Defining Porosity I

THE concept of a porous building might seem like an invitation to disaster. Imagine a gigantic, inverted colander housing the forlorn and sodden who squish-step around moldy desks and couches in a slow motion waltz to a darkling dirge. Certainly stranger experimental building envelopes have been conceived and even built. Architectural porosity should describe a passage through a membrane from unenclosed to enclosed space to avoid the confusion inherent in attempting to redefine a term well established in the language; this essay will focus on the movement of people, rather than rainwater, through a membrane.

Both non-temporal or physically sensible membranes, and temporal, or imagined, intuited or inferred membranes will be considered here. Membranes we can see, touch, hear, smell and taste are physically sensible and include building envelopes of any material, no matter how fantastic, including fabric, water curtain, brick, vegetation, and gingerbread. Membranes that do not register on the five senses include boundaries in the social, imaginative and presumed realms.

Brasiliana Library by Rodrigo Mindlin Loeb +

Eduardo de Almeida

Kevin Lynch, author of the seminal work The Image of the City, wrote about architectural porosity from an oblique perspective under the subheading Designing the Paths in his book. He defined the path thus, " . . . the network of habitual or potential lines of movement . . . are the most potent means by which the whole can be ordered." A path orders, and therefore has the capacity to categorize, create hierarchy, define edges and enclose. These capacities and characteristics manifest in three dimensions with the application of a little imagination; for example, the concept of a path as a two dimensional walking surface needs virtually no imagination to present an obvious line between that to be walked upon and and that not to be walked upon. But if we want to assert that the path creates enclosure, it requires imagination to extrude the two dimensional line upward into an imaginary boundary or membrane.

So, for the sake of coherence, let's limit our discussion to the piercing of membranes by paths. Architectural porosity can also be illustrated by describing what it is not. For example, this discussion of porosity purposely separates itself from Richard Goodwin's notions of porosity; there are two reasons for this: one, the non-temporal scope of this discussion exceeds the scope of his proposed public space versus private space experiments, and two, the temporal scope of this discussion is much narrower than his attempt to define invagination and chiastic space.¹ This discussion will also make a distinction between porosity and transparency to further illustrate the working definition proposed above.

A descriptive contrast between two existing buildings can serve as a case study to illustrate the basis upon which we can build our definition of porosity. The Brasiliana Library by Rodrigo Mindlin Loeb + Eduardo de Almeida, will be used to illustrate one side of the contrast; the other side of the polemic will be revealed in the next installment of this article.

Is That All There Is?

The short answer is no. At this point we have what amounts to a piece of thin plastic swiss cheese behind our interior finishes that likely does not provide an adequate barrier to moist air infiltration. As described below, as the air moves through the wall construction from inside to outside in winter, and outside to inside in summer, it cools. Because warm air can carry more moisture than cool air, the moisture is left behind and collects or condenses on absorbent building materials as the air moves through the wall. The effects of moisture condensation on building materials range from severe to negligible because they depend on a number of variables. For example, the amount of moisture generated by nearby environmental conditions influences the severity of effect; this is why the greatest amount of moisture damage inside walls is usually found around bathrooms and kitchens.

Damage due to water and air infiltration

In addition to wood rot and rust, fiberglass batt insulation has the capacity to retain moisture. When the batts become wet enough, they tend to compress downwards due to the effects of gravity. This downward movement usually creates large gaps in what should be a continuous thermal barrier, which in turn allows a more efficient exchange between warmer and cooler air, and therefore a better environment for condensation and crack loss occur. Even worse, the thermal effectiveness of fiberglass batts depends upon the ratio of air trapped between the fibers to the thickness of the batt, or how fluffy it is; wet, compressed fiberglass batts lose most of their value as a thermal barrier, and instead become very efficient generators of the type of environment that organic growth thrives in. Mold and fungus, for example, thrive in this type of environment. Most people are familiar with time lapse video. Imagine a time lapse video shot from inside a wood framed bathroom wall that's susceptible to significant air and moisture infiltration over a period of ten years; the final scene shows a mound of moldy potting soil replete with happy insects. That's all it takes, I've witnessed the phenomenon.

That is a sample of why I experience such chagrin when an apparently experienced contractor announces that a polyethylene vapor barrier has been installed on every project they've ever worked on.

Bringing Vapor Barriers and Crack Loss Together

So what does crack loss have to do with vapor barriers? Crack loss renders conventionally installed polyethylene vapor barriers ineffective. The principal function of a vapor barrier installed on the inside face of an exterior wall is to limit airborne moisture from migrating from inside to outside. There are a number of reasons for limiting this particular migration, but principal among them are:

1. To limit condensation from forming within the exterior wall construction

2. To reduce heat exchange from inside to outside via crack loss; moist air transfers more heat than dry air.

In order for a polyethylene vapor barrier to function adequately for the purposes enumerated above, it must be virtually air-tight, but in practice, this is impossible. For example, the average fastening pattern for interior gypsum wallboard one encounters uses 50 screws per 48" X 96" sheet; therefore, a wall that is 16'-0" long and 8'-0" high will require 4 sheets of wallboard to cover and will perforate the vapor barrier 200 times. Cut-outs for piping, electrical outlets and other penetrations generally add even larger gaps and holes, and seams and average installation tolerances only make discontinuity an even bigger problem.

Moving from inside to outside, heated relatively moist air passes through the porous plastic sheeting and usually encounters fiberglass batt insulation which has been installed between the studs. It's worth noting here that fiberglass insulation has the capacity to trap liquid moisture. After passing through the fiberglass batts, the moist air encounters the exterior rigid sheathing which is also liquid absorptive and porous due to the same fastening, cut-outs and penetrations the plastic sheeting suffers from. The moist air runs into the spun-bonded polyethylene house wrap. In winter the moist air has usually been cooled and dried considerably by the time it reaches the back side of the house wrap, and because the house wrap has been purposefully perforated by the manufacturer to allow air to pass through it, it does and finally encounters the back side of the exterior vinyl clapboards.

Crack Loss

For the purposes of this article, crack loss describes the quantity of heat lost from a building due to kinetic energy pushing and pulling air through gaps in the exterior envelope. Of course, crack loss is not the only means by which heat is lost from a building, but it contributes the most effective means by which heat is lost from contemporary wood framed residential construction.

Kinetic energy in simplest terms is the energy of motion; anything that moves has kinetic energy. Air has kinetic energy when it is moving, and inside a house it moves almost constantly. When the air inside a home is heated through convection, radiation or conduction, an imbalance is created between the air temperature inside a house and the air outside a house, because we heat our houses when it's cold outside. The physical nature of our environment abhors imbalance, so when we turn the heat off, the air inside the house will always drop in temperature to match the temperature of the air outside to regain balance. This natural balancing is unavoidable at the scale of our observable environment.

Natural phenomena are also inured to the path of least resistance, and an observable crack, hole or other gap that exists between unbalanced temperatures provides the path of least resistance to reestablish balance. For example, the imbalance between 30ºF outside and 65ºF inside a house is an unbalanced temperature phenomenon, and an observable* crack between a window sash and frame

1: Offices 2: Vestibules 3:Industrial 4: Houses 5: Public Buildings

offers a path of least resistance to moving heated air that cannot avoid cooling to balance its temperature with the air outside. Windows and doors are part of a building envelope, and often offer direct paths for air movement from a heated condition to a cool condition, even if closed. This is due to the fact that the seals they employ are often inadequate when new, and almost all degrade relatively quickly through use.

Exhaust fans found in bathrooms and kitchens also offer direct paths, but are even more effective at aiding natural balance because the push large quantities of heated air directly to the outside and tend to create localized negative pressure gradients within a space which can draw-in cooler air from the outside via gaps at windows and doors, and the building materials and components that comprise the exterior walls of a house.

When viewed in isolation, the gaps between building materials and components may appear insignificant, especially because they generally do not align to create direct paths between inside and outside. But this apparent misalignment is a false friend because air movement is generally not restricted by changes of direction like human vision. In other words, the labyrinth of gaps between building materials from inside to outside my be difficult to observe with the eye, but they exist and air with pass through them. It's unavoidable, and in addition, the gaps may be thought of in the agregate. In other words, if one was somehow able to gather all the gaps into one large gap, the aggregate would be on average like having a window open, and sometimes like having a door open in a really leaky house.
*For the purposes of discussion, let's say anything we can see with any device is observable.  Bacteria would be observable, but quarks would not be observable.

Vapor Barrier?

The issue of air and vapor barriers surfaced yesterday at one of my residential projects currently underway here in Massachusetts. There was concern expressed about the lack of a vapor barrier shown in the drawings, specifically polyethylene sheeting, on the inside face of the exterior walls. The concerned party went on to explain that every project he had ever worked on had a polyethylene vapor barrier installed on the interior face of the exterior walls. That last pronouncement just about ruined my day because I didn't realize there still existed still such confusion in the US about exterior wall systems. Well, let me say this about that.

Conventional exterior wall framing.

Mass masonry wall systems, which are exterior walls made entirely of solid masonry* with no cavity or air space, are purposely left out of this discussion and will be addressed in another post at a later date. Most residential construction uses a cavity wall system; this is the type of wall in which one sees vertical wood or metal studs spaced at regular intervals before their exterior side is covered with a rigid sheeting material like plywood and subsequent to that, some kind of finish system. One of the most popular finish systems in the US today is spun-bonded polyethylene, sometimes called house wrap, vinyl siding and some kind of trim. Insulation is usually installed between the vertical members, or studs, and then a vapor barrier and finished wall board of some type is installed on the inside face of the exterior walls. Thus we have, from outside to inside: siding, polyethylene, rigid sheathing, studs and insulation, a vapor barrier and finished wall board--6 categorical layers.

This type of system is ubiquitous in the US and is usually acceptable to most building departments. It is also the cheapest to build due to its popularity; contractors can find cheap labor who know how to put it together, and the material demand allows for an economy of scale in unit pricing. As an efficient building envelope, if it is carefully built and all the layers are installed to exact tolerances, it is mediocre at best. Usually it provides a poor building envelope, and this is due to what's commonly called crack loss. 

* I include adobe, rammed earth, precast and any other type of non-cavity wall system here. 

Primordial Soup

THE expression a born __________ always gets people excited. She´s a born actress. The expression implies natural talent, genius, effortless savoir faire, and a phenomenal secret. The secret starts as a pebble in the walking shoe, a nag that turns shrike in short order. How do these naturals do it? Do I have to be unbalanced by defect like a George Malley or a John Nash character? Does an obscure, virtually inaccessible part of the genius brain activate at the brink of complete annihilation?

For me, drawing is the closest I've ever come to having a phenomenal secret. Ever since I could hold a crayon I have attempted to create some kind of concrete existence for the apparitions that appear out of the murk and mist of imagination. Starting with my immediate family and later including friends and classmates, questions surfaced: who drew that? Did you copy that from somewhere? Strangely enough, I always felt as if it wasn't exactly me who made the drawings in question, that I was acting only as a low voltage wire between what really created it and the paper upon which it manifest. As time went on, my scribbles were met with equal parts delight and derision in most cases, because where at first there was really no decision to draw made on my part, later I debased the secret through decision and used it to discriminate between those that could and could not, and later yet as a secret weapon to aid amorous conquest. There was scant a girl who could resist a flattering likeness Bic-penned on the chipboard backing of her spiral notebook.

It wasn't until I encountered my first architectural floor plan that I became obsessive and religious with drawing. No, I don't dare say that, it's not religious for me, but finally it seemed I didn't have to whore any more. Yes, yes--the apparent delight I saw in the eyes of some was worth it on one level, a high level in fact, but my inner monkey, my baser self was always a link in the delivery chain of these deliberate images.

Working With an Architect V

After the contract is awarded, the construction administration phase begins, the final phase of an architect's basic services. During construction administration the architect's principal tasks are to answer contractor-generated questions, or requests for information (RFIs), review and ultimately approve product submissions and applications for payment, make field observations, and for large projects, provide construction control*.

This project is substantially complete.

It's common and natural that the contractor will have questions or will require clarification regarding specific details of the design after construction begins. For good contractors and architects, these questions are always asked and answered in writing, preferably using a project-approved document or form. Various site visits made by the architect are made according to the contractor-generated and project-approved progress schedule during the course of the work so that the work in place can be assessed via observation and compared to the construction documents for compliance. These site visits usually correspond to progress milestones common to the particular project type underway. For example, for a new house, the first observation would occur when the foundation is complete, the second after the structural frame is complete, the third after the roofing and windows are installed and so on. Most contracts dictate very specific conditions under which the contractor can apply for payment, and the applications can be based on a 30 day schedule, a progress milestone or other verifiable time period. The architect's principal role in the application for payment process is to review the work indicated as completed on the application against what is in place on site. The two often do not match.

Most construction contract documents also stipulate very specific instructions regarding how specified materials to be used on the project are verified. This occurs through the submittal process. Many project materials are submitted solely for record keeping purposes because they comply with the project specifications explicitly. Usually the manufacturer's product data is sufficient to satisfy the project requirements for these types of submittals. Other submissions are too complex for this approach, and often full size mock-ups are required to substantiate the manner in which materials, means and methods, and processes will work and fit together in context. Once a more complex submittal like a mock-up has been reviewed by the project for compliance with requirements and intention, work can continue on that particular part of the project.

It would be unusual for a project not to have a final inspection and close out process. To trigger this process, a contractor will submit a notice of substantial completion, and attach a punch list to his penultimate application for payment. The punch list provides a compilation of all outstanding work and tasks to be completed by the contractor before a final inspection takes place. Examples of outstanding work would be things like final cleaning, installation of minor finish components, submission of project and operations manuals, and a certificate of occupancy. In general, any task or work that would require more time to complete than one payment cycle would be rejected by a good architect, because in that case the work would not be substantially complete.

After the items on the punch list are completed, the contractor submits a final application for payments and attaches to it any outstanding paperwork required by the project. The architect approved the final application, and the owner occupies his new building or home and lives happily ever after. That's the plan, anyway.

* Construction control is a series of building code-required site observations that are made by the architect.  Construction control is usually only required for large, non-residential projects.