[GSBN] Ceiling Air Barrier

Min Hall minhall at xnet.co.nz
Thu Nov 20 16:14:52 CST 2014

Hi Enga - And now another two cents worth. Cathedral ceilings are not that
great for the acoustics in a room and adding the dry wall(gib as we call it
in NZ) certainly helps with this. It helps muffles external noise – rain,
hail, etc. and it can also be part of an absorptive acoustic treatment –
having drywall, then some more battens with 2 inches(50mm) of acoustic
insulation and then your timber ceiling lining with ¼ to 3/8 inch
(6-10mm)gaps between each board. 

Just designing a house for a music buff and we are striking all these




From: GSBN-bounces at sustainablesources.com
[mailto:GSBN-bounces at sustainablesources.com] On Behalf Of Derek Stearns Roff
Sent: Friday, 21 November 2014 10:55 a.m.
To: Global Straw Building Network
Subject: Re: [GSBN] Ceiling Air Barrier


Enga, I was not advocating drywall plus Intello, but rather asking the man
who knows, John Straube, if he could see any reason to use both.  I was also
asking his views on drywall instead of Intello.  Intello is a great product,
from what I read.  I’m not arguing that you should change your plan, rather
I hope to hear more about comparing the options.   


I think the increased fire protection is the strongest reason to consider
drywall.  The roof/ceiling is the most vulnerable part of the building
envelope, from a fire perspective.  A secondary factor would be greater
reliability/success rate in air-sealing penetrations.  These considerations
may also seem like belt and suspenders to some.  My guess is that around
here, the cost of installing airtight drywall, with all materials and labor,
would be a bit less than installing Intello, materials and labor.  Your
situation could be different.   


I’d like to understand your comment about needing to calculate the mass in
the original engineering.  Please say more.  





On Nov 20, 2014, at 1:57 PM, Enga Lokey <enga at thelokeys.net
<mailto:enga at thelokeys.net> > wrote:

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,




Natimuk, Australia
enga at thelokeys.net <mailto: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. 






feile at mudandwood.com <mailto:feile at mudandwood.com> 

www.mudandwood.com <http://www.mudandwood.com/> 



----- Original Message -----

From: Derek Stearns Roff <mailto:derek at unm.edu> 

To: Global Straw Building Network <mailto:GSBN at sustainablesources.com> 

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





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

Thanks for any bits of wisdom.

John F Straube
jfstraube at uwaterloo.ca <mailto:jfstraube at uwaterloo.ca> 
www.JohnStraube.com <http://www.johnstraube.com/> 

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Derek Roff
derek at unm.edu <mailto:derek at unm.edu> 




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