – if they don’t sue - stop recommending you).
Buildings are different from how they were a generation ago. They are different because they are responding to the market that is requiring them to be different. Houses are now larger, but are also loaded with more appliances and features that tend to con-tribute to the increase of humidity levels. We take our cooking seriously; we do more of it and a lot of it on the stovetop. (Pasta rules!) We have hot tubs and whirlpools, steam showers and saunas. Greenhouses abound and so do active wood stoves and fireplaces. We like to think that all the products of combustion (of which water vapor is a large component) go entirely up the chimney, although this may not be the case. We are also more active, so we take more showers. Some luxury homes even have indoor swimming pools. All of this tends to put more moisture into our buildings. On the other side of the equation we are tightening up our buildings. Seems like all the talk about energy conservation is finally beginning to sink in. So we build ‘em tight –especially if we’re building with SIPs. After all, that’s one major reason why we decided to build our project with SIPs in the first place.
The savvy designer can see this coming. If he has done his homework, he knows that the perm rating of the SIP is less than 1 perm – lower than many waterproofing films and coatings. The tight SIP building with a high internal humidity will want to equalize the temperature and humidity of the gas (air) inside with that outside. In the winter, when the delta-T is high and the humidity is low, the pressure to equalize is very high. This is called vapor drive. If there are a few cracks in the building, especially high up where the delta-T is highest, the warm, moist air will find its way out. When it hits the cold outside, the water condenses out and – voila! – rot. The professional designer/architect/engineer knows this. He will design the building to head off this problem in several ways.
On the “supply” side, sources of moisture are controlled and limited. Swimming pools have covers, clothes dryers are vented to the outdoors (yes, especially in the winter!), and bathrooms, steam shower enclosures and saunas are also specified as vented to the out-side. For the energy efficiency extremist, all of these vents are passed through a heat exchanger that wrings out every last BTU before finally exiting the building. In fact, SIP structures are only considered properly assembled when a blower door test shows them to be so tight as to have less than 0.4 Air Changes per Hour (ACH). Many SIP structures test out at less than 0.2 ACH. ASHRAE recommendations, which are referenced by most codes, require supplemental mechanical ventilation for buildings with less than 0.4 ACH, so a Heat Recovery Ventilator (HRV) is almost a default specification for a SIP structure. Strictly speaking, ASHRAE calls for ventilation that will give the building at least 0.4 ACH, the heat-recovery feature is an option.
So the designer/architect/engineer will watch the other components of the building to be sure to control humidity levels within the envelope. From a health standpoint, the ideal is a band between 40 and 50% relative humidity (RH). This helps to prevent the growth of mold and mildew, which require higher levels of humidity to exist. On the “envelope” side of the equation, proper specifications call for the sealing of the SIPs with foams, caulks and gaskets. Ed Stahl of Sunworks in California suggests that intention-al vents be designed into the envelope sort of like “pressure-cooker relief valves” so that the moist air can escape without harm. Joe Lstiburek, preeminent building scientist advocates continuously running exhaust or intake fans working together with such vents as being economically competitive with HRVs.
Presently, the SIP industry is catching some flack about structural damage due to vapor drive. The panel manufacturer is not responsible for the improper application of the product. Let the design community take the responsibility for their own work. SIPs can make you look like a hero, but you have to understand their nature fully in order to use them correctly and with imagination. Specifications must deal with proper assembly and sealing. We, the design community, write the specs.