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Chapel Conversion - wall design again!


RJHumphrey
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Hello, 
I know the subject of wall construction/ design has been covered  previously,  but would appreciate some input from people on their wall construction/design and how it performs.  I am in the early stages of converting a stone chapel in Mid Wales, the walls are 600mm river stone, I was planning to construct an internal timber frame with a vented cavity. i have run a couple of designs through a U value calculator and decided on creating  an aIr tight envelope with t&g woodfibre insulation on the outer side of the frame, filling the stud with Woodfibre batts, but I dont think its going to be an easy build, maybe build stud frames, attach t&g insulation boards, then lift into place, tape/ seal corners. Any advice/ info on wall design would be much appreciated, as the more i research this subject, the more confused I am.

thanks,

Ryan

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Edited by RJHumphrey
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I'm doing a similar thing on a solid stone barn conversion. Original walls are 5-700mm thick lime pointed granite. I've built dwarf walls inside consisting of a marmox waterproof thermal load bearing block with a standard 6" block on top to bring me up to the top of my finished screed height. I've then built 50x100 studwork on the floor with 50mm of sheeps wool inbetween counter battens on the external of the stud, wrapped the back in a tyvek membrane and then lifted each section onto the dwarf wall before packing them tight and bolting them down to the block and screwing up into the posi joists. I've left a 50mm vented cavity between the new stud and original walls . The membrane on each 4.8mtr section of studwork laps through the section joint and is joined to the next on the inner face.

Inside the 100x50 stud I've filled it with more sheeps wool before fixing an "insulated " sheathing board to the inner face to keep my bco happy with any thermal bridging issues. Next I've fixed a smart vapour control layer which is taped and sealed in every way possible to create an airtight envelope inside the building.  I'm currently now fixing counter battens to the front of all of this the create a service void before hopefully fitting my final layer which will be Fermacell to hopefully give me as solid as possible wall whilst still allowing the original stone wall as much of a chance as I can to breath.

It's a fair amount of work but between keeping my bco happy and trying to look after the original building it's the best I could work out to do. 

 

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17 hours ago, RJHumphrey said:

the more i research this subject, the more confused I am.

Me too, and that is because there are conflicting ideas, and conflicting requirements from designers and building control inspectors.

 

They can't all be right.

 

I am currently of the view that no water will get through a 600mm solid stone wall IF it is in good condition. My main concerns with a vented cavity are that it removes nearly all the thermal advantage of that thick wall, creating air movement through the cavity may bring in as much damp as it removes, and bits can fall off the inside over time and bridge it.

 

There is much discussion on here a few weeks ago, and a lot of certainty......not always in agreement, but all interesting, helpful and thought provoking.

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Posted (edited)

Thanks for the reply’s, as you can probably tell from my original post I am not a tradesman/ expert in building techniques, but try to research, listening to all advice and hopefully make an informed decision. I didn’t think of bringing the membrane through the sections where they join and sealing them at the front, thats why I like a forum. A question I have is if you have an airtight membrane/ layer on the external side of the stud, why is it required on the inside, only asking for my benefit, not questioning your method. I have also gone the dwarf wall to finish screed level. I am dreading completing this work and finding the building cold during the winter🤔 At the moment I am looking at wall build from internal, finish board undecided, osb, 150mm wood fibre batt in-between studs, 60mm t&g wood fibre, breather membrane, vented cavity, 600mm lime/stone wall. My BCO seems very uninterested in the spec, not sure if this is a good or bad thing.

As for the vented cavity, I see the plus and the negatives, but can block, restrict air flow if required, I can monitor how the cavity is performing  with an extendable camera. It wouldn’t of been an easy task to create the air vents once the internal stud wall is constructed.

constructive comments welcome…….

thanks,

Ryan

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Edited by RJHumphrey
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My outer membrane is "tyvek housewrap" brerher membrane It's there more to keep the wool from flopping out of the counter battens if the staples decided to give up.  I can't find a picture of my finished dwarfs but the same principle except the bco wanted some kind of a thermal break to the slab. He's favourite subject is thermal bridging, u-values and condensation risk analysis.  Every aspect has had to have manufacturers supply u-value calculation and condensation analysis, right down to each window reveal !20201003_100131.thumb.jpg.1256c8dfd766992c78d3d3832d8e16f9.jpg

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Ouch. I have had that  and not every detail and calc is available. I had very good help from British gypsum re dew points, just last week.

Channel Islands I note. On the island I worked on the Chief Engineer was also the planner and bco. 

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Perhaps I missed a trick with thermoblok, i have decided to go with a wider timber studwork so have shuttered and poured concrete to widen the perimeter dwarf wall. i have been reading up on sole plate sealng/ fixing, looking at either rubber or tapes., whats your plans? Maybe my BCO lack of interest or  guidance isnt a blessing. i thought with the number of stone barns/ structures converted there would be lessons learnt, best practice, do & donts everywhere on the web. I am still hearing the adviice, build a stud wall and fill with rigid insulation, i think things have moved on from this.

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Saveasteading is right, there has been lots of debate on this. He even seems to be coming round! Have a read of the 2 recent threads.

 

Im firmly in "no cavity camp", assuming of course that wind driven rain isnt an issue. Which i guess its not, especially as, i assume you are putting the render back on the outside?

 

Following the renovations on my house, the views of what one might call hardcore lime purists, so no modern materials that restrict moisture movement, do appear, im my case to have worked 100%.

 

How damp is it? Im not sure id have layed concrete. Thats likely to drive the damp to the walls. Depending on how much there is?

 

Have you looked here at their internal wall systems. https://www.lime.org.uk/

 

I found there periodpropert forum very useful specifically for "old stuff".

 

 

 

 

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1 hour ago, RJHumphrey said:

Perhaps I missed a trick with thermoblok, i have decided to go with a wider timber studwork so have shuttered and poured concrete to widen the perimeter dwarf wall. i have been reading up on sole plate sealng/ fixing, looking at either rubber or tapes., whats your plans? 

My sole plate isn't sealed down, the internal airtight membrane is sealed to the block with a tyvek bitumen double sided tape, creating the base of the airtight envelope.

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Thanks, I will have a look at the site. I am planning to leave the front un-rendered as it is relatively protected, just repoint and lime wash, the weather comes in from the back and side, which will be rendered.  I have made some decisions that I have regretted, which would be too costly to reverse, and if I was to take on another project I would do differently, but that isn’t going to happen! I am ok with the cavity, as I can see the logic of the wall acting as a form of cladding, can see the benefit of the fixed direct to wall insulation and cavity route. I have read lots of threads where builders have insulated stone buildings, but can’t think of many where people that live in these properties share how they have performed, so appreciate your building performance update. I know every day is a school day, just would be good to learn from someone’s else’s mistakes, rather than my own (is this something I should of said, or kept it to myself?)

it’s not all doom and gloom and I am enjoying the process…….honestly.

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25 minutes ago, Barnboy said:

My sole plate isn't sealed down, the internal airtight membrane is sealed to the block with a tyvek bitumen double sided tape, creating the base of the airtight envelope.

I will look at bitumen tape, just need to work out how to attach it when I lift the stud sections into place.

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15 minutes ago, RJHumphrey said:

I will look at bitumen tape, just need to work out how to attach it when I lift the stud sections into place.

I wrapped the housewrap around the top and bottoms of the stud so it was protecting the timber and then let the internal vcl do the rest with bitumen tape to the block at the bottom. The housewrap doesn't need to be airtight just the internal vcl.

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55 minutes ago, Roger440 said:

 

How damp is it? Im not sure id have layed concrete. Thats likely to drive the damp to the walls. Depending on how much there is?

 

 

- the building is very dry, the damp areas you see in the right hand side of the photo is from under the old door and on the wall is where the ground is built up against the wall, which is to be removed soon. Where window openings have been made the wall has been very dry.

 

 

 

 

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17 minutes ago, RJHumphrey said:

Thanks, I will have a look at the site. I am planning to leave the front un-rendered as it is relatively protected, just repoint and lime wash, the weather comes in from the back and side, which will be rendered.  I have made some decisions that I have regretted, which would be too costly to reverse, and if I was to take on another project I would do differently, but that isn’t going to happen! I am ok with the cavity, as I can see the logic of the wall acting as a form of cladding, can see the benefit of the fixed direct to wall insulation and cavity route. I have read lots of threads where builders have insulated stone buildings, but can’t think of many where people that live in these properties share how they have performed, so appreciate your building performance update. I know every day is a school day, just would be good to learn from someone’s else’s mistakes, rather than my own (is this something I should of said, or kept it to myself?)

it’s not all doom and gloom and I am enjoying the process…….honestly.

 

I quite like the look of it as it is. Pointing in lime i presume?

 

One of things for me with a cavity is that its is, as you say, just a rainscreen. That does mean, however, the wall is detached from the house and it will always be a cold wall, so will encourage damp and moss growth etc.  Though i cant say i had any practocal experience of that.

 

I did look at a converted barn a decade or so ago. That had been done with a cavity. There were obvious signes of damp near most of the windows, i guess because at the point the cavity is bridged at this point? Didnt buy it so never found out.

 

I spent hours reading, then used some common sense. If i was doing it again, id do exactly what ive done, so no mistakes to share. Which im pleased about.

 

 

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34 minutes ago, Roger440 said:

 

I quite like the look of it as it is. Pointing in lime i presume?

 

One of things for me with a cavity is that its is, as you say, just a rainscreen. That does mean, however, the wall is detached from the house and it will always be a cold wall, so will encourage damp and moss growth etc.  Though i cant say i had any practocal experience of that.

 

I did look at a converted barn a decade or so ago. That had been done with a cavity. There were obvious signes of damp near most of the windows, i guess because at the point the cavity is bridged at this point? Didnt buy it so never found out.

 

I spent hours reading, then used some common sense. If i was doing it again, id do exactly what ive done, so no mistakes to share. Which im pleased about.

 

 

Yes lime to be used for pointing. As for the cavity, there has been no heating in the building for 20+ years and an internal corrugated bitumen lining attached to the walls, which has been removed. I have created air vents through the wall so the air will circulate in the cavity. I can monitor what is happening inside the cavity with a camera  that I can insert through a vent, and will have the option to block or restrict these vents if they are causing problems.

Window returns I feel are a potential weak point if not sealed  and insulated correctly.

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@RJHumphrey What a stunning looking place.

 

This rendering.. do you really have to do it, even if the other walls are half or a bit worse you will knock value off it, not just financial. It would be a real shame to render a beautiful place like that.

 

Technically if you render some walls then you'll have to design for a wall that breathes and others that don't. The interfaces at the corners will be complex to say the least.

 

As an over view always look at how you are going to say wire and plumb the place. Work out how you are going to do this and maintain / alter at a later stage and the practicality if you have insulation placed hard up against the stone. Remember for example that electric cables placed in insulation need to be heavier or need shorter cable runs which may be hard to achieve. It's not easy fitting 4.0mm  cable into a 13 amp standard socket.

 

Yes there are different views on whether you insulate these old buildings with an internal frame, a cavity between the walls and the stone or an insulation system with no cavity. But structurally you often need to tie the old walls into say a mid floor.. often with a chapel and this means complex penetration details at these levels which are hard / costly to achieve.

 

Yes if you have an internal frame with a cavity you to some extent create a bridge layer when doing you U value calcs.. but in practice you are talking about a cavity with trickle ventilation not a howling draft. In the round you may want to take the hit on the reduced U value if you have a cavity but the buildability savings especially at the corners and floor often off sets this. Use these savings to load insulation into the floors and roofs where it is easy to do, more buildable?

 

The key for me.. if that place was mine..well it's not.. but I would look first at the pointing, external drainage and the general weathering details. Pointing is the key as even wind driven rain will be mostly shed by good pointing. If you want to experiment find a bit of that river stone, dry it out in the oven and put it half submerged in a bucket of water. See if the top gets wet and the split it in half. Observe.

 

In summary when you insulate you make the stone wall cooler. Generally the humidity is higher inside the building and this air that contains water gas gets into the wall. At some point in the wall the gas turns to water.. called the dew point. Now the dew point in an old wall is hard to predict as it is a natural material made up of a mixture of random rubble and dressed stone. But you can be sure that you will shift the dew point inwards when you insulate.

 

From a buildability point of view it is very quick to put up an internal timber frame, at the worst case it is also a reversible process and easier to intervene if a localised problem occurs. Look at this in the round.. you can create an energy efficient house but energy efficiency and eco friendly is also about making it easily maintainable and adaptable over its 50 plus year intended life span.

 

 

 

 

 

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Ignore the gypsum internal board( will use different board finished with lime) the airtight membrane between 60mm wood fibre insulation and vented cavity, no VCL as such, not unless I seal all penetrations of the internal osb board.

The VCL debate in a breathable wall is another minefield, I can find every opinion imaginable for having a VCL or not!

 

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If you look back at other discussions there is  a lot of good advice. It is all good whether consistent or not.

Also there were recommendations for technical documents, some of which I had not found despite ninja googling, and I recommend them all to you. There is so much practical advice therein, and also some background to old buildings uses and history.

 

If your gutters are good, and you do really good repairs to the lime pointing, then any water that seeps in will evaporate outwards again.....so no need to parge over the lovely masonry.

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Thanks for the reply, I will have a read through the old threads to see if I have missed anything, any particular one you took most from?

The  photo was taken just after removing the render so the stone looked a lot lighter, and better in my eyes, but after a long winter it’s not looking so good, beauty is in the eye of the beholder and at the moment I like the look of lime washed stone, as it keeps the shape of the stone visible, rather than a render, are you not a fan?

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3 hours ago, RJHumphrey said:

are you not a fan?

The lime wash is attractive, but so is the exposed stone.

 

In favour of the lime wash...it is an additional coating before the mortar  and seals any little gaps.

It emphasises the rough stone construction.

Conceals old repairs.

Relatively easy to repaint if needed.

 

Against: the rough stone face will catch and hold the rain.

 

For the exposed stone, depending on the stone itself, there is less of the absorbent surface.

If pointed near to flush, the rain will run off quickly and only the pointing hold water.

 

It is total rendering/ parging that is most effective but it is also rather plain and functional.

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Another few days of reading several threads and attached links to websites and documents, I have a better understanding of the options available when insulating a solid stone wall, and why I decided to follow the route I have. I am going down the vented cavity route with a timber stud wall insulated with wood fibre. The chapel has stood for 160 years with little/ no heating and the walls are in sound condition, this cavity will act as a similar environment to how it has stood since it was built. The newly constructed insulated stud walls will allow the internal moisture/ air to travel through into the vented cavity. There is a chance of driven rain reaching the internal side of the wall so this is another reason I haven’t fixed insulation directly to the wall, the weather is extreme where the chapel is located, I also require the stud walls to support the first floor joists and didn’t want to fix a wall plate. 
thanks for everyone’s input and direction to other threads.

Ryan 

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On 15/03/2022 at 18:36, RJHumphrey said:

Another few days of reading several threads and attached links to websites and documents, I have a better understanding of the options available when insulating a solid stone wall, and why I decided to follow the route I have. I am going down the vented cavity route with a timber stud wall insulated with wood fibre. The chapel has stood for 160 years with little/ no heating and the walls are in sound condition, this cavity will act as a similar environment to how it has stood since it was built. The newly constructed insulated stud walls will allow the internal moisture/ air to travel through into the vented cavity. There is a chance of driven rain reaching the internal side of the wall so this is another reason I haven’t fixed insulation directly to the wall, the weather is extreme where the chapel is located, I also require the stud walls to support the first floor joists and didn’t want to fix a wall plate. 
thanks for everyone’s input and direction to other threads.

Ryan 

 

Whats the theory behind woodfibre rather than celotex etc, given that its isolated from the main building? Surely thats just more expensive?

 

What constitutes "extreme" regarding the weather? Rain?

Edited by Roger440
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What's the theory behind woodfibre rather than celotex etc, given that its isolated from the main building? Surely thats just more expensive?

What constitutes "extreme" regarding the weather? Rain?

 

Hello Roger,

 

I like this about a forum, where people challenge others decisions/ideas, this is not a sarcastic comment, as this makes me revisit my decision process and potentially stops me making a costly mistake. My understanding  is that a breathable wall makeup regulates internal temperstures, assists with the movement of internal moisture through the wall to the vented cavity, which should prevent mold growth.

 

I have pasted a few snippets from the internet below, I am no weather expert but I have experienced the changeable weather while working on the property, including heavy rain, snow and sunshine all in one day.  no doubt there are stats online for most locations the uk, but the rear of the property is exposed and you can see the driven rain hitting the walls. The chapel is located close to the Sennybridge weather station.

 

Parts of the UK experienced the coldest night of the season on Tuesday, with temperatures of -9.4C (15F) recorded in the village of Sennybridge, Powys,

 

The deepest snow recorded was 30 cm (12in) in Sennybridge, near Brecon, while High Wycombe saw 17 cm.

 

The second named storm of the 2018/19 season brought heavy rain with more than 56.4 mm recorded in Sennybridge, more than half a month's worth of rain.

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