Double futon mattress refers to a standard UK measurement or size. 4'6"/140 cm x 6'3"/190 cm. This size mattress allows two people to sit or sleep comfortably in conjunction with a futon frame.
A double size can mean either a 2 seat or 3 seat futon frame. First of all it's important to note a 2 seat frame is in 3 sections and can sit and sleep 2 people.
Digest the types
Yet a 3 seat frame is in 2 sections and sleeps 2 people but sits 3 people.
Both options can have a double mattress but the 2 seat has a thinner mattress because it has to fold twice. The 3 seat double mattress can be thicker as it only folds once along the length.
It does sound confusing but is very important to digest otherwise you may buy a mattress that is inappropriate for your needs.
Specialist manufacturers tend to offer higher quality and a better choice of filling. Most noteworthy fillings for a double futon mattress from a reputable brand would be Lamb's Wool, Wool Felt, Coir and Latex. There should be no chemicals or toxins used in any of the processes of manufacture. Therefore it's worth checking labels that are a legal requirement when looking to buy a futon mattress.
Covers for futon mattresses
Your futon mattress should have a casing that's 100% Cotton Drill. The cover can be either natural or have a colour.
Most of all avoid covers that are synthetic, these probably will have a very short lifespan. Because you are sitting and sleeping on the mattress for rather long times natural fibres are a better option. It may be in use for many years and consequently it will benefit by being aired.
Care of the mattress
Try and make sure you use your double futon mattress on a slatted frame. Air can circulate and help evaporate any moisture. Furthermore on a hot dry day place the mattress by an open window or outside for some hours