Storm Window Specifications

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Jim Nelson
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Storm Window Specifications

Postby Jim Nelson » August 16th, 2011, 1:01 am

The addition of storm windows is one of the most critical factors in weatherizing restored windows. It should be noted that there is a very wide range of quality levels in storm windows, especially in operable storms. All of the windows tested at the summit, with the exception of one, were fixed storm units, either exterior or interior. The operable storm window tested is considered a High Performance storm window, one that is designed to meet minimum performance criteria for air leakage, water resistance, structural load performance and even acoustical performance. The energy savings related to air leakage, when installing an operable storm will not equal those achieved unless a similar quality level product is used.

The standards, for weatherization, should include minimum performance requirements. The standards listed below are the minimum required for a storm product to be considered High Performance. All of these performance areas are listed in ANSI / AAMA 1002 10-93 "Voluntary Specification for Insulating Storm Products for Windows and Sliding Glass Doors" with test procedures referenced in AAMA 502-90 "Voluntary Sprcification for Field Testing of Windows and Sliding Glass Doors." There are also test standards for optional performance areas involving noise reduction. ASTM E 90-90 "Laboratory Measurement of Airborne Sound Transmission of Building Partitions" and ASTM E 413-87 "Determination of Sound Transmission Class (STC.) (Not necessarily the goal of this standard, but a very important factor to consider with a high percentage of historic windows being located in higher density urban environments. Most people who replace single glazed windows with single glazed storms with approximately 2" of air space with 1" insulated glass units and only one point of separation between interior and exterior experience a significant increase in outside noise penetration.)

These standards are very attainable for manufacturers, yet there are many that do not meet them. Many manufacturers list air leakage performance of up to ten times tighter than the minimum requirements. These standards are meant to be realistic for products that were not specifically produced for a laboratory test, but rather production units, installed and field tested.

A structural load performance class of 20 should be a minimum for residential and most light commercial applications for buildings up to 3 stories tall and in most areas of North America. Consideration needs to be taken on buildings with higher elevations or in higher wind zone areas, especially in close proximity to coastlines. A minimum design pressure (performance class) of 30 is required along coasts. Products must be tested to 1 1/2 times the the design pressure rating. To meet a performance class of 30, the product must test up to 45 PSF.

Performance specifications for High Performance storm windows:


A. Test Unit Size: Test units shall be the sizes listed below. Sill of the test buck shall have a 13 degree slope to the
exterior. (See Appendix “A” for test buck details)

1. Fixed panel and removable panel storm windows: 4”0” wide x 4”0” high
2. Horizontal sliding storm windows: 6’0” wide x 4’0” high
3. Vertical sliding storm windows: 3’ 8” wide x 5’ 2” high

B. Air Leakage Test: The storm window shall be subjected to an air leakage test in accordance with ASTM-E 283-91. Window units tested by an Independent Laboratory shall be glazed with 1/8” clear annealed glass. Air leakage shall meet the following performance requirements.
1. Air leakage for fixed panel and removable panel storm windows shall not exceed 0.15 CFM per lineal foot of net sash crack perimeter at both a positive (infiltration) and negative (exfiltration) static pressure of 1.56 PSF (25 mph wind). Weep holes shall not be sealed during the air leakage test.
2. With the storm sash in the closed position, air leakage in horizontal and vertical sliding windows shall not exceed 0.50 CFM per lineal foot of sash crack at both positive and negative static pressure of 1.56 PSF (25 mph wind). Weep holes shall not be sealed during the air leakage test.

C. Uniform Structural Load Test: With storm sash closed position, the window shall be tested in accordance with
ASTM E 330-91. Apply a minimum exterior positive and negative load of:

30.0 PSF (108 mph wind) Class 20
37.5 PSF (121 mph wind) Class 25
45.0 PSF (132 mph wind) Class 30
52.5 PSF (143 mph wind) Class 35

for fixed panel, removable panel, horizontal and vertical sliding storm windows. Each load shall be
maintained for 10 seconds. At the conclusion of these test, there shall be no glass breakage, damage to
fasteners, hardware or any other damage causing the storm window to be inoperable.

D. Water Resistance Test: With storm sash in the closed position, the window shall be subjected to a water resistance test in accordance with ASTM E 331-86. When a positive static pressure of:

2.0 PSF (28 mph wind) Class 20
2.5 PSF (31 mph wind) Class 25
3.0 PSF (34 mph wind) Class 30
3.5 PSF (37 mph wind) Class 35

has been stabilized, 5 gallons of water per hour per square foot of window area shall be applied to the exterior
face of the window, for a continuous period of 3 minutes. No water shall run over the interior edge of the
sloped test buck sill.

One way for an owner to assure the performance level is met, is to incorporate language regarding field testing. An example:

"POST INSTALLATION TESTING: Upon completion of the installation, the owner may randomly select up to two percent of the total number of storm windows installed and have them field-tested for compliance with the Air-Leakage Performance Requirements. Weep holes may not be sealed during this test. The test area shall be the storm window frame and the exterior mounting surface. Any storm window not meeting the minimum Air-Leakage requirement shall be corrected by the contractor at no expense to the owner. For each unit that fails, the owner may select two additional units to be tested."

A High Performance Storm Window specification will also include important information regarding materials, construction, workmanship, operation and installation. These items will vary widely by manufacturer. Just as there are many methods for restoring wood windows, there are many ways to construct and install a storm window. Some methods are more effective than others, but if the product can meet the minimum performance requirements, at least owners can evaluate the other options knowing that the required performance levels will be achieved.

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Re: Storm Window Specifications

Postby sschoberg » August 18th, 2011, 7:05 am

I think any additional testing on storm windows should include the storm window with the prime window. I do not think additional testing should be done just on storm windows. It seems a good fitting wood storm window with fixed glass will allow the whole unit to perform better than installed with any other metal storm window. So this should be the ultimate standard. The emphasis should always be on the prime window. Weather stripping, which included storm windows should be shown to help the prime window reach a certain performance level.

As a micro manufacturer of wood storm windows, I will need to re-engineer my insert storm window to reach as close as possible the standard performance of a prime window with a fixed glass wood storm window. As will the other metal storm window manufacturers.

I dont want to see a performance standard of a historic prime window with an attached metal storm window.

Jim Nelson
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Re: Storm Window Specifications

Postby Jim Nelson » February 24th, 2012, 1:06 am

The point was more to note that the operating storm window tested was one tested and engineered to meet or exceed the minimum performance standards noted. This is so readers of the test data understand that the same performance levels cannot be expected with any combination storm window - that there are differences in products.

You are right about the fact that a fixed glass storm window should provide the best performance in air leakage. If the storms are built even half as well as the two exterior wood storm samples at the summit, there is no reason they should not exceed all of the standards listed. One storm had fixed glass. The other had removable inserts to the interior. The second showed obvious attention to assuring the insert panels would withstand very high wind-loads and both examples used tried and true anchoring systems. Both would easily pass any of the test standards. Both also utilized the best joinery techniques to assure long life which is at the core of restoration.

The intent was not to specify metal storm windows, but rather to clarify that the differences in quality levels are huge in operable storm windows. The test standards listed were for information since most of the public does not know standards even exist.

Since my original post, even the referenced standards have been adopted. The newest from the American Architectural Manufacturers Association is AAMA 1002-11 Voluntary Specifications for Secondary Storm Products for Windows and Sliding Glass Doors. This document ties in with and references AAMA/WDMA/CSA 101/I.S.2/A440-11, "North American Fenestration Standard / Specification (NAFS) for Windows, Doors and Skylights."

Lastly, the only way to truly compare and evaluate differences between storm window products is through solo testing. Tandem testing is very valuable, but has the benefit of the added performance of the prime window. In the world of historic windows, it would be difficult to find two wood double hung windows with identical air infiltration / exfiltration performance. Solo tests of brand new manufactured products can vary slightly. Specifying a maximum air leakage rate, and backing that up with testing, for a storm window allows building owners to evaluate based on real numbers and even determine payback times.

Tandem tests are, obviously, critical. The whole goal of this project is to prove that restoration and weatherization can perform as well or better than most replacement systems. The tandem test is vital to show the whole window system and prove what we all know - two barriers against the weather, working together is better than a single system with insulated glass destined to eventually fail.

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Re: Storm Window Specifications

Postby johnleeke » February 24th, 2012, 4:39 pm

Steve writes:
I dont want to see a performance standard of a historic prime window with an attached metal storm window.

Steve, in any case this round of testing and developing standards will not result in the setting any performance standards. We are focused on setting standards on methods of repair, maintenance of existing windows, and methods of testing.

The setting of actual performance standards might develop in a future year and edition of the standards.

I'm confident that wooden storms would be included.
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Bob Yapp
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Re: Storm Window Specifications

Postby Bob Yapp » February 24th, 2012, 5:37 pm

The good news is that all exterior storms (Jim's metal, my two variations of wood and John's interior temorary panel) all exceeded the 2012 International Energy Code (IEC) for air infiltration. We're still pouring through Walters report, but Jims comments about the wood storms I built and used in the testing were correct. His aluminum operable storm tested equally well.

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