Airtightness - Permeable or Impermeable Vapour Barriers?

[Matthew Thompson]

We're heavily insulating our barn conversion and making it wind and airtight. Before making a decision on the internal vapour barrier I wanted to find out as much as I could about the different products available, at the moment the three 'brands' that I am focusing on are:-

- Pro Clima
- Siga
- DAFA

My main quandary is do we go for an 'intelligent membrane', permeable or impermeable membrane?

We're having to use PIR insulation to keep the wall build up to a minimum, build up will be:-
exg. wall, c.25mm cavity, PIR insulation, insulation between timber stud, air tight layer, service void, plasterboard.

Any advice would be greatly appreciated.

In addition, pro clima and DAFA use grommets for pipes etc penetrating the air tight layer whereas SIGA use tape, is there any research/opinion on which is the best option?

[Mark Siddall]

Hi Matthew,
I've not come across DAFA before. The other two have been used successfully in Passivhaus projects. As you are internally insulating a barn I would tend to think that you want a membrane that is 'intelligent membrane' / permeable but not impermeable. Also have a look at this thread: http://www.aecb.net/forum/index.php/topic,3601.msg13102.html#new Alan makes some good points.

Mark

[Matthew Thompson]

Thank you for your reply Mark and for the link to the other thread.

As we are proposing to install PIR insulation to minimise the loss of floor area, will an intelligent membrane be of much benefit due to the insulation being relatively impermeable itself?

[Mark Siddall]

I imagine that calcs will try to tell you that it will be okay. In reality it all depends upon how  airtight and windtight you manage to get the faces of the insulation.  It the joints allow air movement then moisture can be transported via convection (rather than diffusion). Reverse diffusion is what would get me concerned (think sun shining on wet masonry driving the moisture inwards - which could then condense in problematic locations). The same convection that transports heat will transport moisture. Have a look at this document to get a few ideas about how to tackle the risks www.aecb.net/PDFs/Impact_of_thermal_bypass.pdf Unless you achieve some very good standards of airtightness and windtightness then a more vapour open option is advisable (even with high standards of airtightness and windtightness you may need more diffusion than an intelligent membrane can offer - to my knowledge the jury is out).

Mark

[Matthew Thompson]

Thanks again Mark, your comments and the link to the information regarding thermal bypass have confirmed the areas of concern that I have.

Initially I thought it may be overkill with taped rigid insulation but do you think that adding a layer of Pro Clima Solitex WA to the external face of the insulation would be beneficial?

It may be that we compromise the U-value to achieve a 'breathable' wall build up!

In either case I might look into putting some form of monitoring within the wall build up

[Mark Siddall]

Matthew,
I'm not the best to advise on specific products, I'd suggest that you contact Niall from Ecological Building Systems or someone else with similar knowledge of these materials.

Also, you may want to think about the buildability - how will you seal the surface that is facing the existing wall. Erect panels on the floor and then lift into place?

Some of the articles by Joseph Little may also be of interest. You can find them here: http://www.josephlittlearchitects.com/papers.html

Mark

[Matthew Thompson]

Thanks for the advice and further reading Mark.

My thoughts are that the ProClima or similar will help with any potential moisture penetration from the external face and in terms of buildability, will ensure that it's easier to seal (i.e. 1 or 2 long horizontally taped joints).

I've lapped the DPM from the floor up so that I can lay the insulation, tape the joints in the insulation and then tape the DPM to the outer face of the insulation before lifting it up into place. It's not ideal in terms of buildability but I want a layer of solid insulation (i.e. not studwork with insulation between) to limit any thermal bridging.

Currently playing around with Build desk and WUFI and SAP has a basic facility for flagging up any risk of interstitial condensation.

Matt