[GSBN] Ceiling Air Barrier

Thu Nov 20 17:05:02 UTC 2014

We detail the airtight membrane stapled to the ceiling joists for the flat ceiling sections, with ample insulation above. As noted previously, hats to seal services passing through are critical. However, if there is space, a nice detail is to allow for a 25 - 40mm (1" - 1.5") service void below the airtight barrier to avoid concern about services breaking through (we always use this detail on walls). The plasterboard/T&G  is fixed to a minimal amount of battens. This is particularly beneficial if anyone wants to add services in the future as they will be independent of the airtight membrane.

On the sloped sections, sometimes it is difficult to get enough insulation within the depth of the rafters. In this situation, we staple the airtight membrane to the underside of the rafters. Then we fix semi-rigid wood fibre boards (e.g. Gutex or Steico) through the membrane into the rafters and fix our plasterboard or T&G to the wood fibre boards. It is an effective detail and has been tested with very good results.

We have also fixed very small lath-like battens on to the membrane in situations where it needed to be a bit more robust, e.g. horizontal battens fixed through the membrane to vertical studs in a timber frame wall where cellulose was pumped in after.

I'm a big fan of Intello as it is a cellular membrane and checks vapour in both directions - so if the weather conditions mean that reverse diffusion is required, this can happen. The micro-porous membranes are not so capable.

You have mentioned being careful at penetrations/junctions - I still use the pen test every time - can you put pen to paper and trace around each section of your building. If you need to lift your pen - this is an area that needs some consideration.

I did hear an interesting point from a services engineer who also carries out a lot of blower door tests. People worry about screws/nails/staples diminshing the effectiveness of the airtight membrane. But the fixings fully fill the holes, so this is not really a concern. However, in houses where there were multiple blower door tests carried out, it was noticed that the results were getting slightly worse every time (now - the figures were probably miniscule). But it turned out that each time the membrane was put under pressure from the blower test, it moved and strained against the fixings, enlarging the holes that little more. 

Regards

Feile

feile at mudandwood.com
www.mudandwood.com

----- Original Message ----- 
  From: Derek Stearns Roff 
  To: Global Straw Building Network 
  Sent: Thursday, November 20, 2014 2:28 PM
  Subject: Re: [GSBN] Ceiling Air Barrier

  It’s wonderful to have your input, John.  I wonder what your opinion is of airtight drywall under (closer to the outside world) the tongue and groove wood ceiling.  The advantages would be greater fire resistance and greater thickness and stiffness that might suffer less damage to the airtightness during the subsequent steps in the construction process, than would an unsupported layer of Intello (unsupported until the T & G is installed).  There are disadvantages as well.  Would drywall plus Intello ever make sense?   

  Thanks,
  Derek

  On Nov 20, 2014, at 6:45 AM, John Straube <jfstraube at uwaterloo.ca> wrote:

    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

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  Derek Roff
  derek at unm.edu

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