Ventilation vs insulation help needed

[JasonDS]

I'm refitting a catamaran which will have 12mm closed cell insulation on all single walled sections (half of the boat is sandwich cored). It has a roof vent above each hull and one in the wash boards.

The worry is i know boats need ventilation to stop mold/mildew but i want to insulate the boat as we'll be sailing cold and warm climates as liveaboards but obviously heating a boat where the ventilation would negate the heating is dumb. I'm also hoping the insulation will help the boat stay cool in warm weather.

I'm also converting the washboards into a door which will make it a sandwich design with closed cell foam inside, not just a piece of ply. Should i still have a vent in it that i can open/close?

I left a damp towel in the boat and when i came back after 2 weeks there was a dry black dust on some of the interior woodwork. This came off with a wipe but has me worried!

Thanks for any advice!

[valhalla360]

Insulation helps if you are heating or cooling the boat.

If you are ventilating the boat, insulation doesn't do much of anything.

In hot weather, air/con helps a lot with humidity and condensation.

In cold weather, it's a bear trying to keep the boat dry. You will quickly find the poorly insulated areas as they will be constantly wet.

[Suijin]

Both are important with respect to managing humidity.

Insulation reduces the temperature swings inside which in turn reduces condensation, beyond making the boat more comfortable.

Ventilation allows the air to cycle and dry out when conditions permit.

That said, you need to understand the relationship between humidity, relative humidity, dew point, and temperature. Whether to ventilate or not at any given time is a function of present weather and the forecast.

You want the option for the most ventilation you can get but that does no mean you use it all the time. My boat has three vents in the coach roof and when I’m north in colder weather sometimes they are open and sometimes closed.

[US1Fountain]

Leave a couple of fans running to keep the air circulating? During the off-season that’s what we do when the boat is closed up for a few months. My understanding is as long as there is air movement, it’s all good. In 6 yrs never had any issues of mold/ mildew when we come back in the spring. We do open all the cabinets and drawers to make sure air movement isn’t restricted.

[Terra Nova]

Originally Posted by JasonDS

...I left a damp towel in the boat and when i came back after 2 weeks there was a dry black dust on some of the interior woodwork...

That boat needs much more ventilation.

[JasonDS]

I work on the boat between 0 and 3 times a week and in this cold weather we have 2-6 Celsius in the UK at the moment, I've been running my diesel heater to keep warm. I also duct taped the vents to keep the heat in which i should have removed before i lock up.

[JasonDS]

What kind of fans? Is this off grid or connected up to shore power?

[ZULU40]

Originally Posted by Terra Nova

That boat needs much more ventilation.

not any more

[CHRIS V CLEMENT]

Look at the Marinco 4" solar powered vent. The model number is N20804S. The are $160 each in the USA from Defender. Try one if in the front of each ama, and one in the aft area. the forward one can be set for intake, the aft one for exhaust. the movement of air will keep the interior clean, dry, and sweet smelling.

[wrwakefield]

Originally Posted by JasonDS

I'm refitting a catamaran which will have 12mm closed cell insulation on all single walled sections (half of the boat is sandwich cored). It has a roof vent above each hull and one in the wash boards.

The worry is i know boats need ventilation to stop mold/mildew but i want to insulate the boat as we'll be sailing cold and warm climates as liveaboards but obviously heating a boat where the ventilation would negate the heating is dumb. I'm also hoping the insulation will help the boat stay cool in warm weather.

I'm also converting the washboards into a door which will make it a sandwich design with closed cell foam inside, not just a piece of ply. Should i still have a vent in it that i can open/close?

I left a damp towel in the boat and when i came back after 2 weeks there was a dry black dust on some of the interior woodwork. This came off with a wipe but has me worried!

Thanks for any advice!

We live on our boat in cold climates, and I have for many decades. One of the most common mistakes when heating the vessel is to close off the ventilation when it is needed the most [typically with hopes of saving heating costs while creating an unhealthy living environment and promoting dampness that permeates everything- in support of mold and fungi... False ecomnomy...]

To answer your specific questions, you need to consult psychrometric charts to understand how much insulation you will need for your intended conditions.

Practical Sailor has a good article about this, and follow-on articles that will answer your other questions.

Additionally, we have shared what we have learned over the years about living on boats in cool climates on our blog if that is of interest.

Best wishes with your projects.

Cheers! Bill

[El Pinguino]

In a hot sunny climate you need all the insulation you can get combined with shading your decks and good ventilation... the outside air during the day at least is invariably going to be cooler than the air in the boat.

In a cold climate...and I don't do serious cold.... about -6*C overnight is as cold as I experience with day temperatures often only getting up to about 5*C and sea temps averaging about 8*C....

....in a cold climate you need ventilation otherwise your boat will end up like a sauna.

I rarely put in my wash boards... only if rain is blowing in from astern... and even then leave out the top one most of the time.

The rest of the time I just have a canvas flap hanging over the hole.....

The secret of survival in the cold is living as close to ambient as you can handle. Do not try and keep your cabin at 20*C so you can wander around in jocks and singlet.

Morning cabin temp for me is often about 6*C... heater goes on until it is 12*C.... and thats it for the day....

Yes, we have to dress warm while below..... but it does make it a lot easier going on deck as it minimises temperature shock.....

And also.... when laying up in the cold I secure the boat with minimum ventilation - dorades are never closed off. First cold night any moisture in the air condenses out and stays that way... a cold dry boat is a happy boat....

[billgewater]

Mold at any temperature only occurs in the presence of moisture. Moist air has a lower density than dry air (that's why the clouds are up there). You can allow this lighter moist air to "escape" by having "highest point" exits that are either curved down at the exit or covered with a dinghy etc. to prevent the ingress of rain. The vents do not need to be large, just at the highest point. My vessel (53 ft) has a single vent area less than half a square foot and we have never had any mold in 40 years in spite of having the boat closed down for 6 month periods - except for a small area in the workshop which does not have ready air flow to the single deck vent. The difference is conspicuous - ceiling mold. Sealing a vessel is a sure way to produce mold.

[El Pinguino]

Originally Posted by billgewater

Mold at any temperature only occurs in the presence of moisture. Moist air has a lower density than dry air (that's why the clouds are up there). You can allow this lighter moist air to "escape" by having "highest point" exits that are either curved down at the exit or covered with a dinghy etc. to prevent the ingress of rain. The vents do not need to be large, just at the highest point. My vessel (53 ft) has a single vent area less than half a square foot and we have never had any mold in 40 years in spite of having the boat closed down for 6 month periods - except for a small area in the workshop which does not have ready air flow to the single deck vent. The difference is conspicuous - ceiling mold. Sealing a vessel is a sure way to produce mold.

Really? So how do you explain fog?

As I say I don't block my dorades when laying up and also... what I didn't mention is that the small hatches in my heads - one frd, one aft - stay cracked open.

We used to have a bit of a black mould on vinyl headlinings in the vicinity of the galley... sorted by wiping down the surfaces involved with a eucalyptus oil solution before leaving the boat...

We leave all lockers and cupboards open... only problem of note there has been white mould on the outside of synthetic jackets... which I don't understand... jackets are now hung in clean air in the saloon when we lay up.. problem solved.

[El Pinguino]

Originally Posted by billgewater

Mold at any temperature only occurs in the presence of moisture. Moist air has a lower density than dry air (that's why the clouds are up there). You can allow this lighter moist air to "escape" by having "highest point" exits that are either curved down at the exit or covered with a dinghy etc. to prevent the ingress of rain. The vents do not need to be large, just at the highest point. My vessel (53 ft) has a single vent area less than half a square foot and we have never had any mold in 40 years in spite of having the boat closed down for 6 month periods - except for a small area in the workshop which does not have ready air flow to the single deck vent. The difference is conspicuous - ceiling mold. Sealing a vessel is a sure way to produce mold.

Which, of course, is why condensation occurs mainly on the deckhead... all other things being equal..

[Pizzazz]

Ventilation is key. I use computer fans... one pushes air in from the bow (through the anchor locker), two more push the air out, one from the stern, one from the cabin top through the dorades.

It is true that moist air is lighter than dry air (has to do with one molecule of water weighing 16, vs. nitrogen/oxygen weighing about 29), so you want to exhaust air from top.

Next, it really helps to use a dehumidifier if you want to keep the cabin in the 70s while the outside air is in the 50s or below. If you have a lot of dust in the boat (either from projects or from nearby construction, etc. then you should add filters. Easiest is to add a filter to the dehumidifier as it does not impact it much. You can also add filters to ceramic heaters but be careful there as either the output will fall (if a PTC element) or you could create a fire hazard (any other element). For organic smells and mold, it helps to use a UV-C light and leave it on constantly. Typically though dust is the biggest problem if the marina is close to a large city and you need filters for that as well as a nice HEPA vacuum cleaner.

Cooking can also add a lot of humidity to the boat. Make sure you have enough exhaust around your cooking area. It also helps to use a pressure cooker - it cooks faster anyway and when you are done you can open it up in the cockpit so that all the steam vents out of the boat.

SV Pizzazz