The most recent blogs at Green Building Advisor

All About Vapor Diffusion

Posted on June 12, 2015 by Martin Holladay, GBA Advisor in Musings of an Energy Nerd

Building scientists talk about several different moisture transport mechanisms. Most of these mechanisms — for example, water entry due to a roof leak — are easy to understand. Other transport mechanisms, like vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. , aren't quite as intuitive.

First, some basic definitions. Water vapor is water in a gaseous state — that is, water that has evaporated. It is invisible.

Water vapor diffusion is the movement of water vapor through vapor-permeable materials. Vapor diffusion happens through a solid material even when the material has no holes.

The Difficulty of Stopping Air Leakage Between the House and Garage

Posted on June 10, 2015 by Allison A. Bailes III, PhD, GBA Advisor in Building Science

A home with an attached garage is usually a home in which people breathe more carbon monoxide (CO). Of course, having an open carport or detached garage is better for air quality (and a feature that usually gets points for you in green building programs like LEED for HomesLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. and EarthCraft House), but what if you don't want to give up that attached garage?

A Straw-Bale Home in Vermont

Posted on June 10, 2015 by GBA Team in Green Building Blog

The owner of Vermont Natural Homes, Chad Mathrani, is building his own home near Brattleboro, Vermont. The walls of the house are built using several different construction methods, including timber frame, straw-bale, and double-stud framing insulated with cellulose.

This video of Mathrani's project was produced by a distributor of building products, 475 High Performance Building Products in Brooklyn, New York.

The Sun Also Rises in the Southeast

Posted on June 9, 2015 by Luis Martinez in Guest Blogs

Anyone who's ever sat out on a Georgia afternoon or wandered outdoors in the bright Florida sunshine knows that the solar power potential in these two Southeastern states is enormous. Now, after a slow start, so is the headway that the clean power technology is making in the Southeast's two most populous states. "In 2011, if you told me we'd be where we are today with solar," says one Georgia solar advocate, "I would have laughed."

How to Insulate a Foundation

Posted on June 8, 2015 by Scott Gibson in Q&A Spotlight

About to start a new house in Climate Zone 5, Nicholas C is working out the details of how to insulate the basement slab and foundation walls. There is more than one type of rigid foam insulation he could use, and it could be applied on either the inside or outside of the foundation.

For a couple of reasons, he's planning on 2 inches of extruded polystyrene (XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation.) beneath the slab rather than expanded polystyrene (EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest.). The XPS would perform better in a wet environment, Nicholas says, and Owens-Corning, one insulation manufacturer, claims it no longer uses a "bad" blowing agent.

How to Provide Makeup Air for Range Hoods

Posted on June 8, 2015 by Martin Holladay, GBA Advisor in Green Building Blog

When Cheryl Morris moved into her new home, she realized that the kitchen exhaust fan was probably too powerful. Whenever she turned on the 1,200-cfm fan, strange things happened. “It pulled the ashes out of the fireplace, halfway across the room, right up to my husband’s chair,” she says. Those dancing ashes demonstrate an important principle: Large exhaust fans need makeup air.

Cost of Passivhaus Compliance Is Sometimes Hard to Justify

Posted on June 5, 2015 by Martin Holladay, GBA Advisor in Musings of an Energy Nerd

In most climate zones, achieving the PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. standard requires more expensive windows, higher insulation levels, and greater attention to air-sealing details than less stringent construction practices. It should come as no surprise that homes with these features cost more to build than homes complying with easier-to-meet standards.

Canned Lighting Conundrum

Posted on June 4, 2015 by Stu Turner in Guest Blogs

In the fall of 2014, I was approaching the rough electrical stage of construction on a (hopefully) net-zero-energy “pretty good house” in the mountains northeast of Los Angeles. My goal for lighting was to specify fixtures to create attractive, properly lit spaces, with minimal impact to my air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both..

Fundamentals of Psychrometrics, Part 2

Posted on June 3, 2015 by Allison A. Bailes III, PhD, GBA Advisor in Building Science

Psychrometrics, you may recall, is the science that involves the properties of moist air and the processes in which the temperature or the water vapor content or both are changed. To understand how all that works, we need quantities and we need them to be well defined. Some are easy to understand (e.g., dry bulb temperatureAir temperature as measured by an ordinary thermometer. and barometric pressure); others are a bit more abstract (e.g., enthalpy). Here we'll take a look at the main psychrometric quanitites, define them carefully, and tell which commonly used term you should avoid.

Friendlier Foam Insulation On the Way, Eventually

Posted on June 2, 2015 by Scott Gibson in Green Building Blog

Builders who have been waiting for a new generation of extruded polystyrene insulation with a lower global warming potential (GWP) than what’s currently available may have to wait a little longer.

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