How to Specify Fiberglass Windows and Doors
A practical guide for architects and specifiers covering fiberglass window and door selection, performance, glazing, finishes, certifications, technical documents and project coordination.
Windows play a major role in how a home or building looks from both the outside and the inside. Frame colour can help windows blend into an exterior façade, create contrast against cladding, complement architectural details, or become a design feature in their own right.
For projects where standard finishes do not provide the right look, custom window colours and split-finish configurations can provide another level of design flexibility. The key is to choose the finish as part of the overall window specification—not as an afterthought.
This guide explains how custom colours and split finishes can be used, what design decisions to consider, and what to confirm before finalizing your Inline Fiberglass window selection.
Fiberglass Window Specification Workflow at a Glance
Project Requirements → Operation & Product Family → Performance Criteria → Glazing → Size & Configuration → Finishes & Hardware → Technical Documentation → Project Review → Final Specification
1. Define the Project Requirements First
Before selecting a fiberglass window or door system, establish the conditions the product needs to satisfy.
At minimum, review:
- Building and project type
- New construction, renovation or retrofit
- Required window or door operation
- Opening dimensions and configurations
- Energy and envelope-performance objectives
- Glazing requirements
- Interior and exterior design requirements
- Finish and colour requirements
- Hardware and accessory requirements
- Applicable certification or documentation requirements
- Installation and building-envelope coordination
- Submittal and approval requirements
A residential project, commercial building and high-performance project may all use fiberglass fenestration, but they may require very different combinations of operation, glazing, technical documentation and performance.
The specification should reflect those differences.
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2. Select the Required Window or Door Operation
The next step is to determine how each opening needs to function.
Inline Fiberglass offers several window and door product families with different operating configurations.
| Required Operation or Product Type | Inline Product Family to Review |
|---|---|
| Tilt & turn or fixed window | 300 Series |
| Casement, awning or fixed window | 325 Series |
| Strip or fixed window | 400 Series |
| In-swing casement, hopper or fixed window | 700 Series |
| Horizontal slider | 800 Series |
| Single hung window | 850 Series |
| Double hung window | 900 Series |
| Fiberglass entrance door | 4500 Series |
| Fiberglass sliding patio door | 600 Series |
This is only the first level of product selection. Exact dimensions, glazing configurations, performance data, hardware and certification applicability should be confirmed using the current technical information for the selected series.
A specification guide should not replace the individual product pages or technical documents. Those sources should remain the reference point for product-specific requirements.
3. Establish Whole-Assembly Performance Criteria
Once the operating type has been identified, establish the performance requirements for the complete window or door assembly.
Depending on the project, relevant criteria may include:
- Whole-product U-value
- Solar Heat Gain Coefficient (SHGC)
- Visible Transmittance
- Air-infiltration performance
- Water-penetration performance
- Structural or design-pressure requirements
- High-performance or Passive House objectives
- Applicable product testing or certification requirements
One important distinction is the difference between glass performance and whole-window performance.
A high-performing insulating glass unit does not determine the performance of the complete window by itself. Frame design, glazing, spacer systems, seals, operating hardware, product dimensions and other assembly characteristics can all affect overall performance.
For that reason, project specifications should rely on current whole-product data wherever the requirement applies to the complete fenestration assembly.
Do not transfer performance values between configurations.
Performance data should be associated with the product and configuration represented by the supporting documentation.
Do not assume that a rating for one series, size, glazing package or operating configuration automatically applies to another.
Where a project has a specific performance target, confirm the applicable configuration before the specification is finalized.
4. Coordinate the Glazing Specification With the Window System
Glazing should be selected as part of the complete fenestration system rather than as a separate decision.
Depending on the series and project requirements, the available insulating glass configurations may differ.
Consider:
- Double- or triple-glazed requirements
- Solar Heat Gain Coefficient (SHGC)
- Solar heat-gain requirements
- Visible-transmittance requirements
- Low-E coating requirements
- Gas-fill and spacer options where applicable
- Glass thickness and overall insulating-glass-unit compatibility with the selected series
A common specification mistake is to call for a particular glazing package before confirming that it is appropriate for the selected window or door system.
A better approach is to define the required performance outcome, then verify which available product and glazing configuration satisfies it.
For high-performance projects, this coordination should happen early enough that the glazing strategy, frame selection and architectural openings can be developed together.
5. Confirm Size and Configuration Requirements Early
Fiberglass can support a wide range of architectural applications, but allowable dimensions and configurations should always be reviewed at the series level.
Do not assume that every operating style has the same: Maximum dimensions, Aspect-ratio limitations, Glazing capacity, Structural capacity, Hardware requirements, Fixed-versus-operable configuration options.
This becomes especially important with custom or oversized openings.
Where larger openings or unusual configurations are part of the design, provide project dimensions and drawings early enough for the proposed product to be reviewed before the construction documents are finalized.
For doors, also identify the required overall configuration rather than specifying the slab or operating panel in isolation. Entrance-door projects may involve sidelites, transoms and other architectural elements, while sliding-door projects may use multi-panel configurations.
The complete opening should be reviewed as a system.
6. Define Finishes, Hardware and Accessories
The architectural specification should also identify the visible and functional options that need to be coordinated with the selected product.
Depending on the product and configuration, these may include:
Colours and Finishes
Define the required interior and exterior appearance and confirm available finishes for the selected system.
Where different interior and exterior colours are required, confirm the applicable split-finish options before documenting the final colour schedule.
Grilles and Simulated Divided Lites
If the design calls for grille patterns or simulated divided lites, identify:
- Desired visual pattern
- Required profile or grille type
- Interior and exterior appearance
- Compatibility with the selected window configuration
These details can have a substantial effect on the final architectural appearance and should not be left as a generic allowance.
Hardware
Specify the operating and visual requirements for window and door hardware, then confirm compatibility with the selected series.
Avoid assigning a particular hardware option across multiple product systems without verifying that it is available for each configuration.
Trim and Accessories
Where applicable, coordinate exterior and interior accessories such as brickmoulds, jamb extensions, casing or other interface components with the surrounding wall assembly and finish materials.
Treat these elements as part of the fenestration package rather than as an afterthought near installation.
7. Verify Certifications and Technical Documents
Technical documentation should support the specification rather than simply appear as a collection of logos or generic claims.
Depending on the selected product and project requirements, the specification team may need to review: Product drawings, CAD details where available, Product specification information, Performance data, Certification documents, Installation information, Glazing information, Finish and option information, Environmental documentation where applicable.
Inline’s Technical Downloads section should be used to review the current documents associated with the relevant product family.
Use product-specific certification language
Avoid broad statements such as: “All fiberglass windows are Passive House certified.”
Certification and testing should instead be associated with the exact product or configuration to which the supporting documentation applies.
A stronger specification approach is:
Selected product and configuration to comply with the project performance requirements and applicable current certification documentation.
The exact product, configuration and referenced document can then be identified as the project moves through technical review.
This makes the specification easier to verify and reduces the risk of carrying an inapplicable certification from one product into another.
8. Coordinate Drawings, Submittals and Installation Requirements
The window specification cannot be developed independently from the wall assembly.
Before finalizing the package, coordinate the fenestration with:
- Architectural opening dimensions
- Wall thickness and interface conditions
- Interior and exterior finishes
- Sill and perimeter conditions
- Water-management details
- Required fastening or attachment conditions
- Field dimensions where applicable
- Installation responsibilities
- Shop drawings and submittal requirements
The project team should also establish who is responsible for confirming field conditions, installation details and final interfaces.
Do not assume that manufacturing, supply and installation responsibilities are identical on every project.
For custom, commercial or high-performance work, early coordination can help identify questions before they become drawing revisions or site issues.
9. Build the Specification Around the Exact Product Configuration
Once the technical requirements have been reviewed, the final specification can identify the selected system more precisely.
A practical framework is:
Product
- Manufacturer
- Product series
- Window or door type
- Operating configuration
Performance
- Required whole-product thermal performance
- SHGC or other solar-performance criteria where applicable
- Air, water and structural criteria where required
- Applicable certification or testing requirements
Glazing
- Required performance
- Double- or triple-glazed configuration where applicable
- Coating, gas and spacer requirements as approved for the product
Appearance
- Interior finish
- Exterior finish
- Grilles or SDLs
- Trim and accessory requirements
Hardware
- Operating requirements
- Finish requirements
- Product-compatible hardware selection
Documentation
- Product data
- Drawings
- Performance information
- Applicable certificates
- Installation information
Submittals
- Product data
- Shop drawings where required
- Finish selections
- Glazing information
- Requested certification documentation
Fenestration Specification Checklist
Before issuing the specification, confirm:
- Building and application requirements are defined
- Required window and door operations are identified
- Appropriate Inline product series have been reviewed
- Opening dimensions and configurations are established
- Whole-product performance criteria are defined
- Glazing requirements are coordinated with the selected series
- Interior and exterior finishes are identified
- Hardware requirements are identified
- Grilles, SDLs and accessories are identified where applicable
- Required certification scope is clearly stated
- Current product documentation has been reviewed
- Large or custom configurations have received project-specific review
- Installation and envelope interfaces are coordinated
- Required drawings and submittals are identified
- Product-specific values have not been copied from an unrelated configuration
- Final product data will be reviewed against the project requirements
CommonFiberglass Window Specification Mistakes
Selecting a series before defining the requirements
Start with the required operation, performance and opening conditions. Then choose the product.
Specifying only the glass performance
Use whole-window or whole-door performance where the requirement applies to the complete assembly.
Using blanket certification language
Certifications should be tied to the applicable product and configuration.
Assuming every series accepts the same glazing or dimensions
Glazing capacity, dimensions and available configurations can vary by product.
Leaving finishes and hardware until the end
Finish, grille, hardware and accessory decisions can affect architectural coordination and should be documented early.
Using outdated technical information
Specifications should reference current product data and current applicable documentation.
Treating installation as separate from product selection
The performance of the specified assembly also depends on proper coordination with the surrounding wall and installation conditions.
Common Questions About Specifying Fiberglass Windows and Doors
Find clear answers to common questions about how to specify fiberglass windows and doors, and how to choose the right solution for your project.
What should an architect include when specifying fiberglass windows?
At minimum, identify the required product type and operation, dimensions or configuration, whole-product performance criteria, glazing requirements, finishes, hardware, applicable certifications, technical-document requirements and installation or interface conditions.
Are all Inline Fiberglass window series certified to the same standards?
No blanket certification assumption should be made. Certification and testing applicability should be confirmed for the exact product and configuration being considered.
Can fiberglass windows be specified with triple glazing?
Triple-glazed configurations are available within Inline’s product offering, but glazing capacity and available packages should be confirmed for the selected series and project configuration.
Where can architects find Inline product drawings and technical information?
Start with the Technical Downloads section for available product information and use the specifier-support path when additional project-specific documentation is required.
When should the manufacturer be involved in the specification process?
Earlier coordination is particularly useful when a project involves custom dimensions, large openings, high-performance requirements, unusual configurations or specific certification requirements.
Does this guide replace the individual Inline product specifications?
No. This guide provides a specification workflow. Product pages, current technical documentation and project-specific review should be used to confirm the final product requirements.
Need Help Specifying Inline Fiberglass Windows or Doors?
Every project has a different combination of operation, performance, glazing, dimensions and architectural requirements. If you are developing a residential, architectural, commercial or high-performance project, Inline Fiberglass can help you review the appropriate product path and supporting technical information.