Posted by: Advanced Glazings Ltd. | March 1, 2010

Is there a trade-off between daylighting and the thermal performance of the building envelope?

By Marie-Claire Veloso & Avi Bar

Traditionally the most thermally efficient buildings contained as few windows as possible. Today, many buildings contain almost 100% glass but at what cost?  In order to achieve the valued daylighting and views associated with glass, there is a compromise. Glass is inherently more expensive and is thermally inferior to walls.

The thermal component of the building envelope should supply an unbroken thermal barrier to maximize energy efficiency. LEED™ provides for 19 Energy Optimization credits and 3 credits are available for daylighting and views.  These three LEED items account for:

22 / 30 (73%) credits required for LEED Certified Buildings

22/40 (55%) credits required for LEED Silver

22/50 (44%) credits required for LEED Gold

22/70 (31%) credits required for LEED Platinum

With sustainable standards calling for “energy optimization” and “daylighting and views” how do we address this balance?”

R-value is measured by the thermal resistance of a surface calculating heat loss by conduction, convection and radiation.  The higher the R-Value, the better the insulation. The thermal conductivity of a good insulated wall would typically be around R20 hr.ft2°F /Btu whereas a high performance low-e window would be about an R3.5.  Single glazing (R1) has poor thermal performance and is appropriate only for applications where interior and exterior temperatures do not vary to a large extent. The vast majority of architectural glazing for store front windows, curtain walls and skylights consists of insulated glazing units. IGU selection is extremely important in the outcome of the building’s thermal performance.

Heat gain and loss also takes place due to thermal bridging through the framing.  Glazing systems typically quote center of glass U value (U Value is the inverse of R Value), which is always better than the total system value, because heat transfer through the framing is not taken into account.  In addition to heat transfer through the framing itself, there is also a decrease in the R value 2 inches around the perimeter of the glass lite due to framing. Aluminum has a very high thermal conductivity.  Often an element of low thermal conductivity such as rubber or plastic is integrated to decrease the flow of heat through the framing. Needless to say, a high quality framing system with particular attention paid to thermal bridging is imperative in reducing heat loss or gain of the complete system.

So, what exactly is the trade-off between daylighting and the thermal performance of the envelope? There no longer needs to be much of a trade off. Why? Our newest product, Solera+Nanogel, has an insulation value up to a center of glass R17, making it the highest thermally-insulated glass unit in the world. This is near to the value of an insulated solid wall yet it still admits full spectrum natural daylight into a building.

Many companies have attempted to achieve the holy grail of integrating aerogel into Insulated Glass Units.  The challenge has been to address the settling of the aerogel as the units expand and contract. Some companies have been able to resolve this in small units. The fundamental limitations are that units cannot be transported beyond immediate distances from their source and their small sizes.   The issue of heat gain/loss through and around the framing can be addressed by increasing the proportion of glass to frame, thus “size matters”.

Advanced Glazings has addressed both these issues, offering the first robust commercially available “Super High Efficiency” Insulated Glass Unit.  Solera addresses these two issues by

a)       Encapsulating the aerogel in our proprietary Insolcore transparent honeycomb, resulting in the aerogels inability to settle.

b)       Solera’s maximum size is larger than any of our competitors’ therefore improving the units overall thermal performance.

In addition to the obvious benefits of being clad in glass, Solera can be glazed into the same framing system you wish to use for vision glazing areas of your project. Solera’s ability to be specified with any architectural glass enables it to accommodate all performance and code requirements.

In terms of design consideration, thermal performance should be of equal importance to the building envelope as the position, dimensions and orientation of the structural frame. With our new Solera+Nanogel insulated glazing units you can drastically reduce thermal heat transfer while providing beautiful glare free daylighting.

For more information on how to integrate Solera into your project, contact your local rep.


Responses

  1. This is stating R17 center of glass, the cut sheets from Solera show between R2.2 and R5. Please provide additional information bridging the discrepancies in insulative value. thank you

  2. Thank you for your inquiry. The information you have refers to Solera L (R2.2) or Solera T(R5).

    R17 applies for our newest product, Solera+Nanogel Insulation.


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