Building Diagnostics Help

“To realize optimum insulation potential, either the framer has to build an airtight frame or the insulation contractor has to deliver an airtight product. A hybrid approach may be the best practical option given the segmented nature of the construction industry.”

There’s a point in the construction sequence where the framer stops and the insulation contractor starts. It’s at this juncture that the average construction plans become vague or incomplete. This lack of continuity increases the likelihood of building and insulation details that allow thermal and air bypasses to occur in the building envelope. The thermal insulation envelope must be continuous with the air barrier to avoid air leakage short circuits and provide the comfort levels today’s consumer want.

The goal is to install void-free insulation into preconfigured, airtight framing bays. Since every building or renovation has its own detailing challenges, not to mention budget and time constraints, finding creative and cost-effective solutions is crucial. We’ve found that no single system, but rather a hybrid combination of materials and practices work well under a variety of situations. It can be a slow process to train and motivate the various sub contractors into redefining their roles and responsibilities. In the mean time builders can do all the work themselves or take steps to improve the details, specifications, and materials to establish a higher standard for their subs.

Gaps Gaskets and Glue
The best time to prevent air leakage through and around framing components is while framing. This sounds obvious but is rarely done. We use a variety of rubber gaskets, typically EPDM , made for airtight construction. These gaskets have good “memory” and superior gap-filling properties. The mudsill gaskets we use cost slightly more but take no more time to install than the conventional, leak-prone products. The joint between the rim joist and the mudsill is a prime leak area too. We’ve tried gaskets, but they tend to interfere with squaring and adjusting the rim. Instead, we run a continuous bead of sealant around the interior joint after squaring the rims, just prior to setting the joists. Gluing sub floor plywood has almost become an industry standard. But to eliminate leakage above the rim joist, we make sure to run a continuous bead of adhesive around the perimeter.

Gaskets also work well under exterior partition plates and add little labor to the framing routine. We staple them to the sub floor before sheathing and standing the exterior partitions.

The tightest window is always susceptible to drafts between the window jamb and the rough opening. Fiberglass isn’t effective in stopping air flow and expanding foam can be difficult to control running the risk of distorting the jambs. Instead, we use a double-tube configuration thin-wall rubber gasket that slides into surprisingly skinny cracks without undue force and does a near-perfect job of air sealing. As the gap between the window and the frame changes from season to season the “butterfly” action of the double tube gasket moves with it.

Exterior wall configuration
Infrared scans of existing walls confirm that cavity insulation works best when it installed in an air tight configuration. To accomplish this we construct an air tight insulation sandwich for all exterior walls and sloped ceilings. Depending on a number of variables, such as siding type, this air tight sandwich is constructed of some combination of exterior and interior air tight sheathing with dry -blown, dense packed cellulose in between.

One option uses taped rigid insulation board as an interior air barrier. The rigid board adds thermal value and also acts as a containment skin for the dense packed dry cellulose. The caveat for this strategy is being aware that the exterior sheathing will be colder and more susceptible to condensation therefore it is crucial to carefully seal the interior air barrier to avoid (minimize) moisture movement into the cavity. An alternative strategy places the rigid insulation board on the exterior and uses the drywall itself as the interior air barrier taking care to install the drywall to the frame using a combination of sealants and drywall gaskets.
Specific configurations may have to be modified for various climate conditions.

Substituting the exterior foam in place of structural sheathing makes more sense as plywood and OSB prices soar. This approach may be limited to buildings that can be re-engineered to accommodate additional bracing and racking considerations. Strategies include using plywood sheathing at building corners and metal bracing attached to the studs and wrapped around the plates as shown.

A wall framed with two by four studs can be upgraded by applying an additional inch and a half interior “strapping”. The resulting wall will accommodate a standard 4 9/16 jamb while offering improved thermal performance over a standard two by six wall. Thermal losses through the studs, headers, and sills are improved by installing cellulose insulation to provide a thermal break.

More leaks and more strategies
Once the exterior seams are sealed and the building is weather tight, we turn our attention to interior leakage locations between the heated space and the attics. The first step is to air-seal and caulk all voids around wiring and plumbing penetrations. These holes are obvious conduits for air movement and are easy to eliminate. Expanding polyurethane foam seals the smaller passages; around larger holes, a combination of foam board, or plywood and expanding foam closes the gap.

Drywall can utilized as part of the air barrier by sealing the perimeter with adhesive or gaskets, prior to wallboard installation.

It’s difficult to properly seal and insulate certain intricate or hard-to-reach framing transitions, like floors under knee walls, cantilevered rim joists, tray ceilings, and other complex configurations. We’ve had good success using a two-component polyurethane foam especially in remedial applications where initial air sealing is absent. The two-component pressurized system is a rapid-high-expansion foam, packaged with a 30-foot hose and applicator, with a 600 board-foot coverage capacity. The foam cures in seconds and makes it simple to seal otherwise challenging configurations in short order. For more on Kneewalls and related key juncture issues click here (doc).

Tight by Design
Our clients come to us specifically because they want a super-efficient, high-performance new home or renovation while realizing a reasonable payback in energy savings. The goal is to build an airtight configuration before the insulation is installed. Air leakage control is not a stand-alone component that comes out of a tube, but rather a total strategy and mind set that is continuous throughout the building process.

The best strategy for avoiding air leakage problems is a comprehensive approach that starts by asking the critical questions at the design stage:

• Where is the air barrier best defined?
• Is it continuously paired with the thermal envelope?
• Where are the framing intersections that represent bypass leakage areas?
• How should these areas be treated?
• Is the configuration moisture friendly?
• How will the insulation placement change the dew point of the exterior sheathing?

By addressing these questions from the start and utilizing a blower door, infrared scanner and test smoke we can produce high performance buildings shells that are tight, well-insulated, and comfortable.