If you are in the market for skylight you have probably figured out that you have two MAIN options when it comes to the glazing of your skylight. Before we can get to the nitty gritty of this subject, there are a few terms that you will need to familiarize yourself with.
Glazing: This is the material that is put or “glazed” into the aluminum frame on a skylight. This material can be glass, plastic and even fiberglass.
Solar Optical Properties (TRA Values): The solar optical properties of a glazing or product are the transmittance, reflectance and absorbence. Generally referred to as the TRA values.
The solar optical properties are further broken down into three categories, total solar, visible and UV.
Total solar properties are used for the analysis of the product/glazing for the entire spectrum related to solar radiation. These values would be used when one is concerned with the solar heat gain (heat) into a space.
The visible properties are the same TRA values but for the visible range of the solar spectrum only. This is the solar spectrum range that the average human eye responds to. These properties would be used for analysis of day lighting, glare, etc.
The UV transmittance is the transmittance of the product/glazing for the UV range of the solar spectrum. This value is used as a rating to determine the product/glazing effectiveness in guarding against UV degradation. The reader should be aware that the UV portion only accounts for approximately 1/3 of the damage. The environmental UV radiation and the heat produced by absorbed solar radiation both impact the degradation to carpet and furnishings.
Shading Coefficient (SC): A value that represents the quantity of solar heat through the product/glazing in question as compared to the solar heat gain through the ASHRAE reference glazing (1/8″ clear single glazing; SC = 1.0), under the same conditions.
Solar Heat Gain Factor (SHGF): A solar radiation level (Btu/hr.ft2 or W/M2) for a given geographic location, surface tilt and orientation, time and day that would enter (transmitted and the inward flowing fraction of the absorbed portion of solar radiation) the space through the ASHRAE reference glazing. This value accounts for the change in transmittance, reflectance and absorbence due to the changing incident angle on the glass.
Solar Heat Gain Coefficient (SHGC): This value was developed to replace the Shading Coefficient due to the fact that the Shading Coefficient is not constant. Specifically for products that have strong angular solar optical properties. ie: louvered shade screen. — The ratio of solar heat gain (transmitted and the inward flowing fraction of the absorbed portion of solar radiation) through the product/glazing to the incident solar radiation striking the surface for a given condition.
U-factor: The U-factor (Btu/hr.ft2.F or W/M2.C) is the heat transmission value for the product/glazing/window which occurs to the indoor/outdoor temperature differential. this can be a heat gain or loss depending on the indoor/outdoor temperatures.
R-value: The R-Value (F/Btu/hr.FT2) is the inverse of the U-value. The value represents the resistance to heat flow due to the indoor/outdoor temperature differential.
Daylight Efficacy Value (Ke): The daylight Efficacy Value is an indication of the amount of light (footcandles) that will enter through a fenestration system as compared to the amount of heat (Btu/hr.ft2 or W/M2). Ke = Visible Transmittance / Shading Coefficient.
Once you are more familier with these terms ( I am sure you have them memorized by now ) you can take a look at our Glazing comparison chart by clicking HERE.
Now …. If you have kept up with our blog, you know that I try to simplify things as much as possible. Sometimes too much. Regardless, here are some easy things to consider when deciding between glass and plastic.
- Must have at least a 2/12 pitch ( can not be installed on a flat roof without modifying curbs for proper pitch )
- Many options for coatings and different glass types to make your skylight perform the way you want it to. Example: allowing the most amount of light while allowing minimal summer heat gain and winter heat loss.
- Easier to maintain. Glass skylights can be cleaned with many standard household cleaners including Windex.
- Longer life expectancy. 30+ years on average.
- Impact resistant.
- Minumum 5 year warranty against glass seal failure.
- Can get expensive depending on the options you choose.
- Can be installed on any type of roof including flat roofs.
- Restricted to only 3 standard color options and no coatings available. The color you choose determines the amount of visible light, heat loss and heat gains.
- Must be cleaned with mild soap and water ONLY. Most household “cleaners” will deterieriate the plastic over time causing it to become brittle and eventually start to crack.
- Average life expectancy of 20 to 25 years
- Specific material can be used to achieve impact resistance.
- Standard one year warranty against defects and workmanship.
- Cheapest glazing material.
I hope you have found this information helpful in deciding which type of skylight you want. Of course you are more than welcome to call us if you need more information or if you need any clarification.