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Size Up Your Structural Options

Wood framing, concrete blocks and SIPs each have pros and cons. Make sure you understand your options before you start building.

WHILE WOOD FRAMING IS THE MOST COMMON AND FAMILIAR TYPE OF HOME STRUCTURE, YOU HAVE OTHER OPTIONS, including insulating concrete forms (ICFs), structural insulated panels (SIPs) and lightweight concrete blocks. Of course, if you’re adventurous, many other systems have been around for decades, including log homes, straw bale, cordwood and even Earthships.

Not every method of construction may be right for your geography, but most technologies can be modified to accommodate your taste and your region. For the purposes of this primer, however, let’s stick to the structural systems your builder is most likely to know and understand.

Wood Framing

Wood, by its very nature, is a green product. If forests are managed properly, trees grow back. How do you know if forests are being treated with respect? Look for lumber that is certified by the Forest Stewardship Council or the Sustainable Forestry Initiative. Typically, energy-efficient builders prefer 2”x6” lumber for vertical studs in wall cavities because the wider space allows for more insulation.

Another more recent wood framing technology is called engineered wood products (EWP). Products such as studs and joists are created in a factory with special water-resistant glues and fibers from leftover mill lumber or fast-growing tree species. They are pressed and glued into lightweight floor joists, rafters or other structural pieces. The green advantages? First, engineered products use more of the tree—there’s virtually no waste. Second, they tend to be more stable and straight than dimensional lumber. The downside? Certain products need to be stored carefully and installed exactly as intended, or they can lose their structural integrity.

Virtues:
Renewable (if forest is well managed)
familiar to contractors
Excellent durability

Caveats:
May create unwanted thermal bridging
Requires skilled labor

 ICF

Insulating Concrete Forms

Poured concrete walls alone have very little insulating value. Yet concrete can last forever, or nearly so, if it’s protected from erratic moisture changes and freeze-thaw cycles. That’s what makes ICFs an excellent structural system. They enclose both sides of a poured cement wall within a water-resistant cocoon of rigid foam. Another advantage to ICFs is that their assembly is quite simple, and the completed walls have an average insulating value of about R-22.

Virtues:
Very little air infiltration
Lightweight forms assemble easily
Thermal mass of concrete slows termperature swings

Caveats:
Exposed foam may need protection
Some brands require additional furring strips to attach to draywall and siding

Lightweight Concrete Blocks

Lightweight concrete is a structural material that’s been around since at least the 1920s. To create these blocks, the manufacturer replaces a portion of the concrete with something lighter and better insulating, such as an industrial waste product like fly ash or petroleum-based polystyrene. Some companies such as Cresco Concrete, creator of Liteblok (www.crescoconcrete.com) use a temporary agent that leaves nothing but air gaps behind. If a product does include fly ash make sure the manufacturer provides data showing that they have carefully tested and screened the material to keep heavy metals and other toxins out of the end product. 

Virtues: 
Easy to handle
Less energy intensive than concrete
Durable and termite proof

Caveats:
May not be locally manufactured
Contractors/masons may need training
Waste components should be tested/verified

Structural Insulated Panels 

 The concept here is simple. Two sturdy panels—typically oriented strand board (OSB)—are glued under pressure to a super insulating layer of plastic-based rigid foam (either polystyrene or polyisocyanurate). SIPs address air infiltration, R-value and vapor permeability, while at the same time creating the home’s structure, and providing a nailing surface for siding and drywall. So why aren’t they seen everywhere? Because they tend to cost more up front than stick framing, and aren’t widely understood by contractors. But if you figure in the benefits in labor savings (up to 60% in some cases), plus the ongoing energy payback to homeowners, you can argue that SIPs come out on par with or lower in cost than wood framing

Virtues: 
Reduces labor time/cost up to 60%
Excellent insulating and air infiltration barrier

Caveats:
Stoarge on site must be dry and flat
More expensive material costs than stick framing
Skilled installation recommended (for the wall to roof transition especially)

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Know the Lingo

  • Dimensional Lumber: Wood that has been cut and shaped from a single tree, typically used for framing.
  • Load-Bearing Wall: A wall that helps hold up the house. Interior walls may not be load bearing, but external ones almost always are.
  • Engineered Wood Products (EWP): Structural
    products made in the factory from industrial wood scrap
    or fast-growing species, assembled with resins under extreme pressure.
  • Oriented Strand Board (OSB): A type of engineered
    wood panel. The thickness of OSB used in most
    SIPs is 7/16”.
  • Fly Ash: Controversial waste by-product from coal-fired power plants. Used as a filler in some—but not all—brands of lightweight concrete blocks.
  • Sound Transmission Class (STC): Refers to how well
    a wall partition attenuates sound. Products such as ICFs have high STC ratings and greatly reduce noise levels
    inside the home.

VISION HOUSE® AT MARIPOSA MEADOWS sponsor

Structural Insulated Panel Association (SIPA)
Since 1990, SIPA has worked to advance energy-efficient construction through the use of structural insulated panels (SIPs). A replacement for wood-frame construction, SIPs are made of foam core sandwiched between two structural facings. SIPs provide builders shorter construction time and less jobsite waste. Homeowners benefit because smaller heating and cooling systems are required with SIP construction. www.sips.org


SIPs on the Roof?

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The point where wall panels connect to a SIP roof is one of the trickier details when building a complete home shell with these pre-made panels. Get it right, and you’ll have a super-strong, super-insulated structure.

Image: www.thermalshellhomes.com