In the last blog we discussed some of the attributes of the double stud wall assembly in a custom home that we are currently building in Haskill Basin. This home incorporates not only above code insulation and sealed framing and drywall techniques for a superior thermal envelope, but this home incorporates a geothermal heat system as well. As this home is on track to be an Energy Star home, on site workmanship is verified by an independent third party verifier utilizing real world performance testing of the home itself.
This double stud wall home incorporates advanced framing techniques of a 24” on center load bearing exterior 2 x 4 wall framed in line with a 2 x 4 non load bearing interior wall. A 12” wall cavity between these studs virtually eliminates thermal bridging of the exterior wall assembly. In addition to the oversize wall cavity being completely filled with blown cellulose, this assembly incorporates numerous intentionally redundant air sealing techniques on the interior and exterior planes of the building.
There are some interesting details involved in the air sealing of this home that are not found on standard homes. On the exterior of the building all sheeting seams are sealed and all Teak seams are sealed as well. All penetrations (outlets, lights, vents, etc.) are sealed with spray foam. After using the modified I building wrap technique and flashing pans (crafted out of advanced flex seal tape) all windows are carefully installed in the center of the opening and all shims are removed after installation. This attention to detail allows for a full bead of spray foam to completely adhere the window and the framing for a superior air seal around all window and door openings.
There are additional air sealing techniques on the interior plane as well. All interior wall penetrations (outlets, lights, vents, etc.) are completely sealed themselves and then sealed to the drywall also. The perimeter of wall and ceiling drywall is completely sealed using mastic and gaskets between drywall and the framing.
Some people will ask, “Just how important is the air sealing component when we have so much insulation in the walls and the attic cavity?” The answer is extremely important! Having a well-insulated building that lacks proper air sealing is much like wearing a sweater on a windy day. The insulation the sweater provides does not help you to stay warm if there is no windbreaker to keep that warm air near your body. And the building community is becoming increasingly aware of this fact.
“Air sealing has resulted in up to a 40% reduction of HVAC energy use in cold climates”
Source: NIST Report “Investigation of the impact of Commercial Building Envelope Airtightness on HVAC Energy Use”, S. J. Emmerich, Tim
McDowell, W. Anis
Another major benefit of reducing the amount of air that moves into the building cavities is due to the fact that moisture from the interior environment travels with this warm air.
“Air Currents…account for 90x more moisture vapor entering a wall cavity than diffusion.”
Source: EEBA conference September 27 2012 by Theresa A. Weston, PhD.
In an unsealed home – this moisture enters the building cavities and often has little insulation to prevent condensation when this moist air cools rapidly. This condensation in building cavities has been proven to lead to mold in the building cavities which leads to lower interior air quality and premature building degradation. Proper air sealing techniques drastically improve this situation and thus lead towards higher levels of interior comfort, better interior air quality, less building maintenance and longer life of the structure.
Not all of the energy savings occur with detailing in the field in the Haskill Basin home however. The design details of the home work towards energy savings as well. There are minimized outlets on the exterior walls. Ceiling penetrations (including can lights) have either been eliminated or completely sealed as well.
Although this double stud wall assembly is field proven and works extremely well – we think it is possible to improve upon this system. Just one way we will do this is by incorporating a vapor permeable 1” of spray foam on the inside of the exterior sheeting.
Another option for a structural system with a superior thermal envelope, minimized thermal bridging, and maximum air sealing is a Structural Insulated Panel (SIPs) frame. Our in-house build crew and team of tradesmen have experience with not only stud framing, but with SIPs building as well. This experience allows us to achieve similar thermal benefits as the double stud wall home, with a smaller wall cavity (resulting in a relatively smaller footprint) and less time erecting the super structure on site, amongst other benefits. Please keep an eye on the blog as we will soon have information pertaining to a SIPs home that we just completed. This SIPs home was both designed and built by our in house team for increased efficiency of the design/budget/build process. The new homeowner have expressed how happy and comfortable they are in their SIPs home as the winter sets in.
More details on SIPs homes coming in an upcoming blog soon!