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.

Working With an Architect IV

One of the most important functions of the construction documents is to provide a basis to validate an owner's project budget and to produce a refined project cost estimate. Because most owner architect agreements require interim progress reviews, it's common for the owner to request that the architect send a progress review set to his construction cost estimator to validate the project budget. A review set that is between 50% and 75% complete is usually used for this task.

The construction documents aspire to provide a high degree of detail and accuracy in order to prevent ambiguity. In a perfect world, the documents would leave no question unanswered and thus allow all bids to reviewed against the simplest possible set of criteria: capability and cost. The documents would also provide prescriptive measures for any foreseeable contingency in the execution of the contract and the building permit. We are not, of course, perfect creatures, so the fourth phase of an architect's basic services comes into play, namely bidding assistance.

The architect can provide a large range of services during this phase, under the headings of bid procurement, review and analysis. Many experienced commercial, industrial and institutional clients maintain facilities departments or have engaged property management companies to provide procurement services so this heading is often limited to residential clients and special situations. In fact, many architects are reluctant to recommend bidders because they prefer to avoid any appearance of conflict of interest no matter how negligible it might be. This is understandable because complex contract relationships exist even on relatively simple projects, and it is almost always better to keep the the lines drawn at arm's length well defined between the project participants.

Bid review is one of the most important tasks undertaken by the architect. The good architect knows the project documents intimately and can therefore recognize and assess omissions or onerous qualifications presented in order to provide a valid basis for comparison of the various bids received. Defects can be highlighted and used by the owner to make an informed decision on which bidder will be awarded the contract. Bid review also provides the opportunity to verify the scope of the project against the owner's budget and verified cost estimate. The result of the architect's assessment and analysis of the bids is called a bid recommendation. The bid recommendation will include the results of the architect's bid analysis which will clearly demonstrate how well the bidders complied with the construction documents when preparing their bid. The recommendation may also include other reasons why the recommended bidder should be considered or not, such as the architect's past experience in dealing with a particular bidder.