In a previous post we’ve discussed the shortcomings of a wood frame wall with fiberglass insulation as a thermal barrier (why R-values are mis-leading). Our focus in this post is isolated solely on the fiberglass itself.
Although fiberglass has been the standard insulation in new construction, it has serious flaws that aren't well known outside the industry. Some of them may surprise you.
Let’s begin with this … its a little known fact that fiberglass insulation loses over 40% of its insulating capacity when outside temperatures fall below 20˚F. LEED confirms through a Canadian cold weather study that fiberglass loses half its R-value below 0˚F. When this happens, an R-19 fiberglass insulated wall performs as if it were only R-9. When you need it the most, fiberglass insulation cannot properly insulate.
Once fiberglass insulation becomes damp its performance decreases dramatically. In fact, it only takes a 1.5% increase in moisture content in fiberglass to reduce its R-value by up to 50%. When moisture is trapped in a conventionally-built wall cavity insulated with fiberglass and sealed with a vapor barrier, insulation becomes damp and loses its ability to insulate. This also promotes mold growth which leads to structural damage.
One reason fiberglass is such a poor performer is that for fiberglass insulation to be effective it must be completely enclosed on all six sides without gaps or air pockets. The material must be installed to be in continuous contact with its surrounding surfaces with no pockets of air left between the framing materials and the fiberglass. This is virtually impossible to accomplish on a job site.
Another problem with fiberglass is that for it to work it must be fully expanded to allow air pockets to perform. If it’s stuffed into an opening or compressed by electrical and plumbing lines it simply doesn’t work. If you have electrical outlets on an exterior wall it’s a safe bet that there’s no insulation between the inside and outside of the wall where these outlets are placed. You simply can’t install a 3” deep outlet box in a 3½“deep wall cavity and insulate properly.
Lastly, fiberglass batt insulation is designed to be used in framed walls. Everywhere a framing member is installed, a thermal break is created between the pieces of insulation that allows air to pass. If 2x4s are used to construct the wall, anywhere a 2x4 is positioned the wall only has an R-value of R-3.5. On average 27% of the exterior wall’s surface is made up of framing members, meaning that 27% of the average wall surface has an R-value of only R-3.5.
Its no wonder The Department of Energy concluded that a solution of wood frame walls with fiberglass insulation "just doesn't work" as an effective thermal barrier.
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