Glass, Solar Radiation and Their Effect on Vinyl Cladding Materials
February 1, 2004
FEATURE ARTICLE | Materials & Components
All common building materials expand and contract when heated and cooled. For this reason, building product design, application guidelines and installation practices factor in normal temperature ranges and their effects on thermal expansion and contraction. All exterior elements of a home will experience significant temperature variation during in-service exposure. Temperature variations of 100°F can occur over a normal year, even in temperate climates.
Companies in the glass, window and vinyl siding businesses have experienced reports of interactions of these components. Design conditions, combined with solar geometry, resulted in extremes of reflected solar radiation that increased vinyl siding temperature, causing the vinyl product to deform. In some cases the deformation became permanent. This article is designed to examine the likelihood of such incidents, explore the key variables and speculate worst case/maximum temperatures.Components exposed to direct solar radiation, depending on their emissivity, or ability to absorb and reflect solar energy, could experience temperature changes of more than 150°F. Solar geometry must also be considered, as reflected solar energy from a nearby adjacent surface can provide significant additive heating, under certain conditions. Many homeowners, when using patios, decks and porches, experience periods of excessive glare and heat because of solar radiation reflected from concrete surfaces, walls and windows.
Solar geometry is a key consideration in looking at potential interactions among building cladding materials and fenestration components. In order to provide those involved in building products, design and construction with a scientific basis for rational product and design choices, PPG researchers undertook an initial study to see “what can occur” in certain design configurations and design conditions. The analysis presented is based on calculation techniques from the American Society of Heating, Refrigeration and Air Conditioning Engineers’ Handbook of Fundamentals and standard heat transfer principles. The key variables include:
- Solar geometry for direct and reflected radiation.
- Geographic location and the resultant incident solar energy.
- Glazing types relative to solar reflection.
- Vinyl siding emissivity, solar and thermal absorption.
SOLAR GEOMETRY AND INCIDENT SOLAR ENERGY
Fig. 1 presents a simple plan view of solar radiation directed onto adjacent but perpendicular elevations of a house. The window and vinyl clad surfaces are noted, as are the paths of direct and reflected solar radiation, and the point at which they may combine to create an additive temperature effect on the vinyl.
The second variable to consider is the amount of incident solar energy. Solar Irradiance data from the 1997 ASHRAE Handbook of Fundamentals was used for the analysis. The authors simulated a geographic location at 40° north latitude, which is representative of a significant climate belt of the northern United States. It was determined that noon on September 21 provided the greatest direct normal radiation, 290 btu/(hr-ft2). Further, on a very clear day, this value can increase by 15 percent, to 333.5 btu/(hr-ft2). Ambient air temperature is assumed to be 100°F.
While these assumptions are at the extreme, anyone familiar with the varied climate in the northern and central United States has experienced conditions such as this during the summer months.
GLASS AND VINYL EMISSIVITIES
There has been a steady evolution of glazing products towards improved performance in both reducing heat loss and enhanced solar control. A key component is the use of low-emissivity coatings on glass that, in addition to reducing heat loss from the home’s interior, improve solar control by both absorbing and reflecting solar energy. For this analysis, two configurations will be evaluated: clear uncoated glass and low-E coated clear glass, both in standard insulating glass units.
Clear glass, by itself, provides minimal solar control; low-E coatings are effective in this role and have increased in popularity for that reason. The various producers in the glass industry provide a range of low-E coated glass products with a corresponding range of solar control properties. For this analysis, a low-E coating with a high solar reflection is used. Table 1 illustrates the total solar reflectance of typical residential IG unit configurations including various PPG low-E coatings.
While thermal and optical properties such as emissivity, absorbtance, and reflectance are most commonly discussed in the building products industry as being energy-saving glass features, other materials, including vinyl, have these properties. And, as with glass, these properties vary for the vinyl products used for cladding and other exterior applications, depending on their composition and color. Table 2 below lists a range of vinyl products by generic color and the respective measured solar and thermal (low temperature) emissivity.
ANALYSIS PROCEDURE AND RESULTS
The purpose of the analysis was to determine the maximum steady-state vinyl temperature, based on the glass and vinyl material properties, and the assumed design conditions. Computations were performed using a heat balance algorithm program to estimate the resultant steady-state vinyl temperature. Cases analyzed were:
- Adjacent Window–None, with uncoated glass, and with Solarban 60 low-E coating on the # 3 surface.
- Vinyl–With solar emissivities of 0.25 and 0.50 representing white and dark colors respectively.
- Design Condition–Typical and Severe as shown in Table 3.
- Analyses were performed at various angles of solar incidence ranging from 0° (normal to the glass) to 85° to simulate the sun transiting the sky. The total solar energy reflected by angle of incidence for the glass products simulated are shown in Table 4 above.
The results of the analyses are summarized in Figs. 2, 3, 4, and 5, and in Table 5.
DISCUSSION OF RESULTS
The relatively limited analysis performed, together with the enormous range of “real world” design conditions and variables, preclude drawing definitive conclusions based on the results. However, the results do support the following observations:
- The impact on vinyl temperature due to the presence of a low-E coated glass in lieu of uncoated glass is less than one might assume, ranging from a maximum of approximately 5.5°F to 9.5°F for assumed vinyl solar emissivities of 0.25 and 0.50 respectively.
- Under the assumed severe design conditions, excessive vinyl temperatures may occur regardless of whether the adjacent reflective surface includes a window or is windowless.
- Under the assumed typical design conditions (analysis performed only for an adjacent low-E coated glass window), excessive vinyl temperatures are not likely to occur.