Under floor insulation

[Bromleyxphil]

I am at the start of my research into the design and build of a narrowboat so am asking many questions.

Why does no one seem to insulate under the floor on a narrowboat, I read lots about older boats rusting from the inside under the floor. When I ask, and I do, the only answer I get is "it's for ventilation". I know you have to have ballast down there but surely that can be worked out and incorporated into the steel floor at build. If the floor was then grit blasted, two pack painted and then spray foamed flush to the spars, surely there would be no need for said ventilation and the rust couldn't get in. Sorry if you are sick of educating folk like me but I can't look at something and not question if I see another solution.

Phil

[system 4-50]

I am at the start of my research into the design and build of a narrowboat so am asking many questions.

Why does no one seem to insulate under the floor on a narrowboat, I read lots about older boats rusting from the inside under the floor. When I ask, and I do, the only answer I get is "it's for ventilation". I know you have to have ballast down there but surely that can be worked out and incorporated into the steel floor at build. If the floor was then grit blasted, two pack painted and then spray foamed flush to the spars, surely there would be no need for said ventilation and the rust couldn't get in. Sorry if you are sick of educating folk like me but I can't look at something and not question if I see another solution.

Phil

I am no one. My underfloor is sprayfoamed with brick & steel ballast sitting on top of it.

It seems to work (that is to say it seems to prevent hordes of spiders infesting the space).

Edited November 9, 2016 by system 4-50

[Alan de Enfield]

I know you have to have ballast down there but surely that can be worked out and incorporated into the steel floor at build.

 

You cannot build a 'fixed' ballast into the steel floor (base of the Hull) as until the boat is fitted out you have no (little) idea how it will need ballasting.

The location of the water tank, fuel tank, pump-out tank, Shower, kitchen, beds etc etc all affect the 'trim' of the boat. Most boats tend to have much of their 'stuff' on one side so little or no ballast is then needed on that side.

 

Having a pump-out tank 'under the bed' when compared to a cassette toilet can affect the boat trim by (maybe) 500kgs.

 

250kgs of engine + 250kgs of fuel at the back end needs balancing by either 500kgs at the front end, or an assortment of weights along the longitudinal axis forward of the C of G, (kitchen, chairs, fire, etc. If these are all on one-side of the boat a similar weight needs to be applied onto the other side to balance the boat side-to-side.

And so on.

 

If you are buying a 'sailaway' hull then the builder can only add in concrete slabs (or similar) so they can be added to or removed once the fit out is complete and you know how the boat will lie in the water.

[Pykebird]

I made sure my floor was insulated when I redid my boat, much to the amusement of most people. But it has made a big difference.

[Bromleyxphil]

I am no one. My underfloor is sprayfoamed with brick & steel ballast sitting on top of it.

It seems to work (that is to say it seems to prevent hordes of spiders infesting the space).

Well "no one" thanks for answering. That sounds like a decent solution do you have a ventilation space above?

 

I made sure my floor was insulated when I redid my boat, much to the amusement of most people. But it has made a big difference.

Thanks Pykebird did you use sprayfoam onto the steel?

 

Can I ask both of you

Did you paint the steel first?

Does it make the boat warmer?

How long ago since you did the job?

 

Thanks Phil

[Bee]

If the steel was grit blasted, which I doubt is needed on a new boat, but painted with 2 pack, (I used plain, cheap, water tank paint on mine) then you are off to a good start, you have quite probably fixed the rust problem so spray foam on top of the steel could be adding unforeseen problems. Leaky pipes / showers / pump out tanks etc could turn a nuisance into a problem, I think drainage channels down either side would be needed as well. As far as insulating the underside of the floor is concerned though, why not? I've never done it but I can't think of a reason why not except that when fitting a boat out most of us want to get the floor down quickly to see some progress. If you are at the planning stage I would try to make as much of the floor removable as possible though, I think it is sensible boat building practice.

[Tom Richmond]

I have renovated a narrowboat, and am currently planning renovation of a 50'x10' dutch barge.

 

With the narrowboat I wire brushed back the steel (no major rust even though it had been full of water for god knows how many years) and painted with primer and blacking. I then replaced the ballast (concrete slabs) and added an inch of cellotex insulation below marine ply and engineered oak flooring. The insulation was raised up on the bearers, tight against the underside of the floor. I cut fairly large (25cm x 8cm) vent holes into the floor in about 8 or 10 places down the length of the boat (43'). I am not sure whether the cellotex has made a difference, but it was not difficult, and not expensive, and the floor is certainly not cold.

 

With the barge I am trying to avoid putting down a plywood floor which will seal the bilges in (see separate thread "floorboards"), but this is mostly because it is an old boat and I want to be able to monitor the condition of the hull. With a new build that had been epoxied I would be less worried, though as Bee says it does seem to be good boat building practice. Currently thinking about how I can insulate under floorboards, while maintaining the ability to lift individual boards to inspect or work on bits of the bilge beneath. Sheets of cellotex will certainly be counter productive!

 

I would definitely not spray foam the baseplate of any boat. If water ever gets in there, it will not be able to run to a suitable place to be pumped out. You need a smooth base plate for that! Any water which collects in the surface of sprayfoam in the bilge would then stagnate and mould and become a habitat for all sorts of undesirable stuff...

[system 4-50]

Well "no one" thanks for answering.

That sounds like a decent solution do you have a ventilation space above?

steel base

then ~2in sprayfoam including over the traverse cross members

then a big enough space to take 56lb steel weights

then an air gap of ?? (1 or 2 inches ??, I don't remember)

then 25mm plywood floor

 

Thanks Pykebird did you use sprayfoam onto the steel?

 

Can I ask both of you

Did you paint the steel first?

The builder of my sailaway painted the floor with a grey primer and then (mostly) a single spray top coat.

A contractor did the sprayfoam on to that.

Does it make the boat warmer?

I can't tell, its the only boat I've owned. But my single centrally mounted 4Kw Puffin SF stove keeps the 60ft NB warm.

How long ago since you did the job?

7 years.

 

The sprayfoam is solid, you can walk on it (I did for many months).

The delay in replying is due to my being away from the internet for a while.

I am a firm believer in sprayfoam but would check the contractor's work much more thoroughly (too many crevices not filled) next time. I used ~ 12 cans of sprayfoam filling up the omissions. Cutting back is also messy, beware of static electricity attracting cut off foam fragments to your eyebrows!

 

Thanks Phil

 

Edited November 15, 2016 by system 4-50

[system 4-50]

I've added a pic but its gone in the wrong place.

The white bits are Waitrose self-seal bags that I've put the weights in.

Edited November 15, 2016 by system 4-50

[DHutch]

The bottom 6inchs of our boat is very cold when heated in winter, although this is only ever for weekends so the boat never fully tested long term.

 

Given we are only on the boat very infrequently over winter, it is a non-event and I expect this is the case for most seasonally used boats. After all, heat rises and a lot of houses have little or no insulation below the floor. I am also 6ft2 and would not wish to reduce headroom from what is currently around 6ft4.

 

I would be very hesitant about spray foaming the bilge, as while very good the foam is never fully closed cell and I would be worried about it taking on water if submerged, say due to a water tank or other plumbing failure. You will always get some condensation, and the likely hood of having a minor plumbing or deck-head leak at some point is high I would say. For condensation avoidance, a good unbroken vapour barrier and no thermal bridges are the order of the day.

 

Therefore if I where building a boat again, for seasonal use, I doubt I would add any. However if I was planning to use the boat all year round, I would likely fit an amount of insulation as a thermal barrier, say 25mm of foil covered PU insulation board, taped at the edges for air tightness. I would then likely add a number of bilge fans, as we have occasionally had water in the cabin bilge and without fans it takes a long time to go. I would also aim for even better paint coverage than we have, which would require better surface prep into the corners than the cursaty shot blasting that was done before the 2-pack epoxy primer and acrylic urethane topcoat was put down.

 

I would also consider the option of have the majority of the balast in the forum of suitable thickness of base plate, thus removing the need for large amounts of loose balast, however for deeper drafted boats this is often not practical. In the case of EmilyAnne we have 2 layers of concrete paving slabs over the majoirty of the boat, around 4inch or 100mm in total, plus a 12mm base plate. Concrete is around a 1/3 of the desity of steel, but this would still mean a 40mm thick baseplate and an amount of loose trimming balast.

 

 

Daniel

[I Spartacus]

I know I have not posted much lately as my build has been put back to next year. I would agree with what has been said

by other members have said regarding ventilation under the floor and the problems in getting the ballast correct. I would agree

that a two pack paint on the internal steel will hopefully resolve any rust problems. Like you I have also looked at this

problem regarding the insulation of the floor and I am planning in putting a SIP floor. this will provide me with 1 inch of

insulation on the floor and also provide me with ventilation to the area under the floor

 

 

Mark