[GSBN] Ceiling Air Barrier

[John Straube]

I am going to disagree with Ian here.  What he has described, we would describe as a roof underlayment, not an air barrier. It is installed in line with the slope of the pitch and is therefore not able to be used as an air barrier if the attic is ventilated like normal wood frame pitched roofs.   The asphalt impregnated paper material is very difficult to seal at laps and joints to make it an air barrier.  when the wind blows, the laps open up and air rushes through.  It is almost impossible to seal tight around pipe penetrations and other joints.  Even if it could be sealed, most materials like this will rip if they are airtight when a big wind gut comes along.  Thankfully they are not airtight, and work great as a temporary water protection, as stated, and continue to catch all the rain leak from holes and laps in the roofing and direct these leaks harmlessly to the outside.  But an air barrier they are not.
An air barrier at the ceiling plane level should be under the insulation and rafter, and ideally there should be some experience with testing the system in real buildings.  The Intello solution is definitely one such solution— tested with blower doors and IR cameras all the time.

On Nov 20, 2014, at 2:33 AM, Ian Redfern <ian at adobesouth.co.nz> wrote:

> Good evening Enga,
> 
> First a question  :  are you installing a fluffy blanket type insulation above and against the tongue and groove sarking   i.e. between the purlins, or if the sarking is under the rafters  the insulation the insulation will be between the rafters  ?
> 
> Assuming this scenario,  we have for decades used a heavy weight black bituminous kraft breather paper building wrap across the slope with generous laps fish scale like to shed moisture during construction  - one advantage is that these mask out any splits or loose small knots in the sarking  as well as providing the essential weather resistance at a critical stage of the build (who wants water stains on their sarking) 
> Another advantage is that it is non reflective so no glare for the building team  - the roof underlay is over the purlins and usually installed by the roofing gang  =  this is another story
> 
> 
> From: Enga Lokey
> Sent: Wednesday, November 19, 2014 20:46
> To: Global Straw Building Network
> Reply To: Global Straw Building Network
> Subject: [GSBN] air barrier
> 
> G'day all,
> 
> I have been getting conflicting info on a ceiling air barrier, so I would love it if some of you that have experience in this realm would like to weigh in on the confusion.... or solution.  I realise that a tongue and groove cathedral ceiling is a potential nightmare for air exchange. I realise that drywall can be installed airtight. It is claimed that air barrier wrap such as Intello can be used between the rafters and the ceiling lining boards to create an air barrier, thus the lining boards leak like a sieve and the roof cavity does not get air from inside. Can anyone confirm if an air barrier used in this position would be effective? With usual detailing of air-sealing at penetrations and wall/ceiling interface of course.
> 
> Thanks for any bits of wisdom.
> 

John F Straube
jfstraube at uwaterloo.ca
www.JohnStraube.com