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Alliance Webinar: Window Energy Efficiency Beyond Business as Usual

On Thursday June 11, the Alliance hosted a webinar for Alliance Associates and others interested in opportunities for window energy efficiency. Moderated by the Alliance’s Vice President for Programs Jeff Harris, speakers representing research, industry and low-income weatherization highlighted options that can minimize window heat loss far beyond common practice. The focus was on high-end R-5 window technologies, but lower-cost products, such as low-E storm windows, and the specific needs of low-income weatherization programs were also discussed. The five presenters’ different perspectives converged in the message that there is a great need for more energy-efficient windows and that advanced technologies and their integration in incentive and weatherization programs can bring far greater savings within reach.

Our thanks to all who attended the webinar; we hope you found the discussion informative. We had many great questions, which are listed in the Q&A section. The presentations can be downloaded below, or you can view a recording of the webinar. If you’d like to learn more about energy-efficient windows and how to apply them to residential and commercial buildings, we invite you to visit the Efficient Windows Collaborative at: www.efficientwindows.org or www.commercialwindows.org.

List of Presenters and Presentations:

Advanced Window Products – A Key Strategy to Save Energy, Save Money, and Mitigate Carbon: Marc LaFrance, U.S. Department of Energy
Highly Insulating Windows: Christian Kohler, Lawrence Berkeley National Laboratory
Advance Window Technolgies: Overview and Benefits: Brandon Tinianov, Serious Materials
R-5 Windows: Tim McGlinchy, GED Integrated Solutions
The WAP Effect: High-Efficiency Windows & The Dilemma (Tyranny?) of Audit Algorithms: John Hamilton, Community and Economic Development Association of Cook County

Recordings of the Webinar:

Watch the webinar in the player below or, for better sound quality you can download a recording of the webinar (9 MB WMV).

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Questions and Answers

Questions and Answers

1. In the NE, is triple glazing needed only on the south side of the building, cost consideration in mind? Top.

Triple glazing is very effective on the east west and north side of the building, where there isn’t as much solar heat gain during the winter to make up for heat loss. On the south side, solar heat gain can offset a substantial portion of heat loss, so that the need for highly-insulating glazing isn’t as big as on the other sides. Because of this and because double glazing can allow higher solar heat gain than triple glazing, some buildings are designed with triples on all sides except the south. This can of course reduce cost, especially if there is a lot of south-facing glazing. Keep in mind though that the effectiveness of the sun to make up for heat loss depends on whether there is good solar access and whether there is enough thermal mass to store the heat for after sundown. And a design for maximizing solar heat gain should also include shading against unwanted heat gain.

For an example of a NE house with triple-pane windows except on the south side, view pages 25 & 26 in the Mar/Abr 2009 edition of Home Energy magazine (1.5 MB PDF).

2. Are R5 Windows becoming a mandate by the DOE? Is R5 equal for northern versus southern climates? Top.

When talking about R5, DOE is presently concentrating on mixed or cold climates. In cooling-dominated climates, solar-heat-gain control is more important than the R-value. DOE will work with manufacturers and buyers to develop interest and lower cost, but Marc has not mentioned anything about mandates.

3. In different (hot, cold, moderate) climates, do triple panes always make sense? Top.

Triple-pane is mainly a cold climate solution. In moderate climates, the need for such a low U-factor depends on energy prices and on whether the full benefits of triple-pane can be reaped (i.e. reduction of the HVAC system size due to less heat loss). In hot climates, it is more economic to use dual pane with low-SHGC low-E coatings because the SHGC is more important than the U-factor there.

4. Are there any triple glazed curtain walls or skylights being developed? What`s soft wall? Top.

Yes, there are triple-glazed curtain wall systems or skylights, either with three panes of glass or suspended film between glass. With “soft wall” I assume you refer to the mention of Southwall, which is a company that produces suspended film for between-glass applications or as applied film for retrofit applications.

5. GED presenter- How can you use 1/2 inch between panes and still use triple panes? Will that fit a double hung DH window in a residential application (can the sashes clear each other?) Top.

The design of the Double Hung window will have to be redesigned to fit a larger overall IG of 1.2” to 1.3“, vs. traditional 7/8 or ¾” pocket. Several designs have been made and are also in the process of being developed to accommodate larger overall width. Some designs project externally, internally or both from the center line, giving clearance for the two sashes.

6. What is the difference in light transmittance between low-e and clear storm windows? Top.

Storm windows over single-pane windows have similar VT (visible transmittance) properties as dual-pane windows. You can find typical VT numbers for different glazing types on the Efficient Windows Collaborative Web sitem. Typical low-E coatings for storm windows have similar characteristics as high-solar-gain low-E coatings in regular windows. As you can see on the site, the typical center of glass VT of high-solar-gain low-E dual pane is 0.75, compared to 0.81 VT for dual-pane clear glass. So at the center of glass, you can expect 5-10% VT reduction from the low-E coating on storm windows over clear single-pane windows.

7. Who manufactures the low-e storm windows? Top.

Any storm window manufacturer can use low-E glass for their storm windows. By switching their glass orders to low-E, manufacturers can quickly make that change. Yet not all storm window manufacturers offer low-E because traditionally there hasn’t been much awareness of this option. But as demand increases due to larger awareness, manufacturers may easily include low-E in their repertoire. You can find a list of storm window suppliers on the Old House Web site.

8. Where do you see window film in the mix in terms of helping to seal a building envelope and working with the glazing unit to deliver energy savings, especially going forward as the newer highly insulating windows hit the market? Top.

On the one hand, window film is used as retrofit applied film on existing glazing, where it can provide solar control and/or security. This is a common application where overheating and glare are a problem and replacement of the glazing is not needed or too costly. Applied film adds little in terms of insulation against heat loss, so it’s more commonly used in southern climates or commercial buildings. In our webinar, we did not focus on applied film because it is already a widely-used option. However, we’re planning on adding specific information on window film to www.efficientwindows.org. Please note that the National Fenestration Rating Council has developed a rating system for window film, which provides certified SHGC and visible transmittance ratings that help to differentiate window films with different properties.

On the other hand, there is suspended film between glass panes, which boosts insulation against heat loss in addition to solar control. Brandon Tinianov mentioned this suspended film in his presentation on highly-insulating windows.

9. How and why does the low-e coating factor into the NFRC u factor (not just SHGC)? Manufacturers of retrofit film claim the film has a u factor as well as SHGC. Top.

Different low-E coatings reflect different wavelengths of infrared radiation. All of them reflect long-wave IR from ambient heat and can thus reduce the radiative heat transfer from one pane of glass to the other, thus lowering the U-factor in double glazing or between windows and storm windows – as long as the coating is between the panes. If the coating is not between the panes (as is the case with a window film attached to the window surface), there isn’t much reduction of the heat transfer between panes, and the U-factor is not reduced much despite the coating.

When it comes to solar infrared radiation, low-E coatings can reflect much of this radiation even if attached on the window surface through a film. But the coating needs to be designed for solar heat control. Films often have solar control coatings, but storm windows usually don’t. This is because solar control low-E coatings tend to be susceptible to deterioration unless protected in insulated glass units or by films.

In short, retrofit window films primarily lower the SHGC whereas low-E storm windows primarily lower the U-factor. For more information on the different types of low-E, see the Efficient Windows Collaborative Web site.

10. Air infiltration rates have not been talked about. Infiltration and the conditioning of extra air has huge impacts on energy usage. Is there a plan in place to uniformly rate the airtightness of different window assemblies? Top.

Building energy codes such as the International Energy Conservation Code require that windows are labeled in a manner to determine that they meet an air infiltration limit equal or better than 0.30 cfm per sq ft of window area as determined in accordance with NFRC 400 or AAMA/WDMA/CSA 101/I.S.2/A440. Such labeling is typically done through certification under the AAMA or WDMA programs. Many window options can reach much lower air infiltration rates than 0.30. However, this is just the initial rate, which can change substantially over a window’s lifetime depending on the durability of the weatherstripping and the tendency of the frame/sashes to expand and contract.

11. How does the 6-9% reduction in air infiltration compare with air infiltration reduction of other products, such as wall insulation, etc? As cost-efficient? Top.

It is difficult to compare infiltration rates of added wall insulation vs. storm window installation as neither would likely be installed due to infiltration concerns alone. Wall insulation typically enhances insulation properties, thereby primarily reducing radiant and conductive heat loss. Low-e storm windows reflect infrared heat, thus boosting the insulation value in addition to reducing infiltration. If air infiltration alone is the concern, the most cost-effective method to reduce air infiltration is air sealing seams, cracks and chase cavities between a building’s interior and exterior. This is typically done with spray foam applied by hand and weather stripping around doors and windows.

12. Who makes fiberglass frames? Andersen, Marvin? Top.

You can find manufacturers of fiberglass windows through the Efficient Windows Collaborative Window Selection Tool.

13. Fiberglass frame windows seem to be capable of high energy efficiency. What is the downside of using fiberglass instead of aluminum? Top.

Brandon Tinianov: The only real downsides are the maximum allowable structural loads and cost. The embodied energy is much lower for fiberglass and the maintenance is the same.

14. What about wood frames? Top.

See information on different frame materials on the Efficient Windows Collaborative site at http://efficientwindows.org/ftypes.cfm.

15. What is the long term migration of argon gas in insulated units? Top.

Studies have shown less than 0.5% leakage per year in a well-designed and well-fabricated unit. In Europe, the norm EN1279 prescribes below 1% gas loss per year. So these gas loss rates are achievable. But this depends on the quality of the seals, the glazing design, and on quality control during the manufacturing process. If seals break, all gas is lost. As Tim McGlinchy mentioned, from 2010 on, the National Fenestration Rating Council and Energy Star will require that the units used in certified windows are also certified under recognized IG certification programs. This is an important step to greater reliability throughout the market.

16. When do you vacuum in lieu of gas in fill in between the window panes? Top.

As Marc LaFrance mentioned during the Q&A session, vacuum glazing is still in development and some years away from possible prime time.

17. So far, there has not been a discussion in vacuum windows. I've read that they will have R-values of R-10 and greater. What is the DOE doing to help develop and commercialize practical and long-term reliable vacuum windows in the U.S.? Top.

Marc LaFrance: DOE supports a long term goal of R10 windows, 0.10 U factors. However, that is in our longer R&D area and is not the subject of near term market products such as today's webinar. We anticipate a solicitation that will be available for a variety of projects such as vacuum glazings. One advantage of a vacuum glazing is that it would provide higher solar heat gain for northern climates compared to a quad pane window.

18. How do gas filled windows work at high altitudes? Top.

This is a topic of much concern among window manufacturers. The complication is that most gas filled windows are manufactured at lower altitudes (under 2000’) and that significant pressure differential to a high-altitude installation site (over 5000’) places stress on the glazing. Glass bowing and seal failure (which would lead to gas loss and moisture entry), can be prevented by using tempered glass, high-durability sealants, and/or capillary tubes allowing the insulating cavity to ‘breathe’. If capillary tubes are used, it is virtually impossible to be sure you retain adequate gas fill at the installation site. Nonetheless, these tubes may be necessary at very high elevations to level out the internal and external pressure. Please contact manufacturers on product-specific high-altitude requirements. Some manufacturers have explored gas filling on-site, but this method is not widely adopted and significantly more expensive.

The role that gas fills play in maintaining a low U-factor varies by window type and is typically more substantial in triple- than in double-glazed windows.

19. For the graphic that showed net energy use for MN- could you show us where the window becomes as good as a wall? Say an R21 2x6. Top.

The graphic that Christian showed (from an LBNL paper on zero energy windows) actually defines a window that’s an energy provider as one that performs better than an R-19 wall. See the graphic on page 8 of the paper (same as in the presentation) – you’ll see that the bold line indicates “neutral energy impact of windows, equal to a building with no windows. The wall insulation assumed for MN is R-19 (see page 22 of that paper).

20. With the cost of high performance windows, how can a savings-to-investment ratio of greater than one be achieved. With increased labor costs in retrofit applications and material costs, will windows only be cost effective in areas that have higher degree days? Top.

The cost-effectiveness of high-performance windows is the highest if heating and cooling costs are a major factor. This does not need be in cold climates, but could also be the case in areas with a lot of solar heat gain. In mild climates, the impact of windows is of course less, but in commercial buildings with high cooling loads, solar heat gain control can be very important nonetheless.

Return on investment in high-performance windows is highest when new windows are needed anyway (e.g. in new construction or when irreparable windows need to be replaced), as in that case, only the incremental cost of high-performance needs to be recouped from the savings. If window replacement or upgrades are done purely for energy-saving purposes, it depends strongly on climate, energy and labor costs and the life-expectancy of the new products whether the savings-to-investment ratio is greater than one. Where the cost of replacement is too high, options such as low-e storm windows, retrofit films or repairs and modifications to existing windows can still be cost-effective alternatives.

21. I live in a poor city with over 4,500 properties on the National Register for Historic Places. Replacement windows are the single biggest challenge for the Historic Properties Commission. Historicly appropriate but energy efficient windows are expensive and out of reach for many property owners. Any suggestions other than low-e storm windows? Top.

The cheapest option and the one that would least interfere with the historical look is to repair and/or weatherstrip the existing windows. That can help reduce air and water leakage, but does usually leave the windows with the poor insulating quality of single-pane glazing. To upgrade the glass to dual panes without replacing the windows, secondary glazing (storm windows or interior glazing additions) might be needed, unless dual-glazing can be inserted in the existing sashes (may be possible with wood windows). If insulating value is less a concern than solar heat gain control, low-E window film can be applied to the existing glazing so as to reflect solar infrared and UV radiation. Finally, some window replacements with historically appropriate windows may be affordable if property owners can take advantage of the federal tax credit for replacement windows (see http://efficientwindows.org/taxcredit/taxcredit.cfm) and of any utility, state or local incentives for energy efficiency offered in your city.

22. Why is the SHGC set at <0.3 even for cold climates to obtain tax credits? Top.

It wasn't DOE who set this specification. Congress decided on one set of criteria for the whole country, partly to keep it simple to administer.

23. What does "wet glazing" mean? Top.

Instead of using a typical double-sided foam tape, the insulating glass unit is sealed with a wet silicone seal/caulk.

24. Desiccants in HVAC systems eventually have to undergo moisture removal. How is that accomplished in a window spacer? Top.

Brandon Tinianov: It is not (industry wide). It is applied at 2X what is needed and backfilled with dry air/gas. It is expected that, without failure, the desiccant never reaches capacity.

25. Why does it seem that industry and the DOE believe that a lower SHGC is much better than a higher one? In my experience designing certified PassiveHouses, a SHGC of .5 or higher is necessary for many climates in North America to drastically reduce energy consumption and to have zero energy homes happen today. Top.

DOE fully supports high solar heat gain for Northern climates. See Marc LaFrance’s presentation at AAMA http://www.govforums.org/E&W/documents/AAMA_Window_Presentation_June_2008.pdf, Slide 49.