As our bus had a small amount of cheap Styrofoam insulation in the ceiling and wall panels, we wanted to add additional installation. We decided that 25mm Recticel Rigid Insulation Board would work best. This fitted nicely within our wall framing and is very easy to cut and use. We had to buy about 20 Recticel boards as we were using it for the walls, celling and floor, this cost a whopping GBP 250. You can get polystyrene sheets for around a quarter of the price but we opted for the recticel due to higher thermal quality, less absorbtion of water rates (should we ever have a leak) and a firmer product (so hoped to cut down on struts if we backed our plywood with something more solid).
We measured each gap we needed to fill, then using a tap measure and a metre rule drew the shape onto the insulation sheet. We then cut these using a Jig Saw with a metal blade so that it didn’t chew up the insulation. The Jig saw was the quickest and easiest way to cut the insulation sheets, but for some of the larger pieces we used a Circular saw bench. The circular saw bench was great for cutting the insulation width ways to get thinner pieces for areas that required it, but expect your garage/garden/street to be covered in celotex snow. Please note that most Stanley knives did not cut all the way through the insulation pieces, but if you only have this option it is still manageable, you just need to cut down both sides, or do one cut and bend it and cut in the bend.
Most of the insulation should fit nicely into the frame work, but if using scrape pieces for smaller areas we held them in place with either sealant or strong duct tape. Bear in mind at this stage every little gap needs filling with expanding foam or metal sealer tape to avoid creating any cold bridges through the insulation.
The next stage is personal choice, with opinions varying wildly: Adding a vapour barrier. The purpose of the vapour barrier over the insulation is to prevent any moisture building up inside the insulation. This may happen as the skin of the van can be very cold and the air inside your van can have a high moisture content from breathing/cooking/washing, and as this air makes its way through the insulation it condenses against the outer skin and can create damp spots. This can happen in ordinary homes, which is why walls and roofs in houses have vapour barriers too. It therefore made sense to us to include a BREATHABLE vapour barrier; one that stops water coming through but allows the insulation to breath. We opted for fabric roofing felt as it is super lightweight, and designed specifically for this purpose. IT wasn’t cheap at $50 a roll, but that is for 50m in length so I imagine we will vapour barrier the ceiling, walls and probably even the floor just for good measure with that amount. This felt comes in large rolls and is easy to apply, you can just unroll directly over framing and cut and staple gun to the wooden framing.