Low-E Exterior Storm Windows

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Description

Before beginning this work, the window should be assessed as outlined within the Ensuring Success tab.

The addition of exterior storm windows to existing windows is a common, commercially available off-the-shelf technology, and is for the most part straight forward.  This work should be completed in conjunction with the window rehabilitation work outlined in a separate guide.  The existing window sill must be in good condition, as it will continue to be exposed to the elements (though exposure will be significantly reduced).

The exterior storm window is fastened to the outer window casing, or in some cases directly to the window trim.  The storms should be sealed with an exterior grade paintable sealant at the jambs and heads, but left unsealed at the sill.  Most storm windows have an adjustable bottom leg to account for variations in the sill height.  This leg should not be caulked, to allow for drainage at the bottom.  Some systems even provide small weep holes or notches in the bottom leg to promote drainage.  Systems with enhanced drainage are recommended.

Exterior aluminum storm showing weep channel

Figure 1 - Exterior aluminum storm windows, showing weep channel Reference

In order to minimize the potential for interstitial condensation, the original window must be made as air tight as possible (Wilson 1960).  Slight ventilation of the exterior storm is typically provided by the weep holes provided at the sill.  If the interior window is made sufficiently air tight, then the slight ventilation of the weep holes of the storm should provide adequate air change to prevent condensation.  If condensation does form, slightly increasing the gap to allow for additional ventilation of the space is recommended.

 

Condensation potential for exterior storms

Figure 2 - Condensation potential for exterior storm retrofits Reference

Ensuring Success

Wood window options for existing homes

Identifying Risks
Prior to any retrofit work being conducted, it is important that the following conditions of the building systems be reviewed:

  • Lead and other hazardous materials
  • Site conditions and project staging
  • Identification of water infiltration concerns
  • Identification of deteriorated or damaged materials
  • Identification of user comfort concerns

Contractor/Homeowner Safety

US EPA: Lead in Paing, Dust and Soil: Renovation, Repair Painting

OSHA: Fall Protection (if window work is to be done at height, from the exterior)

Lead and Other Hazardous Materials
Old wood windows and trim are a common location of lead paint in homes. Any work being completed on the window systems should follow all appropriate state and federal laws regarding handling of hazardous materials.

Site conditions and Project Staging
The home and site should be reviewed to identify impacts and potential risks with completing the work.

If the work is to be done for the exterior, scaffolding, lifts, ladders, or other means to access work areas may be needed. Work done at height may require fall protection be used. Proximity to adjacent property or vegetation may limit access or create unsafe work areas.  Exterior staged work may also damage existing landscaping or vegetation.

If the work is intended to be completed from the interior, consideration should be given to disruption of the occupant and clearances for moving equipment and materials into and out of the space. With any interior work there is always a chance of damage to interior finishes.  Appropriate planning and protection is required.

Identification of Water Infiltration Concerns
Windows, above all other enclosure systems, are a common location of water infiltration issues.  It is important to understand the various pathways for potential water infiltration, and identify current water leakage problems. While the details presented in this document are all intended to improve the moisture performance of the window assemblies, it is not intended to address all possibilities, and is not a replacement for inspection and evaluation of the performance of an individual window.  Existing problems should be identified, and the strategy chosen that will be most appropriate to address the concern. Window systems water leaks can be grouped into four general categories (Figure 1 below):

  1. Between the window frame and rough opening
  2. Through the joints in the window frame
  3. Between the window frame and the operable sashes
  4. Through the joints between the glass and the sash frames

Common window water infiltration pathways

Figure 1 - Common window water infiltration pathways Reference

Prior to any work being done, interior and exterior inspection and monitoring of the conditions of the building should be completed. Water staining, peeling paint or wall paper, and staining on trim or floor assemblies below window systems are indications of water infiltration and/or condensation. Leakage between the sashes and the frame and between the glass and the sash is usually marked by water staining on the interior window frame itself. Condensation on the window frames can also lead to staining of the interior finishes. It is important to monitor the questionable area to prevent a false diagnosis of the water management problem being experienced.

Leakage between the window and rough opening or through the joints in the window itself are typically contained within the wall assembly and may go unnoticed, or could manifest as staining and peeling paint below the window or damaged flooring.

Other problems such as water infiltration at the window head may be indications of failed or missing head flashing. However, other problems not associated with the window system may in fact be the cause of the water infiltration. Care must be taken to properly diagnose the infiltration pathway.

If it is a known recurring problem, then the infiltration problem must be addressed prior to or in conjunction with the window retrofit work.

If no obvious signs of water infiltration problems exist and the window elements and connection wall components are in good condition, no additional work may be needed. However, as stated above, water infiltration problems are often concealed within wall cavities with no outward signs. This becomes more of a concern if the window retrofit work is being done in conjunction with the addition of cavity fill insulation. With the addition of insulation to the wall cavities, water infiltration problems that previously may have had sufficient drying ability, may now lead to prolonged moisture accumulation. Prolonged moisture accumulation can lead to material deterioration. If there is suspected leakage, then further investigation, including but not limited to thermal scans, moisture content measurements, and cutting of investigation holes below window assemblies to look for signs of moisture problems would be recommended.

Unless the problem is obvious, it may be prudent to contact someone with experience with diagnosing water infiltration problems prior to proceeding.

Identification of Deteriorated or Damaged Materials
If damage to existing elements is noted, the materials should be removed and replaced as part of the retrofit. Certain elements will be more critical to the proper implementation of the chosen strategy.

Failed window sill with replacement window installed

Figure 2 - Failed window sill with replacement window installed Reference

The window sill is arguably the most important element of the window assembly, as water will drain downward by gravity either into the wall (e.g., hole through sill) or directly onto the wall (failure of the sill extension). For all proposed measures in this document excluding complete window replacement, the condition of the sill is critical to the performance of the measure. Cracked or rotting sills need to be replaced prior to any work being done.

The window frame including the exterior casings is the next most critical element. If the casing is deteriorating, its replacement may be warranted. This should not be confused with the exterior trim, which is often installed as a decorative element on top of the casing. 

Deteriorating trim may not affect the water management performance of the window however; it may be an indication of other problems and generally creates an aesthetic problem.

Failed window sill with replacement window installed

Figure 3 - Failed window sill with replacement window installed Reference

Depending on the measure being examined, the condition of the window sashes may or may not be a concern to the performance of the measure taken. For window rehabilitation, sash retrofit, or interior storm retrofit, the condition of the sashes is critical to the performance of the window.  For exterior storms, the sashes are more protected from the elements, and the condition is less important from a water management perspective, yet still critical from an energy and condensation resistance perspective. For sash replacement, window insert, or full window replacement, the condition of the sash is irrelevant, as they will be removed. For this reason, windows with severely deteriorated sashes may be better candidates for the latter retrofit measures.

Identification of User Comfort Concerns
As part of the initial review, associated comfort concerns relating to the window systems should be evaluated.  Window air leakage is a significant source of occupant comfort problems.  Unlike other common enclosure leakage pathways, window air leakage is commonly very direct, resulting in distinct drafts. 

Radiation effects from cool glass surfaces are another common comfort problem. This is more difficult to identify, as the tendency is to assume that the discomfort felt when near a window is from air leakage or drafts. This results in some misdiagnosis of the dominant function. A general recommendation is to increase the interior surface temperature of the window system to reduce the radiant heat transfer from the occupant to the window. This is commonly done by adding additional panes of glass (or films) to create an insulating air (or other gas) space between the layers.

Scope

Exterior storm window

A common and long standing window retrofit approach is the addition of storm windows to the exterior of the existing window frames. The most common design is a triple track window that combines a screen with operable upper and lower sashes.  Traditionally, storms have been single glazed with clear glass, but more recently, hard coat low-E glass has become available. Low-E storm windows cost about the same as standard storm windows, but are about 50% more energy efficient than traditional uncoated storm windows (Cort 2013).

As summarized within the Ensuring Success tab, before beginning this work, inspect the interior and exterior of the building, including each window, to identify impacts and potential risks with completing the work. Diagnose water and air infiltration pathways, and choose strategies most appropriate to address each leak. Add gaskets and seals at common air infiltration locations to improve the overall air tightness of the window assembly.

Key points to consider when selecting Exterior Storm Windows are as follows:

  1. interior appearance of the window is maintained; however, the exterior appearance will change. The addition of exterior storms is typically acceptable for most historic preservation projects.
  2. The measure is reversible. This is an important consideration for historic preservation.
  3. This work should be done in conjunction with the rehabilitation work set out in the measure guidel on Window Rehabilitation
  4. This work will improve the energy performance of the assembly by reducing air infiltration as well as thermal conductance through the assembly. Hard coat low-E storm window glazing can provide additional thermal benefits.
  5. This work will reduce the potential for interior condensation problems on the window system, though it adds some risk of interstitial condensation between the original window and the exterior storm (generally an aesthetic/operational concern, as opposed to a durability concern).
  6. Cost is low to moderate depending on the system chosen.

Exterior storm window example

Exterior storm window example

Figure 1 - Exterior sotrm window example Reference

Training

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Compliance

2009 IECC

Section 101.4.3 Additions, alterations, renovations or repairs. Additions, alterations, renovations or repairs to existing buildings, building systems, or portions of buildings or systems must meet the requirements of the code as they relate to new construction.  Unaltered portions do not need to comply. Exceptions are included for storm windows installed over existing fenestration and glass-only replacements in an existing sash and frame.*

2012 IECC

Section R101.4.3. Additions, alterations, renovations or repairs. Additions, alterations, renovations or repairs to existing buildings, building systems, or portions of buildings or systems must meet the requirements of the code as they relate to new construction.  Unaltered portions do not need to comply. Exceptions are included for storm windows installed over existing fenestration and glass-only replacements in an existing sash and frame.*

2009 IRC

Section N1101.3 Additions, alterations, renovations or repairs. Additions, alterations, renovations or repairs to existing buildings, building systems, or portions of buildings or systems must meet the requirements of the code as they relate to new construction.  Unaltered portions do not need to comply. Exceptions are included for storm windows installed over existing fenestration and glass-only replacements in an existing sash and frame.*

2012 IRC

Section N1101.3 (R101.4.3) Additions, alterations, renovations or repairs to existing buildings, building systems, or portions of buildings or systems must meet the requirements of the code as they relate to new construction.  Unaltered portions do not need to comply. Exceptions are included for storm windows installed over existing fenestration and glass-only replacements in an existing sash and frame.*

*Due to copyright restrictions, exact code text is not provided.  For specific code text, refer to the applicable code.

More Info.

Case Studies

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References and Resources*

  1. Author(s): Wilson
    Organization(s): National Research Council Canada
    Publication Date: May 1960

    Document about condensation between the panes, on the inside surface of the outer glass of double pane windows.

  2. Author(s): Cort, Culp
    Organization(s): PNNL
    Publication Date: September 2013

    Research study evaluating energy savings and cost effectiveness of installing low-emissivity (low-e) storm windows over existing windows in residential homes in all climate zones.

  3. Author(s): EIA
    Organization(s): EIA
    Publication Date: January 2009

    Federal statistics about national energy consumption in residential homes.

  4. Author(s): Cort
    Organization(s): PNNL
    Publication Date: September 2013

    Brochure describing use of low-e storm windows as effective ways weatherization programs can increase the efficiency of windows without significant costs.

  5. Author(s): Davis
    Organization(s): Fine Homebuilding Magazine
  6. Author(s): National Fenestration Rating Council
    Organization(s): National Fenestration Rating Council
    Publication Date: January 2010

    Standard providing a procedure for determining fenestration attachment product U-factors.

  7. Author(s): National Fenestration Rating Council
    Organization(s): National Fenestration Rating Council
    Publication Date: January 2010

    Standard for determining fenestration attachment product SHGC and visible transmittance.

  8. Author(s): Brown
    Organization(s): National Research Council Canada
    Publication Date: January 1997

    Research study dcribing an evaluation of selected windows undertaken by IRC researchers at Ottawa’s Laurier House (now being used as a museum) to determine their effectiveness in controlling condensation.

  9. Author(s): Baker
    Organization(s): BSC
    Publication Date: December 2012

    Document providing information and guidance about rehabilitating, retrofitting, and replacing wood window assemblies in residential construction.

Last Updated: 10/01/2013

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