North American High Tops condensationandinsulation

North American High Tops

jacharl_@bell.net

The role insulation plays in limiting condensation

Water Vapour and Condensation

This is particularly relevant to those of us who use our vans in colder conditions.

Let’s begin with a few basic facts of physics:

Energy is involved in any change of state ( solid to liquid or liquid to gas or gas to liquid etc.) of matter.

The latent heat of evaporation of water is 2260 J/g . This means that it takes 2260 Joules of energy to turn 1 gram of water into water vapour (steam eg). This is pretty easy to visualize as we know and can see that a kettle boiling on a stove is using the heat from the stove element and the kettle is pumping out water vapour.

However, the latent heat of evaporation of water works both ways. In order for a gram of water vapour to change back to a gram of water (condense back into water), it must give up 2260 Joules of energy.

Since the concern in using our vans revolves around condensation, then managing the flow of energy is particularly important in managing condensation.

Since metal is a good conductor of energy, if there is a temperature differential on two sides of a piece of metal, energy will be conducted through the metal from the high energy side to the low. I.e. if the inside of our van is at 65F and the outside air temp is 25F, then the metal skin of our vans will be conducting energy through the metal skin from the inside of the van to the outside. If there is water vapour present on the warm side of the metal, then the removal of 2260 Joules of energy will result in the condensation of 1 gram of water.

The role of insulation. If insulation is applied to one side of the metal mentioned above, it will limit the flow of energy to the metal and thus the flow of energy through the metal. If there is limited flow of energy away from the inner surface of a van, due to the application of insulation, there will be limited condensation.

How does a dehumidifier work? A dehumidifier is basically a small refrigerator with the cold surfaces exposed to ambient air, with a fan that circulates ambient air over the cold surfaces – usually coils with fins . As the ambient air is circulated over the coils/fins, they remove energy (heat) from the air, allowing condensation to occur on them – the condensation drips down into a catch basin or drain. The dehumidifier is usually controlled by a humidistat that turns it on when the relative humidity of the ambient air exceeds a set limit. A couple of major factors limit the usefulness of dehumidifiers for us.

1. They consume a lot of energy to run.

2. They do not work well in ambient temperatures much below 60F

However, the use of a humidistat to help manage inside humidity has some real potential in terms of managing humidity and therefore condensation.

A few notes re condensation and managing it.

1. If energy is removed from moist air condensation will occur

2. If the energy “removal” is limited, so will the condensation be limited.

A few effective things to do in managing condensation are to:

- Insulate the inside of the outer skin of the vehicle to prevent the passage of energy from inside to outside surface.

- Manage the accumulation of moist inside air by using a power vent to expel moist inside air. If the vent can be controlled by a humidistat as well as a thermostat, so much the better.

- Try to avoid having “blind pockets” of air that cannot circulate. The Gas laws tell us that the humidity in a vessel will be pretty much uniform throughout the vessel, however, if there is a closed or semi closed “vessel” within a larger vessel, the humidity (concentration of the gas water vapour ) can be significantly different. Therefore, good circulation throughout the vessel – in this case the van, is critical.

- Take advantage of the ability of air to act like a sponge for water vapour. Eg. After returning from travelling in the van from skiing, the inside of the van will be filled with warm moist air, the air being warm will be able to hold a lot of moisture. Before walking away and forgetting about it at the end of the trip, run the vent to expel all warm moist air, thus leaving very little moisture to condense as the van cools down. Better still, after the warm air is expelled and is replaced with cold air, heat that air up and circulate it throughout the van – it will act as a sponge to any moisture left behind (by supplying those critical 2260 joules/gram of water to change it into water vapour) , then expel it as well.

This discussion is particularly relevant now to me as we are about to begin production of new tops from the original (but updated) ADVENTUREWAGEN molds. The original ADVENTUREWAGEN tops installed by ADVENTUREWAGEN were double shell tops. The inner shell has a nice smooth gelcoat finish to it that really appeals to some folks.

However, the cavity that exists betwen inner and outer shell can prove to be problematic in terms of managing condensation. I have an old AW top still attached to the roof clip of a van that was totalled after a collision. When I look down the length of the top in the cavity between inner and outer shells I can see evidence of mould, the result of condensation.

Given the wordy exposition above on managing condensation by managing heat transfer, I am going to recommend the following. For folks wanting to use both inner and outer shells - and there is no reason not to do so, if :

1. The inside of the outer shell is insulated with a closed cell vapour barrier product that will effectively virtually eliminate the conduction of thermal energy to the outer shell itself.

2. There is some provision for circulation of air - at least to some degree - in the cavity between inner and outer shells.

If either the "high" hightop or the ADVENTUREWAGEN top is applied as a single shell, then the inner surface of the top should be insulated.

Of course, if the van is only kept in dry arid climates the entire discussion of condensation inside the top is irrelevant with respect to condensation, however, there are significant thermal benefits to be gained by insulating the inside of the outer shell.