Stone Wall Double Internal Insulation

A double dry-lining project in a random stone-wall cottage

This was a refurbishment of an old cottage built with randon stone walls and a flagstone floor. The rear extension, the old scullery and outside toilet, were modernised to fit a study above and a kitchen below. On a steep hill, the lower half half of the kitchen walls were below ground level.

The inside of the stonework had been cement rendered and whitewashed decades ago, It had crazed cracks, rusty hooks, lumps and bumps, and no damp proof course or tanking membrane. The flagstone floor was uneven and cracked too.

Planning

Advice from builders and council building control was the usual mantras going back to tried and tested methods from the 1950's. Ignoring that, I approached it from an engineering viewpoint, I submitted the design that follows and the response was that it was "not an approved system" so I would have to get it reviewed and approved by an architect. Ignoring that too, I fired the building control service and went ahead anyway. I'm not selling up in my lifetime, and it will deter the asset-strippers after I'm gone that no regularisation will be possible without a lot of pain. The new walls are fine after 4 years of my occupation here, I have no worry about the success of my project so far.

Design

I reasoned that it didn't matter if the old wall was waterproof or not as long as the water went out the same way it came in and didn't enter the dwelling. That removes the need for perfect exterior rendering, saving money there. But I needed a perfect water barrier inside.

I chose XPS for its waterproof properties. Not as insulating as the popular PIR but stronger. 80mm of XPS is as good as 60mm of PIR.(see insulation list).

Materials

As a DIY project, I have much time, enough money, but want to save on complicated installation and sub-contracting. I also wanted very good insulation. The final calculation gave a U-value for the walls of 0.19W/m2K which is way ahead of the current requirement. That value was calculated by the free service provided by Kingspan, as I was using their XPS product Styrozone N300R as the first lining.