The $64million question! In fact, such a difficult one to answer we had to approach our Head Honcho Flooring Guru to distill the pros and cons of these different carpet yarns, or fibres, into an "idiots guide" filtering flask that makes it easy for us all to understand. As he pulled himself out the icy lake (from an undisclosed location) after his daily sunrise 1500 metre swim (to cleanse his soul), he allowed us to record his reasoning. Here's what he said:
Firstly, polypropylene is a man-made, oil derivitive fibre/yarn and currently is cheaper than wool. It is an undeniable fact that wool keeps it's original shape longer than polypropylene and for that reason, in my opinion, for a long term investment wool carpet should get the nod of approval over polypropylene. I do appreciate, however, that there are many other factors to consider when choosing your carpet.
Polypropylene is harder to colour, or dye, than wool. This is a major factor behind why the colours in polypropylene carpets don't tend to be vibrant. They offer functional, earthy colours but not vibrant. The upside to this difficulty in dying polypropylene is that it has a strong colour fastness; so it does not fade and in many cases polypropylene is so stubborn it is actually resistant to bleach. How about that?
So polypropylene is cheaper than wool and is bleach cleanable but it has an inherent tendency to flatten quicker than wool. Polypropylene is as tough as old boots but it will lose it's definition quickest of all fibres used in producing carpet. Better quality polypropylene carpets combat this appearance retention issue by applying what's known as "2 ply yarn" technology, a.k.a. 2 fold yarn. I'll not blind you with the scientific technology behind it but, essentially, it's a produciton tecnhique where two strands of fibre, or yarn, are spun and twisted together and then anchored, or tufted, into the carpet's structure. This makes a hugely positive increase in polypropylene's performance because you effectively have the strength of 2 strands doing what 1 does. Obviously, 2 ply yarn will be more expensive but in relative terms it's not a major difference. In my capacity of Head Honcho Flooring Guru I would always recommend buying a 2 ply yarn polypropylene carpet compared to a single ply one. Granted, there may sometimes be strict budget conditions that affect your decision but I would always recommend 2 ply yarn.
As a Internationally recognised Head Honcho Flooring Guru I have the eyesite capabilities of a Golden Eagle and would be able to determine from a 15 cm x 15 cm carpet sample at 150 metres whether the product is 2 ply or single ply yarn. However, this is extremely dangerous for the untrained to attempt this identification without a safety net - so don't try this at home. What I would suggest is next time you go into a Carpet Retailer and they show you a polypropylene carpet ask them if it's 2 ply and assess their body language as they reply to your knowledgable question with some discomfort.
I would conclude my thesis by throwing a few lifestyle questions at you:
1) Do you expect to stay in your property for sometime? If so, I'd suggest looking at wool or 2 ply yarn polypropylene.
2) Do you have a young family? If so, I'd suggest avoiding the cost of wool and stick to polypropylene carpet, preferably 2 ply if your budget allows.
3) Do you want to make your home look good to sell? Go for single yarn polypropylene with a squooshy underlay. The potential buyers will be won over by the underlay feel which is cheaper than carpet!
4) Are you a Landlord looking to replace the carpet? Assuming your rental property is aimed at the average tenant then I'd suggest a mid price range single yarn polypropylene that has a tonal effect which will help hide some staining that is likely oto occur.
If you need any further advice then feel free to email any of my Pupils.