Mark Bomberg
The building envelope separates two distinct environments: inside and out. Conditions from both environments continuously impose upon the materials that make up the envelope and modify their performance. The interaction between environment and insulation is, therefore, essential to understand how these materials functions. To be able to make a suitable choice one must consider insulation performance under field conditions.
There are two challenges involved in selecting thermal insulation materials. One is to characterize the effect of critical factors affecting performance of the materials -- settlement of loose fills, aging of gas-filled foams, effect of convective air flow on low-density glass-fiber insulations, effect of moisture on thermal performance of all insulations. The second concern is to develop evaluation methods that would produce the results related to the field performance of these insulations.
None of these two challenges are addressed in North America. The main reason being is that each manufacturer competes with others on the basis of archaic, comparative rating standards and has neither linkage nor correlation with the thermal performance of the completed buildings. The responsibility for thermal performance of a building rests with the designer,who, unless he/she hires a consultant do it, does have little (if any) advice from the scientific community.
Arbitrary, comparative tests
As an example, one may quote a section from the ASTM spray polyurethane foam standard on dimensional changes of (measured after three specimens) after exposure to each of the following exposures:
A 28 d at (-20 ± 3) °C, (50 ± 5) % R.H;
B 28 d at (80 ± 2) °C, (50 ± 5) % R.H; and
C 28 d at (70 ± 3) °C, (97 ± 3) % R.H
The percentage volumetric and linear change obtained shall be reported for each exposure and each specimen. The results are to be expressed as a "plus %" when there has been expansion and as a "minus %" when there has been shrinkage.
So for a humid foam and a high level of solar radiation falling on a building one allows 14% of expansion. With other words for a 5 m x 3 m x 4-inch (100 mm) thick insulation (1.5 m3) foam the standard assuming uniform expansion of the thickness, and two dimensions, the standard allows movement of 60 mm. If this was a real case, nobody could use such a material in the construction. Furthermore, it has been demonstrated that a number of polyurethane foams when tested at 95 %RH show only a small fraction of the deformation that would take place under condensation of water since the standard allows it (97+3 %RH) and that the dimensional stability of the foam exposed to thermal gradient is significantly different from that measured under isothermal conditions. One may wonder, why knowing these facts, ASTM is not developing better test method?
The answer is simple - this standard is written for comparing different foam sand not for representing the PU foam field performance. One could go on with many similar cases, but this example is sufficient to highlight the difference between evaluation of field performance and the laboratory comparison of materials.
Foam insulations with captive blowing agent
Several foam insulations such as polyurethane (PU), extruded polystyrene (XPS), polyisocyanurate (PIR) when manufactured have no air inside the cells, only vapors of the blowing agent. The outside air diffuses into the foam and the blowing agent concentration is reduced by the foam walls absorbing the vapors and small, but steady diffusion out the cells. As the cell gas composition changes, so changes the thermal performance of the material. In effect of cell gas pressure changes the foam will expand or shrink, depending on the environmental conditions such as temperature and humidity.
Test methodology for long-term thermal performance (LTTP) was developed in the mid 1990. It used thin (5 to 10 mm) material layers exposed to different environmental conditions to reduce time of testing of all properties related to gas diffusion and solubility of the blowing agent in the foam. Then, based on testing thermal conductivity of thin layers as a function of time and either using scaling of aging time or the computer models for non-homogeneous foams within three to six months the 15 or 20-year field performance could easily be predicted. This methodology was verified against field exposure for different foams yet manufacturers in the NA do not use it. The industry came back to the meaningless 180 days exposure tests because nobody requires them to do more than to address comparative laboratory rating of thermal insulation.
For mineral fiber insulations in the USA, the national acceptance criteria use a very large population of the sample permit an average of the qualifying test to be 10% below the label value (claimed value). On the other hand, Canada requires that an average from 3 tests reaches the claimed value and Europe is using a standard scientific methodology of relating the expected deviation of a sample to the size of the sample. In effect the Canadian rating is within 1 to 2 % of the European rating but the US rating is on average to have 6 - 8 percent lower than it would be if a Canadian or European standard was used. The marketing people say the Canadian weather is cooler so the product sold in the US as R13 becomes R12 when transported across the border. This itself is a good joke, because the thermal resistance of mineral fiber insulation is higher in the cold temperatures.
It is important to highlight, as shown in the above example, that using the same test methods but different statistical interpretation you may get 6-8 % difference. This difference in interpretation relates to the difference between a standard written for an average product manufactured by an industry, and one written to protect an individual customer.
Coming back to the field performance, air movements can also compromise the thermal performance of glass fiber in wall applications. Wind washing, which occurs when wind both enters and exits the façade may also reduce thermal performance. As these pitfalls may be caused by the workmanship on the site, some American manufacturers are producing so-called "high-performance batts" (with higher density i.e., closer to density of batts typically used in Germany or Scandinavia). Applying loose-fill material pneumatically with a water-based adhesive offers another solution. In these blown-in blanket systems the density of the glass fiber is much higher, approaching 24 kg/m3. High-performance batts of glass, slag, and rock fibers demonstrate good field performance, provided that they are protected from ingress of air and moisture.
Thermal insulations fabricated in situ
Despite the move to high-density insulation, glass-fiber batts and blankets and loose-fill insulations continue to measure about half the density reported for these products 30 years ago. This reduction in density (and increased probability of poor performance) has opened a market niche for other materials made on the construction site (such as sprayed polyurethane foam or sprayed fiber insulations) that fill irregular spaces while providing higher thermal resistivity.
The new market, however, is a volatile one, where the main issue is credibility of the installer, rather than agreement between field and laboratory evaluations. Because these materials are manufactured under field conditions to fit the installation, no part of the structure is un-insulated. Yet, this arrangement underscores the need for contractor’s certification.
One exception in these uncorrelated laboratory tests is the cellulose fiber insulation (CFI). Based on technical evaluation and comparison of field and laboratory installations, builders are requested to install the CFI product with a 21 per cent correction for settlement. Thus, accommodating the differences between a product's initial and long-term performance was first incorporated into the Canadian cellulose insulation standards.
Rating tests do not relate to field performance
There is no relation between laboratory comparative tests and the field performance of different thermal insulations when affected to a varying degree by the environmental conditions such as air and water vapor movements.
Mineral fiber and cellulosic materials are affected by the moisture that enters under service conditions. Air entering into gas-filled cellular plastics dilutes the blowing agent and causes reduction of their thermal resistance with time. Low-density glass-fiber products are frequently affected by air flows in the wall cavity. These changes in field performance vary depending on nature of the material and the manner of its installation.
In Europe, building officials have attempted to reduce the gap between field and laboratory performance of building materials by using two measures of thermal properties: declared and design. The declared value, a statistical estimate, is the expected value of the thermal characteristic of a building material or product assessed through data measured at a reference temperature and thickness and stated with a given confidence level. The design value is the value of the thermal characteristic of a building material or product in a condition representing typical installation in buildings according to climate and use conditions.
A Swedish example highlights the use of these two concepts. According to the Swedish Building Code (SBN), the design thermal conductivity of pre-formed fiber insulation boards, quality class A varies with aging, moisture content and normal workmanship conditions. For instance, the standard would permit design thermal conductivity values: 0.038 W/(moK) for boards attached to airtight sheets and used above ground; 0.040 W/(moK) for other uses of the material in above-ground construction; 0.042 W/(moK) for use of the material in the slab on the ground when surface drainage is ensured; and 0.060 W/(moK) for use of the material outside the basement wall when foundation drainage is ensured.
While precision of these corrections may be disputed, these values are much closer to the performance representation than the North American values. Normally, information concerning predictions of long-term field performance is unavailable until after several years of experience and use of the product becomes a tradition. What's more, scientists do not actually predict field performance; they only correlate laboratory estimates with field data.
Closing this review, one may observe the growing popularity of the in-situ applied polyurethane foams in the North American market, despite their high price. These products can perform functions of air barrier, thermal insulation and moisture control and in contrast to fibrous insulations the do not need to be protected from weather (except for UV radiation). Yet, even in this field, a few changes of the blowing agents that started with elimination of chloro-fluor-carbons (CFC) erased trackability of tradition and the currently used comparative test methodology does not provide any real basis for material selection.
On the green material side, the winner is wood fiber insulation boards made with bico-fiber (coated fibers) or polymeric binders. While mineral fiber insulation boards require a significant fraction of perpendicularly oriented fibers, wood fiber boards are very compatible with exterior stucco (plaster). With excellent acoustic and hygrothermal properties these materials are becoming real competitors to foams. The nano-materials and vacuum insulated panels are slowly coming into price consideration for construction, but their price is beyond traditional construction. We will talk about next generation of technology in later issue of this column.
In summary, having spent 30 years in this field, I can say that safety, being a primary concern of the building codes, overshadowed the issues of environmental control. Without larger attention of the designers followed by the demand for performance testing of thermal insulation materials under field conditions the lack of performance data will persist.
Efficiency of thermal insulation
Thermal bridges reduce overall thermal efficiency in proportion to how good is the thermal insulation in comparison to the thermal bridge. We introduce a thermal efficiency index as a ratio between the actual, multi-dimensional heat flow through the assembly to the sum of thermal resistances of all layers i.e., one dimensional heat flow through the virtual assembly without any thermal bridges. Below we show thermal performance of typical wood frame wall without and with external thermal insulation.
Efficiency factor in 2x4 wood frame walls without and with external insulation
The above table shows that the reduction of thermal transmission coefficient is higher when one use more effective insulation in the wall cavity and that a simple way to improve the thermal efficiency of the assembly is to use external continuous insulation. The level of Rsi = 1.0 was selected because it corresponds to 1.5 inch of the typical EPS foam. It is evident that requesting continuous exterior insulation for wood frame walls is fully justified.
Concluding this column on thermal insulation we may remind the reader that since the durability of a material depends on both its nature and the environment in which it is installed, performance criteria cannot exist independently of the construction system. So, architects, designers and specifiers should begin to ask questions about documented actual or adequately simulated field performance instead of using arbitrary material rating standards.
Yet, as long as the design of the building envelope is not based on the cost-benefit analysis (e.g. life-cycle cost), the change in approach to environmental control in design will not be initiated by the codes; it must come through a change of attitude of designers and specifiers.