We have wooden floors in most of our house and there are 2 rooms in particular that have large gaps in between the floor boards. We previously used filler before we stained and varnished them but now the filler is coming loose and keeps being sucked up by the vacuum cleaner.
In the other room the gaps are so big the floor has it's own lighting from the spotlight we have in the floor below.
I know using silicone sealant would be the best for keeping the house warm and because it is flexible but i know it won't be the same colour as the floor and don't want it to look naff after we spent hours of back breaking work making it look great.
If the gaps are larger than 5mm the product you need is called Gapmaster. It comes in many wood colours so you should find a match
The specific problem with Silicon sealants is that they will repel any future finishes you apply to the wood.
before posting, check to see if other posts can give you the answer that you are looking for. one post from only a few days ago deals with this topic - look at the thread entitled "Floor sanding - do it myself, or get someone in?"