Insulation Consternation

My earlier mention of the different types of insulation on offer sparked a rather enlightening email debate between a few friends. It started with my friend Brian offering to help me get my head around exactly how vapour barriers work and the various ways you can place it in the Grand Sandwich of hull, walls, and insulation. Brian’s got an Architecural Engineering degree (albeit not directly in the study of boat insulation, he was very quick to add!) so he was able to give me some fuzzy principles explained in nice drawings that my brain could understand.

His comments went something like this:

I’m skeptical of the insulating paint, honestly; it may certainly help reduce some losses associated with radiative heat transfer, but so will white paint. (Black body radiation and all.) Your biggest concerns – like the rest of us living in structures surrounded by fluids – are conductive and convective heat losses. A more conventional foam-type insulation, like the Celotex or Thinsulate, seems like
what you want – especially as that metal hull is a thermal superhighway between the indoors and out.

I then countered that we’re mostly concerned about water vapour from cooking, breathing, heating, etc condensing onto the cold hull and causing mould on the insulation – not a small concern judging from the amount of blacked rockwool we’ve pulled out of the hotel room walls thus far (and those aren’t even the exterior walls yet!).

He was then able to draw me some diagrams to help me visualise how the vapour barrier would actually work.

A vapor barrier essentially restricts water vapor from
moving beyond it and condensing in a colder place. Here’s a simplified diagram, I think:

Case 1:

Moisture moves from left to right. It can get into the drywall, but can also escape, because the drywall’s the same temperature as the interior air and unrestricted from letting moisture back out. (If the room’s greenhouse-humid, well, that’s another problem.) The vapor barrier only lets water vapor move from right to left, so any that’s in the insulation can escape, but can’t build up. Assuming there isn’t any working its way in from the exterior, your wall’s interior stays dry – especially as the temperature gradient rises through the insulation, so the vapor barrier surface will be less cold, and less prone to condensation, than the bare hull. (Or vapor barrier directly on hull.)

Case 2:

Moisture still moves from left to right. Now it can get into the insulation from both sides, and vapor is bound to condense on the cold side. Given the structure of insulation, it’s nigh impossible to get that moisture back out – after all, it has to evaporate and slowly diffuse out without more condensing. Thus, you’re
looking at microbial growth problems.

I’ve taken this and drawn a more colourful diagram of how our exterior insulation would be set up:

Diagram of our insulation setup

I was feeling pretty clever, so I passed this over to our friend Steve, whose Luxemotor we just visited last weekend. He then raised a whole new round of questions about insulation I’d never even thought about, and frankly, I had no clue how to answer.

At this point I brought out the big guns – I emailed my brother, Steve, who also has an engineering degree, has just completed single-handedly building a massive extension onto his house, and works for the US Navy as a civilian engineer. So he had a bit more recent, practical knowledge and was able to sanity test what Brian had said and fill in a few of boat-owner-Steve’s questions.

The vapor barrier is generally simply a sheet of ordinary plastic, and doubles both as a vapor barrier and a draft barrier. Condensation happens when warm air cools below the dew point, and condensation forms. Rule of thumb is to put the vapor barrier on the warm side. I say “rule of thumb” because the hot and cold side change between summer and winter most places, and half the year, the vapor barrier will be on the wrong side. This is generally OK. What you do not ever ever want to do is put vapor barriers on BOTH sides- one side must be left to breathe. Otherwise any amount of water seeping in will just sit and fester, and water always seems to find a way in.

Ok this all makes sense to me. Boat-owner Steve then took what brother-Steve said and actually improved a bit on it, I think. He pointed out that the celotex actually has its own vapour barrier in the foil facing, and to simplify building you could affix the celotex to the back of your interior wall panels, but do it offset a bit to get a tongue and groove effect, preventing any draughts where the wall panels join.

Brother-Steve was able to give a bit of insight into what the big boys do with their boats, though much more research will have to be done on our part to determine whether we can afford it without a Defense Contract at our disposal…

US Navy ships simply apply the insulation straight to the bulkhead like in the link below. Usually 1” or 2” glass bat, faced with some sort of fireproof cloth. After it’s painted, it’s pretty vapor impermeable on both sides, but navy doesn’t seem to have much of a problem with it… Honestly, I’d just do it the navy way. Tried and true, right?

So we’re still not settled on exactly which type of insulation we’ll be using (though the insulating paint is definitely out and I think we’re less keen on the sprayfoam now), at least we know which order we’ll have to do it in. We’ve got some bare hull in the floor of the Captain’s Cabin bedroom to test any of our ideas on, which will hopefully mean we get some of our mistakes out of the way before blowing the time and money on the grand hotel room hold.

Anyone have any experience with a certain type of marine insulation they’d like to share? This is very much the time for commenting (not after it’s bought and affixed and you tell us what a bad idea it was!

- posted by Melissa Fehr on 8 February 2008, 14:18 in