Air Tightness Membrane

[Trw144]

This was a subject bought up on the old forum...

My timber frame/insulation is 140mm frame, followed by an additional 30mm pir, battens for service void and plasterboard. My question is, whilst I know the general consensus was for the air tight membrane on the inside of the additional 30mm, would it be that bad to put it between the insulation?

The person adding the insulation for me cant do it for a few weeks, after which I have a window of opportunity for first fixes, but it does nt really give me time for adding and taping up the air tight membrane. My additional thoughts are that in places I need more space than the normal void will give me and I can simply take back the 30mm insulation in these places without worrying about the air tightness, additionally the membrane will be protected my the insulation when first fixes etc are taking place and is less likely to be damaged.

 

[Crofter]

Sounds like you have the same wall buildup as me (although my PIR layer is 50mm) so I'll be interested to hear people's views on this one.

Is your airtightness layer also your vapour barrier?

 

You might have the option of taping the joints in foil faced PIR and use that as the barrier, instead of a membrane.

[Calvinmiddle]

I have the same wall I think on my MBC build.

 

its a 140mm frame filled with 150mm of rockwool, then the airtight membrane then 50mm kingspan held tight on with 35mm battens.

This is a buildup offered by MBC and I like the fact the membrane is protected by the kingspan

[Stones]

One of my friends did a low energy revamp of his 1980's bungalow, stripping it back to the frame, which he built out, re-insulated with rockwool, air tight membrane then 50mm kingspan, battens then plasterboard. Has worked very well for him.

[Trw144]

I like the idea of a separate membrane as opposed to taping the insulation and as my car boot currently looks like this, I m using it!!....

Edited May 25, 2016 by Trw144

[Trw144]

Ok, sounds like I can go for it and gives me the weekend and next week to work on it.....

[tonyshouse]

A good plan welt the joins and trap them under a batten, be sure to join to floor and ceiling air tight layers and to window/door frames

[Trw144]

Can I also recommend Passive House Sytems for the supply of tapes and membranes at a good price (gerband). Also, fantastic service - I ordered some for delivery last Friday and they the carrier failed to deliver it. As I was desperate for it for the weekend, the guy at passive house Sytems arranged for someone to travel 1 1/2 on the Saturday to deliver it to me. Life saver

[Declan52]

I used gerband on my build and was well impressed with the quality of both the tapes and membranes. Price wise they came in at half the price of the proclima brand.

[Crofter]

So, membrane before PIR stops the membrane being exposed to damage.

But PIR before membrane allows you to fix the membrane down with battens, which also form your service void.

Does that sound about right? If so I think I will stick with PIR before membrane.

[Calvinmiddle]

No think you misunderstand me.

on my build its:

membrane

PIR

battens

The battens hold the PIR tight to the membrane and the membrane tight to the studs of the timber frame, and also gives you the service cavity.

So membrane is protected and you have a service cavity 

 

[Crofter]

Yes that's what I thought. But presumably means you have to tape or otherwise secure the membrane, as the battens are not directly onto it?

[Rattyjohn]

Just to muddy the waters somewhat, if I recall correctly there was a thread about this on the old forum which I took a great interest in. The drawings for my timber frame build-up (outside-in) show the 140mm timber frame insulated inside and sheathed both sides with 9mm OSB, then the membrane, PIR, service void battens and plasterboard. My instinct was that this was wrong and to put the membrane as far to the inside as possible (ie between the PIR and service void battens) to keep all of the insulation on the outside of the membrane. This was borne out in the thread by a far greater mind than mine (jsharris I think) who cited the risk of interstitial condensation if the membrane was between two layers of insulation, as shown on my drawings. I didn't realise this when I queried the drawings - it just seemed odd to have insulation on the inner side of the VCL.

[Trw144]

It you look at page 9 on this link, this is how Trada show it (different construction I know)....

https://www.trada.co.uk/downloads/publications/Timberframeconstruction.pdf

Can this risk be modelled by someone to see if it is ever likely to happen?

Edited May 27, 2016 by Trw144

[CC45]

Hi all,

 

figure 3.3 is how I'm planning on doing ours.  Why don't you all want to put an insulated service void in?  Obviously where elec runs I will leave the insulation out.  Will put 34mm battons in & then 30mm celotex.  Saves space.

 

cc 

[Trw144]

The issue is about the air tight membrane and whether it can be between the timber frame insulation and the additional insulation, or needs to be on the inside of all the insulation. I believe I read somewhere that it was ok provided the internal insulation was no more than 30% of the total - not sure where I read this or if it is accurate

[Crofter]

It's just occurred to me that I will have to stick to the airtight layer being on the inside of all the insulation, because that's the only way that I can make the layer continuous with the flooring.

[Trw144]

For reference, I just called celotex and they have done a comdensation risk analysis that shows it passing with the vcl between the insulation. The lady said the lines are somewhat off each other and therefore it would be pretty safe going for this route.

[Jeremy Harris]

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.

 

[Guest Alphonsox]

Not sure I follow your argument Jeremy. If I have a cold dry house and a warm damp morning then moisture will be driven inwards until it meets something cold enough to condense on or is stopped by a barrier, worst case it condenses on the back of the VCL itself. Surely this scenario would suggest the VCL barrier should be on the outside of the structure not the inside. (Of course the situation reverses itself for a warm steam filled kitchen on a cold day.)

I'm clearly missing something here.

[Jeremy Harris]

The argument hinges on the fact that with no heating at all the inside of the house won't get cold, it will end up being around the average of the day/night temperatures, probably a bit above that because of solar gain.

This means that it's extremely unlikely that the VCL on the inner face of the house will ever get below the dew point of the vapour concentration that penetrates the structure.  If I were to take a guess, then I'd say a well-insulated and airtight house, with no occupants and in an average UK location won't ever get below 12 to 15 deg inside, yet it could easily be a lot colder than that part-way through the insulation.

Again, it's not something that standard interstitial condensation risk software is that good at, as homes with such high insulation levels and low air leakage rates are not common enough for the "standards" to have caught up yet.

[Trw144]

Well, today I've started adding the additional insulation - it was not a job I planned on doing myself but in all honesty, it isn't too bad as the insulation is only 30mm and seems easiest to cut with a half decent kitchen knife. Managed to do about half of upstairs in a few hours his morning, sorting out the overlaps on the airtight membrane at the same time.

 

This also means the vcl/airtightness membrane will on the inside of all insulation.

Edited May 28, 2016 by Trw144

[Trw144]

Jeremy, you will be pleased to see the membrane on the inner face of all insulation.....

 

 

[Declan52]

That green tape is sticky as. Once stuck its pretty much stuck for life.

[Trw144]

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.