As I began to do some background research for this entry, I was both impressed and humbled by the vast quantity of information available, and a bit disconcerted over the lack of clear consensus regarding actual best practices and an all to common lack of attention to building envelope integrity in actual field work.

   All too often the needed detailing of building envelope systems “fall between trades” and consequently if the general contractor is not assiduous in ensuring that the building envelope is treated as an integral system, that needed detailing is ignored.

    The building envelope can be defined as the integration of systems creating a barrier between the exterior and interior environments of a building. The primary tasks are to prevent liquid water intrusion, prevent unwanted air infiltration, and the control water vapor.

    Water intrusion through the roof system, although problematic, most often becomes readily apparent and therefore corrected before structural damage occurs. The most damaging areas of water intrusion are typically in wall systems at the intersections of roof to wall, or through wall penetrations. These are the areas where the builder must “think like a drop of water”, ie. understand the basics of capillary action, pressure differentials from wind driven rain, and gravity to ensure  that the home will “shed” the water that will assail it during the locations normal weather patterns.

    Any exterior cladding system currently in use is NOT waterproof. Even a masonry veneer wall, when exposed to a lengthy wind driven rain, will allow water to penetrate. Water resistive barriers, either building paper, roofing felt or housewraps must be installed in a “shingled” manner that allows water to shed, and drainage out of the wall system must be allowed for. Any penetrations through the wall system (windows etc.) should be flashed and sealed to prevent water from accumulating in the wall cavity. Ignoring these details can have disastrous consequences. If wood remains at a moisture content of above 19% for a protracted period of time, it WILL rot. I know of a case where a builder failed to implement proper detailing, and by the third year of the lawsuit when I was asked for a third party opinion, many of the exterior walls of the ¾ million dollar home had been at significantly above 20% moisture content since construction. The framing was undoubtedly compost by that time.

   As energy efficiency has become more of a concern over the past years, the “tightness” of the building envelope has received a great deal of attention. It has been stated that up to 38% of the heat loss in a typical house is through air leaks around windows, doors and other voids in the wall system. Sealing under all exterior plates is an inexpensive method to start in the right direction of building an “airtight” envelope. Attention to detail in sealing penetrations for water tightness will also deal with most unwanted air infiltration at these points. The use of housewraps, when properly installed, are a great improvement over roofing felt.

   A number of years ago  I encountered an owner acting as their own GC who decided they would build a completely airtight envelope by installing visqueen over the entire interior framing system. While I am sure the envelope was quite airtight, I am equally sure they learned the hard way (they spurned my unsolicited advice) that they had failed to take into account the issue of water vapor.

   The issue of water vapor is where I discovered the greatest lack of consensus from building scientists. The physics of  how water vapor behaves is readily understood, however the best practice of dealing with it in the building envelope seems a bit of a puzzle. The practice of putting visqueen over interior framing is espoused in homebuilding texts, the idea being that warm moist air inside the building will be stopped before it condenses against the cooler exterior surface. This may be sound practice in North Dakota or Alaska, but certainly not in Mississippi, or the Southeast in general, where the warm moist summer air that could migrate into a wall cavity would readily condense on the cool plastic. The head scratching seems to come with dealing with areas that have mixed climates.

    Water resistive barriers have varying degrees of permeability to water vapor. Even the “vapor barrier” kraft paper on batt insulation is really only vapor resistant. Unless you can build a hermetically sealed wall system, which would be cost prohibitive, the issue of vapor management will be present.

    In my experience there are a number of aspects to this which seem to be working.

One is the use of vents to remove excess moisture from cooking and bathing to prevent its accumulation and eventual condensation on windows or other unwanted places. Another is the use of housewraps that prevent liquid water intrusion but allow for the movement of water vapor. (Yes, I know this also allows vapor in, see above paragraph.) Another aspect that has come into play with the prevalence of blown attic insulation is the lack of a vapor retarder at the ceiling. In colder climates I have heard of excess attic moisture being problematic, but I have not heard or seen this issue in our region with a properly ventilated attic space. The lack of a vapor barrier at the ceiling seems to me to be a good place to allow the inevitable accumulation of vapor, even from breathing (up to 1.5 gallons/day per person) to dissipate without harm.

    I will continue to investigate these issues and report any new information I find.