Building Science Myths

There are a lot of myths about building science. Here are some of the ones we encounter most frequently.

  • icicles are a symptom of heat loss caused by air leakage in a home

    Icicles are a symptom of heat loss caused by air leakage in a home. Air-sealing and insulation in the attic can greatly reduce icicles, ice dams, and the damage they cause.

Myth: Heat escapes a building "horizontally" through windows, doors, and walls.

Fact: Most heat loss is "vertical" through eaves, roof penetrations, and soffit vents.

Myth: You have to be careful not to make a house too tight.

Fact: “Build tight and ventilate right.” All occupied buildings need ventilation. Natural ventilation happens when there is an air pressure difference between indoors and outdoors; the pressure is being caused by wind or temperature differences. Using blower door data, we can calculate the “Natural Ventilation Rate.” The problem is that natural ventilation is unreliable and unhelpful; being driven by temperature differences, our homes are over-ventilated (leaking excessively) when it’s coldest outside. In milder weather, “naturally ventilated” homes tend to be under-ventilated, building up moisture and indoor air pollutants. The best investment is to build or renovate as tight as possible, and then control ventilation with highly efficient exhaust fans or an Energy Recovery Ventilator (ERV).

This originated when builders first started building tight houses in the 1970s, without a clear understanding of ventilation dynamics. The importance of moisture control was under-appreciated. Off-gassing chemicals and other pollutants caused “sick buildings.” Energy tight buildings have to be approached with a whole systems understanding that includes ventilation strategies appropriate for the location, use and occupants.

Myth: Replacing windows is a good investment for saving energy.

Fact: There are many good reasons to replace windows, but energy savings is rarely one of them. In most homes, window replacement is a very poor investment compared to other targeted energy renovations. To start with, glass is airtight, so the leaks are typically associated with the moving parts, the rough opening, sash and trim. An older window can typically be sealed for much less than the cost of a new window. It is not uncommon that a very drafty window in the middle of a leaky house turns out to be fine once the attic and basement are properly air-sealed.

Myth: The “off” button turns it off.

Fact: Many modern household products still consume energy after being turned off. Home entertainment electronics are a major culprit, but the trend extends into seemingly innocuous appliances like coffee makers and window fans. Turning the lights down in many buildings can produce a dozen points of light: devices that are still “on.” Any product that uses a transformer (the little boxes that plug into outlet), like phones, speakers, or chargers, is always pulling current when it is plugged in. Turning off power strips is a common solution, or Smartstrips are a useful approach around the desk or computers.

Myth: Adding more insulation in the attic is the best way to stop heat loss.

Fact: Loose insulation and fiberglass batts do not stop air movement, so adding more insulation may not stop convective heat loss. The best investment in most structures is to air-seal underneath the existing insulation in an attic to stop air leakage. This helps make the existing insulation work better. Air-sealing should always be done before (or as part of) adding additional R-value to the attic.

Myth: Stuffing or packing insulation into a hole stops heat loss.

Fact: Fiberglass and loose insulation products do little to stop air flow. This is why manufacturers choose fiberglass for air filters—the material offers little resistance to flow while doing an excellent job of catching airborne particles. Evidence can often be seen in the dirt and dust that collects on insulation stuffed in holes, cracks and seams—in walls and ceilings, or around windows and chimneys. Proper control of heat loss requires that holes be sealed airtight, typically using products like caulk and closed-cell foam.

Myth: Mold likes fiberglass.

Fact: Fiberglass is hydrophobic, which means it doesn’t hold water very well unless it is soaked. Fiberglass itself does not provide food for biological growth either, although it may help hold water against other materials, like wood or the paper backing on sheet rock, helping to support mold growth. Many people mistake dirty fiberglass for moldy fiberglass. Fiberglass captures dirt when air flows through it, which makes it a good indicator of holes in the air barrier of a building. Once air leaks have been sealed, dirty fiberglass, when properly installed, can recover much of its original R-value.

Myth: R-value is the only performance measure that matters—the more you have the less energy you waste.

Fact: The rated R-value on a package indicates how a material performed under the ASTM laboratory conditions. However, home builders do not build laboratories, so what homeowners care about how the materials work under their roof. This is called the Performance R-value, and it can be significantly lower than the rated R-value. Convective air leakage is the most common reason that rated R-values fail to deliver. However, other significant attributes of insulation are not reflected in the ASTM R-value tests. For example, the time it takes for a given material to reach a “steady state” of heat transfer (from which the insulation’s resistance is measured) does not affect R-value, but can have a noticeable impact on the heat loss and comfort of a building. Finally, radiant barriers can produce huge R-value ratings in the ASTM test but are subject to substantial conductive and convective losses in actual residential applications.