Your Home’s Building Envelope

Sara Grochowski
Do It Green! Minnesota; Original content by Bob Alf Construction

Improving your home’s building envelope is a worthwhile investment as it will save time, money, and energy. Four components of a building envelope ensure maximum control of heat and moisture.

The Four Components:

  1. Weather Barrier: a building’s outer skin that keeps it dry
  2. Air Barrier: limits air leakage or infiltration
  3. Thermal Barrier: prevents movement of heat in or out
  4. Vapor Barrier: controls movement of water vapor

These components may sound like concepts that only the most intense engineer could enjoy. However, building scientists now understand that these are some of the most important considerations for reaching our environmental and economic goals. You can easily understand some of the basics that will greatly help in your home improvements whether you do the work yourself or hire someone else.

Before looking at a few details, we must first debunk the myth that your home is simply a combination of siding, insulation, and drywall or plaster. Homes are complex systems and a small change in one component can have a larger impact on other components. For example, if you were to add insulation to the attic you may find excessive interior moisture (e.g. from showers) condensing or freezing on the underside of the roof, especially during the winter months. This frost may accumulate throughout the winter and eventually melt in the spring, thus causing damage and stains on the ceiling below. Unfortunately, this occurs too frequently. Most people think that it’s a leaky roof problem. Rather, it’s an interior moisture issue that is often solved by the proper use of bath exhaust fans and minimizing the use of humidification systems.

Let’s narrow our focus and consider just the air barrier and thermal barrier which we will refer to as your thermal envelope. In addressing your home’s thermal envelope, the most critical steps are to have a plan and to understand certain rules of building science:

  • Warm, moist indoor air drives toward colder, drier air and surfaces. In the winter, warm, moist air moves toward the outside wall or attic; it does not move around the house evenly keeping your skin from drying out!
  • During the winter there is a stack effect in most homes caused by warm air rising. As warm air rises, pressure increases upstairs (positive air pressure) and leaves a void downstairs (negative air pressure). The valuable warm air is pushed out of the envelope in the upper half of the house while cold air is drawn into the lower levels.
  • Creating air barriers should take priority over insulation. Imagine having wall studs with just fiberglass insulation and no walls or siding. Your warm air would just pour through it! So you need a good air barrier first and foremost, then move to maximize insulation.
  • The more air sealing and insulation, the more you need to control sources of interior moisture. Remember that example of frost on the underside of the roof? It becomes urgent to exhaust bath, kitchen, and other sources of moisture, especially in winter. Failing to do so will likely result in damage to your ceiling come spring.
  • Control depressurization risks and air pollutant risks as the air barrier and insulation are improved. A tight house is effective at saving energy but can result in depressurization that may lead to carbon monoxide poisoning and increased radon. Depressurization can be controlled by adding make-up air to your basement and/or by exchanging your old water heater and furnace or boiler with high-efficiency units. An air exchanger may be needed to provide fresh circulating air. Assessing this will likely require advice from a knowledgeable building-science professional.

Now that you realize that healthy and energy efficient envelopes are complex, you need to start your improvements by first developing a good plan. Your plan should include a drawing of your home’s thermal envelope, outlining all four sides of the home. Identify gaps in the air barrier (an energy audit can do this) and list them on your drawing from the biggest to smallest. Identify the type and quantity of insulation. An energy auditor’s infrared camera can help find insulation voids and weaknesses. Lastly, the plan should include an overall house assessment that looks beyond the thermal envelope so that you can budget for and sequence repairs that may also affect your thermal envelope. For example, fixing rotted wall studs and sheathing is cheaper if done before insulation improvements. Why install great insulation that will continue to get wet, become ineffective, and need to be replaced later?

With your improvement plan in hand, you can now hire professionals to implement the plan or possibly do much of it yourself. In the end, your energy efficient and healthy house will provide you with superior comfort and lower operating costs.

Special Note on Financial Incentives

As part of the new American Recovery and Reinvestment Act of 2009, homeowners who choose certain envelope improvements (e.g. air-sealing, insulation, windows, doors) may qualify for sizable federal tax credits. See for details, which generally include 30 percent credit on qualified items up to $1,500 through 2010. The state of Minnesota is also launching an energy improvement “rebate program” effective October 1, 2009, and ending when limited funding runs out. The program generally covers windows, wall and attic insulation, and attic air sealing. See for more details about the state rebate program. Be careful to follow program guidelines closely and be aware of deadlines and limits.

Act Locally

Bob Alf Construction
St. Paul, MN

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