Airtight Taping OSB vs Smartply

[joth]

We're using OSB lining for the internal airtight layer on our renovation & extension. I had thought Smartply Propassiv OSB3 would be the obvious choice but just discovered the spec only called for OSB3 and contractor has quoted a basic price, not for Propassiv. They have good experience getting great airtightness results taping on the Propassiv but never done it with standard OSB3 before.

I'm getting prices for upgrade to Propassiv, which is my gut instinct to go with, but I'm trying to figure out how much risk we're taking on if we did keep with taping standard OSB? 

Has anyone here taken that route (or got references to anyone that has) and achieved good results? (Or horror stories!?)  Thanks

 

[joe90]

I lined my warm roof with osb3 but bubble glued all joints rather than tape, I worry about tape not lasting (probably wrong) and glue is cheaper ?. Air test result was good (any leaks found was not at osb3).

[ProDave]

Mine too was only specced for taped OSB3.  but I decided to line the whole inside with an air tight membrane and used Protekt Barriair.  The decision was swayed by the builders had already anticipated this and used air tight membrane to detail around the joist ends, something that has become known as a "Tony tray" around here.

[MikeGrahamT21]

I read an article suggesting that standard OSB3 wasn't actually airtight, might have been in passive house plus, they put a section on a wall, taped it, and then put a membrane over the top to ensure only air passing through the OSB3 was registered, and they found it to be poor for airtightness.

 

Personally I can't see how it isn't airtight, as there is so much resin in the mix, but clearly this example showed it wasn't.

 

Also see this: http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9361

[joe90]

I remember reading this on the green building forum and people (including myself) talked about painting the OSB with paint/PVA/stuff but.

 

have to consider water vapour permeability too, if you're looking for airtight but vapour-open(ish).

 

And my conclusion was (for my build ) it was airtight enough, when does vapour open and airtight cross over?

[MikeGrahamT21]

Mu Value for OSB3 is said to be around 500 compared with between 50 and 100 for moisture resistant chipboard, so I would say its pretty much vapour tight anyway

[jamieled]

We have taped with osb3 but until we get an air test done it I can't tell you how well it has worked!

[joth]

Thanks all!   My architect also pointed me to page 9 of https://www.passivhaustrust.org.uk/UserFiles/File/Technical Papers/BRE_Passivhaus_Airtightness_Guide.pdf which has some good info on this too.

 

It seems like there is plenty of track record on taping OSB3 out there, but you need to know you're getting good quality material and [even more] attention to the detail in taping it.  A good part of the attraction of Smartply Propassiv to specifiers and installers is giving a clear brand name and quality assurance around what you're getting and how to work it.

 

8 hours ago, jamieled said:

We have taped with osb3 but until we get an air test done it I can't tell you how well it has worked!

Good luck and let us know how you get on :-)

 

Did you have any trouble with getting the tape to adhere, vs any other surfaces? As ours if a retrofit, I'm already concerned about the effort needed in taping around existing joists, so anything that potentially adds additional points of weakness to the joint is my concern.

 

[jamieled]

As we did it ourselves we could take the time to do it carefully. Making sure the surfaces were muck/dust free undoubtedly helped. We used tescon tape which stuck well to the osb and timber. A month or so later I needed to adjust a velux slightly and the tape was pretty difficult to remove, which is reassuring. 

 

In retrospect we had a lot of fiddly taping which could have been designed out earlier on, but that has nothing to do with propassiv vs ordinary osb. Will let you know how the air test goes, hoping it wont be too far away now.

[MikeGrahamT21]

You can always use the primer if you are concerned about adhesion, guaranteed to stick then!