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Removable Interior Storm Windows

Scope

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

Install removable interior storm windows according to manufacturer’s instructions if commercially available products are used.

Because removable interior storm windows are not designed to improve water management performance, it is important to ensure that the integrity of the original window is structurally sound. If the primary window is so damaged or deteriorated that it allows water to infiltrate, it should be rehabilitated or replaced to enhance its water management performance.

Installation of an interior storm window creates the potential for condensation on the interior surface of the original exterior window. It is important that the interface between the interior storm window and the window frame be as air tight as possible to prevent interior air from contacting the interior surface of the original window. In some cases, other air leakage paths could bypass the storm window (such as through the pulleys for the sash weights), leading to interior air infiltration into this space. These bypasses should be sealed if possible as they can lead to problems even if the interface between the storm and window frame is perfectly air tight. Condensation potential can be reduced by increasing the ventilation of the space to the outside.

Key points to consider when selecting interior removable storm windows include the following:

  1. This approach maintains the exterior appearance of the window; however, the interior appearance will change. The addition of permanent (but removable) interior storms may be acceptable for historic preservation projects. The measure is reversible. This is an important consideration for historic preservation.
  2. Commercially manufactured windows are to be installed according to manufacturer’s instructions.
  3. Depending on the geography of the home and orientation of the window where they are to be installed, it may be desirable to install commercially manufactured storm windows that have low-E coatings.
  4. This work should be done in conjunction with the rehabilitation work set out in the Window Rehabilitation measure guide.
  5. This work will improve the energy performance of the assembly by reducing air infiltration as well as thermal conductance through the assembly. Removable storm windows should be equipped with gaskets that will help to provide air sealing without caulking.
  6. This work will reduce the potential for interior condensation problems on the window system, although it adds some risk of interstitial condensation between the interior storm and original window (which is an aesthetic and operational concern and increases the potential for durability risks.
  7. The storm windows are to be attached in a way that allows for seasonal installation and removal without damage to window framing or walls. Before installation, ensure that homeowners have adequate storage for periods when the storm windows are removed.
  8. Homeowners must be made aware of potential visual impacts to window frames and visible window area.
  9. Storm window installation must allow for code-required egress.

See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Certified Homes, and Indoor airPLUS.

Description

An alternative to exterior storm windows is interior storm windows. Removable interior storm windows are a good option for historic buildings because they are located on the inside of the window; therefore, they do not impact the historical appearance of the exterior of the home. Because they are easy to remove, they are considered a reversible retrofit measure, which is a desirable and sometimes required feature when dealing with historic homes.

Interior removable storm windows are not intended to be permanent installations. They are typically installed on a seasonal basis (usually during the winter in cold climates) because they will limit the ability to open or close the window after installation. However, because of their insulation value, it may be desirable to leave them in place all year, particularly if the home is equipped with air conditioning.

Permanent versions of interior storm windows are commercially available in opening and non-opening models that are permanently mounted to the interior window frame with screws. Permanent interior storm windows may be preferable because they are available with low-emissivity coatings to further reduce heat loss and improve energy performance.

Removable interior storm windows can take a wide variety of forms. Pre-manufactured interior storm windows consisting of fixed or operable glass or acrylic panels in thin metal frames are common. These are usually installed via a metal angle and magnetic track fastened to the window frame. Silicon-framed interior storm windows that friction fit into the interior window space are also available (Figure 1). Lower-cost, but likely less effective options include site-built wooden frames wrapped with films that are friction fit into the window opening or films that are adhered directly to the interior of the window jambs.

Factory-made interior removable storm windows
Figure 1 - Factory-made interior removable storm windows with a flexible silicon frame press into place to air seal and insulate existing windows.

Interior storm windows can improve the thermal performance and air sealing ability of the window system but are not intended to improve the window’s ability to prevent water intrusion. However, to the extent that the interior storm windows reduce air infiltration, they may also reduce water leakage from wind-driven rain.

Interior storm windows can provide a layer of protection for the primary window, preventing moisture inside the home (such as water vapor from showering or cooking) from condensing on the interior surface of the primary window, especially if the primary window is single pane. However, their installation can also create the potential for condensation on the interior surface of the original exterior window if the interface between the interior storm window and the window frame is not air tight (Figure 2).

Condensation can form between the interior removable storm window and the existing window if the storm window is not air tight
Figure 2 - Condensation can form between the interior removable storm window and the existing window if the storm window is not air tight, as seen in the window on the right.

Condensation potential can be reduced by making the interior storm window airtight to the window frame and by increasing the ventilation of the space to the outside. This needs to be done carefully, as increased ventilation to the exterior will result in diminished energy performance of the window and could render the strategy ineffective. It is recommended that the original window be rehabilitated along with this strategy. If condensation problems develop, then the exterior window can be incrementally made to allow in more air until the problem is resolved. This could be done through strategies such as slightly gapping the lower sash at the sill with a shim, or removing the lower sash sill gasket if it exists (though this will have a detrimental effect on airtightness in the summer if the interior storm is removed).

Condensation potential for interior storm retrofits
Figure 3 - Condensation in storm windows can be prevented by air sealing around the storm window and allowing some ventilation to the outside.

How to Install Removable Interior Storm Windows

  1. 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. 
  2. Determine whether you will purchase factory-made glass or acrylic interior storm windows or fabricate interior storm windows on site. 
  3. For factory-made windows, follow the manufacturer’s measuring instructions exactly and measure twice before ordering. Measure the diagonals to determine how out of square the window is. If there is more than an inch of difference between the two diagonals, the windows may need to be made to fit the irregular shape. Consult the product information to confirm that the window frame has adequate depth to hold the storm window.
  4. For site-built interior storm windows, measure the inside frame of each window, subtract ½ inch from each dimension. Assemble a frame using 1x2s and mitered, rabbit, or butt joints that are glued and screwed. Apply double stick tape along the outer edge of the frame. Cut window film about 8 inches oversized. Stretch the film over the frame and press to the tape. Trim off excess. Secure the edges with 2 inches of clear packing tape. Use a hairdryer to shrink the film. Apply ½ inch open-cell foam weather stripping around all edges.
  5. If condensation occurs on the interior surface of the primary window, provide a slight amount of ventilation to the space by shimming the primary window very slightly.
For factory-made interior removable storm windows, follow the manufacturer’s instructions for measuring the window frame
Figure 4 - For factory-made interior removable storm windows, follow the manufacturer’s instructions for measuring the window frame.

Interior removable storm windows should fit snuggly into the window frame
Figure 5 - Interior removable storm windows should fit snuggly into the window frame.

Interior removable storm window sample
Figure 6 - Inexpensive removable interior storm windows are made from wood and plastic film.

Installed interior removable storm window
Figure 7 - Site-built removable interior storm windows can provide some of the benefits of permanently installed factory-made storm windows.

 

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

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

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 

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.

Low-E Removable Interior Storm Windows

Key points to consider when selecting Interior Removable Storm Window options are as follows:

  1. This approach maintains the exterior appearance of the window; however, the interior appearance will change. The addition of permanent (but removable) interior storms may be acceptable for historic preservation projects. The measure is reversible. This is an important consideration for historic preservation.
  2. Commercially manufactured windows are to be installed according to manufacturer’s instructions.
  3. Commercially manufactured windows must have low-E coatings.
  4. This work should be done in conjunction with the rehabilitation work set out in the Window Rehabilitation measure guide.
  5. This work will improve the energy performance of the assembly by reducing air infiltration as well as thermal conductance through the assembly. Removable storm windows should be equipped with gaskets that will help to provide air sealing without caulking.
  6. This work will reduce the potential for interior condensation problems on the window system, though it adds some risk of interstitial condensation between the interior storm and original window (both an aesthetic/operational concern, as well as potential for durability risks).
  7. The storm windows are to be attached in such a way to allow for seasonal installation and removal without damage to window framing or walls.  Before installation, ensure that homeowners have adequate storage for periods when the storm windows are removed.
  8. Homeowners must be made aware of potential visual impacts to window frames and visible window area.
  9. Storm installation must allow for code-required egress.

Climate

No climate specific information applies.

Training

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Presentations

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Videos

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CAD Images

Compliance

The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

Retrofit: 2009, 2012, 2015, and 2018 IECC

Section R101.4.3 (Section R501.1.1 in 2015 and 2018 IECC). Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with this code. (See code for additional requirements and exceptions.)

Exceptions are included for storm windows installed over existing fenestration and glass-only replacements in an existing sash and frame.

Retrofit: 2009, 2012, 2015, and 2018 IRC

Section N1101.3 (Section N1107.1.1 in 2015 and 2018 IRC). Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with this code. (See code for additional requirements and exceptions.)

Appendix J regulates the repair, renovation, alteration, and reconstruction of existing buildings and is intended to encourage their continued safe use.

Exceptions are included for storm windows installed over existing fenestration and glass-only replacements in an existing sash and frame.

More Info.

Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

Case Studies

  1. Author(s): PNNL
    Organization(s): BSC, PNNL
    Publication Date: September, 2013

    Case study about a DOE Building America 2013 Top Innovation on upgrading or replacing windows in existing homes, based on research by Building America research partner Building Science Corporation..

References and Resources*

  1. Author(s): Knox, Widder
    Organization(s): Pacific Northwest National Laboratory
    Publication Date: September, 2013

    Report recording the results of a field study conducted in a single historic home in Seattle, Washington, to document the performance of Indow Windows' interior storm window inserts.

  2. 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.
  3. Author(s): Cort, Culp
    Organization(s): Pacific Northwest National Laboratory
    Publication Date: September, 2014

    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.

  4. Author(s): EIA
    Organization(s): EIA
    Publication Date: January, 2009
    Federal statistics about national energy consumption in residential homes.
  5. Author(s): Davis
    Organization(s): Fine Homebuilding Magazine
    An article about double-hung windows.
  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): Building Science Corporation
    Publication Date: December, 2012

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

Contributors to this Guide

The following authors and organizations contributed to the content in this Guide.

Building Science Corporation, lead for the Building Science Consortium (BSC), a DOE Building America Research Team

Last Updated: 05/15/2014