SIPs

SIPs for those that do not know means "Structural Insulated Panel". Popular in mainstream construction today composed of OSB (orientated strand board, an engineered product) with a polyurethane (PU) foam core that is adhered at the factory with pressure (usually caul plates). The bond strength to the OSB is very strong. There are variations, many, but that is the basics. The whole intent is to combine insulation and structure at the factory for faster than stick framing and insulation at the job site. It certainly has it advantages in a winter build like this. In the build below the manufacture claims a non-tested whole r-value of R-40, the code minimum is R-13. The walls and roof are constructed the same, the Timbers are providing all the primary structure, the SIP a modern world "Insulation Wrap" .

The problem is the seams are moisture traps. You can see the neoprene seal in the photos below. Somehow as the seams are strapped together and assembled this seal should see around 1/3 compression and the gap in the OSB tongue and groove should be designed to allow it. That is the way I have designed seals for the last thirty years and that is standard design practice often found in seal design specs. Well these SIPS panels are put together and the seal is never inspected since it is hidden-blind. It is stapled to the OSB and comes off from assemble forces, or never produces a proper seal. The seal is a capillary seal meaning it is suppose to stop liquid water from getting in the seam, probably not going to happen. Vapor will get in the seams and so will water, it is next to impossible to keep moisture out of seams, wall cavities, proven. Some have tried to inject plastics, rubbers, foams, in the seams that loose memory or fatigue over time. There is no life cycle data, lab, or model data to support any of these theories.

The build below, the RH in this climate zone is high most of the year. The OSB even if taped with a common vapor and air impermeable peal and stick is permeable. Tapes do not last nor do glues, they loose memory and often since they are a polymer with fungi generating food that makes the situation worse. If drying does not occur in 48 hours, and the seams get above 70% RH, heat, generates mold spores that corrode not only the wood, foam, but also attacks the water and ice (another fungi food source) roof underlayment and corrodes the galvanized coating of the steel standing steam roof inner surface. The upper roof is more susceptible, and the repairs very costly.

Some subscribe to a ventilation gap above the upper OSB skins using furring strips or battens. This gap produces a vacuum in the seam if it is vapor open and the flow is correct to attempt to dry it out with air-flow or pressure differentials between the seams and air gap. Again, if the RH of the air moving across the seam is high, drying will not occur in 48 hours, or conditioned air can be lost through the seam into the air streams. Others subscribe to high ventilation rates or whole house dehumidifiers that have no idea what the seam RH is and when to turn off and on, or how to monitor the seam and indoor RH to a comfortable 45-50% for healthy eyes and skin. A new concrete slab like this can add to indoor RH and never dry if not controlled.

Here on a green building site two such roofs have failed in 8-12 years, only theories on a fix, and there are others on the internet: Failures in NH and OH

So essentially what is going on is due to bad design there are lots of theories or what I'll call "band aides" in the form of more manufactured products with no history to support the theories that are all over the place in configuration, to test to depending on whom you talk to or believe to solve the real issue, "The Design".

The only history available the manufactures point you to and try to convince of to falsely validate their bad design by is 30 years ago with a configuration different than the OSB and PU used today. Add all the tape, glues, seals, foam, cost and the configuration is far from having 30 years of proven success. Try less than ten (see above link to GBA) .

See below for old SIPS testing: Old Performance Test

More issues in Alaska, no proven solutions, testing and recommendations: Many failures in Alaska


The obvious answer is a seamless design that are not so sensitive to workmanship. There are natural ones we can recommend that are inert, breathable, healthy.

Summary,

• SIPs seams are still a problem, even with the latest iterations of seam joining that attempts to use compression gaskets. 
• Tapes will likely fail with time, even if they don't vapor can get to the seams via the permeable OSB. 
• There are problems with drying to vents under roofing especially if the vent RH is higher than the seams.
• The whole building dehumidifier approach from below also has problems, including the fact that the RH sensors do not monitor the RH inside the seams.
• The alleged thirty-year track records do not exist because different materials are used today and the assemblies at the seams keep changing. 
• The cause, corrective action, and impact is unknown. 
• There are better seamless designs that are not so sensitive to workmanship. 

For more info give Terry at Paramount Construction a call, 316-644-9941.

Neoprene seal stapled to OSB T&G at factory

Strapping Seams

Open Cell Foam

Walls go up first, then roof

Electrical chases are installed at the factory. Re-routes not easy. 

Holes are drilled for straps to hoist by crane into position, later filled with open cell foam.

water and ice, standing seam cladding

Pine Tonge and Groove  drying direction.