We all know the benefits of insulation; the more insulation, the warmer your house will be right? Well, that's true in a sense but not entirely. The reason for this is various factors including insulating location, insulation type, rates of heat transfer of building materials, and wall and roof assemblies allowing for insulation access.
R-value is a term used when discussing the insulating value of a construction material. It is defined as heat transfer per unit area, or a measure of thermal resistance. The higher the R-value, the better your home will be to resist the warm inside air to move toward the colder outside air (vice versa in warm climates). With that said, R-values are not "etched in stone". For example, damp fiberglass insulation has a lower R-value than its dry counterpart and R-values change in different temperatures or if wind is applied to the equation. Certain insulations such as closed-cell polystyrene foam have a higher R-value per inch (~6.5) versus fiberglass fibers (~3.5), not to mention their air sealing characteristics. Laboratory measurements are always different than the real-world performance.
One must think of R-value as it relates to the entire system; the wall, roof or floor for example. Thermal bridging and the conductance of warm air to cold air (to be discussed later) can drastically decrease what you may think is a super R-value wall. Let's say I have a 2x8 stud wall placed 16" on center. These stud walls extend from the inside to the outside of the structure, essentially bridging one space to the other. Despite filling the voids between the studs with closed-cell spray polystyrene foam (~R-6.5/inch, 6.5x7.5"=R-49!) the whole wall r-value is considerably less because of the combination of the poor R-value of the wood (~R-1.5/inch) and it's poor u-value (the rate of heat transfer-inverse of the R-value).
Another consideration is the location of insulation. The chimney effect or stack effect is the natural movement of more buoyant warm air rising and as a result pulling in cold air inward from below. Having more insulation in your roof (whether unvented or vented) as well as in the slab can significantly increase your home's performance. When talking about the slab that tends to be much colder then the surrounding air above, warm air will travel downward in the direction of the cold, essentially "robbing" the home of heat. Yes, it makes the slab warmer, but is not keeping the warm air where it should be.
Another location to look at are the sill plates or rim joists, top plates, and truss/wall junctions; anywhere there are two building structures coming together. These areas are often ignored in building construction and tend to be the areas where up to 25% of heat loss is occurring in your home. Paying attention to these areas in the design and building of the home can save significantly on heating costs.
As we progress with the building a specific design, I will go through insulating in much more detail.
