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Are Low R-Value Skylights Sapping Heat From Your Building?

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How can building owners achieve code-compliant energy performance and enjoy the benefits of daylighting without spending a fortune on roof deck insulation? By installing skylights with better R-Value. Understanding and investing in better R-Value skylights leads to greater energy efficiency – and therefore makes it easier to meet energy codes, without excessive insulation and the additional costs associated. Here’s how it works:

R-Value and the 3 Fundamentals of Heat Loss

R-Value is the universal measurement to gauge an object’s ability to resist heat transfer. Objects with a higher R-Value transfer less heat. By multiplying the following three factors and dividing by R-Value, one can determine the amount of heat that a building transfers to the surrounding atmosphere (heat loss).

Area: Heat loss increases with the size of the object that heat flows through. Therefore, a 5×6 foot skylight (30 sq ft area) loses nearly twice the heat as a 4×4 foot skylight (16 sq ft) if all other factors were the same.
Temperature Difference: Heat loss increases as the temperature difference widens. The colder it is outside, the more heat is transferred from inside. Conversely in the summer, the hotter it is outside, the more heat load is imposed on the air conditioning unit.
Time: Heat loss increases with time. This factor is especially important for buildings in northern climates with longer winters or for buildings in southern climates with long hot summers. Buildings in temperate areas with minimal heating nor air conditioning loads will not benefit as much by increasing R-value.
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Remember that heat flow is additive, meaning that a person must calculate parallel paths of heat loss individually, and then add them together. For example: Q1 (roof heat loss) + Q2 (heat loss through windows) + Q3 (heat loss through walls) + Q4 (heat loss through skylights) = Qtotal. Note: Heat loss is measured in British thermal units (BTU) or in Watt-hours (W-hr).

Good Skylights VS Bad Skylights VS No Skylights: An Example of Heat Transfer

Builders insulate roof decks with stacked rigid foam polyisocyanurate (polyiso) boards. Generally, every inch of polyiso achieves an R-Value of 5The thicker the layers of polyiso, the more insulated the roof.

Let’s examine the role R-Value plays in minimizing heat loss. The following example uses a 50,000 square-foot building for 24 hours. It is 65 degrees (F) inside and 20 degrees outside, resulting in a 45-degree temperature difference.

In Case 1, the base example, the building has no skylights.
In Case 2, the roof has common skylights with an R-Value of 1.2.
In Case 3, the roof has superior skylights with an R-Value of 4.0.
In cases 2 and 3, skylights cover 2,000 feet, or 4% of the roof area (63 common 4×8 units or 135 Replex units).

As seen in the chart below, when the roof in the base example is insulated with three inches of polyiso, it has an R-Value of 15 and loses 3.6 million BTUs during that 24-hour day. When 1.2 R-Value skylights are added, the roof suffers a 46% increase in heat loss (5.256 million BTUs). To negate the 46% increase in heat loss from the common skylights and still meet energy code regulations, the builder would have to increase the polyiso thickness to six inches, an increase of 2.5 inches of polyiso.

However, with only 3 inches of polyiso and 4.0 R-Value skylights, the roof loses 3.996 million BTUs, just 11% more than the base example. With this minimal change, there would be no need to install additional roof deck insulation. Therefore using R-4 skylights avoids the cost of an extra 2.5 inches of polyiso that will be needed if R-1.2 skylights are used. Builders in some states (e.g. Florida) often use six inches of polyiso to achieve the State required R-Value of 30. In those cases, installing 1.2 R-Value skylights would nearly double (96% change) the level of heat loss in winter (or heat gain in summer), as opposed to the 26% increase spurred by the installation of 4.0 R-Value skylights.

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The need to compensate with ever-increasing layers of polyiso insulation can discourage builders from investing in skylights, preventing companies from reaping the benefits of daylighting and long-term electrical energy savings otherwise needed for lighting. The good news is that higher R-value skylights can minimize the heat gain or loss, without towering inches of polyiso. But what’s more cost effective?

Cost of Heating and Insulating: How BTUs Translate to Dollars Spent

For building owners who want to simply meet energy code regulations for the minimal cost, it is important to examine the cost of wasted fuel through heat loss. To compensate for the heat loss of the low R-Value skylights, the building owner would have to purchase and burn natural gas.

As seen in the chart below, 1.2 R-Value skylights require six inches of insulation to achieve the same energy efficiency that 4.0 R-Value skylights could achieve with just 3.5 inches of insulation.

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How does this affect costs? Polyiso insulation costs roughly 37 cents per square foot per inch of thickness. Thus, to achieve the same performance with the extra 2.5 inches of insulation, the owner pays an extra 93 cents per square foot. For a building with a 50,000 square foot roof, he or she could save $46,000 in avoided insulation cost by installing superior, R-4.0 skylights.

High-quality, 4.0 R-Value skylights enable building owners to meet energy codes with far less insulation and heating fuel than common, 1.2 R-Value skylights. Though installing no skylights at all presents up-front cost-savings, this solution reaps none of the long-term benefits and ROI on lighting costs that come with high-quality daylighting solutions. As daylighting requirements and insulation value are increasingly mandated in building energy code regulations, it’s worth understanding the impact of skylight R-Values on overall heat loss. More importantly, it’s worth it to examine the cost-friendly options to achieve the required heat transfer levels and still enjoy the many positive impacts of daylighting.

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