BSA Historic Resources Committee Meeting Notes for December 2004
Present: Olga Bachilova, Bill Barry, Matthew Bronski, Julie Brown, David Hart, Tarika Harris, John Hecker, Lyn Hovey, Bill Mack, Henry Moss, Sean O'Brien, Susan Schur, Laurie Soave, Malcolm Smiley, Jonathan Smith, Eric Ward, Sara Wermiel
1. BSA Preservation Award Candidates: Sara Wermiel asked committee members to consider thoughtful new buildings and additions in historic contexts they may know of as potential candidates for the BSA Preservation Award. Please send your suggestions for award candidate projects to our HRC awards sub-committee, Laurie Soave (lsoave@brunercott.com ) and Eric Ward (eward@eypae.com ).
2. Vapor Drive 101: Sean O'Brien, a mechanical engineer at Simpson Gumpertz & Heger who specializes in building envelope issues, presented a primer on the fundamentals of moisture migration through building envelopes. Sean's presentation ranged from basic terminology to various analysis methods to problems in buildings.
Absorption occurs as liquid water is drawn into pores, such as a dry brick placed in a puddle. Adsorption occurs when water vapor collects on pores, such as a dry brick placed on a dry tabletop in a humid room. Three potentially separate functions in a wall design are sometimes confused by designers: 1) Vapor retarders (formerly called vapor barriers, e.g., polyethylene sheeting) retard the flow of vapor from inside to outside or vice versa to prevent condensation within the wall; 2) Secondary waterproofing (a.k.a. weather resistive barrier, building paper, building felt, e.g., #15 asphalt-saturate felt paper) controls rain that bypasses the exterior cladding to prevent leakage into the wall or the interior; 3) Air barriers (e.g., Tyvek) retard the flow of air from interior to exterior or vice versa to increase energy efficiency for heating or cooling the building. In certain wall designs, these three functions may be served by three separate components/materials. However, in other wall designs, two components or even one single component may serve all three functions (e.g., a peel and stick rubberized asphalt membrane such as Ice and Water Shield may be used to serve all three functions). An air barrier cannot have holes or breaches, otherwise it is almost completely ineffective. However, a vapor retarder can have holes or breaches and be effective - a vapor retarder that is 5% holes will still be 95% effective.
Vapor pressure, not relative humidity, is the driving force behind vapor flow diffusion. Calculation methods for vapor flow diffusion can generally be classified as Steady State models (1-dimensional, look at one moment in time) and Transient models (1- or 2-dimensional, look at conditions over a period of time, with changing parameters). The graphic wall section analysis method that many of us did years ago in architecture school is a very simplistic, 1-D, Steady State analysis. If your graphic analysis had the two lines crossing, it meant some condensation within the wall on the design degree day (e.g., the coldest day of the year). However, the wall section is not necessarily "no good;" the question at that point becomes how much water condenses in the wall, and does it dry out rapidly (acceptable) or continue to accumulate over time (unacceptable). Steady State analysis does not answer this critical question to determine if the wall is adequate- software for transient analysis is needed. As an example, Sean showed his WUFI software analysis of the typical wood-framed old house wall in New England with insulation but no vapor retarder. Simplistic Steady State analysis shows condensation in the wall. However, transient analysis shows that while a very small amount of condensation occurs on coldest days, it dries out the rest of the year, and no net accumulation occurs. Consequently, these more sophisticated analysis models tell us what we've known all along - these old houses work pretty well and typically don't have condensation problems, even though they lack a vapor retarder and an air barrier.
Asked about the MOIST analysis software, Sean noted that it was a good program in its day and he still uses it occasionally for certain analysis tasks where most appropriate, but he uses WUFI or WUFI 2-D more often, because of their powerful ability to model sporadic reapplication over time of water to wall exteriors (a.k.a. rain!) and take this reality into account in the overall analysis of moisture within, say, a masonry wall. Sean also showed his WUFI analysis of a typical three wythe brick, load-bearing masonry wall with no vapor retarder (the common Boston rowhouse wall) and demonstrated how drying of rainwater within the masonry wall can occur to both the interior and the exterior under certain conditions.
In qualifying for code exceptions, it is useful to have hard analysis of the particular wall in question in its particular climate (which these analysis tools can help an experienced user to generate) to demonstrate the case to the code official. The extremely strict air barrier requirements in the Mass. Bldg. Code make it imperative to ventilate properly, or as the Canadian building technologists often say, "when you build tight, you must ventilate right." Older residential buildings that weren't so air-tight were more forgiving of a lack of ventilation, as air leakage carried excess moisture outside in the winter in northern climates. However, relying on air leakage or opening of windows for ventilation may not be adequate for new, tighter buildings built under current Mass. Code. Oddly, the new extremely strict Mass. air barrier code did not explicitly require increased ventilation (although ASHRAE standards broadly referenced by the code note the importance of ventilation in tight buildings). Sean has recently investigated and is now solving condensation problems in a new residential building near Boston built to new air barrier code that wasn't adequately ventilated in the winter (understandably, Boston-area residents tend not to open their windows in the winter!). Air flow in a humid interior tends to carry far more moisture with it than strict vapor diffusion. Consequently, many of the worst condensation problems SGH has investigated occur in high-humidity environments, such as natatoriums and museums.
3. Rosendale Natural Cement - A conference on American natural cement will take place in Rosendale, N.Y. March 31- April 2, 2005. The conference will include numerous technical lectures, as well as a tour of the cement mine and kiln, and hands-on demonstrations of Rosendale mortar and plaster. http://www.rosendalecement.net/html/american_natural_cement_confer.html
4. Endangered Asher Benjamin Windows, Greenfield MA - The HRC sent a letter opposing the plans of the owner of the William Coleman House in Greenfield, MA to remove their rare and significant Asher Benjamin wood windows and replace them with vinyl windows (copy of letter).
5. Building Additions in Historic Contexts - Ongoing Series, Part 3: In January, the third in our ongoing series on good building additions/neighbors in historic context will continue as we focus the discussion on roof/rooftop additions. At our last session, Jack Glassman's image of a Boylston St. streetscape of numerous roof/top of building additions from various periods (photo below) provoked a great deal of passionate discussion and debate, ranging from hyper specific critiques of individual elements in the slide, to the broadest issues of design review in historic contexts, and the gradual, ad-hoc evolution of streetscapes over time that occurred for centuries prior to the notion of design controls in historic contexts, yet the difficulty of creating such vibrant, hodgepodge urban amalgamations when planned by one hand. Stay tuned: in January, the meeting/discussion format will again be a digital potluck. Please e-mail digital images of good roof/building top additions to Matthew Bronski (mbbronski@sgh.com) by 5:00 PM Wednesday 12 January so that we may include them in a Powerpoint presentation of images for discussion.
Roof/ Top of Building Additions in Historic Contexts
(e-mail digital images for discussion to mbbronski@sgh.com by Wed. 12 January)
8: 00 a.m., Thursday, January 13, 2005
The Architects’ Building, 52 Broad Street, Boston – Fifth Floor
Henry Moss, Matthew Bronski, and Sara Wermiel co-leaders and scribes