Diagram depicting dew point in relationship to elements of wall assembly. This assembly utilizes cork board as the exterior insulation (courtesy Siegel+Strain Architects, Emeryville)
A reader recently queried in response to our recent posting The Brave New World of Insulating Wall Assemblies, “will adding exterior insulation act to decrease the probability of a condensation issue”?
The short answer is, adding exterior insulation will always decrease the risk of condensation within the wall assembly.
That said, in the design of the entire wall assembly including insulation in the stud bay + outboard insulation, we want to design the whole assembly with the dew point outboard of the wall sheathing. That way condensation, in the rare event it does occur, does not take place within the stud bay.
Ratio of exterior-interior insulation to control air leakage condensation. Courtesy Building Science Digests: Controlling Cold-Weather Condensation Using Insulation, John Straube, 03/10/11
The chart at right provides recommendation for balancing the insulation. In a commercial application we can assume indoor RH of 35%. In Santa Cruz, the average for the coldest three winter months (Dec, Jan, Feb) is 49.7 degrees. Cross-indexing these (35% x 50 deg F) results in 0.00. In other words, in our climate zone since temperatures are mild, insulation balancing is not likely to be a consideration.
The most conservative estimate might assume an indoor RH at the highest end of the spectrum i.e. 60%. In the same temperature range (50 deg F) this results in a 24% ratio for the exterior insulation. If we seeking to attain an R-20 wall this means it’s recommended to design R-4.8 into the exterior insulation, and the remainder (20.0 – 4.8 = 15.2) in the cavity. Using conventional insulation, options might include: A. R14 batt + R6 rigid = R20 target B. R19 batt + R1 rigid = R20 target. Since the ratio is less than 24%, Option A is the better of the two, since it is the least likely of the two option to result in condensation within the cavity.
There is a detailed and comprehensive article at Building Science.com: http://www.buildingscience.com/documents/digests/bsd-controlling-cold-weather-condensation-using-insulation. It’s author, Dr. John Straube of the University of Waterloo, is widely considered to be an authority on the subject of moisture transport within building materials and systems.