Building Envelope Systems part 2

Cont.

Moisture Intrusion or Condensation

As building designs and techniques produce tighter thermal envelopes, moisture (from leaks or condensation) has created more and more problems. The water can intrude through a small crack but it is then trapped between the relatively impermeable building materials. Good building techniques typically must deal with both air sealing and moisture retarders to keep moisture from accumulating inside the wall sections. Locating moisture with thermography is often simple because water has both a high thermal conductivity and a high heat capacitance. Determining the source of the moisture, however, can be difficult.

Condensation, rather than leakage, is often the culprit so it is important to identify sources of air leakage that can transport moist air into the wall sections and the cold spots that can result in it condensing.

The classic case is that of warm moist air leaking past the insulation in a metal building or mobile home; as it contacts the cold underside of the metal roof it condenses, and often freezes, causing the occupants the think, mistakenly, that the roof is leaking. Building damage due to condensation includes mold growth, brick spalling, roof membrane fastener corrosion and reduced insulation values.

Construction of Exterior Insulation and Finish Systems (Eifs)

The growing popularity of buildings using EIFS has been accompanied by numerous cases of moisture-related structural damage. Although often attributed to leaking windows, water typically intrudes where sealing or flashing systems were inadequate or have begun to fail. Moisture is then trapped under the relatively impervious foam. In warm climates the decay of structural wooden elements, which can occur very rapidly, is a major problem. Mold growth also often accompanies the moisture resulting in health concerns.

Thermography is an invaluable tool to locate moisture in EIFS. Inspections are best conducted in the early evening from the outside after a sunny day with little or no wind. It may also be possible to locate moisture from inside during cooling or heating conditions. The expanded foam insulation boards tend to become more absorbent to water over time.

HVAC Performance

HVAC systems can be plagued by design and installation problems resulting in excessive energy use and/or uncomfortable buildings. Thermography helps building specialists visualize the otherwise invisible impact of this poor performance as indicated by excessively hot or cold areas.

Once these have been located, the root causes can be determined; these may include reversed or misdirected airflow or poorly placed supply ducts, leading to short-circuiting of air directly to the return ducts. Unfortunately many of these problems are a result of poor design; by the time a thermographer identifies them, it may be too late to correct. On the other hand, some problems (such as reversed diffusers) are very easy to correct.

It is also possible to visualize air flow itself! Several thermography studies have been conducted by hanging layers of plastic netting in a room. As the air flows through it, the temperature of the netting can be imaged. In most comfort-related cases it is probably as useful to simply image the walls and objects in the room a technique that is much easier.

Subsurface Heat Sources

Heating coils are being used more and more either to keep outside walkways or entrance ramps snow free or to provide heat to areas and rooms via hot water or electric cables. Thermography provides a quick way to verify location and performance of these subsurface devices.

Typically the thermal pattern shows up very clearly even when the heat source is embedded in several inches of concrete. Similarly, water pipes in wall sections can usually be located quite readily. Water leaks from pipes, whether in a wall section or under a slab, may also be located using infrared imaging (although airborne ultrasonics may also be an effective method).

To use infrared imaging, a temperature difference must be induced usually by running hot water through the pipe. Note that leaks under concrete slabs may not express themselves because the water is drained away in the sand/gravel base layer and the effects may not be thermally visible from the surface.

As a side note, thermographers are often asked to locate reinforcing steel in buildings. This is much more difficult. Heating the steel is not a simple task, usually accomplished with an inductive coil or direct DC-powered heating. Secondly, the heat is often insufficient to express itself on the surface. Other methods, based on sending and receiving radio signals, are probably more useful.

Verification of Construction Detail

No area of application is more important these days, particularly in commercial buildings, than the verification of construction details and performance. Thermography is being used with great success to verify bond beams and placement of reinforcing in concrete masonry unit (CMU) walls as well as structural elements of pre-cast, tip-up walls. How? The solid portions of the wall change temperature more slowly than the rest.

The inspection can be conducted anytime the wall is in a thermally transient condition - typically in the early evening as it cools down. Because the inspection is conducted during construction, deficiencies can be corrected prior to occupation with relatively minor inconvenience. Verification of placement of insulation in masonry unit walls is also more important than in the past due to high energy costs and health concern regarding mold growth in cold wall cavities.

Sick-building Syndrome

When buildings are too tight or too moist, health-related problems quickly come to the forefront. Grouped together as "sick building syndrome," these can stem from inadequate HVAC performance, moisture trapped in walls, mold growth on cold, damp surfaces and inadequate air change rates. Many of these can be visualized and diagnosed, at least in part, with thermography to help solve these very serious-and common-problems. Diagnosing these problems is often a very complex process and, given the fact that litigation may be involved, it is not something to be entered into lightly.

Facade Delaminations

A great deal of work has been done in SE Asia inspecting the facades of large buildings for delaminations. Failures of these materials can result in serious personnel injury. The masonry tiles used to face the buildings tend to change temperature rather quickly when they begin to delaminate compared to the underlying structure to which they were affixed. The diurnal cycle is typically the driving heat source and inspections are best done in the early evening hours after a warm sunny day.

Roof Moisture Inspections

The inspection of flat roofs, especially built-up roofs (BUR) has been proven for a number of years. The technique allows for the detection of moisture trapped in a roof system; over time this moisture leads to the pre-mature degradation of the roof. The moisture appears warmer at night after a sunny day due to its greater thermal capacitance.

The roof surface must be dry and the technique works best on roofs with absorbent insulation. Inspection work can be accomplished either in a roof-top survey or an aerial survey conducted from a helicopter or fixed-wing craft.

Standards and References

Several important standards exist to support the work of thermographers conducting building inspections. These include, among others, the following:

* ASTM C 1060-97 - Practice for Thermographic Inspection of Insulation Installations in Envelope Cavities of Frame Buildings
* ASTM C-1153-97 - Practice for the Location of Wet Insulation in Roofing Systems Using Infrared Imaging
* ISO 6781 - Thermal insulation, qualitative detection of thermal irregularities in building envelopes, Infrared Method
* American Concrete Institute Design Standard 530 / American Society of Civil Engineers Standard 5, Masonry Building Code and Specification

Another excellent reference, although not a standard, is the Canadian General Standards Board Manual for thermographic analysis of building enclosures (149-GP-2MP). Numerous building related papers have been published over the years; many are collected on a CD-ROM from SPIE, entitled Selected Papers from the Proceedings of Thermosense, edited by John Snell and Doug Burleigh, available from the SPIE organization.

Conclusion

When properly used by qualified individuals, thermal imaging technology can play a powerful role in visualizing otherwise invisible building problems and conditions. Contractors and architects alike are both using thermography to assure the performance of their buildings. Building specialists count on thermography to help them diagnose tough problems that, left unsolved, are costly or dangerous. Owners rely on thermography as a tool for commissioning a new building.

While a foundation of expertise must underlie the successful use of thermography for building diagnostics, getting started with most of the applications is often not difficult. An appropriate infrared system is required, with proper training and experience for the operator as well. Having supplementary knowledge of building sciences or access to that information is also vital. The primary return on an investment in building thermography is gaining a higher level of assurance buildings will perform as intended and occupants will be more comfortable, often at a lower cost.


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