Page 45 - Zero Net Energy Case Study Homes-Volume 1

The building’s new flat roof joist space is filled completely with fiberglass insulation and the slight roof slope for rainwater drainage is created by a tapered polystyrene insulation layer. The result is a total average R-value equal to 32 for the roof assembly. The owners also selected a reflective single-ply membrane roof material that is recyclable at the end of its natural life.
The floor structure of the main level is also well-insulated since it is above the garage space and a large sloping crawl space. The owners decided that they could reduce heat loss through that floor further by controlling outside air leakage in these lower-level unconditioned spaces, so they sealed them and installed a low power fan to ventilate the crawl space minimally to prevent moisture development there. The result was a type of “thermal buffer zone” there.
High performance double-glazed window units were installed per code. The primary orientation of the largest area of glazing is toward the views to the southwest and west, requiring close at- tention to the design of sun control on that façade. Deep overhangs were created above those windows, which provided shading of the glass during the hottest part of the day in the summer.
Building Envelope – Air Tightness
As in the previous case study, the owners recognized the importance of air-tightness to the low-energy performance of the house. When the house had been structurally framed, with the insulated sheathing and windows installed and sealed, but before the insulation and interior fin- ishes were applied, the house was subjected to a Blower Test4. Air leakage points in the building envelope were identified and sealed.
When the final test was performed after the corrections were made, the house measured 1.12 ACH50, an exceptional level of air-tightness. This compares with the Passive House5 standard of 0.6 ACH50 and the California energy standard of 5.0 ACH50. (See the table for benchmarks of air tightness on p.12 of Case Study No. 1, “Corona del Mar New Houses”.)
Natural Ventilation
Natural ventilation is induced on warm and pleasant days by the central stairwell that had to be added for access to the new upper level of living spaces. Extra height was added to this central space, which takes advantage of the natural buoyancy of air to induce air flow from window openings on both living levels to the openable clerestory windows at the top of tall space. Like the case study house at 703 Heliotrope (see Case Study No. 1), this “thermal chimney” design feature provides a natural cooling option with no use of electric energy.
Because the building envelope is well-sealed and constructed with the insulated sheathing board, which also acts as a sound barrier, the natural ventilation “mode of operation” (with open windows) is noticeable because of the difference in the interior acoustic environment. The own- ers find it acceptable for them at this site location, unlike with the case study house at 703 Helio- trope (see Case Study No. 1).
Because of the air-tightness of the construction, indoor air quality (IAQ) is an important factor to consider. There can be a build-up of toxins and chemicals in the air if there is not adequate outdoor air flow that effectively removes them. One option, standard in a Passive House, is to include a separate system for fresh air supply such as an HRV (heat-recovery ventilator) or ERV
4 See this volume, “Case Study No. 1-Corona del Mar New Houses”, p. 12, for a description and benchmarks of air-tightness in houses. See also https://www.energy.gov/energysaver/ blower-door-tests.
5 http://www.phius.org/what-is-passive-building
Zero Net Energy Case Study Homes: Volume 1
(Above) Meter reading of final Blower Door test showing 376 cu. ft. per minute at the blower door fan with 50 pascals of pressure in the house, con- verting to 1.12 air changes per hour (1.12 ACH50).
FORTUNATO HOUSE RENOVATION CASE STUDY NO. 2
(Opposite page) The central space containing the connect- ing stair acts as a “thermal chimney”, providing passive ventilation and free cooling of the house at all times.
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