Choosing the right insulation for our camper conversion

This may well turn out to be one of the more boring and technical posts that we write, but if you find yourself in our van in the Alps when the Beast from the East is blowing you will be grateful! We will start with a disclaimer though, and that is that we are not insulation professionals and as such these should be considered as our personal views on what is right for our own van. In writing this we drew on one of our parents’ experience as a Building Services Consulting Engineer of over 30 years (he designs heating and ventilation systems, including specifying insulation, for domestic and commercial buildings) and we have included references wherever possible.

In this post we explain the different insulation options that we considered and why we have (or have not) used them. One of the key factors in determining our choice was the ability of the insulation to resist heat transfer, after all that is what it’s for, keeping the warmth in and the cold out! The resistance to heat transfer can be measured, which allows different forms of insulation to be compared, and the measurements we refer to are:

  • Lambda value – also portrayed as ‘K-value’ or ‘λ-value’, measures a product’s thermal conductivity in units of W/m°K. The lower the lambda value, the better a product’s insulating properties for a given thickness.
  • R-value – the Lambda  (λ) values, combined with thickness of product, give the thermal resistance – or R value – of each element in a structure. The best insulation will have a high R-value at a low thickness, indicating that it is just as good at reducing heat loss as its thicker counterparts.
  • U value is the reciprocal of the sum of the thermal resistances of the layers that make up an entire building element, including the resistances of the inner and outer surfaces  – for example, a roof, wall or floor. It also includes adjustments for any fixings or air gaps. The lower the U-value, the better insulated the building element.

The second key thing to consider is how heat is transmitted / lost from inside the van, as some types of insulation are better at resisting some types of heat transfer than others. The most common form of heat loss in both campervans and domestic buildings is conduction. Conduction transfers heat via direct molecular collision within a material –  it is the most common form of heat transfer and occurs via. physical contact. Examples would be to place your hand against a cold window or pick up a hot object.

The two other forms are heat transfer are:

  • Convection – When a fluid or a liquid is heated and then travels away from the source, it carries the thermal energy along. An example of this is hot air rising from immediately above a heat source.
  • Radiation  – Thermal radiation generates from the emission of electromagnetic waves. These waves carry the energy away from the emitting object. An example of this is the electromagnetic waves emitted by the sun, and these can travel through a vacuum.

Now we have got that out of the way,we discuss the particular challenges in insulating a campervan, before going through the different insulation material that we considered and their pluses and minus for a campervan conversion. There isn’t necessarily a right answer for which insulation to use, but there are definitely some wrong ones.

Challenges in insulating a campervan

In a conventional building, insulation is commonly either installed externally (such that it becomes the exterior face of the building) or internally. When installed internally manufacturers (for instance Celotex) normally recommend that there is a ventilated cavity between the external wall of the building and the insulation material. This cavity prevents moisture transmission through the structure from a potentially damp outer leaf.

However, the warm air inside the building will contain moisture and, however solid a structure may appear,  this warm air will gradually pass through it, carrying the moisture with it. As it passes through the insulation it cools down (the outside face of the insulation will be colder than the inside face, if it is doing its job) and if this air cools below the dew point the moisture in it will condense out as water, in the same way that moisture condenses on a cold window surface.

However, in this instance it condenses within the actual structure – it is called ‘interstitial condensation’. In a campervan the insulation is commonly bonded to the outside skin of the vehicle, which is a vapour proof metal barrier, so the water will condense out when it reaches the outside face of the insulation / inside face of the van. You really don’t want this as it could lead to your van rusting from inside, and if your insulation can absorb water, this will also make your insulation useless.

There are two ways you can prevent this from happening (there is no harm in doing both):

  • Use an insulation material that is impermeable which will prevent the moist air entering the insulation, so it can’t get to the skin of the van.
  • Install a vapour barrier on the inside face of the insulation. This vapour barrier will prevent the moist air passing into the insulation, so you will not get condensation either in the insulation or against the metal skin of the van. There are potentially some issues with this approach as vapour barriers are typically made from plastic sheeting. You will normally want to fix things like cladding to the inside face of the van, which will mean making holes in the vapour barrier, and this may allow some moist air to reach the insulation.

We considered the following types of insulation:

  • Reflectix foil insulation
  • Sheep’s wool
  • Mineral wool
  • EPS (expanded polystyrene)
  • XPS
  • PIR

Reflectix foil insulation

Reflectix is made of two layers of foil with a middle layer of bubble wrap, it appears under numerous names and is referred to as insulation and / or heat reflector. As it is made from foil, reflectix is an excellent vapour barrier and, as the name suggests, also a good reflector – so good that a similar material was used on the US space shuttle. However, you are extremely unlikely to find a lambda value associated with reflectix as it is very poor at resisting heat loss by conduction. Since, this is the main form of heat loss from a campervan this rules out reflectix as an insulation material for us, although it could be used as a vapour barrier.  

Sheep’s Wool

Sheep’s wool has been keeping sheep warm for hundreds of years and it is environmentally friendly and sustainable, being mostly a byproduct of the meat industry.

  • Positives – It is available in felted sheets which are flexible, so can easily fit to the shape of your van. It is easy to cut with a pair of scissors and the fibres are unlikely to irritate eyes or skin.
  • Negatives – Despite sheep’s wool having very good natural moisture regulation it absorbs and holds water, which, as discussed above, can be a problem in a campervan.

Sheep’s wool has a lambda value of 0.035-0.039 W/m°K.

Mineral Wool

Mineral wool consists of spun yarn made from either glass or rock fibres. It can be formed into flexible rolls or stiffer boards.

  • Positives – It is available in rolls or boards which are flexible, so can easily fit to the shape of your van. It is easy to cut with a pair of scissors. It does not absorb moisture. It is fairly cheap.
  • Negatives – Mineral wool fibres can irritate eyes or skin. Mineral wool has got a lower heat storage capacity than natural insulation materials such as sheep’s wool. This means your van will heat up faster during summer, as the material cannot keep the warmth that long.

The lambda value of this insulation type is 0.03 W/m°K to 0.04 W/m°K.

EPS

EPS insulation is a solid board made from expanded polystyrene.

  • Positives – Boards are lightweight and non-toxic. It does not absorb moisture. It is fairly cheap.
  • Negatives – boards are stiff and will not bend, so may be hard to fit to the shape of the van. It is compressible and can be squashed or broken easily. Cutting it can be messy.

The lambda value of this insulation type is 0.034 – 0.038 W/m°K, dependent on grade.

XPS

XPS insulation is a solid board made from extruded polystyrene.

  • Positives – Boards are lightweight and non-toxic. It does not absorb moisture. It has very good compressive strength and is a good candidate to use in the floor of a van.
  • Negatives – boards are stiff and will not bend, so may be hard to fit to the shape of the van. Cutting it can be messy.

The lambda value of this insulation type is 0.033 W/m°K.

PIR

PIR (polyisocyanurate) insulation is a solid board.

  • Positives – it is the best performing insulation material available (other than vacuum insulation panels which we did not consider as these cannot be cut and must be made to order). PIR is commonly faced with foil, which will prevent water ingress if the boards are taped together.
  • Negatives – boards are stiff and will not bend, so may be hard to fit to the shape of the van. PIR can hold water. PIR is the most expensive insulation we considered.

The lambda value of this insulation type is 0.023 W/m°K

What we decided

Based on the above we decided to go with:

  • A flooring grade PIR insulation for the floor of the van. This is so we can lay the floor directly on top whilst adding the minimum thickness possible to maintain maximum headroom. We will lay a vapour barrier on top of the insulation and then have a hardwood floor on top of this. Since hardwood floors are either tongue and groove or click together, there will be minimal fixings going through the vapour barrier.
  • PIR insulation in the roof of the van. This is because we only have 25mm between the bottom of the metal roof and where we will fix the ceiling panels. As PIR insulation is better performing than mineral wool it is available in thinner thicknesses.
  • A mineral wool insulation in the walls of the van, where we have around 60mm between the outside face of the van and where we will fix the cladding (Knauf Earthwool 50mm, which has a lambda of 0.035 W/m°K). Although this is not the best performing insulation we could have chosen, we decided it was best for us as it will easily fit to the shape of the van; doesn’t hold water and is relatively cheap. Also given our campervan is quite a small space, the difference in thermal performance of mineral wool vs PIR will only equate to around 65W watts heat output (which is about a half of what a seated person will emit).

We hope that you have found this helpful!

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