Page 24 - Zero Net Energy Case Study Homes-Volume 1

CASE STUDY NO. 1
CORONA DEL MAR NEW HOUSES
(Above) Airtightness: attaching the gaskets to the bottom
of the wall frame. (Photos courtesy of John Steed Homes.)
(Above) Airtightness: measure- ment device, called a manome- ter, for testing the number of air changes per hour at a certain air pressure created by the Blower Door. (Photos courtesy of John Steed Homes.)
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For insulation value, the windows at 703 Heliotrope are triple-glazed, providing an R-value equal to 5. As noted above, the solar exposure of the southeast-facing street facade suggested passive solar design using large window areas. To admit solar heat during the winter, the middle glazing layer has a special coating that admits sunlight readily but excludes the ultraviolet light to minimize its damaging effects. (Solar heat gain coefficent = 0.47). In the summer, blinds internal to the window units can be operated to provide shading.
At 609 Marigold, double-glazed windows were installed, which was the more cost-effective choice while still providing good thermal performance in this mild climate.
Building Envelope – Airtightness
One of the keys to minimize energy use in residential construction is airtightness. Building scientists confirmed this in the 1970s and it led to many construction techniques, products and testing methods that are used today. These are essential in ZNE residential structures, as will be discussed in all the case studies in this book. The houses of Case Study No. 1 utilized all recommended methodologies to control energy-wasteful air leakage and to verify via testing the air-tightness level achieved. As such, they are a good set of projects to begin that discussion.
One of the principal techniques to develop airtightness is to seal the wall and roof planes using air/vapor barrier tapes and sheet material, plus carefully seal the joints at exterior windows and doors. Enhanced air-sealing of these planes can be achieved by using closed cell spray foam insulation between the framing members, as was done with the construction of the house at 703 Heliotrope. Open cell spray foam insulation is permeable and does not have the air-sealing char- acteristics of the closed cell product. 609 Marigold is therefore not as airtight as 703 Heliotrope, which was borne out by the results of the testing procedure.
Another important technique is to seal the joint between the framing and the foundation system. This is achieved in both houses by using gaskets manufactured by Strandguard between the foundation and the framing to seal that common source of air leakage. (See figure at upper left.) The gaskets were compressed down to 1/16”, which created an airtight seal.
Good quality (airtight sealing) windows and doors are essential. In both houses, the contractor sealed the joints around these openings with bituthene flashing.
With “airtight” construction techniques employed to the appropriate degree, the houses were tested using a Blower Door1 and associated testing meters and recording devices.
In general, the result of the test is a number that gauges the airtightness of the house, known as ACH50, which is the unit of measurement for the number of air changes per hour at 50 pascals of pressure. The house is pressurized using the blower door and then the pressure difference is measured between inside and outside (after any leak points are located and sealed). As the contractor, John Steed, described in his blog about the projects in 2012, the benchmarks for airtightness of a house can be described as follows:
• >20 ACH50: poor airtightness (i.e., leaky) house
• • •
5 ACH50: adequate tightness per California Title-24 energy standards 2.5 ACH50: “stuffy” house—needs fresh air ventilation system
0.6 ACH50: Passive House2 standard
The result of the Blower Door Test for the house at 703 Heliotrope was 0.4 ACH50, below even the Passive House standard of 0.6 ACH50.
1 For a description of the Blower Door Test, see https://www.energy.gov/energysaver/blower- door-tests.
2 http://www.phius.org/what-is-passive-building
Zero Net Energy Case Study Homes: Volume 1