Insulating exterior stone walls in conversion project

Hi guys,

My name is Russ Hayes and I live in North Wales. I've been trawling this forum for almost a year now although I have only just become a member. May I thank you all for creating and maintaining such a wonderful resource. I'm new to forums in general so apologies in advance if I make any etiquette errors in my first post!

Almost two years ago, my girlfriend and I (both first time buyers!) bought an old property in Penmaenmawr, North Wales, built of local granite. The building is approximately 150 years old and in a conservation area. We believe its first use was as a stables and later on as a quarry hospital. Amongst other things, it has most recently been used as a council depot.

We took a risk on the purchase with a view to obtaining planning permission for mixed residential and commercial use. I run a small commercial recording studio and wanted to combine my home and workplace, and this property is ideal.

After 9 months of battling with the local authority (specifically Mr Conservation) our plans were approved. We are dealing with the residential conversion side of the project initially, which will of course have to be completed to current regulations. The project is being undertaken primarily by my dad who has years of experience in architecture and construction (although has not been practising for some time) and myself. Whilst I have no professional experience, I feel that I've learnt a lot through research and in the work we've completed thus far.

The main issue I'm having difficulty resolving is how to best insulate the exterior walls whilst maintaining breathability and preventing moisture problems. After much deliberation over the past twelve months or so (!) I've drawn a basic cross section of the approach I would like to adopt. (See below)

I have discussed this approach with the local building control officer who was happy with the specification but concerned that moisture might get trapped between the DPM and insulation. Current regulations obviously aren't that helpful when it comes to trying to merge the old and the new, so I would very much appreciate any thoughts or advice that any of you may have.

Many thanks in advance,

Russ

Well if you have been following this forum for a year , then regrettably I have to say you haven't been paying much attention.

In every thread about insulation, the phrase "vapour barrier on the warm side of the insulation" is likely to appear. You however have put yours so the on the cold side so the BCO is completely correct. What did you dad have to say about this ?

In regard to your post, why go on and on and on.......... ? Info about your girlfriend/recording studio/history of the building and location all completely irrelevant to your very simple query which could have been detailed in six lines.

Good luck with the conversion.

Thank you for the warm welcome.

As stated, I'm new to using forums. As a first post, I thought the 'scene setting' may have been a polite way to introduce myself. Apologies for the offense.

My question was regarding the DPM and prevention of water ingress. Perhaps I wasn't clear enough. I'm fully aware of where to place the vapour barrier with regard to interstitial condensation.

Thanks.

"Apologies for the offense. "

No offense taken, just forums are full of posters who are unable to order their thoughts, drivel on at excruciating length and often obscure the question they wish to pose.

"I'm fully aware of where to place the vapour barrier with regard to interstitial condensation. "

So why then does your drawing - which is presumably intended to give us an accurate representation of your intentions - show it on the wrong side of the insulation ?

Edit:

Having looked again at your drawing , I wonder if in fact it is your intention that the Celotex acts as your vapour barrier ? If nicely butted and taped ( can you get T+G in 50 mm ?) that would be fine. As far as the DPM on the outside goes, it seems to be generally accepted ( I'm just repeating here as I am not entirely certain ) that it is not a good idea to sandwich the insulation between two impermeable layers ( although that would be much more relevant with fibre-wool insulation.)

Since, however, the Celotex is waterproof ( I am not bothered about the last 0.01% permeability here ) I don't see any benefit in putting the DPM there, so would suggest you don't bother.

Last comment is that 50 mm really is bottom limit and 100 mm would be better if you can spare the room

Thanks for the reply mointainwalker,

I'll make sure future posts are succinct!

It is indeed my intention that the Celotex acts as a vapour barrier. I should have clarified this.

The principle of running the DPM up behind the studwork is to prevent potential moisture penetration at the point where the studwork meets the slab. (I've circled this detail in the image below). As mentioned, the walls are constructed with lime mortar and so I have to assume that water ingress may pose problems. Running the DPM as high as 1200mm, however, is probably wrong. I was simply considering potential rising damp levels.

With regard to insulation thickness, I could probably stretch to 65mm. The studwork timbers will be 89mm, the idea being to leave space in front for wiring. (The idea of cutting into this extortionately expense stuff makes me feel somewhat nauseous!) I can't afford to lose any more floor space.

Thanks again,

Russ

With the walls as you describe, it may be a good idea to have the DPM. Obviously you will want to have treated battens to resist possible damp: have you considered using metal-channeling ? This would be water resistant and has holes in the uprights to allow easy passage of wiring.

Is it usual for granite house-walls to be "pointed " ?

Consider ebay for insulation, seconds or stuff left over from self builds etc.

Thanks Freddy.

Mointainwalker; I hadn't considered metal channeling. It would be a great solution in this case, although we simply can't afford the quantities we would need.

The original lime mortar joints are in a bad state of repair so yes, the whole building will need repointing with hydraulic lime. (This is also to satisfy the conservation officer as well as breathability).

Simply as an extra preventory measure, do you think running the DPM up 1200mm or so to protect the (treated) timber is necessary? There will be an air gap between the stud and stone walls.

Don't know UK costs for metal but not much more than wooden battens here and only 45 mm deep.

Is your 20 mm gap the same all the way up ? I would expect the wall to taper towards the top thus increasing the gap. I would probably go with membrane for peace of mind since cost is not too great ( remember I don't know your financial circumstances ). You could always use the Celotex as a barrier on this side too and just take a short width of your DPM from "wet" side, under the Celotex and lap it up the inner side to prevent any seepage into floor/battens.