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Author Topic: C.R.A.C  (Read 142283 times)
Tiff
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« Reply #480 on: July 26, 2020, 06:50:54 PM »

I have to submit a full plans application rather than a building notice as we are building too close to a public sewer, that means we need a build over notice and hence an FPA. All in all it means yet another delay facepalm

I reckon shifting bits of paper will have taken longer than it takes for us to build it, now theres a challenge.........

Well at least the challenge has got a lot easier, we're still faffing about with bits of paper bike The build over notice came through after a couple of weeks, Thames water were pretty prompt which isn't surprising considering that we are building a bit over 1.5m from the public sewer so hardly about to cause a collapse or anything. What has been more difficult is getting the structural engineering done while the Covid restrictions are in place. Understandably people are reluctant to visit which has led to some small errors, nothing catastrophic but time consuming nonetheless. It'll all get sorted soon fingers crossed!

Another thing  baffling me is trying to justify the insulation. We are building a 2 storey timber frame extension up the side of a semi detached 30s solid wall house. I wanted to go with a timber frame to save a bit of space but still have a high level of insulation. I was going to fully fill the studwork with rockwool and then 50 mm of EPS EWI system which could carry on over the existing block wall we are building next to. 

BC has said that if you fix EWI to a timber frame you should leave the studwork empty or you risk interstitial condensation, but my argument is that provided we have a good VCL on the hot side of the insulation and good vapour permiability on the cold side why would the situation be any different to sticking EPS on a solid brick wall?  They say it will probably be ok, so can you justify it by calculation?

And there is the rub, I can't, or rather I don't know how to. I know that to control the condensation within wall structures the vapour resistance  should be at least 5 times more on the warm side than the resistance on the cold side of the insulation. Can anyone point me to the maths or methods needed to justify this stuff?

Mart; take note greening up an existing building is bloomin hard, I'm wishing I took my advice and knocked the place down and started from scratch now surrender

Desp

I can't help with the calculation and I agree with your theory, however....

Just be aware that even the tiniest air leak in the VCL will allow a considerable amount of condensation to form on the cold side. Not such a big deal with brick but on a timber frame perhaps an issue - in 10-20 years time.

I have heard plenty of tales of VCL being broken for many reasons, mainly someone coming along at a later date and making holes for cables/services. I have also heard that some tapes used for sealing the VCL have adhesive that does not age well.

I built a timber framed outbuilding with an internal VCL and the insulation the other side of it. It's timber clad so breathes quite well on the cold side, so far so good after 10 years. I'll report back in another 10 years.
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desperate
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« Reply #481 on: July 26, 2020, 09:32:37 PM »

Thanks for the replies,

 Tod there do seem to be plenty of calculators for various build-ups but they are almost always for U calcs or sound proofing purposes, I've not found one for vapour control or thermal gradients through the elements as yet, I will try a couple of suppliers I use and report back.

Tiff, maybe I should use foil backed board when we close up over VCL, as you say we don't want mushrooms growing out of the skirting in ten years. We normally lap the VCL over a stud to minimise our reliance on the adhesives but as you say we will have to take care.

Desp

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A.L.
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« Reply #482 on: July 26, 2020, 10:13:24 PM »

hello,

I have built a wall from your description,  interstitial condensation should not be a problem, see hopefully below.








I have not included the basecoat or finish coat render. A silicon top coat should not cause a condensation problem.  Also basecoats for EPS EWI systems claim to be breathable but I would want figures to be sure


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desperate
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« Reply #483 on: July 27, 2020, 08:11:07 PM »

Thanks A.L.

that's very kind of you to show this example, our build up though doesn't have the block inner leaf, just the 150*50 stud with infill rockwool and 50mm EPS on the outside. Where do you get your vapour pressure figures from?

Desp
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A.L.
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« Reply #484 on: July 27, 2020, 09:21:44 PM »

hello again,

here is a modified build up, no significant change.










These outputs are part of a software package that I used to use. Unfortunately it is no longer publically avaialable.

Here is something that works to the same standards and is available as a 30-day free trial

http://www.builddesk.co.uk/software/builddesk-u/condensation-risk/

This should be an alternative but is going to take some time to learn

https://www.ubakus.de/u-wert-rechner/index.php?
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« Reply #485 on: July 28, 2020, 09:31:19 PM »

Wow thanks A.L. thats really helpful, I might give the thirty day trial a go as we are doing a few more timber frame buildings now home offices are on the increase.

Desp
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« Reply #486 on: August 01, 2020, 08:27:17 PM »

G'day possums,

Well if anyone out there is still interested here is yet another instalment in the saga of Cactusville.

We are still batting details back and forth with out LABC to sort out the engineering although we are now down to the finer details ..........I hope  fingers crossed!  This has been made frustratingly difficult by trying to work at a  Covid safe distance, a few errors crept in here and there that don't get noticed as soon as might be hoped as well as the fact that a lofty on top of a semi with a full hipped roof is awkward to say the least.

The groundworks is pretty simple if a lot of heavy sweat, so this week we dug out all the old drains and crossed our legs for a day facepalm  A nice shiny new access chamber base was connected to the drain pretty much on our boundary and then two new runs installed to connect up the existing stack which will serve the 1st floor bathroom and the lofty shower room. Another run was taken to the downstairs bog, both these runs were in pretty much the same position as the existing, so in breaking out the old we had trenches for the new, but now we dont have a chamber under what will be a new living room. To comply with regs all access has to be outside.

 Next we turn our attention to the footings, sadly the existing garage footing is not really up to the job being only about 50 cm below ground  and 45cm wide. Even though it was only 25-30 cm thick it was too much for my Kango to break up in a sensible timescale, so off we toddled to hire a 29kg Bosch.  "Nah we don't have any of them" says the chap at the hire shop, "but we do have a Hilti version". OK we say , now I'm not exactly a spindly little wimp but this thing needed Samson to lift it into the van and when we get it home it turns out to be a real challenge to control and not topple into our newly built drains. It was mighty impressive at breaking stuff, you could knock planet sized lumps of rock off the surface of a neutron star with it  and within a couple of hours the old footings were reduced to rubble. It still took a couple of days to get it all out the ground and piled up ready for the grab to shift, and that is where we are at now. Hopefully monday the LABC are happy with the excavations and we can plop a bunch of concrete down them on tuesday, then it is on with my brickies hat.

Some piccys will follow as soon as I can work out how to connect  this new computer and my camera.

More soon,

Desp
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todthedog
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« Reply #487 on: August 02, 2020, 05:49:49 AM »

Good  on you Desp, great that you are underway and  making progress groundwork was the bit of any project that I was pleased to see the back  of  fingers crossed!
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« Reply #488 on: August 02, 2020, 08:03:32 PM »

Absolutely Tod, it is great being able to finally straighten the back and start going up.

Desp
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