A Little Addition Achieves Net Zero Energy

Net Zero Energy Addition: An Addition that Subtracts Energy Costs

Our subject for this project was a little house located in the heart of the Takoma Park historic district.  Our objective was to achieve a very high standard of energy efficiency.  Converting an existing screen porch into a sitting room we would increase the living space of the house by 240 square feet (15% of total finished square footage) without adding to the heating and cooling load.  That is, a net zero energy addition.

To achieve net zero energy, the floor, walls and roof were designed with a complex assembly of multiple layers of rigid foam panels, vapor barriers and sprayed foam insulation.  This system took shape as follows:

Step One

We set a layer of rigid foam on the existing porch slab , poured new concrete on top of that and finished with a travertine floor.  This combination of materials makes for a tremendous thermal mass.  Working in sync with south facing double doors and windows the insulated floor captures sunlight during the cold season—converting it to heat for the room.  This trick is frequently used in Abrams additions to take advantage of a zero carbon and zero cost heat source:  the winter sun.  It is an important factor in achieving a net zero energy demand when increasing the size of a home.

Step Two

Walls of 2×6 construction (standard is 2×4) provide a deeper wall cavity making it possible to achieve a much higher R-value when filled with spray foam. The exterior is wrapped with a vapor barrier plus rigid foam insulation, then clad with stucco paneling.

Step Three

The roof deck is sandwiched between two types of insulation. Open cell foam is sprayed between the rafters on inside and sheets of rigid foam are set on the roof deck underneath roofing material.

 

Thermal Bridging

The techniques detailed above are painstaking but they successfully eliminate thermal bridging in the building envelope.  Thermal bridging is a heat transfer that occurs wherever building components connect from the inside to the outside of a building.  So, for example, what happens at the roof is that every joist or rafter will transfer heat by conduction at the points where the structure is exposed to the elements.  So it is important to place insulation strategically to eliminate the “bridging” effect where the roof deck and rafters meet.  Without implementing this technique it would be nearly impossible to achieve net zero energy.

 

And, in terms of creature comfort, this insulation technique eliminates cold spots on interior walls or ceilings that contribute to uneven indoor temperatures in the winter.  So “going green” here results in a double benefit of increased comfort and reduced operating cost.  This approach to building amounts to an abbreviated passive house model.

Energy Analysis

When we designed this project we did a computer based energy analysis that predicted we would meet our objective.  Once the addition has been in use for 2.5 years we felt we had sufficient data from the owners energy bills to determine if we were successful.  After crunching the numbers we found that there was no impact on heating bills even after enlarging their home by 15%!

The following table provides a detailed look at the gas usage before, during and after construction of this 240 square foot net zero energy addition.

* Incidental therms used = normal usage of cooking and water assumed at .4 per day

Daily Temperature Data and Heating Degree Days Data averaged for the entire state of Maryland gathered from www.ncdc.noaa.gov