[GSBN] Ceiling Air Barrier

Enga Lokey enga at thelokeys.net
Thu Nov 20 14:57:24 CST 2014


Thanks all of you for the discussion. Yes and no answers are so useless. They are very unhelpful in fleshing out the subtleties and "why's" of an issue and mostly used by those that don't really know what they are taking about but are strongly opinionated. Feile, your info about multiple blower door tests is something I need to be aware of when design some future studies. Do you have any links to that info? Derek, you are not the first to suggest the drywall idea. To me it sounds like belt and suspenders. Yes your pants are doubly certain not to fall down, but when it comes to drywall and air barrier, the extra drywall is alot of extra materials, time and labour (plus mass that would need to be calculated in the engineering originally). I personally am too lazy for that. But, maybe I just don't understand something important there. Would the Intello not be a redundant and a waste of money at that point?

You guys rock! Thanks,

E

Natimuk, Australia
enga at thelokeys.net





On 21/11/2014, at 4:05 AM, Feile Butler wrote:

> 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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