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Our structural engineer is insisting on me skinning both sides of our lateral walls with OSB for racking strength. It struck me that if I could make the OSB on the inside air tight I would not need to cover it with a membrane. The challenge is making OSB air tight and I wonder if just painting it, either flat before installing it or once installed, with a thick PVA mix, or some such, would do the job. Any thoughts?

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Smartply is what I lined my warm roof with, all joints glued with expanding glue, yet to do a house pressure test but I am very confident it will perform well. I looked into this (on another forum) a few years ago and some there held the view that OSB was “draft proof” but provided a breathable boundary that was “healthy” for the house.

Edited by joe90

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Installed with an expanding Polyurethane glue, OSB is air tight to well over the pressure used in air testing - make a box of offcuts and  attach a hoover to it and you’ll see what I mean ..! 

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9 hours ago, dimpsy said:

Sadly they don't make it in the thickness I require (15mm).

 

8 hours ago, PeterW said:

Installed with an expanding Polyurethane glue, OSB is air tight to well over the pressure used in air testing - make a box of offcuts and  attach a hoover to it and you’ll see what I mean ..! 

You would think so but the scuttlebutt is that it occasionally has porous sections which would be solved by a coating - which is, I believe, what smartply does - well they talk about a coating doing the vapour control in their propassiv technical data sheet.

Edited by MikeSharp01

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I think there's some confusion here between "airtight" and "vapour tight".

 

OSB is, to all intents and purpose, more than adequately airtight as far as a building air test is concerned.  It won't let air through at a rate that would make a jot of difference to the house air tightness in practice.

 

OSB is vapour permeable though, so will allow water vapour to move through it.  As such, OSB is not suitable as a vapour control layer - for that you need to use one of the boards made specifically for vapour control, such as Spano DURÉLIS VapourBlock or SmartPly, or similar.  Our build uses Spano DURÉLIS VapourBlock as the inner skin and OSB as the outer skin for this reason.

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Jeremy, do you have a separate internal membrane in your build-up? I could be wrong, but I thought MBC went through a period of using taped OSB (or perhaps it was VapourBlock or similar) without a membrane. Whatever it was, I believe they found that it wasn't reliably airtight enough, so went back to using a membrane not long before doing our frame.

 

I also read a discussion in (I think) an Irish LinkedIn post about OSB around the same time, where it was concluded that it definitely could not be relied upon for airtightness by itself. I seem to recall that the conclusion was you'd probably be okay, but there were no guarantees.

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2 minutes ago, jack said:

Jeremy, do you have a separate internal membrane in your build-up? I could be wrong, but I thought MBC went through a period of using taped OSB (or perhaps it was VapourBlock or similar) without a membrane. Whatever it was, I believe they found that it wasn't reliably airtight enough, so went back to using a membrane not long before doing our frame.

 

I also read a discussion in (I think) an Irish LinkedIn post about OSB around the same time, where it was concluded that it definitely could not be relied upon for airtightness by itself. I seem to recall that the conclusion was you'd probably be okay, but there were no guarantees.

 

 

No, just the VapourBlock boards with taped joints, but that isn't the airtight layer it's the vapour control layer.  There's confusion in this thread about the two terms;they are different in practice.

 

The idea is to have graded vapour permeability, so that the innermost layer is the least vapour permeable and the outermost layer is most vapour permeable.  Airtightness is a slightly separate issue, as you want the outermost layer to be airtight enough to not cause "wind wash" through the insulation, but not to be vapour tight.

 

In practice, much of the outer airtightness in a pumped cellulose frame comes from the cellulose itself, which in a thick layer that fills every nook and cranny forms a reasonably effective airtightness barrier.  The OSB outer skin helps airtightness a lot, too, as does the protective breather membrane.

 

There have been passive houses built with OSB as the airtightness and vapour control layer, with the edges sealed, and it apparently works OK if you get the design right, so the vapour gradient is from inside to out.  You can do away with the internal vapour control layer if you make sure that the vapour permeability gradient is such that vapour will always want to move outwards over a reasonably long time period, and one house I read of did that by using thicker, denser, OSB with sealed joints as the inner skin and thinner OSB, or something like Panel Vent board, as the outer skin.

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18 minutes ago, JSHarris said:

There's confusion in this thread about the two terms;they are different in practice.

 

I don't think there's confusion, but I agree it helps to have the different requirements set out explicitly.

 

19 minutes ago, JSHarris said:

In practice, much of the outer airtightness in a pumped cellulose frame comes from the cellulose itself, which in a thick layer that fills every nook and cranny forms a reasonably effective airtightness barrier.  

 

In general that may be true, but I'm less convinced about relying on it in areas that are less easy to access. The two areas I've had cause to open in our house (due to the roof leaks and while installing trims for our balcony fibreglass) have both had voids in the cellulose. In one case - and admittedly it's in a really awkward bit of wall - there was a void perhaps 100mm deep, most of the width of the wall, and stretching along nearly half of the width of the window under the cavity closer. Worse, the cavity closer had to be adjusted at short notice when the windows were being installed, so only had a very thin bit of OSB that definitely didn't seal very well to the frame in the area outside the window.

 

Overall, if this region was relying on cellulose for airtightness, it was in trouble! I stuffed the area with some leftover rockwool I had lying around and sealed it up carefully before covering it with the fibreglass trim.

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47 minutes ago, jack said:

The two areas I've had cause to open in our house (due to the roof leaks and while installing trims for our balcony fibreglass) have both had voids in the cellulose.

 

Oh, that's concerning. Very concerning. It makes me wonder if: (i) there is best practice when blowing-in cellulose insulation to minimise voids; and (ii) there can be any post-installation checks to spot them.

Edited by Dreadnaught

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Usually the pressure on the fill hose should be a good indication that the cellulose has penetrated and filled every nook and cranny, but that depends on there being enough fill holes drilled at a close enough spacing.  I remember chatting to the guys pumping our cellulose in and they reckoned that the cellulose seemed to flow a lot further than the distance between fill holes, but I'd guess that there is always the possibility that a particularly awkward area may not get filled completely, or that the installers could fail to spot that they needed to drill another fill hole in a critical area.

 

The four holes I've had to cut through our walls were all very densely filled, and one of those was right in the top corner of a wall:

 

252433450_Holeininnerwall-cellulose.thumb.JPG.8b0d448490aaac3e0629a91032ab3e28.JPG

 

This was awkward to cut out as it was right in a tight corner (hence the very ragged jigsaw hole) but it does show that the cellulose is normally very densely packed into the space, so densely packed that even the jigsaw blade hardly disturbed it and just cut it as if it were solid.

 

This is where the fresh air intake duct for our MVHR goes through the wall - it comes out right under the eaves on the outside of the house.  It was an afterthought, as I changed my mind about the type of MVHR we decided to fit so couldn't put the ducts through the wall before the walls were filled, which would have been the better option.

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Looking at @vivienz MBC house in 

 it looks like they have the Smartply passive boards plus tape on the inside.

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