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boat fit out

I am re-fitting out a 12 year boat 60ft.It had slab insulation which I shall replace.I am going to put 25mm thick foil backed polyurethane between the steel support framework, then fix 50 x 25 wood battens to the sreel frame every third bit of steel framing on the side of the boat. On the roof I will run 3 wooden battens length wise which will be used to fix the oak ceiling panels.I am worried about condensation as some steelwork will be left exposed as will the foil backing on the insulation which could cause condensation to drip on to the wooden roof linings when it gets very cold and I light the wood burning stove. Should I have all steelwork covered with insulation.

Asked by: boatfitout  | 9.44pm, Sunday 13 February

WW says:

Foil backing is not usual practice on narrowboat insulation, but shoudl be ok- though there is a possiblilty of localised corrosion where it makes contact with the steelwork. Making sure the steelwork is in good condition, or actually sticking the insulation to the shell with adhesive, might be useful, as this will stop pockets of condensation between the shell and the insulation.
It is worth ensuring that all the steel is insulated, otherwise, as you correctly notice, you would get cold spots forming, which could stain the wood. Strips of insulation- even thin polystyrene strips- will help reduce this.
It would also be worth coating the back of the wood with a very stong sealer, to prevent water ingress from condensation. a two-part epoxy resin treatment would be ideal, and would ensure long-life. Failing that, an aluminium primer would work- varnish usually degrades after a while. Burgess Hydrosol also works well, as is an aquesous resin solution, which soaks in well, then seals the wood.
Just a note of warning, ensure that any electrical cables you run are not in direct contact with the insulation, but are in a conduit- polystrene will react with PVC coated cables, breakig down the insulation, and increasing the resistance (voltage drop) of the wires, which would lead to localised heating.

Mark Langley  | 10.37AM, Monday 14 February

Ideally, the insulation should be placed over the steel framework so that it forms a continuous barrier to heat loss. Assuming that you are fixing the timber battens on the inside face of the steelwork, the battens will provide some insulation, although you might get a slight 'pattern stain'. This is caused by the reduced insulation value of the battens which makes the lining mature differently where they occur.
Some builders fix the battens to the side of the steelwork in order to give more internal space in the boat. In this case, it is important to leave the battens slightly proud of the steelwork so that there is a gap between the steel and the inner face of the lining. This can be filled with thinner insulation for good measure.
On the roof, I assume that the steel framework runs across the boat in which case you could cover it with insulation as this will be about the same thickness as the battens.
You should not get condensation on the foil backing because the insulation should keep the temperature of the moisture vapour in the boat above the 'dew point' at which it forms.

Graham Booth  | 10.43AM, Monday 14 February

I have found that the best way of avoiding the dreaded drips is to fit a vapour barrier after all the insulation measures are taken. This served me very well on the last three boats ive owned and fitted. Its a bit trying to complete but cheap and well worth the effort. The idea is to prevent condensation by stopping the moisture laden air inside the boat from comming into contact with the steel shell. I used a 4 metre wide roll of 500 guage transparent DP membrane from the builders merchants, I found that stapling the sheet on to the timber grounds with a hand stapler worked well. A centre line was marked on the roof bearers and the centre fold in the polythene roll was stapled to this before working out and down with the staples at about 100mm intervals, fastening round the window frames etc. (the transparent sheet allows you to see the bearers easily)The sheet wont reach all the way to the floor so an overlapping sheet will be needed below the gunwale. The sheet is allowed to overlap about 50mm at the end bulkheads which are each sheeted with a separate piece. This method need a sense a humour and patience to complete but I would certainly use it again if using conventional insulation rather than spray foam. Apart from the time and effort involved I havnt found any real drawbacks to this system. Good luck with the refit.
Mike Jordan

Mike Jordan  | 8.55AM, Tuesday 15 February

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