Remodeler Heather Thompson, supervisor Mark Pollard, and architect Rachel Conly were holding their breath as Harvey Johnson set up the blower door, connected the controls, and started the test. But it was high fives all around as the blower door told its tale: the three-bedroom, two bath house they had been remodeling all winter was blowing away their expectations (see Slideshow).
"I'm hoping for 1.0 ACH50," Pollard said privately before the test, "but I'm telling everyone we're aiming for 1.5." As the digital controller settled down, however, Pollard realized that the home's performance was far surpassing those guesses. At 50 pascals of depressurization, airflow through the door measured about 120 cubic feet per minute (CFM) — somewhere between 0.4 and 0.5 air changes per hour (ACH). The team wasn't trying to meet the Passive House standard in this project. But at least where airtightness is concerned, the project has surpassed not just the Passive House EnerPHit blower door target of 1.0 ACH50, but even the Passive House airtightness spec for new construction of 0.6 ACH50.
Architect Rachel Conly watches as Harvey Johnson adjusts the settings of the blower door control.
Rachel Conly and Ed Muennich find and seal air leaks.
After a few minutes of celebration, the crew set about chasing leaks, trying to drive the number even lower (see slideshow). Blower door tests serve at least two purposes: One is to quantify the home's leakiness, which is important not just to demonstrate compliance with codes or program standards, but also to verify that the building's heating and cooling loads won't overtax the specified equipment. Another purpose, however, is to enable crews to identify and plug any leaks, large or small. Chasing leaks during a blower-door test can help bring the building's energy performance into line with its mechanicals, especially if large leaks are identified. And it also enhances durability, by helping to eliminate situations where an air leak may expose moisture-sensitive materials to indoor or outdoor water vapor carried to a cold condensing surface on small currents of air.
In this project, most of the leaks were tiny and hard to find. But the test did shine a light on a few problematic vulnerabilities in the building's air pressure boundary. The walls and the roof had no significant issues. But the basement floor, basement walls, and the intersection between the basement and the main house frame showed up as concerns.
The original house had been built with a masonry block foundation, set directly on soil with no footings. There was no basement slab — instead, rough irregular ledge formed the basement floor, bringing the ceiling height to around five feet at the rock out-cropping's highest point. In the remodel, Thompson Johnson had excavated under the perimeter block and installed concrete spread footings by trenching, forming, and placing just a few feet of footing at a time. Then they had placed a concrete slab in most of the basement, but not all of it — the estimated cost of excavating the ledge in the center of the basement proved too much for the budget, so the ledge remains in place in part of the basement, and the new slab terminates against this irregular bulge of stone.
Inside the block walls, masons repointed the walls and applied a cementitious parge. Then, the crew installed foil-faced polyiso insulation against the block, sealing the joints in the insulation with tape.
Even after all that work, the basement was not quite airtight. The irregular joint between the fresh slab and the existing ledge allows some soil gas into the room. And the parged masonry block walls, exposed to the outside air, are still porous. The taped foam on the basement walls is largely airtight; but with the blower door running, the crew was able to feel air seeping in at the top of the foam-faced walls, below the sill beam of the first floor frame. Pollard and his crew attacked that gap with canned foam sealant. The leakage around the slab will be addressed through the building's active radon control system: perforated pipe laid in coarse gravel, with an exhaust stack directed through an outside wall and equipped with a powered exhaust fan.
At the end of the test, point sealing had brought the blower door reading down to 112 CFM — easily low enough to surpass any standard in the market.