How to Protect Structural Insulated Panels from Decay
Sealing air leaks at SIP seams can be difficult, especially when roof panels will be finished with interior boards
UPDATED on July 7, 2011 with comments from Tedd Benson
Jay Hersh is building a house in northern Vermont that will have a roof of structural insulated panels. Although his plans are fairly advanced, Hersh is still stumped about detailing the SIP(SIP) Building panel usually made of oriented strand board (OSB) skins surrounding a core of expanded polystyrene (EPS) foam insulation. SIPs can be erected very quickly with a crane to create an energy-efficient, sturdy home. seams to prevent the migration of moisture-laden air from inside to outside. He's also looking for a foolproof way of heading off any leaks from the outside.
This strategy, as he explains in this Q&A post at GreenBuildingAdvisor, involves the installation of a membrane (as yet unidentified) between the tongue-and-groove ceiling and the roof SIPs, and another membrane between the top of the SIPs and the standing-seam metal roof.
Suggestions to date for the roof underlayment on top of the SIPs include asphalt felt, RoofTopguard, or Grace Ice & Water Shield. He's not sure what type of membrane to use between the ceiling boards and the SIPs.
Hersh asks for advice, and the resulting discussion is the subject for this week’s Q&A Spotlight.
Air infiltration can be a problem
SIPs consist of an inner core of foam and two outer faces of oriented strand board (OSB). While the assembly can perform well, writes Albert Rooks, it’s also “extremely sensitive” to moisture in seams where the panels meet.
Rooks seconds GBAGreenBuildingAdvisor.com senior editor Martin Holladay’s suggestion that the most important step is a good air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both., including spray foam at all SIP seams plus a high-quality tape on the interior at all seams.
Moisture problems can still develop over the life of the roof in three ways, Rooks adds:
- By piggybacking on migrating air, water vapor originating inside the building finds its way into the internal network of wire chases and then to the exterior OSB skin of the panel.
- Building movement over time can degrade the interior foam joints, opening up new pathways for moisture-laden air.
- Lack of an airtight joint at the seams between the panels and the foundation can allow air into the chase network, and once the water vapor hits the exterior OSB, it can condense.
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He recommends foaming all seams and installing Siga Rissan tape to seal seams at the interior. (Albert Rooks is an importer who sells Siga tapes.)
Rooks adds that it is not advisable to use an impermeable roof underlayment. Instead, he advises installing "a waterproof material with a high diffusion rating” to allow drying to the outside, specifically Siga Majcoat — one of the products he imports and sells. “The key to the assembly is keeping the permeability increasing as you go up in layers,” Rooks writes. “It will increase the OSB’s ability to dry out any internal moisture.”
Follow the Majcoat with battens and counter-battens or sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. to carry the metal roofing, he says.
Dealing with the tongue-and-groove ceiling
One complication to air sealing on the interior is the tongue-and-groove ceiling.
“The SIP installer intends to put the T&G ceiling boards down on top of the frame before the roof SIPs go on since it's a lot cheaper, faster and easier to do it that way than to install them on the underside of the SIPs,” Hersh says. “Also, it means the SIP doesn't get perforated.
“So I'm confused about the suggestions to tape & foam the interior, since I don't see how the interior seams can be accessed if the SIPs are going down over the T&G boards. I'm pretty sure that's why they recommended some type of material be placed over the T&G boards.”
That’s going to make it tough to get a good air seal, Holladay says, because you won’t be able to tape the interior seams of the panels.
“Experience has shown that foaming the seams is insufficient to attain high levels of airtightness,” Holladay writes. “Although taping the exterior seams is a good idea, you could still end up with interior air circulation through convective loops at your SIP seams; when this looping air contacts cold surfaces — the exterior tape — condensation and rot can occur.”
Although adding a layer of Tyvek above the T&G boards might work in theory, the idea has some practical shortcomings: it will take a lot of fasteners to hold the slippery Tyvek in place, each one of which is a potential air leak, and the Tyvek would have to be bonded to the air barriers in the walls perfectly. “That idea contains a lot of opportunities for failure.”
Instead of Tyvek, Rooks suggests another product he sells: Siga Majpell, with a perm rating of 0.68, or a similar CertainTeed product, MemBrain, held on not with nails but with double-sided tape.
A method for taping from below
The problem of moisture-laden air migrating through what look like well-sealed joints is familiar to Chris Koehn, who says he first started installing SIPs over timber frames in 1988 in Wisconsin.
For years, Koehn used expanding foam to seal seams between panels from above. On a few occasions, he later noticed ridges in shingles in cold weather and discovered frost on the bottom of the shingles directly over panel joints.
Working with Insulspan, he developed another approach. By draping tape over the tops of timbers, then installing the panels, the seams could be sealed on the inside. When a T&G ceiling is going in, Koehn would add spacers on top of roof framing to create a gap between the framing and the panels. T&G boards could be added later.
Or, look for a roof alternative
The practical problems associated with preventing moisture damage in SIPs prompts this suggestion from Brett Moyer: “Skip the SIP walls and roof,” he writes. “Why the hell do people use these? They are super-expensive and far from green. I’m so tired of these topics popping up on GREEN building websites. You all know they are pumped chock full of petrochemical foams, right? Use a double wall and frame the cathedral ceilings with raised heel scissor trusses... Airtight Drywall Approach for your air barrier. I guarantee this will be a cheaper and certainly much greener strategy.”
Although Hersh explains the careful evolution of his SIP strategy, Moyer asks whether anyone has done a cost analysis of SIPs vs. other wall and roof assemblies. “I can confidently say that there are more cost-effective ways to achieve these R-values.”
In fact, writes Al Cobb, just such a study has been done, which is available at the website of the Structural Insulated Panel Association. “The study identified higher initial cost and embedded energy to build with SIPs,” he adds. “It also shows the return on investment to be relatively short and dependent on the region you build.
“A higher performing system (like SIPs or double wall construction) almost always starts with a higher price. However, just as you frame a double wall for lower overall cost, many elect to use SIPs for the same reason.”
Our expert’s opinion
The comments below were provided by Tedd Benson of Bensonwood Homes.
Jay’s query about proper detailing a SIPs roof installation over a timber-frame is an excellent one and the essence of the advice offered in this thread adds up to an extremely impressive response. In fact, I haven’t seen so much well-considered information about this important topic in one place in my 32+ years of working with SIPs. So the first thing I want to do is to give a note of gratitude to the Green Building Advisor. This forum has become one of the most important information resources and exchanges for high performance building practice and is helping to fill a big void. Many thanks to all!
I first learned about SIPs at Advanced Cooler Manufacturing Co. in Halfmoon, NY, in 1978. They specialized in walk-in coolers and refrigerated warehouses. Since good insulation was the basis of their business (and the reason I was there), they had a dedicated laboratory and a small team of engineers to study the performance of different types of insulation, their assemblies, and panel connections. Years of making walk-in coolers and ongoing research led them to eventually develop an insulation assembly more commonly called “stressed-skin” panels at the time. The Advanced Cooler panels had thin metal skins and a polyurethane foam core. When they took me to see a refrigerated warehouse with their panels mounted over a Butler steel frame, I squinted and saw a future in the potential marriage of stressed-skin technology and timberframes.
But that wasn’t the end of the visit. They gave me a tour (led by their president, Ed O’Hanlon) of the laboratory and showed me why making the joints airtight in the installation is just as important as making good panels. They were able to simulate pressure and temperature differentials with laboratory mock-ups and otherwise had lots of experience with cooler installations around the country that revealed why leaks at the panel seams could be so significant. For my layman benefit, they called the issue the “pinhole-in-a-balloon effect,” making the point that a nearly perfect assembly can actually exacerbate the problems caused by the imperfections, however slight. One of the engineers claimed to have seen an icicle sticking 3 in. horizontally out a panel seam on the exterior of a walk-in cooler.
So, the first thing I learned about the product most people now call SIPs is what I already knew about timber frames, which is that most of the challenge, science, and craft is at the connections, because that’s where failure is most likely.
Of course, the consequences of leaky seams in a walk-in cooler are small in comparison to what this kind of problem can do to a building with high-quality finishes inside and out. It’s something we simply have to get right.
No single solution
To prepare some comments for this forum, I asked a few of my associates to give me their thoughts about the opinions and ideas expressed in the thread. The following therefore includes some “collective intelligence” from my discussion with Paul Boa, Jay Lepple, and Hans Porschitz. This is not the first time I’ve benefited from their experience and expertise.
From our point of view, Martin Holladay gave a good succinct answer at the beginning: redundant solutions; spray foam between panels; vapor closed sealing strategy on the building interior; vapor open strategy on the exterior. If the thread had ended there, it would have left Jay with good guidance.
Albert Rooks’ comment about SIPs being “extremely sensitive” at the seams echoed what I learned from the Advanced Cooler engineer years ago. That point is perhaps the most important thing to know about SIPs. Rooks goes on to suggest a comprehensive solution, utilizing SIGA products. We are fans of the SIGA collection of air-sealing products and have been integrating a few of their tapes into our assemblies. SIGA could have saved us a lot of heartache if they had been available 30 years ago. The proposal that Albert and Patrick Haacke promote follows Martin’s basic outline and is smart in how it controls vapor movement and diffusion. It’s not the cheapest solution, but since we all agree failure is not an option, a system that would nearly ensure success is worth a lot.
Chris Koehn’s suggestion for applying boards from the inside after taping and sealing is the way we would go if we had to install the panels individually. His proposal was to use a 7/8-in. furring strip to allow space to insert 3/4 in. boards after dealing with the seams and edges. It’s more labor-intensive, but you also have assurance about having achieved a good interior seal. It should be noted that the same strategy can be used for installing and sealing SIP wall panels. In that case, we would use 5/8-in. “packers” to allow 1/2-in. drywall to be hung after sealing.
Foam alone not adequate
On another note, I will differ with Al Cobb and say that I don’t believe a good, dependable seal can made with spray foam alone. There are several potential pitfalls. One is simply the foam flow in the joint that you can’t see. In the right conditions, in the hands of a professional with good experience, it will fill and seal quite well, but are those conditions always perfect? Of course not. Just a flake of panel foam broken off would inhibit the correct fill and seal.
The other condition that’s a big variable is weather. When the foam-pack requires pre-warming in the pickup cab, and the outside temperature is below freezing, it’s time to worry. Any system that requires unknown conditions to be perfect isn’t good enough on its own. That’s why foam plus tape makes so much sense to us. Do the best job you can with the foam injection, but don’t depend on it.
Our SIPs roof panel system has evolved over the years, as we’ve constantly tried to make it better and more efficient to install. We like to make big panels that are as complete as possible. First, we cut the panel to the roof dimensions and geometry. It’s important at this point to fill all the exposed voids between the bun stock pieces that make up the larger panels (a significant problem with some of the manufacturers!). Then we tape the interior OSB seams with SIGA Sicrall, after which we attach the finish ceiling material -- usually T&G boards -- directly to the panels before installation.
When the panels are installed on the frame, the vertical seams between the big panels always align with rafters. To ensure a good seal between the panels, we use a Trelleborg gasket toward the inner side of the SIP connection, just above the OSB. (This little detail is similar to the solution I first learned at the Advanced Cooler company. Since they wanted both a good seal AND for the coolers to be demountable, they used only substantial gaskets, like those on any freezer door, to seal between the panel seams.) When the panel installation is completed, we foam the joints as we have for years, and then apply the SIGA Wigluv tape over the exterior seams, just as Albert Rooks described. The final layer is Grace TriFlex, which provides a watertight, but vapor open barrier.
Our projects are probably split 50/50 between warm and cold roof applications. We prefer to use the cold roof as a general rule, but most budgets have limits, and more so these days. But either way, this is a SIPs roof system that should last for a very long time.
Keep building components separate
One thing that should be evident from this exchange is that creating a sustainable high performance envelope isn’t simple, nor inexpensive. Therefore, a building built this way ought to be extremely durable and should be expected to last hundreds of years. One important aspect of that, we believe, is to employ strategies to disentangle the shorter term building parts from the longer term. In general, the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials., or shell, should be separate and distinct from the space plan, mechanical systems and finishes, which can also be separated from each other. The shell should be designed for permanence; the infill elements for change and churn.
With this as kind of flexibility and durability as a paramount objective, wires and fixtures should be separated from high-performance wall and roof systems whenever possible. We add a mechanical chase layer to our walls always, and to our roof panels when there is a heavy mechanical demand in that area. Otherwise, we have dedicated chases at the building peak, at the eave and other strategic locations when necessary. The point is that after sealing a building in the way we’re talking about in this forum, we need to also make changes and upgrades easy and provide a path other than one that violates the integrity of the envelope.
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