Insulation and the truth behind R-value

What is the correct R-Value to be installed in a vented attic?

When speaking in terms of R-Value as it relates to insulation, we are speaking to the speed at which heat conductively travels from the hot side towards the cold side .  The higher the R-Value the slower the speed of heat transfer.  Note R-Value does not stop the transfer, merely slows it down.  An example on your kitchen stove is the time it takes to warm an aluminum pan verse the time to warm a cast iron pan; we will still cook in both cases.   Keep in mind heat travels in multiple methods and wave lengths including airflow.  Conventional insulation can only address the conduction transfer and ignores the transfer of heat by radiation and airflow (convection).  

The solution, an air barrier. When measuring the effectiveness of insulation, R value is the only factor most insulation manufacturers talk about because it’s all they offer. R value measures the ability to limit conductive heat flow (heat transferred through it). But the primary method of heat transfer is not conductive heat flow — it’s air leakage (convection). Air leakage can account for up to 40% of a building’s energy loss, and it cannot be stopped with traditional insulation. R value is important, but an airtight seal is equally important for achieving energy savings.

Icynene spray foam insulation provides insulation R value and air-sealing in a single step. This is the only way to truly provide your clients with the most energy-saving value from their insulation costs.

The Wikipedia link https://en.wikipedia.org/wiki/R-value_(insulation) indicates R-Value as thermal resistance.  The higher the R, the more effective.  However there is a space requirement and a balance which should be recognized when choosing the R- Value.  Space is required to add R-Value.   In discussing the batt insulation product 8” is required to achieve a R-Value of 30.  If one were to compress the R30 batt into a 5” cavity this result is a denser product but also a lower R-Value which is near 19.

When is Too Much R in a vented attic a poor concept in hot humid climates?

A vented attic is designed to allow air flow to enter and escape.  Conceptually, heat rises and ridge vents and whirly birds support the relief of heat in an attic.  This process reduces some of the heat, depressurizes the attic and draws new or cooler air in through existing soffits or form the conditioned air of the home.  It is important to have reasonable air flow to prevent other issues which can cause mold or mildew.

Essentially, we are cooling the attic, yet the air is then released to the world.  AC duct systems work in this environment and may be subject to sweating if the metal or aluminum conduit of the duct system is cold and the attic air is too hot.  Restating the importance of air flow in a vented attic, we realize as we increase the volume of insulation to the deck of an attic we reduce the ingress and egress of air flow.  Many times over insulating an attic deck promotes higher dew points and sweating because the attic become even hotter.  Although it is separated from conditioned living by more insulation, reducing the loss of cool air the attic bakes at a higher temperature.  This concern becomes especially evident as the roof slope of an attic is low.

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