Air Tightness of Ply

[Ruairi Kay]

Quick question on ply:

How thick does WBP ply need to be to be airtight? I am currently working on a project where we are laying ply over an existing suspended floor to create a new air tight layer and the contractor wants to use 3mm ply, I don't think it will be airtight or robust enough for that matter.

R

[Nick Grant]

Hi Ruairi

Spotted your question on Twitter!

Used 9mm WBP on our house and am pretty confident that is not where any of our air leaks. I expect ply better than OSB for given thickness but i think 15mm OSB often quoted as min for OSB (sure this has come up before on forum).

Suggest good tape for joints, ie ProClima or Siga.

However as it is a floor and coming fresh out of CPD with Niall from Ecological Building Systems, I'd suggest getting some advice re moisture movement. Refurb is not my expertise but good to check with someone who can advise. My approach is to go for air and vapour tight on warm side and vapour open on outside with no rising damp – possible on new build but I'd worry about your joist ends.

Interesting to see what Niall says.

Nick

[Mark Siddall]

Glad a soltion has been reached. Canadian experience is that 15mm is acceptable, 9mm is less ideal but could do the job. Germans use 18mm OSB successfully in PH's (it seems that they don't tend to spec thinner due to cultural attitudes). Joints are the major concern – therefore, as NIck suggests, good taping is critical.

In this detail location, timber floors, moisture is a real concern.

M

[Tom Foster]

Joints are the major concern – therefore, as NIck suggests, good taping is critical.

Any reason why gapfilling glue-and-screw, all edges supported on a shared timber (or dryliner's flatstrap or angle), wouldn't do as well as tape, short-term? (sure it would do better long-term).

[Mark Siddall]

Quality assurance and remediation. How do you sp;ot gaps in glue joint? How do you remediate for long term fix?
Unless you have all the tools for undertaking microscopy within the joints and syringes to remediate then you'll have a tough time.

Mark

[Tom Foster]

That's exactly the same problem as with sticky tapes. Visual inspection has exactly the same limitations in both cases. Or is this something that there's actually evidence for? For example is leakiness a known problem with plywood boatbuilding? Anyway, if leaks are located later then the remedy wd be the same – more sticky tape!

[Anonymous]

For example is leakiness a known problem with plywood boatbuilding?

That's probably not a good example. Plywood boats had a very poor reputation until the advent of resins and especially epoxy. The wood provides various aspects of strength but it's the resin that provides the watertightness. The resin covers the entire inside and outside surface of the structure, not just the joints. And pinholes anywhere are a problem.

n.b. that doesn't say anything about the merits of tape versus glue for building airtightness.

[Mark Siddall]

Visual inspection has exactly the same limitations in both cases.

Errr….no it doesn't. At least not if you do the pressure test at the right time i.e. when the air barrier system is exposed. Tapes are on the surfaces and can be seen. The glue joint can not be accesses in the same manner esp. with t&g boarding. In the case of glue with tape remediation the two gaps will not be coplanar (this is particularly likely to occur at corners). As a consequence the leak can not actually be mitigated (though it may be reduced).

Mark

[Tom Foster]

Fair enough, Dave and Mark. We shall see, shortly.

[Nick Grant]

Coplanar, I like that!!

[Tom Foster]

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6184&page=1#Item_8

[Author]

Posts