Info

Source: Adapted from www.coloradoenergy.org/ procorner/stuff/r-values.htm

Source: Adapted from www.coloradoenergy.org/ procorner/stuff/r-values.htm

The R-value of a studded cross section is less than a studless section because every stud is a thermal break that conducts heat faster than the insulated sections. Eliminating unnecessary framing has a direct effect on a wall's overall thermal performance, in addition to reducing the amount of wood used. When framing walls on 16-inch centers, as is most common, 15% to 25% of the wall's total surface area is wood (not counting windows and doors). When framing on 24-inch centers, this number shrinks to 10% to 20%. Of course, all of this depends on how the wall is built—for example, how many window jacks, extra studs in corners, trimmers, and jack studs are installed. How well the insulation is installed (no compression and no voids, and touching all six sides of the cavity), what type of insulation is used, and air leakage will also influence whole-wall R-values.

To determine the actual R-value of the walls in your home design, use the calculator at www.ornl.gov/sci/ roofs+walls/AWT/home.htm. This program allows you to specify the wall type, the cavity insulation, thickness of any foam sheathing, and exterior finish type.

building science

Exterior rigid foam board insulation with taped seams reduces airflow and moisture migration, and adds to R-value.

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.)

Problem Areas

A difference in temperature, air pressure, and humidity between the inside and outside of the home will create a pathway for warm, moist air, which will condense as it contacts colder surfaces. If this is not addressed quickly, moisture damage within a wall is inevitable, and the occupants may not be aware that mold spores, like rotting studs, or saturated insulation may be developing.

Pathways for airflow can result where two dissimilar materials come together in a building envelope, i.e., where the often-uneven concrete foundation ends and the wood-framed wall begins or where a recessed light fixture sits within a framed and drywalled ceiling. That's why the integration of the building materials—often more than the choice of product itself—are crucial in preventing this and other problems.

In all cases, the control mechanisms for heat, air, and moisture must work together. Additionally, wall designs, materials, and system choices need to be climate- and site-specific.

In this 1980s-era home built in Ontario, Canada, moisture-laden interior air is exfiltrating through and around an electrical outlet on an exterior wall and condensing on the back side of OSB sheathing, leading to material degradation.

In this 1980s-era home built in Ontario, Canada, moisture-laden interior air is exfiltrating through and around an electrical outlet on an exterior wall and condensing on the back side of OSB sheathing, leading to material degradation.

Courtesy Energy, Mines, and Resources Canada & Building Science Corp.

building science

Whole-House Mechanical Ventilation

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.

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.

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 (see ASHRAE recommendation in the "Ventilation" sidebar);

• 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 ([email protected]) 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

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