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Practical Steps to Creating a High Performance Building Envelope

5/27/2015

There is a lot of talk about high-performance building envelopes. As the outer shell of a home, the building envelope encompasses the roof, exterior walls, and foundation. A “high-performance" building envelope is one that most efficiently facilitates the building's climate and moisture control, thereby increasing comfort and energy efficiency, (which reduces energy costs), and lowers the home's overall energy footprint. All good things.

There are many ways to increase performance – some produce only minor gains while others can significantly improve it. Whether you aim to simply meet code, achieve various certification levels, or go significantly beyond, here are some steps that are practical and fairly easy to implement on every project you undertake.


1. Roof

Roofs are subject to extensive heat, heavy rain, and/or snow, depending upon the zone. Its job is to resist water infiltration and protect the rest of the home.

• Good roof design cannot be overstated. A steep pitched roof with a wide overhang will naturally move water away from the walls and foundation beneath.

• Be mindful of the climate with regard to the roofing material that is chosen. Asphalt shingles, for example, are very durable but may have low solar reflectance. This can be addressed by increasing the insulation used in the attic.

• Proper flashing is always crucial, especially where roofs intersect with walls. The materials selected depend upon what you are flashing, the climate, and the other materials used on the roof.

• If attic spaces remain unconditioned with proper ventilation, air seal and insulate the attic floor properly. Check the code standards for your climate and determine what is required to exceed those standards.

• If attic spaces are conditioned and unvented to allow housing for HVAC equipment or to accommodate complicated ceiling designs, air sealing and ample insulation should be installed on the roof deck to thermally separate the roof from the attic. Doing so prevents moisture build-up and the formation of ice dams.


2. Exterior Walls

The biggest issues for walls are related to moisture penetration, particularly wind-driven rain, and heat loss.

• Water infiltration through windows, doors, and deck attachments that are not properly sealed is a common source of moisture. Attention to detail and even some redundancies here can make a big difference.

• Choose siding that is back-vented to allow moisture that penetrates behind to drain rather than get trapped. This allows the surface to dry.

• Air barriers should not be thought of as a single product but as a combination of products that work in concert to prevent air leakage: drywall, sheathing, housewrap, high-performance windows/doors, tape, caulk, spray foam, gaskets, and weatherstripping are prime examples.

• The role of insulation is to slow heat transfer. In colder climates, it is recommended that insulation should not be installed just in the stud cavities, but should also be installed outside the box by using rigid foam sheathing. When this is done, thermal bridging can be reduced greatly.


3. Foundation

There are two important facts about foundations and moisture control: The ground surrounding the foundation is always wet and foundation walls cannot dry to the outside. Once moisture hits the porous concrete, pressure differentials between the outside and the inside create capillary action that can pull moisture from the foundation all the way up to the roof, if the foundation is not properly protected.

• Good drainage and drainage systems are crucial. Grade the site properly, install footing and wall drain systems, and add gravel.

• Insulating the exterior of the foundation will not only block moisture from hitting the porous concrete in the first place, but also will keep the concrete warm. Warm concrete prevents air humidity from condensing on the wall.

Finally, the “tighter" the house, the more crucial it is to have adequate airflow/ventilation systems in place.

With just a few practical changes to standard building practice, the houses you build will be high performers for their homeowners and for your bottom line.

Related Tags: Thermal | Performance | Moisture