Wall insulation extending under floorboards

[rreanor]

If you are insulating walls internally does this need to extend underneath the floorboards?

I'm looking at internal wall insulation for some solid wall rooms, and have been reading some threads which indicate that you shouldn't stop at the floor, and that you need to insulate underneath the floorboards to avoid thermal bridging and damp damage to joists.

This seems entirely impractical, as even if you put insulation underneath the floorboards, unless you cut the joists back and then wrap them in insulation they're still going to be exposed to the outside wall.

 

[^woody^]

it would be a waste of time in this instance.

Ideally, you should insulate the floor too, otherwise you'll get significant heat loss via the floor, and the condensation risk is increased.

 

[rreanor]

it would be a waste of time in this instance.

Ideally, you should insulate the floor too, otherwise you'll get significant heat loss via the floor, and the condensation risk is increased.

Even if you have thick underlay and carpet?

 

[mikeey84]

You should for the 1st floor, but not the ground floor as woody says. It can cause issues with cold joist ends in the wall, so important to get the details right on the design

The idea is to have continuous insulation so no warm damp air from the living area can find a cold surface to condense on.