Stone Wall Double Internal Insulation

A double dry-lining project in a stone cottage

This was a refurbishment of an old cottage built with thick random 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. Outside the paintwork was cracked in places and flaking off the mortar between the stones,.

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

A cavity wall would mean building an inner leaf. but there was only sandy clay under the floor flagstones. I couldn't get a decent U-value just with dry-lining. So I went for a double-lining, which is like a filled cavity wall. I reasoned that it didn't matter if the old outer 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 removed the need for better exterior rendering, saving money there. But I needed a perfect water barrier inside. Tanking as the wall was part underground. So for my first skin 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). I chose mineral fibre between steel stud as the second lining. Easy to run services inside that. I decided not to use a vapour barrier under the plasterboard, allowing moist air to return to the room through the board. The studwork and fibre arent affected by moisture and the middle XPS means the dew point will never reach the inner wall. I submitted this specification to the free service provided by Kingspan, as I was using their XPS product, and their calculation gave a U-value for the walls of 0.19W/m2K which is way ahead of the current requirement. Building Control later forced me to add a vapour barrier under the plasterboard but never mind.

I lifted the floor flagstones and relegated them to the garden path. I dug out 300mm of sandy clay and laid 100mm of drainage gravel, clean chips without silt. Over that I dropped a thin layer of grit and layed 100mm sheets of XPS. Did a bit of dancing on it until it settled, added a membrane (see later) and had 70mm of ready-mixed fibre-rich special flooring screed spread on top, which I later tiled.

The membrane edges I ran up the inside of the wall's central XPS lining before I built the inner metal studwork. Then I built the stud wall between an upper U-channel fixed to the ceiling joist and a lower U-channel raised on 50mm blocks on the floor XPS. The floor screed locked the lower channel and the top of it was the level guide for the screeding float. so the channel is now embedded in the floor.

The edges of the damp-proof-membrane were now trapped between the two wall linings, not very securely, so I battened the edges and hung them from the inside of the metal studs with wire so they can't slip down or fold.

Materials

The inside render was hard sand and cement so would easily support the Kingspan Styrozone N300R I chose for my inner lining. This came in 600 x 1200 rebated sheets. I discovered "insulation supports" on the same supply web-site. These are 8mm nylon nails or spikes with a wide flat head and come in various lengths. I bought a box of 110mm spikes and nailed 80mm XPS onto the wall by drilling 8mm holes on a 300mm grid then hammering the spikes into the cement render. The XPS sheets fitted snugly with their overlapping edges and the sheets didn't warp, split or buckle. XPS is tough stuff! Easy to saw too. No dust, no mask needed.

The fronting lining I chose 50mm metal I-stud which is stronger than the usual C-stud, Doesn't corrode, has service slots punched htrough for pipes and cables, and I could bed it into the floor screed yet still slide the studs along the channel.

I filled the inner lining with 50mm rockwool cavity batts after all the services were in place, cutting them batts to size with cloth shears. I hung the unneccessary polythene Vapour Control Layer in front and screwed 12mm fireline plasterboard to the studs with dry-wall black screws. Without a jig, this was hard work, so I pre-drilled 2,5mm which helped.

Available approved tanking materials seem archaic to me. I needed a DPM between the floor XPS and the screed above it, this had to run more than a metre up the walls to meet the underground room rules, between the XPS middle lining and the metal inner studs. I decided to use Firestone EPDM rubber as it does not interact with either XPS or screed and can stretch 300%. The room was small enough so I could tank it with a single sheet, dog-ear the corners and stuff them behind the stud and hang the edges up on thin plywood strips between the studs. I folded the rubber over the strips and stapled generously. I had to cut out three door entrances and form them with offcuts which I glued to the main sheet with the special weld solvent and adhesive specific to EPDM.

Strangely, the manufacturer's team couldn't show a BBA certificate for this construction method. They have approval for swimming pools, roofs, or whole-house wrapping where the EPDM is outside a new-build. The only explanation I could find was that solvent weld/glue use is expected so EPDM cannot be used indoors for health reasons. My tiny room needed only a single sheet and a little glue so I wore a mask and kept the doors open. I'm at home. not at work, so the H&S acts don't apply.