Not every new home built has the budget to be high performance. That doesn’t mean some energy efficiency measures and decisions based on sound building science can’t be added to a design to build a better house.
Recently, I was involved in a project that had a tight budget, but we were able to squeeze an additional 3% in to accommodate a few upgrades that I felt would reduce operating costs, result in more comfortable conditions, and improve the durability of the structure. This build happened during the summer/fall of 2020, right at the time of the lumber price hikes. Luckily, most of the materials had been ordered before the increases.
The home is located in northern Minnesota, climate zone 7. It’s a slab-on-grade, three-bedroom, two-bath, 1550-sq.-ft. ranch-style house. There is an attached, conditioned garage with an additional 720 sq. ft. The main source of heat is in-slab hot water served by a natural gas boiler, which also supplies the domestic hot water. A minisplit air-source heat pump supplies supplemental heating and as well as cooling. There is also a sealed-combustion gas fireplace sized large enough to heat the entire home should the boiler fail. A heat recovery ventilator (HRV) supplies fresh air and is used for spot ventilation—bump switches in the bathrooms increase the HRV’s fan speed when the bathroom is in use.
During the initial planning, I conducted a simple energy model using the Department of Energy’s BEopt 2.8 software. I was looking at two different upgrades. The first targeted energy efficiency; I knew it was possible to improve the airtightness from 3ACH50 to less than 1ACH50. The second was to see how adding exterior insulation—done to eliminate the need for a class I or class II vapor retarder—would affect the performance. Because this home…