How to Make a SIP Roof Better

first_imgOur expert’s opinionHere’s GBA technical director Peter Yost’s take:SIP assemblies have little inherent moisture forgiveness, regardless of the source of the moisture. Therefore, you need:Really good flashing details, connecting the weather resistant barrier to any and all flashings at penetrations.Really good air sealing details at any transition, penetration, or panel joint.Back-venting of all claddings to strongly promote convective drying.Really good mechanical, spot ventilation, and a hygrometer installed for homeowners to KNOW their interior relative humidity.Trade contractor scopes of work that require ANY workers who penetrate the SIP building envelope to properly air seal and flash that penetration.When Nathan Yost, Steve Baczek, and I wrote the Winter Panel Installation Guide, we did so with all five of the above points in mind, because that is what the president of Winter Panel (now owned by Vantem), Dave Gauthier, insisted upon. Many of the Winter Panel high performance SIP details are available on the GBA website.So, choose a SIP system with really good joint details, always back-vent your claddings, and set up the home to keep wintertime interior relative humidity around 30%. (Running an HRV will dehumidify in just about all climates during the winter so long as effective spot ventilation is also in place.)I like the idea of using a top-side layer of rigid insulation such as mineral wool. If the insulation has adequate vapor permeability, this approach would maintain drying potential to the exterior (in combination with back-vented claddings) while providing a thermal break at the many SIP panel joints and rough openings with through-framing.While I don’t think that EPS rigid insulation has a low enough vapor permeability to present a problem, that really depends on how much moisture the roof assembly is trying to manage to rid itself of. This goes back to how good the water management and air sealing details are. The foam will help protect the roofGBA senior editor Martin Holladay likes the idea of using foam over the roof panels. “Installing a layer of rigid foam on top of your SIP roof to address thermal bridging is an excellent idea,” Holladay writes. RELATED MULTIMEDIA Structural Insulated Panels Podcast: Air Barriers vs. Vapor Barriers RELATED ARTICLES Roger Lin’s Washington, D.C., house will have a roof of 12-inch-thick structural insulated panels (SIPs). By most standards, that’s a well-insulated roof. But Lin wants to add 2 inches of rigid foam on top of the panels to reduce thermal bridging.He’s uncertain about the details. He has already installed roofing underlayment over the panels. Can he put expanded polystyrene foam on top of the underlayment and cap it with metal roofing? Or does he need a layer of plywood or furring strips over the foam before the metal roofing is installed?His question, posted in GreenBuildingAdvisor’s Q&A forum, is the topic for this week’s Q&A Spotlight.The discussion quickly turns to the risk of rot in the oriented strand board (OSB) skins of the SIPs, and two competing points of view emerge. No, the foam may lead to rotTo Philipp Gross, it’s never a good idea to “outsulate” SIPs because the added foam increases the risk that moisture will be trapped against the OSB. “Depending on your climate, you might be OK, but the risk that the outer OSB will rot away over time exists. Wrapping material that can rot (e.g. OSB) between foam, which slows down drying potential dramatically, does not make sense in my opinion,” Gross writes.Thermal bridging shouldn’t be a huge problem, he adds, and if Lin wants to improve the thermal performance of the roof, he’d be better off using thicker panels.Holladay suggests that air leakage, not thermal bridging or vapor diffusion, is the usual cause of decay in SIP roofs. But Gross believes it’s wrong to completely disregard the risk of vapor diffusion. He adds that moisture problems could go undetected for a long time.Albert Rooks also warns Lin away from his plan. “Use mineral wool instead,” Rooks says. “It has the insulating value that you need, and the vapor permeance that the SIP needs. There is zero tolerance in a SIP system. Placing the OSB layer between two impermeable layers is not a good idea.“Martin, with all due respect, Phillip is right on this one,” Rooks adds. “Moisture could even be present when the assembly is overlaid with a non permeable material at this time of year. There are enough ways that moisture could find it’s way into the OSB layer over the assembly’s life. Once there…“It’s like riding a motorcycle without a helmet. Sure most times you’re fine. It’s only takes one occurrence for catastrophic results!“I know we like data in this arena. There is plenty of it around: take a SIP and just add water … the results are not pleasing!”Clarifying his position, Holladay notes that he has no illusions about SIP problems. “A SIP roof has several weaknesses: OSB is prone to rot; the outer layer of OSB is cold; there are often internal framing members that act as thermal bridges; and the seams often leak air,” says Holladay. “SIPs make be nervous, and there are many questions concerning the longevity of OSB. … That said, I stand by my statement: an outer layer of rigid foam will lower the risk, not raise the risk, of OSB rot.” The foam layer should be airtight, he adds, so seams should be sealed with high-quality tape. Metal roofing should not be installed directly over the foam but on furring strips or a continuous layer of plywood. And if plywood is used, then Holladay recommends that Lin install 2x4s from soffit to ridge between the foam and the plywood to create ventilation channels.“These ventilation channels will give you a cold roof — cheap insurance if you are going to the trouble of getting your details right,” he says.The bottom line: the extra layer of exterior foam will keep the OSB warmer and dryer, “and therefore safer.” Green Basics: Structural Insulated PanelsHow to Protect Structural Insulated Panels from DecayForget Vapor Diffusion — Stop the Air Leaks!Rigid Foam Insulation Learning from past SIPs mistakesFor John Brooks, the question prompts another look at research from building scientist Joseph Lstiburek of Building Science Corporation.Lstiburek inspected SIP homes in Juneau, Alaska, to figure out why they were showing signs of decay. In his report on the problem, Lstiburek concluded that air leaks at panel seams allowed moisture to escape and condense on the OSB, eventually leading to areas of rot.Lstiburek’s suggestion was to make panel joints airtight and “provide a mechanism for moisture removal such as a vented over-roof.”So, asks Brooks, what about it?Good idea, says Gross.Good idea, says Holladay. Add a layer of rigid foam over the SIPs to reduce air leaks and thermal bridging, he says. Then add 2x4s from soffit to ridge to create ventilation channels, and then another layer of plywood. “This helps address air leaks and thermal bridging, while creating a cold roof,” he says.In the end, Holladay says, the complications of overcoming inherent weaknesses of a SIPs roof might be an argument for choosing another roof system altogether. Options include a PERSIST roof, a hybrid design that includes rigid foam over the sheathing and dense-packed cellulose in the rafter bays, or an uninsulated roof over a vented, unconditioned attic with a deep layer of cellulose on the attic floor.Yet another possibility comes from a commercial Passive House project in Portland, Oregon. In a summary of the technique, Rooks describes the use of a highly permeable (but air- and water-proof) membrane coupled with cellulose that allows drying to the exterior.The system uses no foam. GREEN PRODUCT GUIDE Video: Lstiburek’s Rules for Venting Roofslast_img