Wood vs. Steel
Cost
The US Department Housing and Urban Development conducted a test in 2001 where they built two identical homes side by side, one with wood framing and the other with cold-formed steel framing. They then issued several research reports, one on the construction cost, and one on energy efficiency and performance. The material cost of the steel framed house was 14% higher and the labor cost was 4.5% higher than for wood framing.
Since that report was issued the price of steel has more than tripled, while the price of dimensional lumber has crashed. This suggests that wood trusses are likely to be more economical for most applications.
Steel trusses have traditionally been used in commercial and industrial construction. However, wood trusses and framing may offer superior cost and performance benefits in most applications. Many people are misinformed about the benefits of wood versus steel.
Fire Resistance
Myth #1: Steel framed buildings are more resistant to fire and easier to get approved for fire code.
Though steel is less flammable than wood and doesn't contribute fuel to a fire, it still performs poorly in fires. Any urban firefighter can tell you that when steel gets hot it expands and twists, which often causes the roof system to fail and fall down - endangering the lives of firefighters and people trying to escape from the building.
In a fire, the hot air collects at the top of the room or building, heating the truss system to 1500F - 2000F. Since steel loses 80% of its strength at 1000F, this often causes the roof and floor systems to collapse much sooner than in a wood structure under the same conditions.
By contrast, wood retains its structural strength even at temperatures above 2000F and while it is burning. When coated with intumescent paint wood structural members can withstand temperatures of 2,000F for up to one hour without failing.
Even without intumescent paint, wood performs better than steel in a fire because it tends to burn at about ¼" per half hour. Therefore the thicker the structural members, the longer they will last in a fire. Large laminated wood beams have the best fire performance of any commercially available product.
Cost of Intumescent Paint
Though intumescent paint is expensive compared to regular latex paint, you have to look at the cost of the whole truss system. Exposed interior steel has to be painted anyway, so there is a paint cost there to begin with.
Intumescent-painted wood trusses are still less expensive than a steel truss system with or without paint. The cost of intumescent paint is about four times the cost of regular paint because it costs twice as much and has to applied in double the quantity to be effective; but you have to look at the percentage of the price of the entire truss system. Paint is one of the smaller costs.
Many fire codes will allow un-painted wood to be used in an exposed roof truss system, which can be . Wood looks and feels "warm" compared to steel.
Longevity
Many people think that steel lasts longer than wood. However all building components degrade in the presence of moisture.
Unlike wood, steel "sweats" because it conducts heat very efficiently. In the humid Appalachian Summer climate, steel structural members conduct heat from the air into the foundation of the building. This lowers the temperature at the surface of the steel members and causes condensation to form on them. The condensation then drips down into the connection points as well as any insulation in contact with the steel. This lowers the thermal efficiency of the insulation and also provides the conditions for mold to grow on dust trapped in the insulation. Moisture in the connection points causes corrosion and rust and shortens the effective life of the building.
Steel trusses and roofing also tend to produce condensation in the winter. Humidity moves from warm to cold, so water vapor leaving the building encounters the cold steel members and condenses on them.
Steel skinned or framed buildings that will contain walk-in freezers or refrigerators often have exacerbated condensation problems that can create puddles on the floor and damaged ceiling tiles.
Condensation problems are so common with steel framed or skinned buildings that an entire industry of products has grown up to retrofit and fix the problem.
Mold
Companies selling steel buildings often claim that wood is prone to mold and steel is not. While mold can grow on wood, the most common food for mold in buildings is dust trapped in the insulation and on surfaces inside the walls. All it takes to grow mold is moisture plus dust. These ingredients are common in all buildings.
As shown above, steel buildings are prone to moisture problems unless insulated with special products to prevent condensation on the steel surfaces.
Wood that has been painted will generally not mold at all. If a wood-framed building has been properly designed to prevent condensation in the walls, mold is not likely to develop.
The key factors to prevent mold from developing in a wood-framed building are:
- Make sure the basement or crawlspace is sealed to keep moisture out.
- Properly insulate the walls and ceiling to prevent sharp temperature gradients that cause condensation.
- Install an HVAC system that maintains a slight positive pressure inside the house. This prevents dust from being sucked in through the walls and coating the insulation. Dust is the primary food for mold.
Other Considerations
A truss system using regionally grown and manufactured wood from Dominion Truss can contribute up to 10% of the LEED and Healthy Built Home points required for certification.
Wood sequesters carbon in the building itself. Steel has a high carbon cost in manufacture and shipping.
Our trusses use Southern Yellow Pine for the webs, which is grown and manufactured locally.
We can provide a cut list to show that the trusses were optimized to save wood (1 HBH point).
In the HUD performance test mentioned above, the steel truss and framing systems conducted heat so efficiently that they negated the effect of insulation, resulting in attic temperatures about 15 degrees higher than in the wood framed home. Wood trusses do not conduct heat to the roof or through the walls.
Green Roof Systems
Dominion Truss can provide trusses for . We specialize in hard-to-design projects, and would be happy to work with any architectural firm or building contractor who needs trusses for a green roof project.
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