Opinion on installing flashing to fill gap (stop draft) from between floor boards?

Please forgive my ignorance in the following, as I'm just after an opinion:
The previous owner had a number of floorboards lifted when restumping the house a few years ago. The boards were then polished, etc; however there are gaps between some of the boards (approx 5mm), through which there is a bit of a breeze in winter and windy conditions (also get a little light near the edges the house). I was planning on filling these with some Silicon based filler (to avoid cracking should movement take place in the future). I'm a little worried about the finish and also likely need to get under the house to put masking tape or similar along the gaps in the boards before I fill them to hold it in place. Given I'm getting under the house I was wondering whether I should just install flashing instead. This won’t actually fill the gaps but will stop the breeze or light entering the gaps/spaces between the boards. Obviously I'm not going to rip up the floors to do this and was planning on simply cutting the flashing to size (to fit between the joists) and stapling it in-place using a staple gun.
Apart from solving my problem, is there any advantage or disadvantage to doing this? For example, could this create a space (between the boards and flashing) for pests such as termites, etc?
Regards, Darren

I did this at my place. I used coloured water-soluble gap filler (Chalk in Colours). I filled the gap from underneath, then went up and cleaned up any that squeezed out with a wet rag. I think that is your easiest option and gives good results. If it's hard to get to underneath, just do it up top and clean up as you go.

Is that a silicon based filler or a hardset type of putty? Did you fill it level with the boards on the top?

Also out of curiosity, did you match the board colour?

The stuff I used is a product by Fuller called Caulk in Colours. It is an acrylic sealant that comes in a tube like silicone - water clean up. The colour range is not very comprehensive, the one I used is called cedar, which is just a mid-brown. I think you would want to use something flexible for this.

The way I did it, it wasn't seen except in a couple of places where the tongue on the board was missing. I did it from underneath and all I wanted to do was to seal the gap between adjacent boards, so I was only filling up the gap between the edge of the groove on one board and the rebate behind the tongue on the next board. From the top side, it just looks like it always did, with some boards still having a gap.