Replacement Window GuideThe frame is a large part of a window and a major factor in how well it insulates. The Efficient Windows Collaborative notes that the sash and frame represent 10 to 30 percent of the total window area, so frame material significantly influences total performance. Wood, vinyl, and composite frames insulate far better than bare aluminum, which conducts heat readily. Read each material against your climate and maintenance tolerance.
Why frame material matters
The material a frame is made of governs both its insulating value and its physical properties. The Efficient Windows Collaborative explains that the material choice has a major impact on the thermal characteristics of the window.
Because the sash and frame make up a meaningful share of the opening, that impact is not trivial. The Efficient Windows Collaborative notes the sash and frame represent from 10 to 30 percent of the total area of the window unit, so frame properties significantly influence total window performance.
Wood and clad-wood frames
Wood is a traditional frame material that insulates well and remains common in historic settings. The Efficient Windows Collaborative reports wood frame U-factors in the range of 0.3 to 0.5 Btu/hr-sq. ft-F, and notes that well-built and well-maintained wood windows can have a very long life, though wood is susceptible to rot.
Clad wood reduces the upkeep. The Efficient Windows Collaborative describes wood frames clad on the exterior with vinyl or aluminum while keeping an interior wood finish, giving lower maintenance requirements, with color options generally available in a limited number.
Vinyl and composite frames
Vinyl is a widely used low-maintenance frame material. The Efficient Windows Collaborative reports that vinyl thermal performance is comparable with wood depending on frame construction, that vinyl has good moisture resistance, and that it is effectively maintenance-free because the color goes all the way through.
Composite frames aim to match or beat wood while resisting decay. The Efficient Windows Collaborative states composites are very stable and have the same or better structural and thermal properties as conventional wood, with more decay resistance, and can be textured and stained or painted much like wood.
Hybrid frames and reading the label
Manufacturers increasingly mix materials in a single unit. The Efficient Windows Collaborative notes that hybrid frames combine multiple materials to produce a complete window system, but that this makes thermal properties increasingly difficult to estimate.
Because you cannot judge a mixed-material window by its parts, the collaborative recommends consulting the NFRC label that provides the thermal properties of the overall window. ENERGY STAR describes the label's U-factor rating, which measures how well the window insulates, with values ranging from 0.25 to 1.25 and lower numbers indicating better insulation.
Aluminum frames and thermal breaks
Aluminum is strong and durable but a poor insulator on its own. The Efficient Windows Collaborative states that aluminum readily conducts heat, greatly raising the overall U-factor of a window unit, and that in cold climates a simple aluminum frame can become cold enough to condense moisture or frost on the inside surfaces.
The standard fix is a thermal break. The Efficient Windows Collaborative explains this splits the frame into interior and exterior pieces, and that current technology with standard thermal breaks has decreased aluminum frame U-factors from roughly 2.0 to about 1.0 Btu/hr-sq. ft-F.