Sunday, January 3, 2010

Zero Energy Buildings

Getting to ZERO

So, What is a Zero Energy Building ???

The goal of the project is a zero energy building, or "ZEB".  That is a building that produces all the energy it needs -- both thermal as well as electrical.  In this climate, thermal loads for a small super-insulated building are all on the heating side, as no active cooling should be needed.

A zero energy building starts with what we term "Near-ZEB Construction".  Essentially, that means a conscious effort at every phase of design and construction (or renovation) to lower the thermal and electrical loads of the structure, and all of the loads used inside that structure.  Once those loads are as low as possible, making the heat or electricity required becomes a more manageable task.

The basic question to consider when building a new structure or home, is "How much energy (fuel) do you want to buy?"  Since fuels to heat (and cool) homes are expensive, and certain to get a LOT worse in the near future, one option is to buy none, or ZERO.  Thus, the term Zero Energy Building...So, how is this possible?  Well, for the most part, our buildings and our building codes set a very low standard for thermal energy performance.  If we take the time and care to look at all the details, and carefully engineer a building to require dramatically less energy to meet its design heat load, we're a long way toward that goal of zero energy.

Near-ZEB construction is the starting point, so I'll attempt to explain what that means:
  • First, consider a new building built to current building codes.  That hypothetical building we'll call code-built construction.  The heat load for such a structure is, by definition, 100% of the thermal load for a building of the same design and floor plan.
  • Next, consider a building that is uses 20% less energy, or 80% of the energy consumed by the code-built home.  In this climate, that 20% threshold is required for Energy Star designation.  Thus, we'll define Energy Star Construction as a building that uses 80% of the fuel required by a Code-Built building.
  • Near-ZEB Construction is a building that uses 70% less than the code-built home, so it requires only 30% of the energy of the typical code-built home.
  •  ZERO Energy Buildings, or ZEB's, make all of the energy required for the building load.  Without Near-ZEB engineering, planing, and design, getting to ZERO will be very difficult, or very expensive.
Q:  But isn't Near-ZEB performance expensive?
A:  NO!  Not for new construction.
 Here's a very important fact to consider:  For new construction, spending about 10-12% more on the project (and putting that into the thermal envelope of the building) can save you 70% on heating and cooling costs -- FOREVER!  

Really -- as in, no kidding.  In other words, a code-built $200,000 house in this climate will need about 1,000 gallons of oil for heating.  Spending an extra $20,000 now will bring that down to 300 gallons -- a 70% savings.  As the price of oil (or whatever fuel) rises, the "payback" on this investment gets smaller and smaller.  Over time, it can save you hundreds of thousands of dollars, and can make all the difference in surviving oil shocks and rough economic times to come. Thus, the thermal performance driven by our current building code is very VERY low indeed, and Energy Star is only a small step in the right direction!

Achieving Near-ZEB performance in retrofit situations is far more difficult, and will be more expensive.  For down-to-the frame gut/rehab jobs, its very cost effective to thicken the walls and get as much insulation and air sealing in there as possible.

What does Near-ZEB construction look like?  Usually, near-ZEB performance means thicker walls and higher performing windows and doors with lower U-values (higher R-value).  It also means careful attention to all the little details of air movement, and air barrier integrity to achieve low infiltration values.  How thick the walls need to be depends on the type of insulation being used, and the design heat load of the building.

The design heat load is the # of Btu's required, per hour, to maintain a constant desired interior temperature, under the coldest conditions usually faced in the building's location.  That's not the record cold temperature, but the lowest temp's seen 98% of the time.  The assumption is that designing for the record low would be overkill for most of the time.

Thermal Performance for near-ZEB Buildings in Northern New England:  John Unger Murphy, of Murphy's CELL-TECH uses the following numbers to describe ideal near-ZEB specifications:
"5-10-20-40-80"  (note that each number is twice the preceding number).
  • 5 = 0.05 air changes per hour at 50 Pascals (VERY tight)
  • 10 = R-10 Windows & Doors.  (That's a U-value of 0.10, so pretty high performance!)
  • 20 = R-20 on all below grade surfaces, including basement floors
  • 40 = R-40 above grade (pretty high -- 2 x 6 and R-19 fiberglass actually performs at roughly R-11)
  • 80 = R-80 in the attic & roof.
Meaning:  If you do this, or get as close as possible, you can create a NEAR-ZEB building!

Wall Thickness -- (Remember the wall thickness question?...)  The wall thickness to achieve roughly R-40 is about 11" for dense-packed cellulose (R 3.8/inch), and about 6" for high-R spray foam (~R-6.8/inch, initially).   While most people haven't designed a home with 11" thick walls (or thought about how that would look and feel), getting to R-40 will cost about 3 TIMES as much with spray foam as compared to cellulose, which does level the playing field a bit.

So -- how will this bale house perform?  Well, time will reveal all.  We won't make the above goals, but we will be testing the air infiltration and seeing how close we can get to ideal Near-ZEB performance.  That will be the topic of a future post.

For now, Happy New Year, and Stay Warm!


The Gray Monk said...

Looks like a very interesting project. My primary concern in using straw for building would be the fire properties and smoke production in a fire involving this structure. Extinguishing a fire in piled straw is very difficult and the smoke is highly toxic.

That said, you folk use more timber in construction than we do and the straw will not significantly increase the fire loading so I guess it can be managed. Are you fitting a fire suppression system internally?

BPVietje said...

Excellent Point Gray Monk.

RE: Fire Safety -- Right now, with straw exposed, the fire danger is quite high. We're heating with a wood stove, so we have to be very careful. Interior plasters are being mixed today, BTW.

However, once the plasters are applied, straw bale buildings resist the spread of flames more than 3 TIMES as well as conventionally-framed structures. Fire has a tough time getting through the 1-1/2" thick (~3.5 to 4 cm) plaster, and if it does, it chars the straw and smolders, but robbed of oxygen, it doesn't tend to spread. This means firefighters have a much longer time to respond to a fire alarm, and save the structure and its inhabitants.

I'll publish a brief summary of fire safety studies when I get some time to address it. As a general rule, properly built straw bale buildings are far safer than wood frame structures. In our case, the timber frame would likely burn and collapse long before the bales gave way -- a horrible thought.

Ben Graham said...

It should be noted that most of the strawbale home fire incidents are indeed during the construction phase. Another colleague experienced a fire from a nearby compressor that was located in a difficult to reach site for firetrucks. They were still easily able to put out the fire easily with only minimal damage as the bales will only slowly smolder. The major damage being the demolition of finishes to expose the underlying bales to visually assess fire condition.
Strawbale wall systems have also achieved 1 hr fire rating with ASTM standards and probably could have achieved 1.5 hr rating.
Any smoke spread ratings should be applied to the finish coat. For the interior on this structure we are planning to use clay paint.

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