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📋 Part of Starveil’s Technical Fenestration Library

Related: Commercial Aluminum Window Manufacturing · Architectural Facade & Curtain Wall Systems · 5 Critical Climate Specs for Aluminum Windows

When full window replacement is undesirable because of historic fabric, occupant disruption, existing construction or project budget, aluminum storm windows can provide a secondary glazing layer over an existing window.

The value is not simply the aluminum frame. A secondary window changes the thermal, air-sealing, acoustic and moisture behaviour of the existing opening.

For architects, contractors and procurement teams, the correct approach is to evaluate the existing primary window, the secondary system, the cavity, the glazing and the installation interface together.

AAMA 1002-25 is a dedicated North American specification for secondary storm products, while AERC provides energy-performance rating programs for secondary windows. These should not be confused with ASTM test methods or building codes.

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Quick Summary

The key procurement point is simple:

A storm window is a system upgrade, not a generic “extra pane of glass.”

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The Building Physics of Secondary Fenestration

An aluminum storm window creates a second glazing layer separated from the existing primary window.

Unlike a factory-sealed IGU, the space between the two windows is part of the installed assembly. Its performance depends on cavity geometry, leakage paths, glazing properties, frame conductance and the condition of the primary window.

That means a secondary system should be evaluated as a composite assembly.

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1. Air Leakage: Seal the Opening, Then Verify the Result

Older windows can have significant air leakage caused by deteriorated weatherstripping, loose sashes, perimeter gaps and movement of the existing frame.

A properly fitted secondary window can create an additional air barrier and reduce direct airflow through the opening.

However, the amount of improvement is project dependent. It should not be presented as a universal 40%, 60% or 80% reduction.

ASTM E283/E283M is a test method for determining air leakage through fenestration products under specified pressure differences. It does not by itself establish a universal allowable leakage value.

For residential storm windows, the current ENERGY STAR storm-window specification uses air-leakage criteria of ≤1.5 cfm/ft² for exterior storm windows and ≤0.5 cfm/ft² for interior storm windows. Those values belong to the ENERGY STAR certification framework and should not be presented as a universal commercial requirement.

For commercial projects, specify the applicable product requirement and request the corresponding test evidence.

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2. Thermal Performance: The Existing Window Still Matters

A secondary glazing system can reduce heat transfer by adding another resistance layer between the interior and exterior environments.

A clear secondary pane can increase the thermal resistance of the opening. Low-E glazing can further reduce long-wave radiative heat transfer.

But there is no single U-factor that applies to every aluminum storm window.

The resulting performance depends on:

• existing primary window
• secondary frame
• glazing type
• cavity depth
• glass emissivity
• frame-to-glass ratio
• perimeter conditions
• installation

AERC’s secondary-window programs use defined rating procedures and baseline configurations, which is why certified ratings should be tied to the actual product configuration rather than copied as a generic industry number.

Low-E selection requires configuration control

Hard-coat Low-E can be appropriate where the coating surface is exposed to a secondary cavity, but the correct specification depends on the glass construction and coating orientation.

Do not write:

“Only pyrolytic Low-E is allowed.”

A better specification is:

Specify a Low-E coating and surface orientation approved by the glass manufacturer for the actual secondary-glazing cavity and exposure condition.

This keeps the requirement performance-based instead of turning one product technology into a universal rule.

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3. Acoustic Performance: Cavity Geometry Matters

Secondary glazing can also improve sound isolation because the existing and secondary panes are separated by an additional air space.

A wider cavity generally changes the acoustic interaction between the panes. Glass thickness asymmetry and laminated glass can further influence sound transmission.

However, acoustic performance is highly configuration dependent.

ASTM E90 is used to measure laboratory sound transmission loss, while ASTM E413 is used to determine STC and ASTM E1332 for OITC classification. The resulting rating should therefore be taken from a laboratory report for the actual glass, framing and cavity configuration.

Do not specify:

“A 3-inch cavity always gives STC 40+.”

Specify:

Required STC/OITC performance + tested glass/cavity configuration.

That is a much safer procurement specification.

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Exterior vs. Interior Aluminum Storm Windows

The installation location changes the moisture and preservation strategy.

Exterior Aluminum Storm Windows

Exterior units are installed outside the primary window.

Typical applications include:

• weatherization
• historic buildings where exterior changes are permitted
• existing wood or steel windows
• multifamily retrofit projects

The critical issue is drainage.

ENERGY STAR specifically excludes exterior storm windows without weep holes or another method of allowing moisture to drain from the space between the storm window and primary window.

The exterior installation should therefore include:

• properly designed sill drainage
• manufacturer-approved weep paths
• compatible perimeter sealant
• suitable substrate anchorage
• corrosion-resistant fasteners
• allowance for movement

Avoid simply sealing the assembly completely around the bottom edge and trapping water inside the cavity.

Historic appearance

Historic-preservation decisions are site specific.

The National Park Service identifies storm windows as one possible approach for improving the thermal performance of historic windows and notes that storm-window frames may be made from materials including aluminum, while unfinished aluminum should be avoided.

Local landmark or preservation authorities may impose additional requirements.

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Interior Secondary Glazing

Interior secondary glazing is installed inside the existing window reveal or surrounding trim.

Its main advantage is that the exterior elevation can remain visually unchanged.

For historic buildings, this can make interior secondary glazing attractive where exterior alterations face preservation restrictions.

The critical technical issue becomes the interior air seal.

Warm interior air should not be allowed to move freely into a cold secondary cavity where it can encounter a colder primary pane and generate condensation.

Therefore, specify:

• continuous perimeter sealing
• compatible compression gaskets or subframes
• controlled cavity conditions
• appropriate drainage/condensation strategy
• manufacturer-approved installation details

Do not specify an artificial “0.00 CFM/ft² perimeter” requirement. The correct requirement is a continuous, effective air seal verified using the applicable product or project performance test.

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Three Common Secondary-Glazing Configurations

Fixed Secondary Panels

Best suited to openings where maximum simplicity and minimal moving hardware are priorities.

Operable Secondary Windows

Useful where the existing opening still needs ventilation or access.

Depending on the design, systems may use sliding, removable or multi-track configurations.

Commercial Custom Secondary Systems

Large commercial openings, historic steel windows and irregular existing frames may require custom master frames and removable or operable inserts.

For these systems, drawings and field measurements become as important as the profile itself.

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Standards and Testing: Keep the Hierarchy Clear

ASTM E331, for example, determines resistance to water penetration under a specified static pressure difference; ASTM explicitly states that the test pressure must be selected with the actual envelope loading conditions in mind. It is therefore a test method, not a universal “15 psf” requirement.

This distinction should be preserved throughout the specification.

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B2B Procurement Matrix

The manufacturer should also identify the tested size and configuration.

A test report for one product configuration should not automatically be presented as proof for a substantially different size or assembly.

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Retrofit vs. Full Window Replacement

A secondary system can be attractive when the existing window is salvageable and the goal is to improve performance without removing the original frame.

It is not automatically the better solution. Existing-frame condition, required performance, historic restrictions and project economics should be evaluated first.

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Installation: Where the Retrofit Either Works or Fails

The installation details should be treated as part of the product specification.

Exterior

1. Inspect the existing substrate and blind-stop/casing condition.
2. Verify that the frame can support the secondary unit.
3. Follow the manufacturer’s approved perimeter sealing details.
4. Maintain the designed sill drainage and weep paths.
5. Use corrosion-resistant fasteners suitable for the substrate.
6. Provide electrical/galvanic isolation where dissimilar-metal contact requires it.

ASTM E331 also notes that installed performance depends on proper installation, adjustment and supporting construction, reinforcing the point that laboratory product performance does not automatically equal field performance.

Interior

1. Survey the reveal for square and plumb.
2. Provide a continuous perimeter air seal.
3. Use manufacturer-approved gaskets, receivers or subframes.
4. Control the relationship between interior humidity and the cold primary glazing.
5. Verify that the final configuration can be maintained and removed when required.

Do not invent universal gasket-compression percentages or fastener spacing. Those values should come from the system manufacturer or project engineering.

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Aluminum Storm Window Specification Checklist

Before issuing an RFQ, verify:

• Existing-window condition documented
• Opening schedule completed
• Field dimensions confirmed
• Exterior or interior configuration selected
• Secondary frame system identified
• Glass build-up defined
• Low-E coating/orientation confirmed
• Safety-glazing requirements checked
• Thermal-performance target defined
• Air-leakage requirement defined
• Water-performance requirement defined where applicable
• Acoustic target defined where applicable
• Drainage details approved
• Anchorage/substrate requirements defined
• Finish specified
• Test reports requested
• Installation drawings requested
• Warranty terms reviewed

This converts “commercial grade storm window” into a specification that can actually be compared between suppliers.

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What to Request in the Manufacturer Submittal

For a serious B2B purchase, request:

1. Product data

Frame, glazing, hardware and finish.

2. Shop drawings

Head, jamb, sill and interface details.

3. Test reports

Air, water, structural or acoustic testing as required.

4. Thermal documentation

AERC rating or applicable calculation/test data where available.

5. Installation instructions

Substrate, anchorage, sealant and drainage requirements.

6. Configuration statement

The exact frame, glass, dimensions and hardware represented by the test or rating.

This creates a traceable chain:

project requirement → proposed system → tested configuration → evidence → installation

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Frequently Asked Questions

What are aluminum storm windows?

Aluminum storm windows are secondary window systems installed over or inside an existing primary window. They can improve air sealing, thermal performance and sometimes acoustics without requiring complete removal of the existing window.

Are aluminum storm windows good for historic buildings?

They can be. The National Park Service identifies storm windows as one option for improving the performance of historic windows while retaining original fabric. The acceptability of an exterior aluminum storm window remains dependent on the building and local preservation requirements.

Are interior or exterior storm windows better?

Neither is universally better. Exterior systems can provide a useful weather and thermal layer while preserving the primary window, but they affect the exterior appearance and require effective drainage. Interior systems can minimise exterior visual changes but place greater emphasis on the interior air seal and condensation control.

Does AAMA 1002-25 apply to storm windows?

AAMA 1002-25 is the North American specification specifically addressing secondary storm products. It should be distinguished from NAFS, which applies to covered primary fenestration products, and from ASTM test methods such as E283 and E331.

What air-leakage target should a storm window have?

The target depends on the applicable product standard or project requirement. For residential ENERGY STAR storm windows, the current criteria are ≤1.5 cfm/ft² for exterior units and ≤0.5 cfm/ft² for interior units. Commercial projects should use the applicable project/product specification rather than automatically applying those values.

Can storm windows improve U-factor?

Yes. Adding a secondary glazing layer can increase the thermal resistance of an existing opening. The final U-factor is configuration dependent and should be verified from a certified rating, test or project-specific calculation rather than assumed from glass type alone.

Can secondary glazing improve acoustics?

Yes. The additional cavity can change sound transmission behaviour, and glass thickness, laminated construction and cavity geometry all influence the final result. Specify the required STC/OITC value and request laboratory evidence for the actual configuration.

Do exterior storm windows need weep holes?

Moisture drainage is an essential part of exterior storm-window design. ENERGY STAR excludes exterior storm windows without weep holes or another mechanism that allows moisture to drain from between the storm window and primary window.

Should I replace the primary window instead?

That depends on the condition of the existing window, historic requirements, required performance, installation disruption and project budget. Secondary glazing is most attractive when the existing primary window is worth retaining and the project can meet its performance objectives without full replacement.

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Bottom Line

Aluminum storm windows should be specified as engineered secondary fenestration, not as a generic add-on product.

The correct procurement sequence is:

Existing Window → Retrofit Configuration → Frame → Glazing → Cavity → Air/Water → Thermal → Acoustic → Installation → Test Evidence

For historic projects, preserving the original window may be a major part of the decision. For commercial retrofits, the key question is whether the secondary system can achieve the required performance without the disruption and cost of full replacement.

And for every project:

do not compare a marketing number with a tested number.

Compare the same configuration, same test method, same performance requirement and same installation scope.

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