Gaps between exposed floorboards

[Ben-]

Hi all - I'm planning on sanding & oiling my living room floorboards after lifting the carpet, but there are small gaps in between the boards - maybe 3-5mm throughout. Below the floor there's a decent void (3-4 feet) before soil, so smells rise up but I also have access to the underneath. I'd like to seal the gaps, but I need to ensure that this is relatively air tight, somewhat flexible and not obvious from above.

My current options:

Cut fibre board to size and nail from underneath
Problem: joists are irregular so this will leave gaps at the edges and there's not enough space under the floor boards to use a saw or handle full size sheets so it would be a pain

Tape from underneath
Problem: pet hairs and general dust will stick from above

Insulate with glass fibre from underneath
Problem: not airtight

Caulk from above
Problem: difficult to get it to stick in the wider gaps and wouldn't be the best look against the wood when viewed from above

As it stands, my best idea is to tape from underneath with a few staples to hold the tape tight and then caulk from above, trying to keep the caulk well below the top of the wood and using something cut from plastic as a spudger to smooth it out, staying below the top level to reduce how visible it is.

Anyone have any better ideas?!

 

[endecotp]

Push slivers of wood in from above and plane down level, before sanding. (You can buy slivers specifically for this.)
And insulate from below.

 

[foxhole]

Floor needs good ventilation to avoid rotting timbers, this also prevent smells .

 

[Ben-]

Cheers, what are the slivers called and where can I buy them from? The void has air bricks, but the soil is damp so smells are guaranteed.

 

[foxhole]

Cheers, what are the slivers called and where can I buy them from? The void has air bricks, but the soil is damp so smells are guaranteed.

Are the air bricks blocked? you would only get smells with poor ventilation [and rotting timbers].

 

[Ben-]

No, they're open. Timbers are fine as they're a good distance from the soil and well above dpc. The smell is just slightly musty, nothing too bad but might as well fix it while I'm doing the job.

 

[endecotp]

Slivers are called slivers.
Google "slivers for floor board gaps".

 

[joe-90]

If you can get the boards up you can use a floorboard cramp to push them tight against each other.

 

[Ben-]

If you can get the boards up you can use a floorboard cramp to push them tight against each other.

Thanks for the idea but this would end up in a lot of work to lift the boards without damaging them, recycle the old nails and then find matching boards to fill the gap left over!

 

[geraldthehamster]

If you can get into the void, you could fix 25mm or 50mm foiled Celotex across the bottom of the joists, and seal round the edge with expanding foam. The idea being completely to separate the warm side above from the cold ventilated side beneath the floor.

 

[Ben-]

...25mm or 50mm foiled Celotex...

Thanks for the suggestion, but this would be very visible from above.

 

[geraldthehamster]

...25mm or 50mm foiled Celotex...

Thanks for the suggestion, but this would be very visible from above.

How? Assuming your joists are 4 inches deep, the board will be four inches below the floorboards, in the dark.