Envelope
Air Sealing
Houses are notoriously leaky. In fact, Energy Star reports that air leakage accounts for 25–40% of the energy used for heating and cooling in a typical home.
DC row homes with their turn of the century construction details and changes due to age are more difficult to effectively air seal. However, 311 Varnum received a delux air sealing treatment which included:
> Low expansion spray foam was used around doors, and windows.
> Regular expansion spray foam sealed new penetrations to the exterior, floor to floor penetrations, behind band joists, major holes that occurred over the years in attics and in walls and more.
> Caulk was used to further seal all the framing seams, joints, cracks, sill plates, and corners.
> Drywall was glued to the studs and joists to provide a further layer of air sealing.
> Exterior rigid board was overlapped and tape sealed.
> Lastly, walls were packed with dense pack cellulose, which adds a further layer of air sealing in addition to what was already done with foam sealant, caulk and glue.
High Performance Windows and Doors
Windows
WINDOW PERFORMANCE
Insulation filled vinyl frames
Low-E Argon filled glass
U-factor = .2 (lower is better)
Solar Heat Gain Coefficient = .2 (lower is better)
Visible Transmittance = .5 (higher is better)
Energy Star and NFRC (National Fenestration Rating Council) Certifications
EXTERIOR DOOR PERFORMANCE
Insulation filled fiberglass
Low-E Argon filled glass
U-factor = .25 (lower is better)
Solar Heat Gain Coefficient = .25(lower is better)
Energy Star and NFRC (National Fenestration Rating Council) Certified
WHAT DO THESE NUMBERS REALLY MEAN?
U-Factor
U-Factor is a description of the rate of heat loss through a window assembly, or put more simply, it describes how well a window insulates. The lower the U-factor, the greater a window's resistance to heat flow and the better its insulating properties.
Interestingly, while the U-factor is used to express the insulation value of windows, R-value is used for insulation in most other parts of the building envelope (walls, floors, roofs). R value and U-factor are directly related. To compare R-value and U-factor, divide 1 by the U-factor number, E.g.: a 0.25 U-factor equals a 1/0.25 = 4 R-value.
At U-factor .28, 311 Varnum's double-pane windows are high performance. The highest performing windows, typically triple pane windows, can achieve U-factors as low as 0.15.
Solar Heat Gain Coefficient (SHGC)
The SHGC describes how much solar radiation (heat) passes through a window. It is a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits.
311 Varnum's SHGC is .3 which is a typical value for high performance double hung windows.
Visible Transmittance
The visible transmittance (VT) indicates the amount of visible light transmitted through a window. It is calculated by including not only the glass, but the frame as well. The higher the VT, the more light is transmitted.
While VT theoretically varies between 0 and 1, most values among double- and triple-pane windows are between 0.30 and 0.70. 311 Varnum's windows are .5.
Many thanks to the Efficient Windows Collaborative for these definitions. Please see www.efficientwindows.org for more detailed information about windows performance.
Insulation
All exterior walls are insulated to between R25-R35 through the use of dense pack cellulose or dense pack cellulose and polyiso board.
Also, the attic is insulated to minimum R60 with blow in cellulose at a depth of 16-20".
Third Party Performance Testing
After all the air sealing work is done, the building's leakage rate can be measured by doing a blower door test. A blower door test pressurizes a house and measures the amount of air that escapes to the outside.
Tight houses lose less heating and cooling to the outside...and therefore use less energy. The leakage rate is therefore a crucial number for understanding how well the house performs from an energy use standpoint. It is a critical input in the energy model.
The target leakage rate is below 4.25 ACH50. The final test has not been performed yet.
The exterior skin of a building is where the majority of energy efficiency work occurs. Read below to understand the major components of making a house energy efficient.
