What is the building envelope?

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All houses are an interface between us and the climate we live in.

For most of human existence shelter has been evolving, From caves and lean-to shelters, we moved to built structures. The first concern was relief from water, cold, and heat. Later, as buildings became more sophisticated and the building became tighter, stopping drafts became a concern. Today’s houses are very tight. That very tightness has become one of our great difficulties; close up a house and add insulation, and you wind up with reduced ability to deal with water that manages to get inside the building.

Methods of construction are closely related to the technologies available. In the past 75 years plywood and insulation have been introduced. Both of these have been a great boon to comfort, but have been difficult in terms of water and air quality.

Both plywood and insulation increase the likelihood of water problems in a home. The trouble is that drafty home that no one wanted; changing from shiplap sheathing to plywood, especially when combined with new air barriers, stops air from having easy infiltration and exfiltration. Insulation slows air movement further and creates a reservoir to hold water.

In the past 30 years we have begun to learn better the ways that water and air move into and out of houses. The initial thought that the water that was rotting out houses in the 1980’s and 1990’s was from poorly detailed siding and the weather resistant barrier (WRB) such as asphalt impregnated felt paper, Tyvek, etc. This turned out to be partly true- old windows were fairly well detailed, with metal pans and integrated sloped sills. With the new flanged windows inserted into holes in the sheathing, there was paradoxically a greater ability for water to get into the framing, even though the windows themselves were fairly waterproof, and unlikely to be damaged by water. Added to this was a common misconception that the window flanges acted as flashing, which led to a lazy attitude that, in turn, led to billions of dollars spent on repairs.

Another failure came from the absence of understanding of water in air. Bulk water is what we think of most- rain falling onto siding. Bulk water is certainly the biggest issue, and the most obvious. That is what roofs and siding, and the WRB deals with most of the time. All work to keep water moving down to drain away from, rather than into the house and foundation.

At the same time we were closing up houses, we were also bringing more water into the house. In the 1940’s people washed their hair an average of once every 2 weeks. Today Americans wash their hair approximately 4-1/2 times per week. Washers and dryers became more common in the home. Bigger stoves and microwaves. Bathrooms multiplied in number and size, with more sinks, bigger tubs. Central heating had an impact- warm air holds more water, and having a larger home with more water inside translates to previously unknown amounts of water vapor moving from inside the house to the outside.

Water vapor isn’t a problem when it’s around warm surfaces, but it condenses on cold ones. In houses this means that the water vapor is going to condense on whatever surface is cold, something that changes depending on the climate. A house in Dallas, Texas, where most days it is going to be hot outside, is going to experience condensation on a different surface than in Seattle, where most days it will be cold outside. In Seattle, water will condense on the sheathing. In Dallas, hot air will condense on interior surfaces- on windows, etc.

So as we’ve gotten better at knowing what the vapor is doing, we’ve had to come up with new methods and materials for installing windows, doors, insulation, siding, electrical outlets, lights: in short, everything that interfaces with any surface that interacts with both the interior and the exterior.

In today’s building envelopes, the siding deals with the majority of the bulk water. The WRB handles what bulk water manages to get through the siding- and all siding is assumed to leak. All wall penetrations are flashed in such a way that any water that gets to them is directed onto the exterior side of the WRB. And that WRB is now a vapor permeable product, meaning that any vapor (water suspended in air) that reaches the WRB can pass through it, as opposed to being stopped and likely condensing on that WRB.

Asphalt impregnated felt paper is still used today, and has its place. Most buildings today get some version of a synthetic WRB such as Tyvek or Typar. Many jurisdictions now require some sort of rainscreen, which technically would be an air gap that prevents capillary action, but many of the WRB’s that qualify as a rainscreen would better be called “drainscreens” as they create the drainage plane by crinkling the paper or adding some sort of texture; this does not create a gap large enough to prevent capillary action, but does allow for faster, easier drainage of any water that gets to it.

Various different types of flashing tapes have been created to allow waterproofing of window, door, or other wall penetrations. The first generation were asphalt-based flashing tapes that worked well, but could also react to the plasticizers that are used to make PVC and Vinyl windows flexible (most of the new windows installed are PVC or Vinyl), and also could react to the sealants (caulks) used to help waterproof them. The next generation tapes were Butyl, a synthetic rubber that wouldn’t react to plasticizers or sealants. The most recent flashing tapes are Acrylic tapes that perform very well.

Another method of creating an air barrier are liquid applied flashings, sealants, and WRB’s. Liquid Applied WRB’s have been used for several decades inside houses as waterproofing in showers and bathrooms. Advances in chemical creation has allowed for reliable vapor permeance in these barriers when applied at the right thickness. Commercial buildings (low and high rise structures) have been using Liquid Applied barriers for a few decades, and have performed very well. As they have become more widely used the price has dropped enough to become competitive in the more price-conscious residential markets.

Along with liquid WRB’s come liquid flashings are designed to work with the WRB, and are designed to cover gaps (between sheets of sheathing) and plane changes (wrapping around corners and into window/door openings). All of the earlier variations required mesh tapes to bridge gaps and at corners, but the newer generations are formulated with fibers or particles to allow them to span gaps.


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