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 of 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).

Designing for High Performance Slab-On-Grad Part III Controlling Air Flow

The 1″ Zip System R-Panel is made up of the widely used Zip System wall sheathing and a 1/2″ layer of Polyisocyanurate foam insulation. Each of these materials are well known air barriers on their own, but have been brought together into 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 known 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 the 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 the 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.

- Written by Chris Laumer-Giddens, Architect, HVAC Designer, Building Science Professional, Certified HERS Rater, LEED AP

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5 Responses so far.

  1. Dan Bovinich says:

    Very good article. I like all the details.

  2. Thanks, Dan! I appreciate the feedback.

  3. Good depictions and details…. in addtion to the above wall construction we have added the following details to our advanced framing system and have noticed improved performance:

    1) We install on both the top and bottom wall plate inside faces 4″ of sill seal foam strip. We do this for all walls – interior and exterior. This creates an absolute air tight seal to the stud wall bays, due to the minor variances to framing lumber, and prevents any possible latent air leakage from unconditioned attic areas. The sheet rock compresses the seal and eliminates any minor convection air currents that could result in the stud wall. Believe it or not, convection air currents can still develop even within in closed cell stud wall cavities, behind the sheetrock.

    2) We actually have had better air tight results at the frame wall/concrete connection using construction adhesive in lieu of the foam sill seal. As the sill seal does not always compress adequately due to workmanship finish tolerance at the top of slab or concrete stem wall. Then as an additional measure, we run a final bead of construction adhesive at the base plate and subfloor edge on the inside just to make sure that we have sealed all possible connection air gaps — no matter how small.

    3) Finally, all recessed can and light boxes are caulked sealed to the sheet rock cut outs.

    4) All exterior wall drywall used is 5/8″ not 1/2″. Very small expense but adds moderately to internal thermal massing.

    These steps are really not all that expensive, but pay huge dividends for building energy efficiency. We have successfully incorporated these steps with our framers, insulation and drywall contractors for a relatively insignificant premium.

    Along with your tight wall details, these added procedures have added less than 1 1/2% in overall total construction cost — and well worth the time and effort. It also allows downsizing of mechanical equipment, lowering cost on that end. The results are present in finished product blower door tests which can be demonstrated to homeowner customers — and makes a terrific sales demonstration differentiating us from the competition!

  4. […] the energy efficient and durability design and preservation of the original house. All moisture, air and thermal control will be done on the outside of the structure to keep it protected from the […]

  5. […] a leaky house. Simple and proper air sealing details can make a home go from a sieve to an airtight chamber, and can reduce the heating or […]