Air Tightness Membrane

[Declan52]

Don't get any on your arms if you are hairy, it's not nice removing it.

[Jeremy Harris]

2 hours ago, Trw144 said:

Yes - managed to stick it to my iPhone and pull it apart trying to get it off! Still, not as bad as dropping my old blackberry into a tin of gloss paint on my last house.

Great tip!  I have a job to do replacing the battery in an iPad, and was not looking forward to prising the case apart, even though I have a set of tools to do it.  I reckon a couple of bits Siga tape stuck either side should be a great help in separating the two halves of the case.

[tonyshouse]

Or heat it up a bit in an oven!

Regarding the tape, how can you tell how long it will stay stuck for? And how will you join wall air tightness to floor and ceiling/roof membrane.

[Trw144]

Well I m hoping it stays stuck on my house for a good few yrs! I have/will tape it to the "tony tray" sat below the first floor and roof joists and to the dpc floor membrane .

[Jeremy Harris]

9 hours ago, tonyshouse said:

Or heat it up a bit in an oven!

Regarding the tape, how can you tell how long it will stay stuck for? And how will you join wall air tightness to floor and ceiling/roof membrane.

There's been a great deal of extended life testing on this stuff, just as there is with all plastic membranes, like DPCs, airtightness membrane, external breather membrane etc.  The stuff lasts many decades, and has been shown to, so is not a problem.  I'd be far more concerned about the durability of other materials in the house, like the PVC insulation around the wiring, than I would about the airtightness tape, as the latter seems to have been the focus of a great deal of accelerated life testing.  Ordinary polythene and PVC membranes are also likely to have a shorter life if they are exposed to movement, as both will become less flexible and more brittle with age.

There are a lot of tapes that last for decades, with no failure of their adhesive at all.  Years ago one of the places I worked at had a very large aluminium sheet ground plane, outdoors, for measuring antenna characteristics.  It had been built at the end of WWII and all the joints between the sheets were sealed with heavy duty self-adhesive aluminium tape.  50 years of exposure to sun, rain, snow and frost hadn't caused any of that adhesive tape to fail, and we still had spare rolls from when it was built that were just as sticky as when they were new.  If we had the technology to make very long-lasting adhesive tapes more than 60 years ago, then I can't see that we've managed to lose it somehow.

 

[Trw144]

Thinking ahead to battening this out, do I attempt to do anything where the battens will be screwed through the membrane or will this have neglible effect and therefore is nt worth worrying about?

[Jeremy Harris]

Our battens are just nailed through the membrane, it seals up around punctures very well, as it's pretty tough and tends to shrink tight around punctured holes, it seems.

[gc100]

On 27/05/2016 at 17:03, Jeremy Harris said:

I would advise caution, because all of the commonly available interstitial condensation risk analysis tools are, as far as I'm aware, non-dynamic.  As you improve the overall insulation level dynamic effects assume greater significance, and these are not predicted well by the normal condensation risk software.

Most of the risk comes from water vapour penetrating from outside, so the idea is to have the VCL at a point where it will ALWAYS be at a temperature that is above the local dew point for any condition, particularly the case where you have a cold night and a cold house (on holiday perhaps) where the structure cools, followed by a warm damp morning that creates a high external humidity level and drives water vapour inwards through the structure.  You have to be absolutely certain that under such conditions there isn't going to be a condition where moisture could form.  Once formed, moisture will take much longer to vaporise and travel back out, because of the relatively high energy needed to drive the phase change from water to vapour.

I've looked at some of the interstitial condensation risk software and all that I've looked at has been sub-optimal (in one case dysfunctional) when used for a higher than normal level of insulation in a  structure. 

My personal view is that the easy and safe way to approach this is to always put the VCL inside ALL the insulation, as this then removes the risk, to all intents and purposes, irrespective of what a bit of software says.

 

My buildup has been specified much the same as the original poster.

I think your comments all have merit. However I really like the idea of the vcl/air tight later sandwiched between the main insulation and a thin layer before the service void, mainly for protection against accidental penetration but also being sandwiched there is going to be less stress on the table glue from movement and therefore less prone to failure over time. I get the point about a cold house and warm damp day outside. However this is only likely maybe a couple of times a year (unless we install AC due to climate change!!!). So even if there was some condensation won’t it still be able able to escape (slowly) so not really a real risk given the occurrence rate?