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Pacifica raises the roof

A Pulp & Paper Canada Special Report

As with most pulp and paper plants, Pacifica Papers’ PM 11 at its Powell River, BC, mill operates in a hostile, high-humidity environment, placing severe demands on the performance of the roof and roof insulation systems. To function efficiently, thermal insulation must remain dry. When roof insulation becomes wet, or partly so, due to roof leaks, water vapor migration from within, or faulty protection prior to and after installation, problems likely will occur — as they did on the roof of the PM 11 machine room.

Up to 70% of roof system damage is caused by moisture intrusion. If insulation absorbs just 4% moisture, the material may lose as much as 70% of its insulating value. Eventually, these moisture problems may spread to the underside of the roof deck, resulting in membrane system failure and corrosion or deterioration of the roof deck.

On the PM 11 machine room roof, the original mineral fibre insulation installed in the built-up roofing system “had totally failed in less than 15 years,” according to Gordon Milne, a senior structural engineer with NLK Consultants, Inc., of Vancouver, BC. “The mineral fibre insulation had absorbed a tremendous amount of water,” said Milne. “It’s R-value was zero, it was spongy it had completely lost its strength.” NLK specified the new roofing system for the project, did the field engineering and supervised the installation

In addition to total loss of thermal efficiency, the failed mineral fibre insulation was creating other serious problems within the paper mill. “Holes had punched through the membrane support system and had destroyed it,” Milne said. “The steel structure was breaking down, too, due to the added moisture attack created by the lack of insulation. Water condensing on the steel was dripping down, causing potential damage to electrical equipment and paper rolls below. Each of the rolls weighs 0.75 tonnes and is valued at US$700 so the leaking roof had the potential to cost the mill a tremendous amount of money in damaged paper alone.”

Compressive strength

Besides the roof insulation’s thermal and moisture-resistant properties, another issue was the compressive strength of the roof insulation. The roof of the machine room would be subjected to considerable loads (workmen, equipment, material) during insulation application. And, after the roof replacement, servicing of rooftop equipment would mean additional foot traffic. The insulation system would be required to have the necessary strength and rigidity to combat any tendencies to slump or compress under these weights. Otherwise, troublesome insulation voids and sagging could occur.

Another concern is the reaction of the insulation system to temperature variation. Surface temperatures on the typical roof membrane vary considerably from surrounding air temperatures. For instance, on a clear, winter night, roof temperatures can plummet 15F to 20F below air temperature. On a clear summer day, the temperature of a roof membrane can soar 60F to 70F above the air temperature.

Therefore, the insulation should be as dimensionally stable as possible to reduce any tendency to swell, warp, shrink or distort, due to extreme temperature differentials. It’s preferable that the specified insulation has the lowest coefficient of thermal expansion and also having the highest resistance to moisture migration.

With resistance to high internal humidity levels, compressive strength and dimensional stability key concerns, NLK Consultants selected Pittsburgh Corning’s FOAMGLAS cellular glass insulation for the re-roofing project. Its moisture resistance, dimensional stability and compressive strength (average compressive strength of 87 psi) allow it to provide a firm, stable base for modified bitumen, built-up, or adhered elastomeric membrane systems. If the membrane is punctured, water cannot penetrate the closed-cell structure of FOAMGLAS as it can with porous insulating materials.

“Basically, we specified FOAMGLAS because it is impervious to moisture,” said Milne. “It’s one of the best insulation products for preventing moisture intrusion. It’s also a very strong material with superior dimensional stability and high compressive strength (making it better to walk on), which are important because of the heavy demands placed on the roof.” FOAMGLAS does not absorb water, Milne added, it is basically blown glass.

During the re-roofing project, a styradiene butadiene styrene (SBS) exposed membrane system was installed. This was composed of, first, a 1.5-in. layer of FOAMGLAS fastened to the metal deck. A vapor barrier was hot mopped in place. Then, a second 1.5-in. layer of insulation was applied with hot asphalt. Finally, a base sheet was hot mopped over the insulation and a cap sheet was torch-applied to the base sheet. The cap sheet contains fine gravel making it non-slip and safer to walk on.

“Since installing the FOAMGLAS system, the results have been excellent,” said Milne. “There have been no problems with the insulation system whatsoever. We expect that the FOAMGLAS insulation will outlast the membrane system. That’s unusual for an environment such as a pulp and paper plant because the demands on the insulation system are so severe.”