Decision time: 3-glazed or 2-glazed windows in zone 4A?
New Construction: walls = R-26; unvented roof = R-65.8; slab = R-10. Hip roof with 4’ eaves all around. Aiming for airtight. Energy rater estimates load for both floors will be less than two tons (“1 ½ could get the job done”).
Main floor = 2155 sq.ft; doors & windows = 534 sq.ft = 24.7% of floor space
Main floor WALLS = 1992 sq.ft.
Lower floor (shop/future use/resale value) = 1549 sq.ft; door & windows = 265 sq.ft. = 17%
Lower floor walls = 2058 sq.ft.
Intus windows Arcade series, which provide Ug only:
2-glazed windows = 0.27 Btu/sq.ft.
3-glazed windows = 0.1264 Btu/sq.ft.
Note that Lstiburek et.al. recommends <.3 U for high R-value enclosures in zone 4 (http://buildingscience.com/documents/bareports/ba-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones/view) Cost increase from 2-glazed to 3-glazed = $6,800 = 13% increase Electricity costs $0.088/kWh Q1: Does it make sense economically? Q2: Will it make a difference in comfort? Plan is to use 1-9K & 1-12K minisplit on Main floor; 1-12K on Lower, all with short ducts. Since the air movement will be minimal, will the lower Ug make a difference is temperature stratification or variability across the space of a room? Q3: Recommendations for alternative choices for the windows?
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