dnoble

Airtightness and first fix

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I've encountered a bit of confusion around this. MBC smartply lined house, intended passive house standard (not certified)

The airtightness team are coming next month to airtight-it and blow in insulation

 

I've had an electrician doing first fix mains + lighting circuits including some wires which are fixed to inside of external walls in a ring main. Tacked on with normal cable clips.

 

Apparently this is not supposed to happen until after the airtightness (as it may compromise the test)

It's not clear if it's OK to attach the wires in this way afterwards, or if not how they would be attached.

They go in a void created by 25mm battens (which are also nailed on) and then plasterboard on top.

 

I wouldn't have thought a small hole with a nail tight in it, which may not even penetrate the board would make a major air leak would it? Especially if there will be blown cellulose packing the inside void plus plasterboard and plaster over the top.

 

The person I spoke to at MBC was sure I shouldn't have had them attached thus but didn't have an obvious alternative strategy. Did suggest attaching them to the battens, though they aren't all orientated in the right way and wiring afterward mean drilling loads of holes through the width of the battens.

 

Any ideas anyone?

 

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Ideally, you should avoid attaching anything to the  Smartply. If you do penetrate the Smartply with pins or screws or accidentally, then you need to seal the penetrations, however small, with tape or sealant. Any such penetrations will make a difference to the integrity of the Smartply for both vapour control and air tightness. 

 

All cables  (and pipes)should be attached to the battens. All  back boxes should be  fixed to noggins that are either glued to the Smartply or fixed to the battens - not screwed to the Smartply. . One of the jobs that should be tackled first in 1ST Fix and before the airtest and cellulose filling,  should be any  conduit penetrations from internal/external that you have planned for external lighting, satellite dishes, alarms, CCTV, etc etc.. It is  very difficult to make these penetrations and ensure good seals AFTER the cellulose has been installed. It is  useful to have these penetrations installed, sealed  and tested as part of the airtest [

 

I've attached some photos of my 1st Fix to give you an idea. MBC didn't use Smartply on my house but a less robust system using  SIGA  vapor control layer membranes/tapes over OSB that required more work and superseded /improved with the use of Smartply.  

IMG_1558.jpg

IMG_1585.jpg

IMG_1541.jpg

Edited by HerbJ

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There's nothing to say that you can't attach stuff this way, just not before MBC have done their airtight work and test. It's your house and once MBC have fulfilled their contractual obligations, you can do as you please with it.  That said, putting holes into something that's just cost a lot of money to make airtight doesn't make sense to me.  On the smartply board, it's the green coating on the board that makes it airtight, not the board itself and so if you've gone through this, you've compromised the airtight quality of the board.

 

I'm at the same stage as you with MBC due back on 5th November to do all the airtight work and they've been very clear all along that first fix shouldn't start until after this so that if there is an issue, it's clear whether it's down to MBC, and they have to fix it, or a third party, in which case they don't have to fix it.

 

I would be inclined to remove any nails/clips/screws/board penetrations and see if there's some sort of temporary support that you can give them, then seal all the holes that have been created so at least you won't get any arguments if the airtest gives a substandard result and it's unclear who's fault it is - at least it won't be yours.

 

 

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Thanks for these replies

I see HerbJ you've attached the electrical boxes to horizontal noggins between the battens

Does this not cause them to stick out too much from the plasterboard, or did you need especially shallow ones?

 

The battening is nailed to the OSB so doesn't this compromise the inner skin in exactly the same way as nails in the cable clips?

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Mine was like a sieve when they came to do the air test mainly down to the windows not being fitted properly.  We were under pressure to get the air test done so mbc could get finished and paid.  All they did to resolve the many leaks in my building was put tape over the gaps and tell me I should get it sorted..........it was many many months later before I was able to resolve the window issue and  get the place airtight, had a fab cert though.  The air test people are not programmed for ‘failure’ you will get your cert to the contracted level or better if you have holes/gaps they will temp tape them to get the result and then you will need to sort it out properly afterwards.

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Good point about the battens being nailed through ply. Is this not the same?

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24 minutes ago, dnoble said:

The battening is nailed to the OSB so doesn't this compromise the inner skin in exactly the same way as nails in the cable clips?

Ideally anything going through an airtight layer is screwed and they won't loosen over time. In your case if the battens were screwed to the structure through the OSB that would be better.

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18 minutes ago, dnoble said:

The battening is nailed to the OSB so doesn't this compromise the inner skin in exactly the same way as nails in the cable clips

 

8 minutes ago, Oz07 said:

Good point about the battens being nailed through ply. Is this not the same?

The battens are nailed through the Smartply or in my Vapour Control membrane/OSB,  but all battens  are only positioned at the main structural frame lines so even then penetrate the Smartply they are nailed into the main frame  supporting it and this effectively seals the nail penetrations. The problem arises when you puncture the Smartply by itself...

 

24 minutes ago, dnoble said:

I see HerbJ you've attached the electrical boxes to horizontal noggins between the battens

Does this not cause them to stick out too much from the plasterboard, or did you need especially shallow ones?

 

The battens have been installed with recesses as necessary to ensure the boxes did  not protrude through the plasterboard - you  have a depth of 50mm (batten   +12.5 (plasterboard) = 62.5 mm depth to install the boxes plus the noggin, recessed as necessary.

 

For clarity, the only 1st Fix items we completed before auirtest was the penetrations ( conduit or MVHR Ducting) required for external equipment  and these penetrations were pemporarily sealed for the airtest by MBC.

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FWIW, it's vapour tightness that is far more critical than airtightness, as the very last thing you want is water vapour from inside the warm house migrating through the VCL and into the colder timber structure, where it may condense and cause rot.

 

We ran all cables and pipes that went down outside walls alongside a counterbatten, with none fastened to the VCL board (ours used Spano Durelis, rather than Smart Ply, but it also has a green vapour tight coating).  Where back boxes has to be fixed to the board, I glued pieces of thin ply to it with quick setting PU adhesive and also applied sealant behind every back box around the screws (easy to do, just blob sealant on the holes and screw through it).  This was all down after the air test, but frankly a few screw holes on the board aren't going to make a blind bit of difference to airtightness, what they may do is let water vapour migrate out over a period of years, though, which isn't at all a good thing.

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Thanks for that perspective Jeremy

I suppose it might be worth levering out the cable clips and putting a blob of sealant in the hole and nailing them back in to try and  re-establish airtightness/vapour-tightness? The nails aren't actually long enough to penetrate the full thickness of the board (though I understand it is the green inner skin that provides the barrier)

 

Having said this where the green OSB has been nailgunned to the underlying frame by MBC there are numerous breaches and cracks in the green skin. None of the nailheads appear to have an airtight join with the green surface, often where the nail has gone deeper than the surface.

Edited by dnoble

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It's debatable as to whether it's worth the trouble of pulling things out and putting sealant in, TBH.  In our case all the nail heads where the boards were fixed to the frame were covered with airtightness tape, but our lining is 12mm Spano Durelis, rather than Smart Ply.  At a guess I'd say that Smart Ply may well be more tolerant of nails going though it.

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I thought there was tape you could put between the battons and the smartply that sealed the holes from the nails / screws.  Doesn't help with the cable clips though. Also notice in one of the previous photos there was a horizontal cable run halfway between bottom of window and floor to the socket.  Surely this wouldnt pass muster?

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50 minutes ago, redtop said:

Also notice in one of the previous photos there was a horizontal cable run halfway between bottom of window and floor to the socket.  Surely this wouldnt pass muster?

That cable running horizontally from the socket under the window is in the safe zone created by that socket.  It then turns upwards and runs up alongside the window to a 1G back box, so is in the safe zone created by that accessory.  Nothing wrong with that at all.

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11 minutes ago, ProDave said:

That cable running horizontally from the socket under the window is in the safe zone created by that socket.  It then turns upwards and runs up alongside the window to a 1G back box, so is in the safe zone created by that accessory.  Nothing wrong with that at all.

ah got ya.  not fully up to speed with the safe zones 🙂 guess its something to do with being unable to run the cable vertically due to window.  obvious really lol

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7 minutes ago, redtop said:

ah got ya.  not fully up to speed with the safe zones 🙂 guess its something to do with being unable to run the cable vertically due to window.  obvious really lol

Safe zones run vertically and horizontally from every accessory.

 

All my sockets are wired horizontally around the room from socket to socket. As long as there is one socket on each wall you can go right round the whole room.  Leave a little bit of slack cable in each leg, and at any time in the future you can cut another hole in the plasterboard and add an extra socket anywhere you want to.

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2 minutes ago, ProDave said:

Safe zones run vertically and horizontally from every accessory.

 

All my sockets are wired horizontally around the room from socket to socket. As long as there is one socket on each wall you can go right round the whole room.  Leave a little bit of slack cable in each leg, and at any time in the future you can cut another hole in the plasterboard and add an extra socket anywhere you want to.

every day is a learning day 🙂 is that how they usually do it? any idea on how you avoid making holes in the VPL when fixing vertical cable clips (we will be using propassiv OSB)

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2 minutes ago, redtop said:

every day is a learning day 🙂 is that how they usually do it? any idea on how you avoid making holes in the VPL when fixing vertical cable clips (we will be using propassiv OSB)

It is how I usually do it for the reason already stated.  Not everyone agrees.  One build I did, the joiner building the house just would not accept I could do that, to the point we had a stand up finger wagging argument about it. I don't normally get worked up, be he was basically trying to physically stop me drilling the studs to run my cables through.

 

You avoid penetrating the VPL by nailing the cable clips to the sides of the battens.

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Aye, we've the same passive wall with airtightness on Monday. We're planning to do any external penetrations immediately after the airtightness test but before the cellulose gets blown in. We've been told it can be done after blowing it in, but I believe doing it inbetween just makes the most sense.

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22 hours ago, Visti said:

Aye, we've the same passive wall with airtightness on Monday. We're planning to do any external penetrations immediately after the airtightness test but before the cellulose gets blown in. We've been told it can be done after blowing it in, but I believe doing it inbetween just makes the most sense.

and i would think if you get a good result it will improve once the cellulose goes in.

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