Discussed insulation with a friend, which made me think a bit about the issue. Surely there is no subject that has so many “truths” given by confident bloggers (none of whom actually have any deeper knowledge in the field, background as a civil engineer or similar… ) I would love to hear your views on my “assumptions”.
Vapour barrier : I have read different opinions about the Vapour barrier, where some say that you shouldn’t have one as the metal exists. But according to building engineering, the sheet is on the “wrong” side of the temperature gradient, so that in itself is hardly an argument. The argument is also that you build moisture into the wall, which is true, but… The volume of one “side” of my van (metal to inner wall) is about 3.5x1.9x0.05m => 0.33m3. On a normal day that means there is about 4grams of water in the wall, a volume that hardly even if it falls out 100% causes any problems with mould or rust. Given that the Vapour barrier is tight, I see no problem whatsoever with 4g of water being in the wall. Far from any amount that could create rust problems. An interesting question is how much rust 4g of water can theoretically create, don’t know, but can’t be much?
If you don’t use a Vapour barrier, you are constantly feeding new moisture that can fall out, which can then lead to problems given that you have a point in the wall (metal or insulation) where condensation occurs. Of course, you can use insulation material that does not consist of air or can absorb moisture, but to succeed in filling a wall with cavities etc. to 100% I see as difficult.
Insulation: Nothing to do with moisture per se, but the insulation material creates a temperature gradient (from the outside temperature to the inside), where moisture (given that there is moisture) will fall out when the temperature gradient passes the dew point. In theory, solid materials without air in them are better, but since there are cavities in the car that can’t be filled, the material doesn’t really matter, given that you have a Vapour barrier. But of course, if you don’t have a Vapour barrier, there may be a point in having that type of solid material as there will be no moisture where the dew point is. Maybe two different design philosophies but with the same result. But can you really say from a theoretical/practical point of view which one is better? Even if you have a solid material, you have to ensure that no moisture can get into any cavity or hit a cold surface.
What are your reflections, are there “truths” that are in fact myths, things that are purely theoretical but never become a practical problem?