Location and type of airtight barrier

[davidc]

I am trying to asses the pros and cons of having the air tight barrier inside or outside the main wall/insulation (that is for TF, SIP and similar build types). And also if it is better to opt for tapes and membranes or to rely on sheathing boards and mastic.

 

As I understand it a benefits of having the layer outside (i.e. close to the exterior) of the insulation is that draughts are less of a factor undermining the thermal efficiency of the insulation (perhaps less of a factor for blown high density type) and the amount of airtight barrier punctures (or potential punctures) for services is reduced as the layer is not in close proximity to most of the service runs.

 

At least one package provider I have looked at uses mastic and external boarding outwith both the TF wall and roof to form a continuous airtight layer and I wondered how this is viewed as up to now I have only encountered the tape and membrane solution placed on the inside of the wall and roof (with a service void inside).

[gravelld]

I think you're mixing two things: air tightness and wind tightness. You should have both, and the former should be on the inside of the insulation.

 

I would be wary about mastic, instead favouring taped wind tightness membranes.

[K78]

On the insulhub course they had a purple paint that guarantees air tightness. 

[gravelld]

Blowerproof.

[Dreadnaught]

1 hour ago, davidc said:

At least one package provider I have looked at uses mastic and external boarding outwith both the TF wall and roof to form a continuous airtight layer

 

Sounds like Touchwood Homes. That is their approach. I have visited two of their builds, both good. The other advantage of having the airtightness layer on the outside is the reduced chance of it being invisibly punctured during second fix.

Edited May 7, 2019 by Dreadnaught

[Ed Davies]

4 hours ago, gravelld said:

air tightness … should be on the inside of the insulation.

 

Why?

[gravelld]

38 minutes ago, Ed Davies said:

 

Why?

 

Well, more complicated answer is it depends on the insulant and the characteristics of the barrier(s) in terms of air and vapour tightness but the default is to have the air tightness inbound because it's more tolerant of the choice of materials (which may often be swapped on site etc).

[TerryE]

See our Dew Point thread and specifically

 

[Ed Davies]

A layer can be, by any building standard, airtight yet still be vapour open enough to be put on the outside of any reasonable construction.

 

For example, the Protect TF200 Thermo I have on the outside of my gable walls has a water-vapour resistance of 0.45 MNs/g which equates to a vapour permeability of 2.22… μg/N·s. AFAIK, most membranes just resist gasses, they're not picky whether the molecules they're stopping are H₂O, O₂, N₂ or whatever.

 

50 pascals for an hour over an area of 1 m² equates to 50×3600 = 180 kN·s which, multiplied by the permeability would let through 0.4 g/m²·h. Air has a density of around 1.2 kg/m³ so that would be an air leakage rate of 0.000333… m³/m²/h. Somewhat less than 0.6 m³/m²/h so, if you can seal it up tight, that membrane while quite breathable and suitable for being on the outside of the building would make a perfectly acceptable airtight layer.

 

More practically, Tom Foster has been designing houses with the airtight layer being the OSB sheathing. With careful control of the brand of OSB used (there seems to be wide variation in the airtightness and vapour permeability of OSB from different suppliers) and thorough gluing of the joints (as he seems to achieve with use of the right builders) that seems a very good approach to me; combining the windtightness and airtightness layers in a bit of the structure well clear of marauding sparkies and plumbers.

 

It's good that you can do the airtightness testing and any correction required very early in the process, as soon as the house is weather tight, while things are still easily accessible. Also, you get to actually test the windtightness, something that's not normally done.

 

That outer layer can be airtight enough without being so vapour tight that an inner vapour barrier (which would then be a de facto airtightness layer) would be required.

[davidc]

Thanks for everyone's input. I suppose because I have only seen the airtight layer implemented as membrane with tapes and on the inside never sealed sheathing on the outside I wondered if there was some fundamental reason why the latter would necessarily be inferior to the former.  If the vapour control caveat is taken into account and the sheathing and sarking joints are carefully sealed it sounds like it could be advantageous.