Tag Archives: exterior insulation

Thermal Insulation: Which is Best?

There are plenty of excellent insulation materials on the market today. Many of these have been around for quite some time. Each of these insulations have their own ups and downs. As a result, when deciding which insulation material you should use, you should be sure to be aware of which material would work the best in your situation. Considering differences like R-value, price, environmental impact, flammability, sound insulation and other factors, here are the 5 most common types of insulation materials:

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The 5 most common types of insulation on the market.

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All About Rainscreens

To lower the risk of wall rot, it’s increasingly seen as sensible to provide a ventilated air gap between the siding material (also known as cladding) and the structural sheathing. As rainscreens become more common, mainstream builders are more often being compelled to ask, “What is a rainscreen? How do I know if I need one?”. This article will discuss the most common questions about rainscreen gaps between siding and sheathing.

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Stopping the rain that gets past the siding. In most cases, the investment in a rainscreen siding installation pays dividends in paint longevity and siding durability.

What is a rainscreen?
A rainscreen siding installation is one that includes an air gap between the siding and the water-resistive barrier (the asphalt felt or housewrap). Every well-designed rainscreen wall needs:

  • a water-resistive barrier (WRB).
  • an air gap between the WRB and the back of the siding.
  • flashings at all penetrations and vulnerable areas.
  • weep holes at the bottom of the wall.
  • ventilation openings at the top of the wall on a case-by case basis.

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The Dew Point in Insulating Wall Assemblies

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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.

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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.