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.
