The R-Value Myth
The Great Myth of Modern Home Construction is the ‘R-value’ measurement. R-values have been peddled to us for so long that they have taken on a ‘Chiseled-In-Stone’ status, even with some otherwise knowledgeable contractors and architects.
The saddest part of this is that the R-value by itself is almost a worthless number, as it is impossible to define insulation effectiveness by this number alone.
WHY IS THE MYTH PERPETUATED?
It definitely favors ‘traditionally used’ fiber insulations made by large International Corporations, who regularly lobby & contribute to keep it in place, despite peer reviewed science that shows its limitations.
What would the R-value of insulation be after it has been submersed in water or had a 20 mile per hour wind blowing through it? Obviously the ‘Effective’ R-value of fiberglass, cellulose, wool and/or cotton insulation would go to zero, but Spray Foam Insulation would be largely unaffected. Again, R-values are ‘funny’ numbers; they are meaningless unless we know all the relevant factors.
The use of R-values as the only criteria for insulation is absolutely ridiculous, yet we have Government Agencies mandating R-values of 30 to 50 today, despite any actual science to support these levels. Traditional insulation having an R-25-value installed in a wall/floor/ceiling, but not properly sealed will allow air to travel through it and/or moisture to saturate it – providing no real R-value at all.
R-values are a result standardized testing of heat flow through insulation in a sealed test fixture under controlled laboratory conditions, but these tests are not even remotely associated with the real world environment where insulation is commonly installed.
To calculate the actual installed performance of insulation, we also need to know its resistance to air penetration, to internal air currents, to liquid water, and to water vapor drive. What really matters is the ‘effective’ R-value when insulation is subjected to these real world conditions.
Fiber insulation is generally assigned an R-value of approximately 3.5 per inch, yet it can only achieve this value when tested in an absolute zero air-movement and zero moisture environment. Zero wind and zero moisture are not real-world conditions, as all homes leak air and water vapor from the outside air, showers, cooking, breathing, etc., constantly moving through our homes.
If the attic is not properly ventilated on a fiber insulated home, the water vapor generated inside a house will very quickly semi-saturate the insulation above the ceiling and in the walls. Even small amounts of moisture will cause a dramatic drop, as much as 50 percent or more, in fiber insulation’s performance.
VAPOR BARRIERS
Because such small amounts of moisture have devastating effects on fiber insulations performance, it is recommended that fiber insulation have a vapor barrier installed on the warm side of the insulation.
This recommendation poses a completely new problem; which is the warm side of the wall?
Obviously, the warm side changes from summer to winter, even from day to night. If it is 30 degrees outside, the inside of an occupied house is certainly the warm side, conversely, during the summer months, the warm side is the outside of the wall. This can also change in the winter, when the sun is shining, on south facing walls, heating them up from the outside.
Occasionally, a novice will put vapor barriers on both sides of the insulation, but this proves to be disastrous because the vapor barriers will stop most of the moisture but not all. Small amounts of moisture will still move into the fiber insulation, between the two vapor barriers and become trapped. Condensation will accumulate as the temperature swings back and forth, altering the direction of vapor drive forces, leading to significant mold growth, health problems, building rot and increased energy costs.
Fiber insulation must be ventilated on one side or you can end up with serious mold growth problems, as illustrated in these images. This required ventilation also allows air movement within the fiber insulation, significantly reducing its effectiveness AND R-Value.
