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Underfloor insulation

I am starting to fitout a narrowboat with a planked oak floor. Is it possible to put some insulation or thermal barrier under the floor?

Asked by: Chris Plumpton  | 7.45pm, Monday 3 October

WW says:

Often narrowboat floors can be quite cold in winter, since it is the deepest in the water and not insulated like the sides. The floor is usually quite thick, with boards over the ballast and floor bearers, and a smart floor finish on top.
It should be possible to put insulation between the floor bearers before the floor boards are laid; this could be glass fibre wool, but rigid high density foam sheets would be better as they do not retain moisture. The sheets should be fixed to the floor panels so that the insulation lies between the floor bearers. If there is insufficient space above the ballast, some packing strips will have to be fitted on the floor bearers (check the head room first).
Another option would be to lay fabric insulation over the boards before fitting the oak planking. I am not sure whether this would work since it would be squashed flat. It might be possible to lay insulating foam sheets under the planking with battens to stop it being squashed. Strips about a foot wide, with battens between, at right angles to the oak planks. This will only work if the oak planking is strong enough to spread the load, and extra support might be needed at entrances and points of high traffic.
Rupert Smedley

Rupert Smedley  | 9.36AM, Tuesday 4 October

It is also very important to ensure that there is adequate ventilation in the bilge space- so ensure that you don't fill all the space with insulation. You need a through-flow of ventilation (ideally a low level vent near the front of the cabin, linking to a high level (or deck level) vent aft. This should help ensure any condensation that forms in the shell (and so collects in the cabin bilge) can evaporate. Otherwise, a considerable volume of water can build up over a period of time, corroding the shell from the inside out.
However, if you have thick floor boards, and then carpet or wooden ovraly on top, you would probably not gain much by insulating the floors further- the air gap between the ballast and the boarding is sufficient. You won't decrase the U-Value (thermal transmission) by much, adding a layer of insulation- wood is already quite a good thermal barrier- and as "heat rises"... But if you do add insulation, then take care with the insulation.

Mark Langley  | 10.04AM, Tuesday 4 October

Readers say:

Many thanks for your reply. Will be of great value in the fitout.
Chris Plumpton

Chris Plumpton  | 2.20PM, Tuesday 4 October

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