GLOSSARY OF INSULATION TERMS AND SYMBOLS

Solving insulation problems depends on a knowledge and understanding of the terms and symbols used in calculation methods. Included here are relevant terms and their definitions along with appropriate symbols to help simplify your thermal calculations.

BRITISH THERMAL UNIT (BTU): A unit of heat defined generally as the quantity of heat required to raise the temperature of one pound of water one degree Fahrenheit.

CALCULATED THERMAL VALUES: An arithmetic summation of the resistances for all the components making up a building section including surface air films. This method of calculation does not consider the thermal short circuit effect of structural members, fastener population, compression of insulation material at structural member and may indicate lower heat loss values for a building envelope than may actually develop in the finished structure.

DEGREE DAYS (DD): A measure of the severity of a heating period, usually an entire season, based on climatic conditions. It is found by determining from weather records for each day the difference between 65 degrees F and the mean temperature for that day. The sum of these differences for all the days in the heating season is the Degree Days for that locality.

DENSITY (D): A measure of the mass per unit volume defined as the number of pounds per cubic foot of volume.

DEW-POINT: The temperature at which cooled air, with no change in pressure or amount of water vapor, becomes completely saturated and forms visible condensation (attains a relative humidity of 100%).

GUARDED HOT BOX: Test procedure used to accurately determined thermal conductance or heat flow through a non-homogeneous building section. The construction of the Guarded Hot Box and the procedure for test is governed by ASTM Test Method C236. Where accurate measurements of heat loss (U-value) through a building envelope are required, it is strongly recommended that this method of determination be used.

HUMIDITY, RELATIVE: This indicates, as a percentage, the amount of moisture that air of a given temperature is actually holding in vapor form as compared to what it could hold at that same temperature when fully saturated.

LIGHT REFLECTANCE: The measure of light reflectance is that fraction or percent of available light which is reflected by the surface. Greater percentages indicate more light being reflected from the surface in question.

OVERALL HEAT TRANSMISSION COEFFICIENT (U): A unit expressing heat passage through a complete building section, including air films. Technically, it is heat transmission in BTU per hour, per square foot, per degree Fahrenheit of temperature difference from air to air for a composite building section. It is the reciprocal (i.e., divided into 1) of the total resistance of the section, 1/R. It is used as a basis for determining transmitted heat loss. The lower the U-value, the better the insulating value of the building section.

PERM: A vapor transmission rate of 1 grain of water vapor per square foot, per hour, per inch of mercury pressure difference (1 pound = 5,760 grains). The lower the perm, the better the resistance to water vapor transmission.

SOUND ABSORPTION (Noise Reduction Coefficient): A measure of the ability of surface to absorb a fraction of the randomly incident sound. Measurements are taken at 125, 250, 500, 1000, 2000, and 4000 cycles and NRC is the average of coefficients at 250, 500, 1000 and 2000 cycles. The higher the NRC value, the more sound has been absorbed by the surface.

SOUND ATTENUATION (Sound Transmission Class): A measure in decibels of the reduction in sound as it passes through a wall or roof. The higher the STC value, the better are the sound attenuation properties of the construction in question.

SURFACE AIR FILM COEFFICIENT (f): The amount of heat flow in BTU per square foot per hour between an exposed surface of a material and the adjacent air, fi - inside surface film coefficient, fo - outside surface film coefficient. It is a measure of the conductance of heat through the air film that clings to all surfaces. The resistance of such films is expresses as 1/f. Values can be found in the ASHRAE Handbook of Fundamentals.

THERMAL CONDUCTANCE (C): A unit specifying the amount of heat, in BTU’s per hour, that passes through a square foot of material which has a given thickness and one degree Fahrenheit of temperature difference between its surfaces. Used for comparing insulating efficiencies of materials with varying but stated thicknesses or those which are composed of two or more basic materials. The lower the conductance value, the less amount of heat is permitted to pass through the non-homogeneous section.

HEADING?: Vapor retarder facings for insulation are available in a variety of styles ranging from plain white vinyl film to laminated composites containing various combinations of aluminum foil, plastic films, kraft papers and reinforcing yearns. The facings vary in water vapor transmission rate (WVTR), color, strength, light reflectivity and noise absorption properties.

VINYL: Elastic, bright white and available in several thicknesses, vinyl films provide an attractive installation at a relatively low cost. Easy to handle during warm weather, vinyl films will become brittle and difficult to handle in colder temperatures. Vinyl film insulation facings have a WVTR of 1.0 US perms or greater.

FOIL/SCRIM/KRAF: Foil/scrim/Kraft (FSK) offers excellent WVTR and strength properties at a reasonable cost. FSK is usually available with two foil thicknesses: 0.00035 and 0.0005 inch, along with several yarn patterns. Before installation, FSK with a 0.00035 foil has a WVTR of 0.02 US perms. Because of poor abuse resistance, normal job site handling and installation can raise the WVTR well in excess of 1.0 US perms. Some manufacturers apply an elastomeric barrier coating to improve abuse resistance and installed perm values.

POLYPROPYLENE/SCRIM/KRAFT: A new whiter, more opaque polypropylene film has been developed for use on polypropylene/scrim/Kraft (PSK) facings. Due to the greatly improved opacity of the new film, PSK’s will now be offered with both metalized films (WMP-10 type facings) and non-metallized films (WMP-10B type facings). The new "B" type PSK’s are designed to provide improved WVTR and strength properties for applications that would otherwise use facings with a WVTR as high as 1.0 US perms.

POLYPROPYLENE/SCRIM/FOIL: The new, more opaque polypropylene film will also find use in polypropylene/scrim/foil (PSF) facings. Manufactured using 0.0003 inch foil thickness, PSF facings provide a WVTR of 0.02 US perms and have excellent light reflectivity.

VINYL/SCRIM/FOIL: Vinyl/scrim/foil (VSF) facings offer good aesthetics, a 0.02 US Perm WVTR, and work in most pre-engineered buildings applications. VSF facings range from extremely light (0.8 mil vinyl/0.00035" foil) to heavy duty (3 mil vinyl/0.0007" foil) with the type and pattern of reinforcement varying widely. The heavy duty versions offer durability and abuse resistance.

VINYL/SCRIM/POLYESTER: Vinyl/scrim/polyester (VSP) has a highly abuse resistant construction, excellent appearance and a 0.02 US perm WVTR. One of the most expensive facings available, it is usually constructed of 3 mil vinyl film, scrim, and metallized polyester. The accompanying chart (page 27) gives a general description of the most facings. The primary purpose of a facing is to keep the insulation dry. WVTR is a vital property because it impacts the overall effectiveness of the insulation system. The lower the WVTR, the better the vapor retarder and the drier the insulation. With changes in temperature and humidity, a difference in water vapor pressure exists between the inside and outside of a building. This vapor pressure differential acts as a driving force, causing the water vapor to seek equilibrium. During winter months, a building’s interior heated air can hold much more moisture than the cold outside air. When water vapor in the interior heated air is allowed to enter the insulation, condensation may occur on the colder steel structure and sheathing. Since water has a much higher conductivity than insulation, excessive moisture within the insulation can adversely affect insulation properties.

Facings with WVTR greater than 1.0 US perm aren't considered adequate vapor retarders for industrial or commercial building insulation applications.
(Refer to ASTM C-755: "Selection of Vapor Retarders for Thermal Insulations.")

The following calculations outline the relationship between perm rates and the quantities of water that can potentially be transmitted within large buildings. They are based on a building containing 50,000 sq.ft. of facing, no leaky seams, inside and outside temperatures of 73 degrees F., with 50% RH inside the building an 0% RH outside.

Variations in temperature and humidity can increase or decrease the quantity of water transmitted. These figures demonstrate, under standard conditions, that quantities of water can be transmitted with a 1.0 US perm facing, a 0.09 US perm facing and a 0.02 US perm facing.

As the chart illustrates, there is a substantial gap between the permeance of vinyl facings, compared to that of polypropylene or foil facings. According to the calculations, vinyl facings can allow as much as 490 pounds of water to permeate the insulation per week.

Polypropylene and foil facings, on the other hand can allow as little as 10 pounds of water to collect. These figures, based on test conditions, show the difference a facing can make in retarding moisture.

Today’s energy-conscious builders have discovered than a pre-engineered building is a highly energy-efficient, cost-effective structure. An important part of maximizing this energy efficiency is selecting the correct facing for the job.
Insulation facings can enhance insulation system performance and prolong the life of the building, without adding excessive cost to the project. Researching and selecting facings for their performance values can improve the quality of the building and the reputation of the builder or designer.