Insulating the engine space?

[blackrose]

My cruiser stern widebeam has a large deck area which is completely uninsulated underneath. I once got down there and found the undersides dripping with condensation. In the 12 years I've been on this boat I've only ever seen it happen once, although it may have occurred more often and not been noticed. The engine hadn't been running at the time. 

Anyway, I've got some spare sheets of 1" Cellotex and I was wondering whether it was worth insulating the undersides of the deck and perhaps a foot down the sides. I would just stick the Cellotex to the steel with Stixall and then insulate all the 2" perpendicular angle stiffeners with some foam pipe insulation. But it's quite a lot of work and I'm wondering if I'd be wasting my time? The Cellotex is aluminium foil lined and I can't help thinking that it might prevent heat escaping from the engine space and perhaps the reflecting heat might even make the engine run hotter?

Edited September 12, 2017 by blackrose

[Detling]

Insulation by itself will never prevent condensation, as the bilge will inevitably have some moisture, all the insulation will achieve is stopping moisture condensing on the insulation, it will then just condense on the sides and the hinges and the prop shaft etc, whatever is the coolest surface. The insulation may also prevent the suns heat on the deck warming up the airspace and drying out somewhat. The best thing to prevent condensation is to improve the ventilation, to allow moist air out and provide a bit of heat to keep the air above dew point.  But is it worth it?

[Bloomsberry]

I've put insulation on the ceiling of the engine bay of my trad, more to keep the engine room insulated during cold snaps than preventing condensation.

[blackrose]

2 hours ago, Detling said:

Insulation by itself will never prevent condensation, as the bilge will inevitably have some moisture, all the insulation will achieve is stopping moisture condensing on the insulation, it will then just condense on the sides and the hinges and the prop shaft etc, whatever is the coolest surface. 

The best thing to prevent condensation is to improve the ventilation, to allow moist air out and provide a bit of heat to keep the air above dew point.  But is it worth it?

I'm not sure about that? Residual water or moisture laying in the bilges and condensation forming on cold surfaces are two different things. Anyway, my bilges are bone dry and the only condensation I saw last time was on the undersides of the deck and down the sides about a foot. The rest of the engine space was dry. You're correct that all the insulation will do is prevent the condensation on the insulation - that's precisely the idea! A lot of people seem to have this rather odd idea that if you insulate a surface then moisture in the air will move around "seeking out" the next best place to condense! That's not really how it works.

As far as ventilation, I don't really understand how vents keep the air temperature above the dewpoint? But anyway I have big open vents on both sides of the boat and I'm not intending to cut any more holes in the hull. 

Edited September 12, 2017 by blackrose

[Murflynn]

ventilation causes the condensed water to evaporate in the moving air which will normally be at less than 100% humidity and therefore able to absorb more water.  I don't think it is related to temperature differentials.  But it probably requires some forced air movement to be really effective.  If you're on a shore lead then a small fan should suffice, perhaps on a time-switch a few hours a day at the times when condensation tends to occur.

[blackrose]

If as Detling suggests, insulation isn't effective unless you insulate every surface, then why do we insulate our cabin spaces? I assume most of us don't insulate the cabin bilges?

The point is that warm moisture laden air will rise and that's why we tend to see condensation on uninsulated ceilings and walls rather than on the floor, so insulation in those areas does work, otherwise we wouldn't do it. I don't care if I still get a bit of condensation on the engine, gearbox and prop shaft because that won't produce anything like the amount of condensation I had running across the underdeck area and down the engine hole walls.

My question was simply about whether it was worth insulating an engine space where I've only rarely seen condensation. I guess it only occurs under certain environmental conditions. The only real downsides that I can see are the amount of work involved and the potential for the engine space to get too hot when the engine is running.

Edited September 13, 2017 by blackrose

[Old Son]

I wouldn't worry about the heat. Some engine are housed in an insulated cocoon and they don't get too hot.

I have a large bilge fan connected via a thermostat  and ducted to the outside air in case the engine bay gets too hot. My engine bay has insulation on the underside of the cruiser deck. The fan has never switched on to my knowledge.