Page 63 - Zero Net Energy Case Study Homes-Volume 2

STRATTON-LEE DIY HOUSE
CASE STUDY NO. 8
A slightly different approach was taken with the roof structure, whose rafters were made only with 2X6 framing members. In the living space (Phase 1), since the attic was removed to create a larger volume, 2X12 lumber was attached to each 2X6 as part of strengthening the structure. This also allowed the insulation value of the roof to be raised substantially by providing space to add more insulation. The same 3” of dense polyisocyanurate insulation was installed adjacent to the exterior sheathing and roofing, and 6” of fiberglass insulation filled the rest of the enlarged cavity before applying the 1” of rigid polyisocyanurate insulation board. The net result was a total roof R-value of R = 46. (See sketch on the opposite page.)
In the bedroom area (Phase 2), since the attic space was retained, the owners opted for a conventional ventilated attic design. The ceiling of this area was well-insulated by adding 14” of blown fiberglass insulation on the floor of the attic, and good ventilation of the attic space was achieved by installing rafter bay vents and a continuous ridge vent.
The original windows had already been replaced some years before with double-glazed vinyl windows. The owners decided that changing all the windows would only be marginally effective and, in the interest of maintaining very low cost to the improvements and to save time in the DIY work schedule, the owners opted to leave the windows in place.
Building Envelope — Airtightness
Airtightness is a key design strategy for any home targeting ZNE and zero-carbon performance. For this project, the air-sealing had to be done from the inside and without the advantage pro- vided by reframing. It was, therefore, a painstaking effort. Spray-foam or acoustical caulking was used to seal gaps between the insulation and the studs.
The blower door test was done several times during the final stages of construction. Preliminary tests yielded 18.3 ACH50 before sealing anything and 8 ACH50 after Phase 1 was completed. The entire house achieved 4.1 ACH50 in the final blower door test.
The crawl space was encapsulated. With the encapsulated crawl space, usually it is wise to use either a supply duct from the air conditioning system or a humidity-controlled exhaust fan to keep the humidity within desirable limits to prevent excessive moisture levels. Chris installed a relative humidity recording device to see if either was necessary. It was recorded that the relative humid- ity never got as high as 60% so neither action was taken.
Heating, Ventilating and Cooling Systems – Domestic Hot Water
The house was retrofitted with a completely new heating and cooling system. The system is comprised of a heat pump mini-split system and an energy recovery ventilator (ERV)3. There are two mini-splits, one in each of the two parts of the house, with an exterior compressor. Each
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mini-split system is rated at 1.0 tons and the wall units are rated at SEER 26.1 . The mini-split
serving the Phase 1 living area is ductless, but the second mini-split has a supply duct to each of the bedrooms. The exterior compressor for both mini-splits is 1.8 tons.
3 See M. Holladay, “HRV or ERV”, Green Building Advisor, (Jan., 2010), https://www.green- buildingadvisor.com/article/hrv-or-erv; see also D. Boyer, “Choosing between an HRV and an ERV”, Ecohome, (Feb., 2014), https://www.ecohome.net/guides/2276/choosing-between-an- hrv-and-an-erv/
4 “Seasonal Energy Efficiency Ratio” or SEER is the ratio of the cooling output of a heat pump in Btu over a typical cooling season divided by the energy consumed in watt-hours. The higher the SEER ratio, then the higher the energy efficiency of the system.
Zero Net Energy Case Study Homes: Volume 2
(Above, top) Insulating the roof in Phase 1 living area with the 6” fiberglass batts.
(Above, middle) Sealing the gaps with caulking to make the house airtight.
(Above, bottom) Encapsulating the crawl space.
(Photos by Wen Lee)
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