Designing for High Performance: Slab-On-Grade, Part III – Controlling Air Flow

In an earlier post, I talked about 3 of the most important strategies for making a home energy efficient. They were; 1. Air sealing, 2. Air sealing, and 3. Air sealing.

Air sealed, or tight, homes do a better job at containing the conditioned air coming from the HVAC system where it belongs…inside. Tight homes also keep the unwanted hot/humid or cold/dry outdoor air where it belongs…outside. Both of these keep your HVAC system running less, which ultimately reduces your energy bill. In other words, the home is more efficient.

Fresh air should be brought in to the home through mechanical ventilation, which maintains proper levels that fresh air, and can also filter and condition it depending on the type of ventilation strategy employed in your home.

By the way: Tight homes also do a better job at keeping moisture, pollutants, sound and insects from getting!

Stopping Air Flow

If you’ve read Part I and Part II of this series, you’ll remember this detail (below). In the earlier posts, we point out our strategies for controlling moisture and heat flow at the slab-on-grade condition of the Proud Green Home at Serenbe. Today, it’s all about air sealing. (Note: this detail is for a home in North GA, or Climate Zone 3A. Local conditions vary, and each home should be designed for it’s local climate)

The 1″ Zip System R-Panel is made up of the widely used Zip System wall sheathing and a 1/2″ layer of EPS foam insulation. Each of these materials are well know air barriers on their own, but have been brought together in to a single product to act as a structural component, air barrier and thermal break. As is done with their sheathing, Zip Systems Tape is applied to all seams between each panel and at all penetrations. Windows are given extra attention to properly flash so as to prevent air and/or water from finding it’s way back in to the building.

The Zip System Tape not only performs very well at all sheathing seams and penetrations, but it also adheres very well to concrete. In fact, it is one of the better products for this application. We are applying a continuous strip of the tape at the bottom of the R-Panel where it meets the foundation wall. This will prevent air flow at the base of the panels.

The sill gasket shown below the bottom plate, in conjunction with the continuous Zip Tape at the exterior seam between the plate and slab, works well at an area know for a lot of infiltration. We’re also going to be applying a continuous strip of the tape on the inside of the wall at plate-slab junction to provide insurance for a tight seal. Similar attention will be paid to the top of the wall where we will have double plates. Another prime location for air leakage.

Above the slab

In the walls, you will see we will be completely filling the cavities with expanding open-cell spray foam. When installed properly, spray foam will not only insulate our walls but will provide an excellent air barrier in and around the entire cavity.

Finally, we’ve chosen Marvin’s Integrity Windows because of their durability and low air leakage rate. We will install sealant on the back side of the window fin before placing it in the opening. Then, we will use expanding foam between the window and the sill, jambs and head.