4 Resources Help Draw the Shades on Poor Window Performance
Predicting performance and rationally selecting window coverings—from awnings to films to cellular shades—is incredibly challenging, but real help is on the way.
There is a lot of interest in just how much (and at how low a price point) window coverings can improve building thermal performance.
Both the U.S. Department of Energy (DOE) and the U.S. Environmental Protection Agency (EPA) have been working on this issue; electric utilities would like to know how window coverings can fit into their efficiency programs; and both building professionals and consumers need objective guidance on how to compare window coverings—to each other and to window replacement.
Where does our industry stand on assessing thermal performance of window attachments, or coverings? There are four new or emerging resources that paint a more complete picture.
For the last several years, BuildingGreen has been working with Lawrence Berkeley National Laboratory (LBNL) on evaluating window coverings and their role in energy savings (as documented in our EBN feature article “Making Windows Work Better”). This effort was financially supported by DOE and was driven in no small part by a dedicated group of about 15 industry professionals in a project Advisory Committee.
Similar to the Efficient Windows Collaborative website, the centerpiece of the Efficient Window Coverings website is a selection tool. The main difference between the two selection tools is complexity.
Selecting windows is far from simple, but the number of attributes evaluated is relatively small and is exactly the same from window to window. Evaluating and selecting window coverings is complicated by the wide range of coverings—interior/exterior, adjustable/fixed, fabric/plastic/wood/metal—and the numerous functions the window coverings can serve or feature, as well as the choice of window over which coverings are installed.
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The Efficient Window Coverings website selection tool currently covers 19 types of coverings and includes 18 distinct attributes.
Some of the attributes are assessed qualitatively (such as view, privacy, ease of egress), but others—like thermal performance, including insulation, airtightness, and solar heat control—will move from qualitative to quantitative assessment.
For some classes of products (such as venetian blinds, vertical louvered blinds, roller shades, solar screens, cellular shades, window films, interior and “storm windows,” and awnings), modeling programs like EnergyPlus and WINDOW have the capability of accurately predicting solar-optical, thermal and energy performance. However, for several other classes of products (such as pleated shades, drapes, Roman shades, roller shutters, louvered shutters, etc.), this capability is still under development. That leads us to the next new resource on evaluating window coverings.
Energy Savings from Window Attachments
LBNL recently completed a major modeling study: “Energy Savings from Window Attachments.” The study evaluated the thermal and optical performance of eleven window attachments for twelve cities, four residential building types, two HVAC systems, and three baseline window types.
Simulations began with the use of WINDOW and THERM for calculating thermal and optical performance of baseline windows with coverings, and then these results were pulled into EnergyPlus for modeling at the whole-building level. All of this required years of development work to bring the often thermally and optically complex window coverings into modeling capability.
Given all of the variables in this modeling study, it is not surprising that the results are quite varied and complicated. Here are some generalizations:
- In northern and central climates (where heating energy is greater than cooling energy), interior window panels, exterior storms, and cellular shades save the most energy.
- Combined critical design parameters of window attachments (designated “A” through “D” in this modeling study) were developed because of variability of design parameters between window attachment types and even within any one attachment type. These parameters were emissivity, reflectance, transmittance, and deployment (position for adjustable or operable attachments).
- In southern climates, not surprisingly, the combined design parameter “A” (low emissivity, high reflectance, low transmittance) provided the greatest energy savings.
- What is a bit surprising is that interior window panels and exterior storms maintained good energy savings in southern climates, with all exterior shading devices (solar screens, awnings) also doing well.
- Deployment had a huge impact on results for operable/adjustable attachments.
This last generalization is completely expected, but how did the LBNL modeling study address the sticky issue of just how occupants use adjustable window attachments? That leads to the next new resource on efficient window coverings.
Residential Windows and Window Coverings: A Detailed View of the Installed Base and User Behavior
The LBNL modeling study of window attachment energy performance used the results of this DOE-sponsored window attachment market and user behavior study conducted by D&R International. Two very compelling results of this study are:
- Inexpensive horizontal blinds are by far the most common window attachment (62% of installations).
- While dependent on time of year and day, adjustable window attachments simply don’t get adjusted very much; they tend to stay up if they are up and stay down if they are down. 75%–84% of window coverings remain in the same position during the day, and 56%–71% of households don’t adjust coverings on a daily basis.
This study has big implications for energy savings:
- Poorly performing, inexpensive horizontal blinds can be replaced for significant energy savings.
- Assumptions made about user behavior of adjustable window attachments, or opportunities to improve energy savings by users more optimally adjusting their window attachments, can significantly affect the energy savings window coverings represent.
And while automation of window coverings was not a part of either study, it is easy to see that automation will have a big impact on energy savings of adjustable window coverings (and price, of course).
Has all this work on assessing window attachment performance really helped folks select the right window covering? Not yet, but that leads us to the latest development regarding efficient window coverings.
Certification and Rating of Attachments for Fenestration Technologies (CRAFT)
DOE has released a Funding Opportunity Announcement (FOA) for the development of CRAFT. From the FOA:
“The awardee will develop performance verification, and labeling standards for residential and commercial fenestration attachments. The successful applicant will also develop a Program to rate fenestration attachment energy performance and provide accurate and useful product comparison criteria, allowing end users in residential and commercial markets to assess the relative energy cost/benefits of fenestration attachments.”
As a part of this process, LBNL will continue its extensive modeling of window coverings to support CRAFT.
And at some point, we hope that just as the Efficient Windows website has become the main tool that the building community uses to select NFRC-rated windows, the Efficient Window Coverings website and selection tool will become the main way that building professionals select CRAFT-rated window coverings.
Published February 15, 2014 Permalink Citation
(2014, February 15). 4 Resources Help Draw the Shades on Poor Window Performance. Retrieved from https://www.buildinggreen.com/blog/4-resources-help-draw-shades-poor-window-performance
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