I've got a retrofitting idea that I might start doing in one room this summer. Our daughter is housebound with CF/ME and spends most of her days in her "happy place" doing artwork in her bedroom. We want to make the space as friendly as possible, while at the same time increasing its warmth and dryness in the winter (it's at the cold south end of the house and NZ building standards with regard to insulation and thermal bridging are Absolute Shite). Rather than tearing into wall and ceiling cavities to add insulation, I want to increase the thermal mass on the interior surfaces and my plan is to use some light earth panels fixed to the existing structure.
So, the big idea is to get coir matting, such as that used on roading cuts to prevent erosion, cut it into manageable pieces of a square meter or so, and coat it with clay slip. When it's dry, I'll screw the panels to the existing studs (find them first), then skim coat with earthen or lime plaster. This will create a bit more insulation, but more importantly will add a fair bit of mass on the occupied side, and possibly help with moisture management to a degree. It's that last bit that I'm not completely comfy with: since the normal modern stick-framed house depends on the interior paint as part of the moisture barrier, will an earthen interior be effectively stranded from its function of humidity control? Should I strip off all latex paint first? This puts a huge work increase into the project and I'm already time poor.