Internal Insulation of Granite House as Part of Kitchen Re-Build

[JKBDIY]

Moved this from the 'Your Projects' forum as I now see this is perhaps the better place:

Hello,

I am at the early stages of a full kitchen rebuild that began as “paint the cabinet doors, new countertops, and re-tile the backsplash”.

This is all happening in a granite home in Aberdeen, built c 1840. The room in question though is a more ‘modern’ extension, likely still a century or more old. This is granite on all four walls with stone floors and only a couple of windows, neither attracting much natural light. So it’s a cold cave much of the year up here!

The first major diversion from the original plan was when, during tile removal, I discovered the extend to which the current kitchen, installed in the early 1990s, reduced the size of the room by blocking the old fireplace including full height alcoves either side. I’ve since removed the wall to expose that fireplace and we’re set on recommissioning that and installing a wood burning stove.

This is the gable end and is partly subterranean; the wall appears to have some sort of tanking material applied to it but you’ll see from the pics that some amount of moisture has been allowed to pass. I’m not overly concerned as the kitchen has been in place 32 years in this way and we encountered a bit of mildew on only one cabinet (admittedly, near the sink), and a bit of ongoing wet to the wall base plate at the mouth of the chimney. The other three walls had minor damp that I’d attribute to build up due to a lot of rubble from degrading lathe and plaster.

I now plan to tear out all three remaining walls, back to the granite, and frame these out new, allowing me a chance to insulate. I’d like any advice here on my plan as follows:

Tear room out to blank granite and all four walls, stone floors. Clean and re-tank gable (chimney) wall only. Frame whole room, including alcoves but not fireplace, in new 4”/2” timber with PIR insulation between studs. I believe I need another thin PIR over studs as a thermal break but please confirm. Then plasterboard over top.

For the floor I’m proposing plastic sheet at a moisture barrier (or should this be topical DPM?), then floating floor comprised of PIR, sheathing, new laminate floor.

Should be a brand new room inside an old facade.

I’m aware of the pitfalls of adding internal insulation to a stone house. Does my method, assuming a 50mm, air gap between stud wall and granite, seem ok? I’m also curious to hear other’s advice on whether insulating 3.5 walls and a floor is all a big waste of time and money if I’m leaving a large stone fireplace exposed!

I’ve typed all this on a phone, on a bus on the way to work so I appreciate I’ve glossed over some required detail. For now, that’s it, and here’s some photos.

Thanks in advance for any input, it’s all appreciated.

JB

 

[Mr Chibs]

Think this is a tricky one as the stone walls are quite a feature, but you also want to live in warmth and damp free.

The gable wall seems to have copped for more than it's fair share of damp.
Previous solution seemed to have built a stud in front of said wall and hope for the best.

You could do similar again, but you are only covering the problem.

Think if it was mine and I wanted to keep the stone fireplace (which I would), then I would be looking at putting a barrier to the soil from the outside.
When you watch these grand designs on TV, with part subterranean rooms, they put polystyrene/insulation & DPM on the outside before putting the soil back. I realise this is not as easy as sorting from the inside but in my mind you've removed the largest likelihood of transferred moisture.

Tanking may be a solution, but not something I know of.

Celotex's method for walls with damp was dpc placed behind batten which is screwed to the wall, then PIR foam screwed to the batten (this gives 25mm gap, cables can go here), boarded and skimmed. No need for a bridging layer.

I did this in and old converted solid brick room, all is good and haven't noticed any issues.

Good luck, it's going to look cracking when completed.

 

[JKBDIY]

Thanks Mr Chibs,

I am set to tear out the last of the lathe and plaster this weekend so hope to embark on 3 of the 4 walls shortly.

The celotex recommended method you describe above makes some sense but I think I'll have a torrid time trying to screw anything into granite so I'm set on building a stud wall instead.

With that, there are a few things I'm still wondering about:

I plan to build a 4x2 stud wall all round, insulate between studs using the thickest PIR I can afford, then vapour barrier (which I assume is just a plastic sheet), then plasterboard. Not sure here why everyone seems to recommend skimming over plasterboard when it can be taped/floated then painted??

Bits I'm not clear on:

• should vapour barrier be on the warm side or the cold side of the insulation (as celotex seem to suggest in your reply) and why?
• is the thermal bridging issue through the studs a big concern? If so, is it best to apply sheets of thinner PIR over top of everything described above, then plasterboard? I'll be hanging kitchen cabinets so assume I shouldn't go too thick on this sheet PIR over top of studs?
• would it be an idea to install a roll of damp proof membrane underneath base plate of new stud wall, just to keep damp from stone slab at bay?
• I am assuming a gap of 50mm or so between granite and stud wall - I see others recommending no gap? In any case there'll be some gap due to the wonkiness of the inside of the granite wall but I wonder what is the logic behind no/small/large gap under these circumstances?

Thanks,

JB

 

[Mr Chibs]

From my understanding, the VCL is on the warm side of the insulation, and helps stop warm moist moisture from the air inside the room being transferred into the wall and promoting interstitial condensation.

If you are using PIR foil backed, and it's taped up, this forms a vapour barrier that will reduce moisture transmission.
For belt and braces after infilling your stud work, you would overboard the structure in another layer of PIR to reduces the chance of cold bridging etc.

You are already on the back foot, as the wall is carrying moisture in from outside, due to earth build up, by trapping this behind a stud, you might be creating issues further down the line, mould/condensation etc.

Make sure you either add more studs or some plywood areas, where you intend to hang wall units, as they can't hang off plasterboard and insulation!

As for the sole plate for your studs, you could lay a course of engineering bricks & DPC strip and sit the sole plates on top of this.

A lot of folk don't like jointing and painting and prefer skimming, horses for courses, plaster finish is better, but not if your plasterer is not very good.

Sounds like a job, good luck