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Author Topic: Tricky insulation issue on barn conversion  (Read 3153 times)
jtp10000
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« on: January 18, 2018, 07:13:57 PM »

I am going round in circles trying to get a solution to this problem. Any input / ideas would be very gratefully received.
The project is a big barn conversion. We have a dwarf wall around the perimiter with 24 piers that stick in further. On top of the dwarf wall is stud work that will be insulated with quilt. The problem is how to insulate the dwarf wall. Once the screed & floor insulation is down it will only be about 400mm high but I still need to avoid any condensation / damp problems.
There is no DPM under the slab so it will go under the floor insulation. We then need to bring this up the walls so it can be taped to an airtight membrane.
To get any half decent join taped up we need a nice clean DPM edge to tape the intelli pro to. The problem is because the piers (all 24 of them) each with 2 outside and 2 inside corners it will be horrible with all the taping and cutting and inevitably 'sub-optimal'. Additionally still just inside the perimeter wall are 24 new posts for the roof that the DPM will also need to go around (unavoidable). This means we can't batton out the dwarf wall much even if we didn't mind losing the floor space.
My idea was to pad out the recess between the dwarf walls with PIR insulation and then have a nice clean edge wall to run the DPM up. I.e. the main body of insulation would be outside the DPM. We could then run another 25mm of PIR on the inside of the DPM overlapping the higher stud to make sure no obvious cold bridge across the sole plate.
I am told that it is a 'bad idea' to have the 100mm PIR outside the DPM. It would fully fill the void and there would still be a clear run of 25mm on the inside of the DPM.
The blockwork has been damp. But we are on sand, gutters have only just gone on, we have Black-Jack DPMed the whole dwarf wall on the inside, will seal it on the outside and will dig a trench and shingle all around the outside wall so it stays as dry as possible. So is it really a risk of damp coming in? I don't think so.
So then it is only potential condensation that is an issue. We are Vapour controlled on the inside with the air-tight membrane and there shouldn't be a cold bridge, although the piers will only have 25mm of PIR insulation.
Do I have to worry? Given it is only 400mm high?
If i can't do as I hope the only alternative is a lot of fiddly DPM cutting and taping which will have its own problems as well as a hard job getting an edge to bring the airtight membrane taped onto. I can't see which is worse.
Any ideas or advice very gratefully received! Thank you
« Last Edit: January 19, 2018, 07:32:09 AM by jtp10000 » Logged
JohnS
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« Reply #1 on: January 18, 2018, 07:26:08 PM »

I do not have solutions, but a couple of comments.

Firstly:  I am told that it is a 'bad idea' to have the 100mm DPM outside the DPM.  Suggest you edit this to change it to PIR, if that is what you intended.

Secondly:  For the 25 mm PIR inside the membrane, consider using Spacetherm Aerogel from Proctors (Google will find it).  Not cheap but excellent and if it is only 400mm high, not expensive in the big scheme of things.

Thirdly:  we have Black-Jack DPMed the whole dwarf wall on the inside, will seal it on the outside
I cannot help but think it would be a bad idea to seal it on the outside.  How will it dry when the inevitable moisture gets in it?
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offthegridandy
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« Reply #2 on: January 18, 2018, 08:55:13 PM »

Just a couple more thoughts,

Could you perhaps use slightly thinner PIR foam board, and set it to the front face of the piers,  This would leave a void between the brick work and the insulation?

Do you have any kind of membrane/DPC between the top of the wall and the sole plate? If moisture is present in the wall could it get drawn up into the timber?

Andy

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jtp10000
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« Reply #3 on: January 19, 2018, 07:33:06 AM »

I do not have solutions, but a couple of comments.

Firstly:  I am told that it is a 'bad idea' to have the 100mm DPM outside the DPM.  Suggest you edit this to change it to PIR, if that is what you intended.

Secondly:  For the 25 mm PIR inside the membrane, consider using Spacetherm Aerogel from Proctors (Google will find it).  Not cheap but excellent and if it is only 400mm high, not expensive in the big scheme of things.

Thirdly:  we have Black-Jack DPMed the whole dwarf wall on the inside, will seal it on the outside
I cannot help but think it would be a bad idea to seal it on the outside.  How will it dry when the inevitable moisture gets in it?

Corrected thanks
Will look at Spacetherm
And yes I now agree that we don't want to seal on the outside.
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jtp10000
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« Reply #4 on: January 19, 2018, 07:34:01 AM »

Just a couple more thoughts,

Could you perhaps use slightly thinner PIR foam board, and set it to the front face of the piers,  This would leave a void between the brick work and the insulation?

Do you have any kind of membrane/DPC between the top of the wall and the sole plate? If moisture is present in the wall could it get drawn up into the timber?

Andy


Thanks I could leave a gap but it would not really be able to breathe anyway as it is all enclosed, and I would lose insulation.....
Yes there is a DPC on the top of the blockwork so that should be ok.
« Last Edit: January 19, 2018, 08:34:39 AM by jtp10000 » Logged
Warble
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« Reply #5 on: January 19, 2018, 12:55:10 PM »

It is best to have the insulation on the outside of the DPM in a Cold Climate.

It might be better to use XPS instead of PIR if it is going to get damp as it is a bit more durable.

You will need to be careful to avoid condensation at the bottom of the wall as the floor slab and dwarf walls are outside the heated envelope.
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titan
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« Reply #6 on: January 19, 2018, 01:01:57 PM »

You don't mention if there is a damp course in the wall, rising damp may well become an issue since the internal floor will become impervious. Could the wall be rendered on the inside ( and possible outside) with an insulating lime render like Isochanvre. Not comparable with 100m of PUR but a compromise and better for moisture control on the dwarf wall
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knighty
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« Reply #7 on: January 20, 2018, 01:15:11 AM »

dpm the walls first, one long ring around the room, have it run onto the floow about 6 inches - or more, whatever works for you when you're putting it down

the do the floor after, overlap it 6 inches up the walls an seal them together


there's all kinds of ways to join/seal dpm together

normal way is with 'special' tape, but there's other fancy ways - spray on stuff.... which might be good/easy for you to use and work well ?


because the dpm is overlapping and you're going to seal the two bits together you can cut it much easier for the corners - cut the wall one way and the floor another so the cuts don't overlap... if it goes wrong and you don't like abit of it... seal another bit in front/behind it
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TT
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« Reply #8 on: January 20, 2018, 09:32:18 AM »

dpm the walls first, one long ring around the room, have it run onto the floow about 6 inches - or more, whatever works for you when you're putting it down

the do the floor after, overlap it 6 inches up the walls an seal them together


there's all kinds of ways to join/seal dpm together

normal way is with 'special' tape, but there's other fancy ways - spray on stuff.... which might be good/easy for you to use and work well ?


because the dpm is overlapping and you're going to seal the two bits together you can cut it much easier for the corners - cut the wall one way and the floor another so the cuts don't overlap... if it goes wrong and you don't like abit of it... seal another bit in front/behind it


But the part thatís tied into the room  ie wrapped around it would need to be tied into the wall dpl
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knighty
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« Reply #9 on: January 20, 2018, 06:26:30 PM »

as long as it overlaps the wall dpm/barrier on the inside then there's no need to seal it up

any moisture/condensation will run down the wall behind the dpm


just checking I have this right

concrete floor, no dpm
400mm premiteter wall
stud work ontop
air tight barrier inside stud wall

what's the ground hight outside like ?


the dpm you're putting under the floor insulation will need to run up the walls a little anyway (and a safe amount above the outside ground height)  - so that no moisture coming in from outside can get above the height of the dpm and then run/pool on top of it  (and soak your insulation ruining it)


easiest way I can think is dmp around the wall, dpm on the floor, join them

at least then you can cut it in the corners (the floor and the wall dpm)
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jtp10000
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« Reply #10 on: January 22, 2018, 02:03:15 PM »

Thanks everyone.
We have DPM'ed up the dwarf wall and fitted insulation into the recesses, there is a DPC under the studwork on top.
It wasn't such a big deal in the end but the one thing we did spot is that the vapour barrier was originally only going to run to the top of the stud wall and join the DPM. This would have meant the insulation on the dwarf wall was inside the vapour barrier. This was clearly wrong so now the vapour barrier will run down to the screed level and be sealed to the floor there.
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TT
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« Reply #11 on: January 22, 2018, 08:24:42 PM »

Outside
Wall construction
Insulation
Vapour barrier
Plasterboard

Is this how your vapour barrier is fitted as it should be on the warm side
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gb484
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« Reply #12 on: February 25, 2018, 10:53:02 AM »

Coul you use hempcrete?
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