Window and door performance and frame details

One-line orientation

Window performance has three separate measures: U-factor for heat conduction, SHGC for solar heat gain, and air leakage for drafts. Do not treat them as the same thing.

Key points

• High-performance window — three factors, three heat paths:
  • U-value / R-value → conduction: how readily heat conducts through the assembly (lower U-value = better insulator).
  • Low-e glazing: generally reduces heat transfer and solar heat gain.
  • Air leakage → infiltration: air moving through the assembly, measured in CFM.
• Key metrics to know: U-value, Visible Light Transmission (VLT), and SHGC. VLT is the daylight/views metric (visible light only), distinct from SHGC (total solar energy).
• Frame material: vinyl has the most thermal expansion; detail the frame so movement and thermal bridging do not undermine the assembly.
• Door frames — anchoring into the wall:
  • To masonry: anchor tabs that sit between the courses.
  • To steel-stud framing: dedicated stud connection.
  • To wood-stud framing: dedicated stud connection.
  • The frame must be positively tied to the surrounding structure for each substrate type.

Confusions / comparison

Window metric	What it measures	Heat path	”Better” direction
U-factor (U-value)	Conductive heat transfer through the assembly	Conduction	Lower = better insulation
SHGC	Solar heat admitted through the window	Solar gain	Lower in hot climates; higher can aid passive solar in cold
Air leakage	Air infiltration through the unit	Infiltration (CFM)	Lower CFM = tighter
VLT	Visible light transmitted	(daylighting)	Depends on daylight/glare goals

Hollow-metal door frame — anchoring by substrate:

Wall type	Anchor method
Masonry	Anchor tabs that sit between the courses
Steel-stud	Dedicated stud connection
Wood-stud	Dedicated stud connection

Related

→ The four enclosure control layers (this topic): openings are where control layers must stay continuous · Vapor retarders: perm classes and placement (this topic): condensation control at glazing · Glass glazing types (Construction): annealed/tempered/laminated/insulated glazing that fills the frame.