Basics to Building a Better Green Home

August 22, 2008

Insulated stud frame wallEditor’s Note: This post was provided by one of our paid sponsors, Solar Energy International (SEI), a USA non-profit organization whose mission is to help others use renewable energy and environmental building technologies through education. SEI teaches individuals from all walks of life how to design, install and maintain renewable energy systems, and how to design and build efficient, sustainable homes. SEI offers trainings online and in 22 locations around the world.

Homes built today are generally twice as efficient as their 1980s counterparts. Improved window technology, more efficient heating and cooling equipment, better control of air infiltration, and greater use of insulation are helping decrease energy use in today’s homes. But building science—the physics of optimizing building performance and understanding why buildings fail—also plays a pivotal role.

Building science encompasses the study of heat transfer, airflow, and moisture movement through building enclosures; and how those factors affect the building’s performance, durability, comfort, and air quality. It predicts and measures the relationship people have to the controlled environment of buildings. Building science encompasses home design, construction, diagnostics, repair, and operation—all pitching in to make better buildings.

Dealing with Heat Flow

Insulation controls the flow of heat through a building assembly by slowing the conductive heat transfer through the envelope. Wherever floors, walls, ceilings, windows, and doors are exposed to differing inside and outside temperatures, heat conduction takes place.

The more insulation you pack into the envelope, the more heat transfer will be slowed. Building techniques have a particularly important role in this. In typical stick-frame construction, the large amount of framing material (i.e., studs, footers, and headers) not only means less opportunity for insulation but more conductive heat loss and gain. Framing members act as thermal bridging, reducing a wall’s R-value by 10% to 25% in an average home.

To avoid thermal bridging, insulation should be installed in a continuous, unbroken manner from foundation to roof. Conventional stud-framing does not allow this, but wrapping, taping, and sealing rigid insulating sheathing around the 40exterior can offer one solution for a stick-frame house. Whole-wall systems that combine insulation and structure in one unit, such as structural insulated panels (SIPs) and insulated concrete forms (ICFs), are becoming a more common approach to providing a continuous layer of insulation, to reduce air leakage and thermal bridging.

In addition to reducing conduction, insulation also plays a role in keeping the wall assembly dry. For example, another benefit to wrapping the exterior walls with rigid board insulation is that if (or more likely, when) a small amount of moisture enters a warmed stud cavity, it would not come into contact with any cold surfaces and condense.

Addressing Airflow

Building wrapped in Tyvek insulating materialWhat insulation does not do is stop air leakage into and out of a building. Although caulk, gaskets, and weather-stripping will reduce unwanted air leakage, understanding how air (heated and/or moisture laden) moves through wall assemblies will help you develop a more systemic approach to cut leakage.

Use a three-pronged approach to gain control over the movement of air in and out of your home:

  • Seal the leaks (both the obvious and not so obvious). During construction, no one typically is responsible for sealing penetrations made in the framing or structure. Never assume that the electricians and plumbers are thinking about reducing the air leakage, so be diligent about sealing these leaks. Notorious air leak areas occur around electrical outlets, recessed light fixtures, and anywhere dissimilar materials come together—a window frame and wall, for instance, or where concrete meets wood.
  • Provide a continuous air barrier that can resist air pressure differences. This is usually done with impermeable “house wrap,” but can be done in conjunction with a continuous layer of insulation. Batt and blown-in insulation do not act as air barriers, so one should always be added. Spray foams and rigid insulation (with sealed joints) are the only insulation types that are also air barriers. Gypsum board and exterior sheathing that are properly installed are also effective air barriers.
  • Control air pressure. Sealing leaks and providing an effective air barrier can translate into an efficient house that saves both energy and money. However, “tight” houses need to be ventilated to exchange stale, moist indoor air with fresh outdoor air. Controlled or mechanical ventilation systems can be exhaust-only, supply-only, or balanced (a combination of the two). However, don’t assume that just because you have one of these systems, you have control over your air pressure. Any ductwork and air-handling equipment must be sealed against air leakage to prevent unequal pressure in of your space. Your HVAC contractor may or may not have done this—it’s your job to follow up!

Managing Moisture

Moisture accumulates in a building envelope when more enters than exits. When moisture accumulation is greater than a material’s ability to store the moisture, problems result. There are three general strategies for avoiding moisture damage in a home:

  • Control moisture entry at the roof, walls, and foundation
  • Control moisture accumulation when a material gets wet by allowing it to dry
  • Remove moisture by source control and ventilation

These strategies are best used in combination. However, making them work together effectively can be quite a balancing act.

The first step to controlling moisture is to shed bulk water away from the building. Grading the site to channel water away from the structure and using appropriately sized roof overhangs, gutters, and roof flashing are the major methods. However, for maximum efficiency, this principle should also be applied to dormers, windows, doors, skylights, balconies, decks, and railings. Keep even more moisture at bay by designing simple roof structures instead of complex ones, locating the building, overhangs, and landscaping to protect against prevailing winds and rain, and making sure moisture-management architectural specifications are followed.

For moisture that does manage to work its way in, have a plan in place for draining bulk moisture out of the building. Drainage planes, such as building paper installed in shingle fashion with properly installed flashing, can be effective drainage tools.

Controlling water vapor is more complicated, and every climate calls for a different strategy. Many designers and builders don’t understand the vapor profiles of the wall assemblies they specify. Instead, they rely on the use of impermeable membranes, which, when breached, trap moisture in assemblies and often cause the exact problem that they were trying to avoid.

Vapor barriers and retarders, such as foil-faced insulating sheathing or extruded polystyrene (thicker than 1 inch), are two technologies used in vapor control. Vapor barriers stop the movement of water vapor or are impermeable to water vapor through the wall system they are applied to. Vapor retarders are considered semipermeable to water vapor, allowing a small, measurable amount of water vapor to pass through them. They are made from a variety of materials,including polyethylene, foil, rigid-foam insulation, and even vapor-retarding paint. When to use vapor barriers/retarders depends on a variety of climate and site factors. In general, vapor retarders are most commonly used and are most effective in the more extreme hot and cold climates, where the differences between indoor and outdoor temperatures are large and humidity is great. In cold climates, installing vapor control to the inside allows moisture to dry to the outside. In hot-humid climates, installing vapor control toward the outside of the wall assembly allows moisture to dry to the inside.

In climates where you get a bit of both seasonally or if you live in a “mixed” climate, design the wall to dry to both sides. One common approach is called the “flow-through” method, which allows water vapor to diffuse through the wall assembly without accumulating. (See Access.)

Moisture is often easily controlled by managing it at the source. For example, perhaps the site’s drainage is directing rainwater toward the home’s foundation or residents aren’t being diligent about using bathroom or kitchen exhaust fans, causing excessive humidity to condense on windows and rot the sills.

Exhaust-only or spot ventilation systems are strategically placed exhaust fans that remove moist and polluted air. The fresh air either comes from open doors or windows or random leaks unless you have a tight house, where vents that passively open with negative air pressure can provide the needed fresh air.

Supply systems use forced-air heating or air-conditioning systems to supply fresh outdoor air through existing ductwork. These systems involve control mechanisms, and do not necessarily reduce energy consumption. Diligent use of exhaust fans is still needed.

Balanced systems capture the heat from exhaust air to condition the supply air, capturing the majority of otherwise lost energy.

The American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) recommends that mechanical ventilation be no less than 50 cubic feet per minute (cfm) with 50 to 100 cfm requirements for kitchens and bathrooms.

In “tight” houses, a controlled ventilation strategy controls air pressure but also helps maintain consistent interior humidity levels and provides fresh air to the occupants. The keys to a successful controlled ventilation strategy are to:

  • Know how many cfm of air you need to move;
  • Avoid overventilating in the winter, which can increase heating needs and bills; and underventilating in the summer, which can cause rooms to be stuffy and uncomfortable;
  • Provide ventilation only when the building is occupied.

Occupants want their homes to be comfortable and healthy, and not enslave them to high heating and cooling costs. Our household environment should not have to suffer for poor planning, dysfunctional designs, and a short-sighted approach to home building. By understanding the basics of how our homes operate and why they fail, we can move beyond “green building” and make superior building performance standard practice.

Access

Rachel Connor (rachel@solarenergy.org) teaches, coordinates, and develops sustainable building curricula for the online and hands-on programs at Solar Energy International (SEI • www.solarenergy.org).

Building Science Corporation • www.buildingscience.com • Building guides to various climates by Joe Lstiburek

Green from the Ground Up, by David Johnston & Scott Gibson, 2008, Paperback, 336 pages, ISBN 978-1-56158-973-9, $25 from The Taunton Press, PO Box 5506, Newtown, CT 06470 • 800-888-8286 • www.taunton.com

This article was originally published in Home Power magazine.


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