Although the right design allows for many different materials, nothing beats a hardwood conservatory. Among timber, Western Red Cedar is THE traditional material for wooden conservatories and green houses.
Red Cedar (Thuja Plicata) contains natural oils which make its wood resistent to insects and moisture. It expands and shrinks only slightly when subject to moisture, cold and heat over the seasons and insulates much better than aluminium or plastics. Above it all: Few other woods and no plastic alternative age so graceously. Exposed to sun and weather cedar wood over the years developes a lovely silvery patina, which provides an additional protection. A treatment with hardwood oil can prevent or reverse this effect if you wish. If affordable, a well-designed hardwood conservatory is superior to any alternative. Compared to soft woods I found this to be especially true if non-hardy plants are supposed to find shelter during the winter, and a certain amount of heating is necessary to keep the conservatory frost-free. As there is always an unavoidable amount of condensation, after 10 years pine wood (initially a good bit cheaper) shows the first signs of degradation in some wall elements, esp. some not so well-ventilated corners (which is actually made worse by acrylic paints). Cedar will cope with such conditions much better.
Another traditional material becomes popular again in conservatory building: Seasoned oak. Its strength and stability are known to survive centuries, and in many cases it is locally available.
Some of the recently promoted woods like Meranti are common in Asia and well-tried there.
So, if you can afford it, chose a hardwood conservatory. Even in cheap conservatories at least the base is often made from cedar wood.
