What Affects the R-Value of a Building?
This is part two of our posts on R-value. Read the first post to get a definition of R-value and find out what it means to you.
Doors and Windows
As much as we love, and need, doors and windows, they are not good for a building’s insulation value. Doors open and close, allowing air to move freely inside and out. Additionally, it can be tough to completely seal the gap around a door, causing even more air leakage when the door is closed. Windows are not very insulating, as we saw in our last post. Recent technology and the use of multiple panes of glass have improved the insulating properties of glass a bit, but it still can’t compare with other materials.
Just to further confuse the issue, windows have to have their own label to measure their insulating value – the U-factor. U-factor is the inverse of R-value, so the lower the number, the better insulation the window provides. Another characteristic that affects the insulating ability of a window is SHGC (solar heat gain coefficient), a measurement of the amount of heat caused by sunlight that is allowed through the glass. The lower the SHGC, the less solar heat the window transmits. It is important to note that both of these measurements only apply to the glass in the window, not the frame.
How can you increase the insulation value of a window? Options include: insulating window shades, window tinting or film to prevent solar heat gain, and the use of multiple panes of glass with insulating gases (such as argon) inserted between the panes.
Air gaps are the bane of insulation! As much as the insulation and other building materials try to keep the outside out and the inside in, gaps between materials, or holes within materials, defeat their purpose. The fix is easy – no air gaps! Completely sealing the space where materials meet and making sure that no holes are allowed will help decrease air gaps substantially. They can’t be eliminated completely, but the less the better.
Bridging, or more formally thermal bridging, can best be described through the phrase “taking the path of least resistance.” In other words, air temperature will travel through the areas of a roof or wall with the least R-value, providing a “bridge” for the hot or cool air to reach the inside of the building. This may seem a bit confusing. An example from Green Building Advisor will help:
Suppose your walls have 6″ fiberglass batts… Every 16 inches on center in that wall is a 2×6 or 2×4 stud. The fiberglass has an R-value of around 3.5 per inch, and the stud is around R-1.2 per inch. The wood studs allow heat to flow through the wall assembly at a rate that is 3 times faster than the heat flow through the surrounding insulation. While the advertised R-value for a 6-inch fiberglass batt is R-19, the building assembly’s effective R-value is about R-3 lower.
What a bummer! How do we fix this? A few options are available: use advanced framing techniques to limit the number of studs, install insulation between the studs and the inside or outside surface to break the bridge, or use alternative building materials that do not require the use of studs, such as straw bale or SIPS (structural insulated panels).
R-value isn’t the only characteristic to consider when choosing building materials, but it can play an important role in improving the efficiency of a building. Wall and roof assemblies should be evaluated at their weakest point, and steps taken to combat the infiltration of air or temperature whenever possible.