external insulation External insulation on a 500mm+ solid stone wall

[Chris HB]

I’m renovating a small mission hall on the Isle of Lewis in the Outer Hebrides. Due to it’s already small internal size (7.500 x 4.500), I was thinking of looking at external insulation rather than internal?

 

Its a solid wall construction (approx 500mm+) currently with an external pebbledash finisish. Any thoughts or recomendations?

[Chris HB]

I understand, internal insulation might be a cheaper route to go, but that’s a lot to loose off each wall in an already small space!

 

[Gus Potter]

Hi @Chris HB

 

That sounds great. My Sister lives on Tiree.

 

Questions..

 

What kind of stone do you have. Is it granite / imported stone from the mainland or local stone off the beach?

 

What are the walls founded on? sand or rock?

 

Wall construction? Have you exposed what is under the roughcast?  Is the outer leaf dressed but just weathered hence the render? inner core of random rubble and rough stone on the inside?

 

What is the roof made of, construction and how does it sit on the walls?

 

Lastly don't go hacking off the render before you have fully understood how the walls / their bearing (hesitant to say founds)  are constructed and their condition.

 

Post more if you can.. photos would be great and anything else you can think of.

 

 

 

 

[saveasteading]

I look forward to pictures too, if possible. This will make it all so much clearer to respond usefully.

[Chris HB]

OK, I’ll upload some more pics in due course… I don’t know all the answers to your questions @Gus Potter and @saveasteading but so far this is what I have discovered…

 

It was community built, date TBC, but could be around late 1930’s, but have heard other earlier dates mentiond as well? Pebble dash exterior, slate roof over main hall, composite tiles over the front small entrance porch. I have glimpsed timber sarking boards in the roof through a loft hatch, but not had chance to get in the roof space currently. Raised tie roof construction.

 

From what I can see/tell, it is a solid wall construction from local stone (assumed). Where I have exposed a couple of areas, it seems to have been internally rendered with a hard white coloured render (see sample chunks) which I’m thinking is more likely to be an early concrete rather than a lime render? It has subsequently had a basic timber frame and plasterboard finish later on at some point fitted over this fixed to the wall.

 

Suspended timber floor/wooden joists, unsure as yet what they are resting on. Boards all mostly good.

 

PS; yes it includes its very own disused bus shelter at the side of the building too! 😂

 

 

[saveasteading]

Brilliant. Very Exciting.

 

My  initial thoughts.

 

Obviously the damp bit to be dealt with. 

 

Not high enough for 2 storey (?) although could get a gallery perhaps.

The walls are very thick, and provide a lot more insulation than the calculation programmes allow.

You will be converting so do not have to reach any specific standard for insulation. However it is in your own interests to do what you can.

Therefore I would put high quality but skinny insulation and stud on the walls, and make up for it in the floor and ceiling/roof.

If the floor is concrete or stone, you can keep that in place and insulate/screed over it. I suggest UFH and Air source. This also avoids undermining the walls, which may be very shallow.

 

You will be living within a plastic tent, with no draughts, which is possibly more important than insulation levels.

 

Dampness from the walls has to be dealt with. I am happy with our solution for 600mm solid granite, although one or two other BH members disagree. Later.

 

You have to look properly at the roof timbers. Prepare for bad news, although the roof doesn't seem much out of shape.

 

External insulation would mess with the geometry, hence my suggestion above. I think you need to sacrifice 125mm on each wall. perhaps 100.

 

Try to add a porch as it will be a heat buffer when the winter winds blow.  No reason why planning shouldn't allow that.

 

Apart from the roof, it seems to be an inside job, with a few service holes.  So if you are into DIY this looks promising.

 

[Onoff]

That looks like a mission...

[Chris HB]

48 minutes ago, Onoff said:

That looks like a mission...

 

… and the tape will self destruct in five seconds!

[Chris HB]

1 hour ago, saveasteading said:

Brilliant. Very Exciting.

 

My  initial thoughts.

 

Obviously the damp bit to be dealt with. 

 

Not high enough for 2 storey (?) although could get a gallery perhaps.

The walls are very thick, and provide a lot more insulation than the calculation programmes allow.

You will be converting so do not have to reach any specific standard for insulation. However it is in your own interests to do what you can.

Therefore I would put high quality but skinny insulation and stud on the walls, and make up for it in the floor and ceiling/roof.

If the floor is concrete or stone, you can keep that in place and insulate/screed over it. I suggest UFH and Air source. This also avoids undermining the walls, which may be very shallow.

 

You will be living within a plastic tent, with no draughts, which is possibly more important than insulation levels.

 

Dampness from the walls has to be dealt with. I am happy with our solution for 600mm solid granite, although one or two other BH members disagree. Later.

 

You have to look properly at the roof timbers. Prepare for bad news, although the roof doesn't seem much out of shape.

 

External insulation would mess with the geometry, hence my suggestion above. I think you need to sacrifice 125mm on each wall. perhaps 100.

 

Try to add a porch as it will be a heat buffer when the winter winds blow.  No reason why planning shouldn't allow that.

 

Apart from the roof, it seems to be an inside job, with a few service holes.  So if you are into DIY this looks promising.

 

Thanks. I want to keep things fairly minimal and a little industrial aesthetic wise around the edges,  possibly adding a mezzanine level in at one end, but this could be phase two of the plan! 
 

I need to find a way to make the very small lobby into a WC as well in an ideal world! Unfortunately the space for that is very tight! 🤔

 

Interested to hear how you approached damp with your solid walls? 

[saveasteading]

1 hour ago, Chris HB said:

how you approached damp with your solid walls? 

Haven't done it yet, but start in the next week or so.

Fundamentally we are using the properties of the wall. It is made of inner and outer granite masonry and a central core of rubble and mortar.

Very little dampness gets through all that, according to numerous studies. The little damp that inevitably exists can be dealt with by 3 methods.

1. waterproof the inside face...not a good idea because it will come off in time.

2. build inner insulated wall with a cavity, and ventilate it. This is a common solution, but in my opinion, defeats the insulating benefit of the thick wall.

3. ditto but not ventilated.  There will always be some ventilation through the wall anyway, especially along the core.. The bottom metre is the only part which has significant damp so we are tanking that, and allowing the rest to breathe. (I borrowed this from another project which had a blog. Also it fits with technical info on tested buildings.

 

come back if this works for you, but await other comments that number 2 is the only way.

 

Our steading has similarities. Although it is big, the individual wings are narrow, so we had the same concerns about losing room width.

We have dug out the floors (the ground is sand) and will put in 125mm of PIR. Walls we are thinking 50mm PIR and 50mm rockwool.  attic spaces Fibreglass quilt. Exposed roof tried some PIR, and now changing to cavity batt.

Insulation levels will exceed new-build requirements.

 

You urgently need to know the wall construction. I am anticipating the end extension is modern and block.

[Chris HB]

40 minutes ago, saveasteading said:

You urgently need to know the wall construction. I am anticipating the end extension is modern and block.

it seems to be the same thickness, so I was assuming the same construction, but I’m sure I’ll find an area where I can check…

 

I like the idea of just a line render or earth plaster to just allow the wall breathe as that’s how the wall would become dry over time? I could live with that. 
 

I’m not sure how you’d get the U values for a stone wall or if on their own they’d meet current standards? I’d prefer to have them exposed internally, but not sure that’s a permissible option?

[saveasteading]

3 hours ago, Chris HB said:

if on their own they’d meet current standards?

Not by a very long way. Scottish standards for a conversion ask for 'reasonable efforts'.  So perhaps you can convince them that the floor area is crucial, and that a U value of 0.6 in the walls is ok because you will have  stunning insulation to floor and roof, etc

 

Also for a floor area of under 50m2 some  rules are less strict but I have not read in detail.

 

Conversions - in the case of conversions, as specified in regulation 4, the building as converted shall meet the requirements of this standard in so far as is reasonably practicable, and in no case be worse than before the conversion (regulation 12, schedule 6).

 

[Chris HB]

Yep, when you only have 35 sqm to play with, every little bit really does count!

[Gus Potter]

10 hours ago, Chris HB said:

Yep, when you only have 35 sqm to play with, every little bit really does count!

Hi Chris.

 

Had a look at the photo's.. what a great project.. many challenges but the main thing is to enjoy the journey as well as the end result.

 

Just to touch on some basics.. you may have got a handle on this already.

 

What about things like drainage.. septic tank soak away or biodigester. Get stuck into the ground and find out what you have under the top soil. Don't quite know your exact location so have not had a look at this end but some places have deep peat (or really soft sands) interspersed with granite dykes (called igneous intrusions).. great for building on but really hard to excavate for modern sewage tanks to achieve ground porosity for soakways. Have a look about the island and ask about. Local knowledge coupled with some basic research should give you clues here. See link below.

 

https://mapapps.bgs.ac.uk/geologyofbritain/home.html

 

The photos of the internal render are interesting. Hard to tell from the photos but do you think they have a bitumen content.. like an early attempt at waterproofing.. like mastic asphalt? You may actually be still able to smell "oil / hydro carbons" when freshy split or put the sample in a bit of petrol and see what dissolves.

Don't taste it.. stick to the whisky.

 

I would really try and find out more about the walls, the founds, there may just be a partially failed damp course! .. could be a slate / pitch (tar) one. Also what is under the floor..  are there any solum vents? may be hidden but look closely. In the 1930's things construction methods were changing fast and costs were getting driven down so don't assume that just because it is a mission hall it will be more traditional and that it is dressed stone outer layer / random rubble with a rough stone inner skin. In fact if it was a mission hall they would have maybe aimed to get the cheepest building on the market at the time so they could spend their money on the "mission" thus this could be an ideal case where you may find that innovation has occured. Do research here on the original intended use and who paid for it to get built. You may find this in the deads.. but that is cheating!

 

You may just be able to squeeze a quazi mezzanine / gallery up there but post a dimensioned cross section. It will need a bit of design flair and an understanding of the regs etc but don't rule it out for now.

 

Lastly what about these out buildings.. any chance of doing something with these? Could you link them to the house? If done sympathetically you just may be able to incorperate this into the habitable space with a link concept at some stage. Is any part listed grade C say?

 

Try and really spend time being "Columbo" now and it will pay dividends later when you come to the design of the insulated envelope and subsequent structure / finishes.

 

Plenty folk like @saveasteading and myself for example are interested in this so please keep posting.

 

Lastly keep a photo diary of this as it will great great interest to all both now and in the future.

 

 

 

 

 

 

[Chris HB]

I’m trying to take things slow and discover as much about the building in its as is state, just so I understand what I’ve got and where I’m starting from.
 

Unfortunately (in some ways) the regulations will probably require me to make some fairly big changes to bring things up to current standards. I understand why this is the case, just that it might end up turning the building/project into something that I wasn’t really intending or after… 🤔

 

Good news is that services are all nearby so mains connections are all feasible if I want. The picture probably makes it look more remote that it actually is. 
 

Roof pitch is about 50° so with the increased height from being an old mission hall, I was planning on opening the roof space up and having a mezzanine level at one end, then keeping the other full height. 
 

The other buildings is a cool disused bus shelter with store for the funeral bier(?) behind it.
 

People might be right about the front porch part being newer. I’ll remove some of its plasterboard and see what’s going on behind it… 

[saveasteading]

8 hours ago, Chris HB said:

discover as much about the building in its as is state

Excellent. Gus's points are very good.

What were they building with in those days? I don't think concrete blocks were around yet, so stone seems likely.

And yet it has that coloured render. 

More pictures would be good.

 

When I started researching stone steadings, I knew there was a lot to learn. It has been fascinating and also very important. 

If you can find local information that would be  a good start, from talking to people / asking at the library or whatever.

[Chris HB]

15 hours ago, saveasteading said:

Excellent. Gus's points are very good.

What were they building with in those days? I don't think concrete blocks were around yet, so stone seems likely.

And yet it has that coloured render. 

More pictures would be good.

 

When I started researching stone steadings, I knew there was a lot to learn. It has been fascinating and also very important. 

If you can find local information that would be  a good start, from talking to people / asking at the library or whatever.

 

There is a good local archieve, but unfortunately not much is know. They are looking into it for me though… I’ll pursue the CoS as well, but initially, they didn’t seem that interested.

 

Intersting removing some of the plasterboard in the porch. There was a mix of some walls in stone, some in concrete block, and some a mix. Looked like some evidence of maybe some earlier/lower roof rafters? Maybe an early form of a concrete wall plate sitting above this? Feels a shame that it would need to get covered up with insulation to make it meet standards if you want to live in it!

 

I will be doing a photo diary of sorts with it all, but electric is not connect as yet, so that may have to wait a little while. I’ll stick it on my design blog if anyones interested in following… https://www.passion4.co.uk/

 

Having met with a local person regarding warrant drawings etc. external insulation was not recommended. Good to hear some local expert insight. This makes the next big challenge (of many!), designing a tiny bathroom/entrance area whilst trying to navigate local accessability space allowances. I’ve been advised this is probably not possible!

 

The ‘nuclear option’ is to convert one of the main hall windows into a front door/entrance. I’m loathed to go this route, because if you did this, you could just design and add a big side extension to the building and make everthing simple. At that point, it might as well be someone elses project…

 

 

[saveasteading]

Stone is certainly more interesting than block. This is not the work of a master mason! There doesn't seem to be any coursing, more of an undulating pattern.

Do you know what kind of stone is used locally? 

Because we have granite, we use the kind of lime that goes quite hard, and has to be used before it sets, which was a surprise after reading so much about lime staying soft for days.

7 hours ago, Chris HB said:

CoS didn’t seem that interested.

Most people wont be interested it is  'just a building'.

But there may be an estates manager, or a retired clerk of works who could be interested to help.

 

Also try to find what other conversions have been carried out recently, as there may be information to be had.

[Modernista]

It would be too much of a coincidence for the Isle of Lewis mission hall advertised below in 1970 (Aberdeen Press and Journal) to be your one - and assuming it's not - it might be worth seeing what happened to this one in the seventies.....anyway might be some hooks into further research (and apologies if you already have this)

[Chris HB]

20 hours ago, saveasteading said:

Stone is certainly more interesting than block. This is not the work of a master mason! There doesn't seem to be any coursing, more of an undulating pattern.

Do you know what kind of stone is used locally? 

Because we have granite, we use the kind of lime that goes quite hard, and has to be used before it sets, which was a surprise after reading so much about lime staying soft for days.

Most people wont be interested it is  'just a building'.

But there may be an estates manager, or a retired clerk of works who could be interested to help.

 

Also try to find what other conversions have been carried out recently, as there may be information to be had.

I understand that it was ‘built by the community’ so yes, more utility than master craftsmen 🤣