How long does it take a layer of R-30 fiberglass (or cellulose) insulation to become saturated with heat so that it begins transferring the heat?
Answer:
At the risk of sounding flippant, how high is up? The amount of time it takes heat to penetrate R-30 of mass insulation depends upon several factors, principally the temperatures involved and the amount of convection air that surrounds the mass.
In the test to determine the R-Value of the material, the mass is placed between two surfaces — one hot, one cold (i.e. 100 degrees, the other 25 degrees) so that there is a 75-degree difference between the two. After about one hour the material reaches a steady-state temperature and the calculation is made.
Once the same material is installed in a building everything changes — temperatures, air spaces, convection and radiation, so the R-Value and time required to reach steady-state is not testable.
Hope this answers your question.