A different question from windows
uPVC against aluminium for windows is a well worn argument. Sliding doors change the terms, because a door panel is much taller and heavier than a window sash, and span becomes the deciding factor rather than insulation alone. The honest answer is that the two materials are good at different things, and the right choice depends mostly on how large the opening is.
Where aluminium has the advantage
Large spans and slim sightlines
Aluminium is far stiffer than uPVC, so an aluminium section can be slimmer and still carry a tall, heavy panel. If you want a floor-to-ceiling opening with narrow frames and as much glass as possible, aluminium does that more easily. On very large openings it is often the only practical answer, and for the minimal-frame look it is effectively the only answer.
Very heavy glazing
Thick laminated or large double-glazed units are heavy. Aluminium's stiffness means the frame deflects less under that load, so the panel keeps running true.
Structural confidence on exposed elevations
On a high floor or an open frontage with significant wind load, an aluminium system of equivalent cost will usually carry a larger unsupported panel than a uPVC one.
Where uPVC has the advantage
Heat
This is the big one in most of India. Aluminium conducts heat readily, so a plain aluminium frame in direct sun becomes hot and transfers that heat inward. uPVC is a poor conductor and does not. Aluminium systems can solve this with a thermal break, which is an insulating strip set inside the frame separating the outer and inner halves, and good thermally broken aluminium performs well. But a thermally broken system costs considerably more than a plain one, and plain aluminium is what most budget comparisons are actually quoting.
If you compare uPVC against plain aluminium, uPVC wins clearly on heat. If you compare it against thermally broken aluminium, they are much closer and the decision moves to span and cost.
Condensation
A cold conductive frame in a humid room collects water on the inside face. In air-conditioned rooms in humid cities this shows up as damp sills and, over time, staining on the wall below. uPVC frames stay closer to room temperature and do this far less.
Corrosion near the coast
uPVC does not corrode. Aluminium does not rust in the way steel does, but coastal salt air pits anodising and degrades powder coating, and corroding aluminium at a fixing point is a real failure mode within a few kilometres of the sea. Along a coast, uPVC is the lower-risk material.
Noise and sealing
uPVC sections are hollow and multi-chambered, and the systems generally use generous EPDM gaskets, which helps on sound. Aluminium can be sealed just as well but it needs to have been designed and installed for it.
Maintenance
Both are low maintenance. uPVC needs washing and nothing else, and has no coating that can fail. Aluminium's powder coat is durable but is a coating; once it is chipped or chalked, repair means recoating rather than cleaning.
Cost, stated carefully
There is no single answer, because the comparison depends on which aluminium you mean. Broadly, plain aluminium is the cheapest option, good uPVC sits above it, and thermally broken aluminium sits above that. Prices move with profile specification, glass and hardware far more than with the material label, so a uPVC quote and an aluminium quote can land either way round. Compare the full specification, not the material.
What actually decides it
- Opening up to a normal balcony door width: either material works. uPVC is usually the better value for comfort, because you get the thermal performance without paying for a thermal break.
- Very wide or floor-to-ceiling, slim frames wanted: aluminium, and budget for thermally broken if the opening faces real sun.
- Coastal location: uPVC, unless the span forces aluminium, in which case specify marine-grade treatment and expect maintenance.
- Air-conditioned room in a humid city: uPVC, or thermally broken aluminium. Plain aluminium will sweat.
- West-facing with long afternoon sun: uPVC, or thermally broken aluminium. The frame temperature is noticeable.
Things that matter more than the material
Whichever you choose, these decide whether the door is pleasant to use:
- Rollers rated for the real panel weight, with adjustment and with replacement parts available.
- A bottom track that drains outward. Standing water in the track of a large door is the most common complaint about sliding doors in either material.
- Interlock sealing where the panels overlap, which is where noise and dust come through.
- Multipoint locking, because a large sliding door is the most accessible opening in most homes.
- Toughened or laminated glass, which is a safety requirement at door height, not an upgrade.
- Installation. A large sliding door that is out of level will never run properly, and no hardware will fix it.
Decide the span first. The span usually decides the material, and then the specification decides whether you are happy, in either material.