The Green Thumb: Cover Story (Part Two)
Last month, we saw how sheathing is used as a multi-purpose building material and discussed the environmental concerns-past and present-for the manufacture of two of the most widely used residential sheathing materials: plywood and OSB. This month, we'll continue the series with a look at the bigger picture, where wood-based sheathing (plywood and OSB) and the environment are concerned.
Environmental concernsWhile there may be other concerns for sheathing, such as shear resistance, fastener pull-out, moisture intrusion, cost, etc., for sheathing materials, the environmental concerns-both for the natural and built environments-boil-down to the following:
- Source of raw materials
- Energy consumption
- Pollutants generated in both fabrication and installation
- Volatile organic compound release (off-gassing potential)
- Product lifecycle (durability)
- Recycled content
We'll discuss the source of raw materials for each type of sheathing product in upcoming articles in this series. Let's first discuss one of the most important environmental aspects of wood-based sheathing products. Embodied energy represents all the energy required/used to produce a finish product. For wood-based sheathing, this must include:
- Forest harvesting operations
- Transportation to mill
- Processing raw lumber into a finished product
- Additional components(i.e. binder)
One of the nice things about trees, besides the shade, oxygen and beauty they provide, is the fact that they convert free solar energy into the energy they need to grow to maturity. Though some wood sheathing products use waste-wood fibers for energy during the manufacturing process, the processing of trees into a finished sheathing panel product and harvesting operations require use of fossil fuels and electricity.
The large, specialized machinery and vehicles used in the tree-harvesting process are the main source of fossil-fuel usage. Of course, the 18-wheelers that deliver the raw logs to the mill and distribute the finished panel product are also an important component of fossil-fuel consumption in the EE equation for wood-based sheathing. The geographic distance from the harvesting site to the mill and then distribution of the finished product to suppliers is a variable factor.
In the short term, clear-cutting is the most efficient harvesting method for trees but it is also the most eco-disruptive and a major cause of soil erosion. Where wood-waste fibers are concerned-either from harvesting and/or from the milling process (mill waste)-little or no energy is required for harvesting but some energy for transport must be considered if it is waste derived from the harvest site.
EnergizedProcessing raw logs to create a finish panel requires two forms of energy:
It is thermal (heat) energy that is used the most for processing. For processed lumber, consider the fact that kiln-dried lumber uses about five-times as much heat energy as does air-dried lumber. For processing plywood, the thermal energy required is greater than the heat required for kiln-dried lumber. Heat must be used to dry veneers, pre-condition logs (before peeling) and cure binders in the pressing stage.
From a mechanical energy perspective, the energy required to process plywood is about the same as it is for lumber. Since plywood and OSB require a binder (typically phenol formaldehyde), the EE for the binder itself must be considered in the overall EE equation for the finished panel product. Since PF is derived from petroleum (phenol) and natural gas (formaldehyde), extracting, transporting and processing these raw materials into a PF resin is a critical factor.
Penny savedBesides the energy consumed, energy reclaimed during the manufacturing process forms a significant part of the EE equation as well. Plywood plants and lumber mills often generate heat energy from their own waste-wood as mentioned previously. They also sell by-products of their manufacturing process as raw material for particleboard, medium density fiberboard and/or for other commercial uses (i.e., sawdust). The U.S. Green Building Council's LEED green building rating/certification program offers credits/points for using "regional manufacturers" (within 500 miles of the building site). This serves to lower transportation costs and reduce the finished product's EE.
As well, use of Forest Stewardship Council-certified wood products is recognized by LEED and helps preserve the dwindling resource of large diameter logs-a particularly acute problem in the Pacific Northwest. OSB is made from newer, less commercially viable small diameter tree species, such as aspen, so its impact on forest resources is less so than for plywood. Such small diameter trees are growing faster than they are being cut down (twice as fast in Canada). However, it is the virgin, old growth forests with large diameter trees, such as Douglas fir, that are fast disappearing in North America.
Best and worstUsed primarily in residential (combustible) construction, wood-based sheathing provides little-to-none fire resistance. With plywood, a variety of grades provide differing levels of:
- Moisture resistance
- Exposure performance
For the former, both plywood and OSB will swell, warp or rot when directly exposed to liquid moisture. Therefore, great care should be taken to keep it dry during storage, transportation, installation and in place over framing members. Nowadays, a secondary weather/moisture barrier in the form of two layers of 15 pounds felt (tar) building paper or one layer of 30 pounds roofing paper (the better choice) is installed over plywood or OSB to prevent it from getting wet. Plywood is 100 times more vapor permeable than OSB, so it is the much better choice where vapor permeability is concerned.
However, both plywood and OSB will promote mold growth if their surfaces and/or edges come in contact with liquid moisture. With barrier or face seal EIFS, the assumption is that no water can penetrate the face (membrane), therefore the substrate, plywood or OSB in residential construction would not get wet. If there are "flanking paths" (windows, doors, chimneys, etc.), this will allow water to get behind the EPS insulation and make contact with the bare wood panel substrate. Even with drainage and rain screen systems, this secondary barrier is typically required providing redundancy against moisture intrusion.
Another common problem is the method of installation of the felt paper. It must be installed horizontally from the bottom up in "shingle fashion." This means horizontal lap joints of 2 inch minimum and vertical lap joints of 6 inch minimum. All too often, felt paper is installed with simple butt joints; this provides a flanking path for moisture to penetrate the building paper, thus rendering it irrelevant.
Where wood-based sheathing is most effective is in providing shear strength and racking resistance. It also provides a solid substrate for nailing siding materials. Attached directly to wood or metal framing, wood-based sheathing will resist-for the life of the structure-wind (air pressure differential), seismic (earthquake) and other applied loads (perpendicular or parallel) to the plane of the wall.
In part three, we'll take a generic look at the other types of sheathing products available for building construction.