Getting back to our Dummerston, Vermont farmhouse this week, I’m reporting on our use of a relatively new product for air-sealing homes: EcoSeal from Knauf Insulation.

First some context: In the building science world, there is growing interest in achieving a robust air barrier at the sheathing layer of a house, with layers inside of that able to dry toward the interior and layers on the outside able to dry to the exterior. To make that work, the sheathing layer has to be tightly air-sealed.

In our house, we used Zip System sheathing from Huber Engineered Wood as the sheathing layer, taping the joints. Zip System sheathing is an oriented strand board (OSB) sheathing that has a coating to improve weather resistance and reduce permeability — so it makes a great air barrier. The version of Zip used for wall sheathing is green and the version used for roof sheathing is a reddish color. Huber also distributes a high-performance tape — the tape is manufactured by 3M — that’s used for sealing joints and edges of the Zip sheathing.

It’s tricky to air seal an old house

In working with an older house like ours, there are inevitably some irregularities that make air sealing with the sheathing more difficult. With our house (originally built in the early 1800s), for example, there are beams at the top of the eave walls that extend 4 inches out from the wall plane (oddly), and we had to box those in with the sheathing. There may also be some air leakage at the joints, despite the taping.

So to achieve an airtight sheathing layer, it helps to add some air sealing from the interior. Some builders use a “flash and batt” system for this: a thin layer of spray polyurethane foam (SPF) is applied against the sheathing from the interior (up to about an inch thick) and the cavity is then insulated with batt or other cavity-fill insulation. The SPF is great at air sealing but pretty expensive as an insulation material, so flash-and-batt is a reasonable solution.

Another solution is to use one of two new products for sealing just the joints and cracks at the sheathing layer. Owens Corning makes the EnergyComplete system, and Knauf makes EcoSeal. (Warning: this last link opens with an installation video that has a somewhat jarring soundtrack).

An acrylic air-sealing system

EcoSeal is an acrylic product that is applied using high-pressure paint-spraying equipment. The installer arrived with two 5-gallons buckets of the bright-blue acrylic material that was the consistency of very thick paint. The system comes with a long, 200-foot hose, so the pump and bucket can stay in one place in the house while the work proceeds. The pump is very quiet.

The installer started on the first floor and worked methodically around the room sealing all the joints and cracks, and then moved upstairs. We had arranged for someone from Efficiency Vermont to come down with a blower door (a device used for testing the airtightness of a house) and run the blower door during the EcoSeal installation.

Here’s how a blower door works: a fan in the blower door depressurizes the house enough to maintain a 50 pascal difference is air pressure between the inside and outside — as measured by an integral manometer (air pressure gauge). Instrumentation in the unit calculates the cubic feet per minute (cfm) of air flow going through the fan to maintain the 50 pascal pressure difference.

The blower door, as we used it, did two things: first, it exaggerated the air leakage so the installer could feel cracks that needed sealing; and second, it allowed us to measure the success of the air sealing.

EcoSeal doesn’t expand as it is installed (as do foam sealants), and it takes up to day to fully cure. The cure time depends significantly on the environmental conditions — temperature, humidity, etc. Our house was fairly cool during installation, so the cure time was significant. The material can span up to about a 3/8-inch gap, according to Knauf, and it remains flexible.

If EcoSeal gets on surfaces where it doesn’t belong (as occurred once during our installation when some got on one of our windows), it easily washes off with water. We were in the house throughout the installation and could barely smell it, so I’m confident that it has low VOC (volatile organic compound) emissions.

Significant measured improvement

When we started EcoSeal installation, the blower door was showing 950 cfm of air leakage at 50 pascals (cfm50). During the course of about four hours of work on the air sealing, that air leakage rate dropped to 640 cfm50. That’s an improvement of a third — not bad.

Given the volume of the house, 640 cfm50 is equivalent to 1.6 air changes per hour at 50 pascals (ach50), which is very respectable for a new house, let alone a renovation.

EcoSeal costs about $1,000 to $1,500 per house

Knauf Insulation introduced EcoSeal in January 2011. It’s still a very new product, with only 100 installers nationwide, according to Brett Welch of the company. He estimates that about 2,500 homes have so far been sealed with the system.

In houses where there hasn’t been as much attention paid to air tightening (no taped sheathing), a more typical tightness achieved is between 2.5 and 3.0 ach50. Welch said EcoSeal has also been used in a few Passivhaus projects, where airtightness of 0.6 ach50 must be achieved.

EcoSeal costs $200 to $250 per 5-gallon bucket, according to Welch, with 2 to 3 buckets typically required for a house. He estimates 6 to 10 hours of labor for an installation, bringing the total installed cost into the $1,000 to $1,500 range. It can be installed at temperatures ranging from 20°F to 115°F, though at the low temperature range, the material in the bucket must be fluid and the cure time is longer. It can be stored at 35°F to 120°F.

Similar system from Owens Corning

While Knauf’s EcoSeal is a one-part system, Owens Corning’s EnergyComplete is a two-part system that is foamed in place. It expands slightly as it is installed and sets up very quickly — in less that a half-hour. I don’t have personal experience with EnergyComplete, but visited a Passivhaus building under construction in Palo Alto, California, in late 2010 that had just been sealed with the system, and was impressed with it.

[Editor’s note: For a comparison of EcoSeal and EnergyComplete, see Air Sealing With Sprayable Caulk.]

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. In 2012 he founded the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.