The time has come to start putting some insulation into the van so in this video I explain my process and thinking and I go off on a bit of a rant about the uselessness of sticking anything on top of foil “insulation” products.
The thing is – and I clearly didn’t explain it that well in the video as I got a handful of angry comments about it on YouTube – foil is not an insulator. Note specifically, I’m talking about foil here, not “foil in bubblewrap” as that’s two different products combined and I’ll come back to that in a minute.
Rather, foil – just foil! – can be used a radiant barrier which is to say it reflects heat that has been transmitted as infra-red light. In order to do this, the foil has to “see” the heat coming towards it and for that to work there has to be an air gap in front of the foil.
So if you line the van with foil but then cover it up with another product such that there’s no air gap, you’ve blocked any infra red light transmission so the foil becomes useless.
Many of the radiant barrier products sold are not simply foil though, that would be too fragile. What you actually buy is bubblewrap lined with foil. The exact same principle applies here in that it must have an air gap in front of the foil side (facing whichever way you wish the heat to be reflected back to, such as inside the van for winter or bouncing heat back out during summer) for the radiant barrier side of it to do any work.
Whilst foil has no insulating properties, I have read (on a British Board of Agrément certificate) that bubblewrap does have a (quite reasonable) lambda value of about 0.033 which puts it on a par with some loft insulation. The trouble is that at a thickness of just 3-5mm, which is typically how these products are sold, that’s so little as to have almost no discernable insulating effect.
Would you accept 5mm of loft insulation to keep you warm in a house?! Heck no, you’d have 100-200mm at least. Similarly, to get any sensible insulation from bubblewrap, you’d need several layers of the stuff, maybe 50mm (10 layers!) at least. What you tend to see in many van builds are 2-3 layers at best but often just the one.
Whilst that will, certainly, have some insulating value because (as just described) bubblewrap does indeed insulate, a couple of layers is a very, very small amount of insulation. These products tend to hide that in small print, along with the need for the foil side to have an air gap.
Thus foil-wrapped bubblewrap has limited usefulness in a van unless a) you leave at least an inch air gap between the foil and whatever the next layer is (whether that’s the ply lining or carpet or whatever) and b) use several layers of the stuff to build up the bubblewrap to a useful thickness.
A lot of van builders swear by bubblewrapped foil and will say “well my van’s very warm, thank you” but when queried, admit they also stuffed conventional loft insulation on top of it, in which case it’s the loft insulation that’s keeping them warm, not the bubblewrap or the foil!
As referenced in my video, Greg Virgoe’s excellent channel is here: www.youtube.com/gregvirgoe which includes two terrific videos about insulation, amongst many others.