Cold-climate thermal guide
Which windows are warmest: FRP, PVC, wood or aluminium.
The warmest window is the verified combination of frame, glass, spacer, seals, opening type, size and installation with the lowest suitable whole-window Uw and controlled air leakage. Frame material matters, but it cannot decide the result by itself.
Short answer
Start with whole-window Uw for the actual or representative size. Then compare airtightness, glass-edge temperature, spacer, opening type and installation. Full-FRP, insulated PVC, engineered wood and thermally broken aluminium can all reach demanding targets when the complete construction is designed and tested for them.
Use Uw, not a material slogan.
Uf describes frame heat transfer and Ug describes centre-of-glass heat transfer. Uw describes the assembled window and includes the frame, glazing and edge effect for a defined size. A low Uf does not guarantee a low Uw if glass, edge spacer or proportions change.
Ask whether the figure is calculated or tested, which standard was used, and whether the quoted opening matches the reference. For operable windows, air leakage and seal compression affect heating demand and comfort alongside conductive heat loss.
| Frame route | Thermal design characteristic | Verify before selection |
|---|---|---|
| Full-FRP | Low-conductivity structural profile without a continuous metal path | Joints, fire route, weather tests and exact whole-window build |
| PVC | Multi-chamber frame with steel or composite reinforcement | Reinforcement, dimensions, colour stability and cold-temperature behaviour |
| Wood | Low-conductivity solid or engineered sections | Moisture protection, coating maintenance and joint quality |
| Thermally broken aluminium | Metal faces separated by an insulating zone | Break geometry, connection, glass edge and whole-window value |
A practical comparison sequence.
Set the required whole-window performance from the project climate and regulation. Compare the same representative size, opening function and glass build-up across systems. Then check air, water and wind evidence, internal surface temperatures and the installed junction.
- Define climate, design temperature and target Uw.
- Hold size, opening type, glass and spacer constant when comparing frames.
- Review airtightness, water and wind together with thermal performance.
- Model or test the installation junction and perimeter sealing.
Where FD90 fits.
PINDÉ FD90 is a 90 mm full-FRP window system. Passive House Institute certificate 2491wi03 records Uw 0.78 W/(m²·K) for the identified configuration at class phB for the cool, temperate zone. That is useful evidence for the certified build, not a value for every size or glass option.
Frequently asked questions.
What is the best number for comparing warm windows?
Use whole-window Uw for the same size and glass build-up, then compare airtightness and installation. Uf or Ug alone describes only one part of the window.
Does triple glazing always make a window warmer?
It often lowers Ug, but whole-window Uw also depends on frame area, edge spacer and size. Glass weight can also change sash and hardware requirements.
Why can two windows with the same profile depth perform differently?
Internal geometry, reinforcement, glass position, spacer, seals, joints, sash proportion and installation all affect the finished result.