Why do SIPs need so much drywall on them for fire protection? Just put that fancy paint stuff on them like they do with other wood products and call it good.
Well, not really. First off, the building codes require that thermal plastics, the white stuff in a SIP, be covered with an approved 15-minute thermal barrier like ½” gypsum.
Next is the hourly rated assembly discussion. Stud construction can meet the hourly requirement with a single layer of gypsum because the exterior surface of a stud will char in a fire. This charring protects the inner core of the stud. This inherent protection of the inner-core of the stud allows the stud to continue carrying structural loads even though the exterior of the stud is charred. A single layer of gypsum will provide the additional fire protection the stud needs to meet the one-hour requirement.
In a SIP the EPS is the core. The EPS melts at higher temperatures, so the gypsum protects the skin of the SIP from getting too warm and melting the EPS core. Gypsum has molecular water bound in its chemistry. When gypsum burns, it releases the molecular bound water. This process keeps the surface of the SIP cool.
Have you ever been camping and boiled water in a paper bag? As long as the water is in the bag, the paper won’t burn. Once the water is boiling, then the paper burns. You should not hold the bag at this point. SIPs react to heat in the same manner. The EPS needs to stay below the melting point to continue supporting the skins from buckling.
Depending on the thickness of the gypsum, it usually takes two layers to provide enough water to keep the EPS cool enough to last the one-hour fire test. Fancy paint coatings don’t contain water, so they are not able to keep the temperature of the skins cool for the one hour test.
Hopefully, this discussion sheds a little light on why the industry has yet to find a material that is as cost-effective as gypsum when it comes to protecting the SIPs from fire.
View SIPs in action– DIY Network Deconstruction show features Premier SIPs and demonstrates (their way) some pretty cool strength testing, including a fire/burn segment.