It is personal choice - and it comes down to what a customer is looking to achieve from their project. The comment above about only getting properly used in summer is complete valid, as too is the overall size of the doors, where they are located on the house, what the view is like out of the property...indeed there is so much to consider we wrote and entire blog on the subject.

Hi Kathryn, we manufacture bi folding doors to suit any purpose. Regarding sound proofing we use the best seals on all joints. The joints between doors have 2 seals. Our standard doors are 54mm thick which does prevent most sound traveling through. However if you want extra thickness we can increase them to 68mm and install acoustic glass. You can contact us on 01633 740636 or view our work here http://timberbifold.co.uk

Put a door bolt in the middle of the door at the top

If you cant find a bi fold option on line anywhere then there are two options. buy a 33" door and make it a bifold or pack out the frame sympathetically to allow a 30" option allowing for the gaps needed to open and close. All depends on if you are capable of the job in hand yourself or if you will need to pay for the work to be done by others.