I was taught many years ago by an older fiberglass insulation contractor the importance of a quality insulation job. His biggest piece of advice, FLUFF, DON’T STUFF! Poorly installed insulation can have a big impact on the overall performance of a home. In this blog, I’m going to show you mathematically how a small insulation deficiency can have a big effect in how a home performs.
This thermal image is a good example. This was a brand-new home where I was performing a code required blower door test. I ran around the home with my thermal imaging camera before starting the test and thought I might be able to use this image to show thermal bridging, which it does a good job at, but if you study the pic a little closer, we see several areas where the fiberglass insulation was poorly installed. Compressing (not fluffing) the product creates areas of cooler temperatures in the insulation batts. The dark area (at the ceiling on the right side of the photo) shows an area of missing insulation. What effect does this poorly installed insulation have on the overall (effective) R-value? We can show the results mathematically.
Before we get into the math, lets define a high-quality insulation job. Because batt insulation is a common product used to insulate walls, floors, and ceiling, let’s use that in the example. There are three grades professionals give to an insulation installation, grades I, II, and III plus there is an uninsulated designation.
Grade I is the best possible with insulation in contact with all six side within the insulted cavity. There is minimal compression with no pathways through the insulation from the inner side to the outer side of the wall, ceiling, or floor. The insulation is tight fitting around any objects, (such as electrical wires and boxes, plumbing pipes and fittings, etc…).
Grade II installation allows for more compression in the installation where the insulation surface does not contact all six sides, but there still cannot be any open pathway from the inner insulation surface to the outer side.
Grade III installations will have more deficiencies including excessive compression and pathways leading between the inner face and outside wall, ceiling, or floor surfaces.
So, how big of an effect will a poorly insulated floor, wall, or ceiling have on the overall performance of the home? Let’s use an unconditioned and vented attic as an example. The attic itself is insulated at the attic floor to a grade I standard, is insulated to R-50, and has 1000 square feet of surface area. But there is one problem, the contractor forgot to insulate the attic hatch, which is 5 square feet. That area only has the ceiling drywall for insulation, which is R-.5. This is something I run across on occasion when performing energy audits and assessments.
Before we get into the math, lets define R-value and U-factor. R-value is the resistance to heat flow, conductive heat flow to be specific. With R-value, the higher the number, the more resistance to the movement of heat. U-factor is the rate of heat that transmits through a building assembly or product. The lower the U-factor value, the slower the heat moves. In other words, higher R-value and lower U-factor numbers are good, they are what we want in our homes with regards to energy efficiency and comfort.
To calculate the overall or effective R-value of the attic given 99.5% is insulated to R-50 and 0.5% is insulated to R-0.5, we use a formula called area-weighted average. First, we have to convert the R-values to U-factors by using the formula U = 1/R.
The insulated attic area is R-50 which converts to a U-factor of U-0.02.
The un-insulated attic area is R-0.5 which converts to a U-factor of U-2.0
Now we multiply those U-factors by the area of the ceiling each is insulated to. The formula is:
U-effective = (.995 x U-0.02) + (.005 x U-2.0)
U-effective = 0.0199 + .01
U-effective = 0.0299
Now we can change that U-factor back to an R-value by the formula R = 1/U
1 / 0.0299 = R-33.44
The overall effective R-value of the ceiling is R-33.44, nowhere near the R-50 we thought we had. All because somebody forgot to insulate an attic hatch that represented 0.5% of the total ceiling area. Do you think missing or poorly insulated areas make much of a difference? Hopefully this simple example shows you it does.