Wednesday, January 16, 2013

Modular Home Factories Are Best 'Net Zero" Choice

Modular home factories and their builders can make a substantial impact on Net Zero Housing. The inherent building methods found on the factory floor such as building in a controlled environment and assembling the house from the inside out allows much better air infiltration and insulation methods. Standing on the production floor installing these items is much more efficient than trying to install them on the job site where the most inexperienced workers are usually tasked with doing these jobs.

Here are some of the basic construction techniques that modular factories can do better than their on-site brothers:
  • Continuous air barrier from the basement slab to the attic floor. Each transition of materials needs to be carefully detailed. That means the basement slab to the basement walls needs to be air sealed with a small crack resulting in a lot of air leakage. The foundation to the floor system needs to be air sealed, whether through housewrap or another material; and the walls to the attic floor need to be air sealed.
  • Advanced framing is one of those rare win-wins for builders. Less framing can typically be accomplished through good structural design. Less framing means less framing costs and more room for insulation in the shell. 
  • The need for proper insulation around openings, like outlets and junction boxes is essential. One of the issues that comes up is compressed insulation around openings or the insulation is missing altogether, particularly around outlets. Make sure everything is  both air sealed and insulated.
  • Draft-stopping. When using fiberglass or cellulose wall insulation, it should be enclosed on all six sides. Double walls, ceiling height changes, and areas behind tubs and showers are often missed. The insulation should be sheathed with a rigid material and sealed around the edges. 
  • Recessed light fixtures can be big air leakers. Make sure the airtight gasket is installed during the trim stage.
  • Attic hatch sealing is very important. Make sure the attic opening is fully gasketed and that the pull-down stairs, or door, come into full contact with the gasket to provide a thorough seal,” says Dickson.
  • Detailed scopes of work are the bread and butter of modular construction. Air sealing should be treated as a production management issue rather than a technology issue. Modular factories that successfully get tight houses from air sealing do so by having good contractor scopes of work and responsibilities laid out.

After the factory sends the home to the builder, they must step up and complete the work needed to earn a Net Zero House designation. One important area is the HVAC System.

The biggest mistake that builders make when designing the mechanicals for their high-performance homes is over-sizing the systems. Right-sizing the HVAC equipment is vitally important. The builder should always check to make sure that the Manual J (heating and cooling load calculations) that was done for the house is based on the specific location and the updated building envelope features that you’re using.

Getting ventilation right is critical since ZEHs are virtually airtight. There are three primary strategies for ventilating a home: exhaust ventilation, supply ventilation, and balanced ventilation.

But the real trick to pulling off a net-zero energy product in the modular building environment is to consider renewable energy last. Only after the building envelope is as airtight and well insulated as possible, and things like building orientation and HVAC systems are fully optimized, should decisions be made on the energy production requirements.

As a rule of thumb, factories and builders should plan on getting to a HERS Index of 55 or so with the thermal envelope and then close the gap to zero with renewable energy. It’s an ever-changing scale, but the way the models work, that’s the point where other structural solutions start becoming more costly than renewable energy. If done properly, the cost to go Net Zero should be in the 5-7% upgrade price range over a non-ZEH produced home.

When it comes to solar energy solutions, builders should consider hybrid systems. There are hybrid solar electric/solar thermal systems that produces two-and-a-half times more energy per square foot and about 50 percent more energy per dollar than conventional solar. It’s a tremendously dense system that can create a bunch of energy to get from HERS 55 to zero in a cost-effective, space-efficient way. 

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