Building Envelope & Air Sealing Design

 1.

Optimize framing to reduce thermal bridging effects and add rigid exterior insulation. The changes from typical framing practices needed to accomplish this would include:

• Employing specific framing layouts to eliminate unnecessary framing members, using correct sized headers, and locating openings to match framing members
   
• Using optimum value engineered (OVE) techniques – such as turned stud corners, stand-off wall intersections, optimized window locations, etc – to reduce thermal bridging in corners and wall intersections
   
• Panelizing wall sections to minimize or eliminate framing confusion in the field
   
• Using exterior insulating sheathing

 2.

Another practical method to increase wall performance is to increase the dimension of the wall framing, typically from 2x4 framing to 2x6 framing. This effort will require a more extensive design change to the house framing but, with planning, can have limited cost increase compared with 2x4 framing. Features of this design could include: 

• In-line framing at 24” o.c.
   
• location of first floor headers to the band area,
   
• incorporation of optimized framing techniques to limit framing member count,

Planning: In terms of cost, these framing techniques can add anywhere from $0 to more than $1,000 to typical construction costs. In order to contain costs as much as possible, consider these practices:  

• Begin your planning at the design stage; lay out the walls and window location to optimize lumber use, and specify on the plans the OVE features that will be used; specify the air sealing and water management approach for your exterior walls – if you’re using panelized walls, some air sealing can be done in the plant which will lower your cost and simplify field installation
   
• Determine with your framer/panelizer the best structural approach to incorporate exterior insulating sheathing while limiting cost increases
   
• Meet with your trades before construction begins to discuss the importance of having the wall system perform both structurally and thermally; establish trade scopes of work that identify all aspects of the thermal enclosure that must be reviewed jointly by the builder and trade at strategic points throughout the construction process
   
• Use specific testing protocols – such as a blower door test – to ensure design goals have been met

Benefits: These optimization techniques should impose minimal disruption to your typical building practices, especially if you begin to implement the plan during the design phase. At the same time, they can significantly increase the building’s efficiency independent of its HVAC efficiency – allowing more total efficiency options for you to market to the homebuyer. Because these practices also work best when approached as a team effort between you, your suppliers, and your trades, they can also foster a more focused, invested workforce that is motivated to suggest and/or implement additional innovations of their own.

Challenges: Despite the significant upside of using these techniques, there are some aspects to consider, namely:  

• Since all building materials require different field handling and care, be sure to understand these requirements for any new materials you use
   
• You will need to select siding that has rated attachment profiles to accommodate non-nail-based sheathing and 24” o.c. framing
   
• You will need to plan for extended jambs and sills due to the increased wall dimensions
   
• Trades will need to establish/document a method to communicate your scopes of work to new employees; or changes in scopes to existing employees
   
• Greater cost increases should be expected until you fully realize all optimization opportunities in the techniques you select
   
• You will need to train your sales staff on the value of these high-performance features so that they can communicate that value to customers appropriately and effectively

 1.

Consider sill plate gaskets designed to provide both a capillary break and an air seal
 

2.

Use house wrap as the exterior air barrier

• Tape all vertical and horizontal seams and penetrations
• Seal top and bottom edges of the house wrap to the structure
• Install the wrap using appropriate “shingling” methods to maintain the water barrier characteristics

 3.

Use a structural insulated sheathing system as the exterior air barrier
 

 4.

Use an airtight drywall approach as an interior air barrier

• Consider using continuous gaskets or drywall adhesives that are also sealants
• Seal top and bottom plates and around rough openings of exterior walls
• Seal top plates of interior walls
• Seal all ceiling penetrations at the attic

 5.

Isolate the attic from exterior walls and conditioned space

• Seal interior drywall and exterior sheathing at top plates using gaskets or sealant
• Consider spray foaming over top plates from the attic
• Install gasket or sealant for all ATIC recessed light fixtures at the attic
• Install gaskets for attic access panels or stairs

 6.

Seal rim joist areas (critical for both interior and exterior air barrier designs)
 

 7.

Seal exterior wall bottom plates to deck using caulk or gaskets
 

 8.

Seal window and door rough openings
 

 9.

Seal garage-side drywall at conditioned space

• Top and bottom plates
• Around doors and electrical boxes

 10.

Seal all framed cavity air barriers at the thermal air barrier (e.g., fireplaces, attic knee walls, dropped ceilings and soffits, behind tubs, cantilevered floors, garage draft stops)
 

 11.

Seal all HVAC ductwork (ideally, all located in conditioned space)
 

Minimize size of holes for mechanicals through framing and air barriers
 

 13.

Seal around and behind electrical boxes (both interior and exterior)
 

 14.

Consider holding interior wall framing back one inch from exterior wall to provide continuous drywall using approved top plate bracket 

Planning: To make the most of your air sealing efforts, be sure to consider the following: 

• Develop a whole building design approach that considers all of the air sealing, insulation, structural, and energy efficiency systems and details of how the components work together
   
• Integrate the design and energy performance measurements of a home through a quality management system; should include a detailed list of responsibilities for each trade partner and scopes of work
   
• Identify any necessary construction monitoring points required to review, test, and document what may not be in the standard specifications and incorporate these points into the construction schedule
   
• Complete the Energy Star Thermal Bypass Checklist (a good idea whether you’re seeking Energy Star certification or not)
   
• Perform a blower door test to check for any missed leaks that can be repaired or minimized, and to check the actual air-tightness of the home
   
• Conduct long-term monitoring of the home’s actual energy use

Benefits: Complete and proper air sealing is generally very cost effective and has a number of benefits when trying to produce a truly high-performance, energy-efficient home.

• Results in a significantly smaller amount of energy required to heat and cool the home (30-40% less per DOE estimates) due to reduced air leakage and infiltration and minimized wall convection losses
   
• Contributes to lower HVAC equipment and operating costs
   
• Reduces potential for moisture accumulation inside the wall cavity, thereby reducing the risk for mold, decay, and structural deterioration
   
• Optimizes insulation effectiveness by reducing thermal bypass
   
• Helps prevent contaminants (e.g., from garage, crawl space, attic, wall cavities) from entering the home
   
• Allows for controlled mechanical fresh air ventilation

Challenges: In general, the benefits far outweigh the costs for air sealing, but there are some things to keep in mind.

• Be sure to consider vapor retarder code requirements during wall design
   
• Do not inadvertently create a vapor barrier where one is not desired
   
• Always get your trade partners on board early during the project design so they understand the importance of air sealing and their critical roles
• Do not ignore the details – air barriers must be continuous to be effective.